The Aerospace industry has many varied requirements for moisture measurement. Whilst these can be for similar processes as for many other sectors, the measuring ranges can be very low for some of these requirements, and the accuracy required is more stringent. Typical applications for moisture measurement are welding, heat treatment, exotic materials, sintered metals production, fuel moisture content, aircraft landing tyre moisture content, emergency oxygen supply, blanket gases and other gases including nitrogen, oxygen and hydrogen.
Many industries that supply into the aerospace sector operate quality systems that are compliant with The NADCAP program and adhere to AS9000 guidelines (Aerospace Basic Quality System Standard). NADCAP (National Aerospace and Defence Contractors Accreditation Program) is an industry-managed approach to conformity assessment of special processes using technical experts from various sources who establish requirements for approval.
NADCAP also provides independent certification of manufacturing processes for the industry, which often covers moisture measurement. Aerospace Quality Systems (AQS) is used throughout the global aerospace supply chain to achieve NADCAP accreditation. NADCAP audits are a rigorous technical assessment of compliance with customer requirements and industry standards conducted by industry experts.
Moisture measurement requirements vary, and applications require both checks with portable equipment and continuous online measurement.
Welding for the Aerospace Industry
Many components required for the aerospace industry undergo welding processes. Moisture or other hydrogenous material in a welding arc will be broken down into hydrogen and oxygen. Molten metal will absorb this hydrogen from the arc atmosphere. Any hydrogen content of welding consumables or parent metal may also add to this hydrogen absorbed from the arc.
The principal sources of hydrogen in the arc atmosphere are moisture or hydrogenous compounds in the welding consumables (flux etc.) and oil, dirt, grease and hydrated oxides (e.g. rust) on the surface of the welding wires. Any hydrogen or water in the shield gas will also provide a source of hydrogen, and there can also be a small contribution from moisture in the ambient atmosphere.
Moisture and air contamination into the shielding gas from leaking hose fittings or bulk gas distribution systems is common and a potential source of crack-producing hydrogen in the welds. Diffusion through the hose material is also a source of contamination. In one case, the measured relative humidity of shielding gas in shop piping was as high as 98%. This moisture contamination, as a source of hydrogen, can cause delayed cold cracking in welds. Welding gas quality is controlled by checking the amount of moisture in the shielding gas by checking the dew point, which must be below -60 °C.
To check the moisture level in the shield gas for a welding process, it is essential to understand the operation of the equipment. The test should be conducted under standard welding operating conditions, including factors such as the flow rate of shield gas and the equipment set-up. The measurement is usually carried out with a portable dewpoint hygrometer by taking a sample of the shield gas. Alpha Moisture Systems’ portable dewpoint hygrometer is ideally suited to this application as the sensor is kept in dry conditions in a desiccant chamber before being exposed to the shield gas, and the test can be carried out quickly and accurately.
Heat Treating for the Aerospace Industry
Heat treating is essential for high-quality aerospace components. In the aerospace industry, stress reduction on metal parts to enhance component strength and fatigue life is critical to ensure components stand up to the extreme demands of aerospace applications. Heat treating is an essential step in the production process to enhance strength and meet aerospace applications’ precise demands and specifications.
Heat treating is the application of heat or cold to alter the metallurgical properties of a metal part. The treatment is applied to harden, soften, or relieve stress on the metal without changing the part shape. Heat treating can also improve machining, formability or restore ductility after a cold forming operation.
Meeting the heat treating needs of the aerospace industry requires the ability to treat multiple materials and use various heat treatment methods, including heating parts in controlled atmospheres and vacuum heat treating.
There are many conditions in heat treatment, and the processes are numerous. Protective atmospheres are a widespread application for monitoring moisture. These gases or gas mixtures are primarily used to prevent oxygen and water from contacting the metal when it is hot.
Protective atmospheres mainly consist of two types of gases. Firstly, those that are non-reactive to the part being treated primarily displace moisture, oxygen, and corrosive gasses; secondly, gases that can scavenge or combine with the oxygen to keep it from contacting the part. These are often also fuel gases since they burn to combine with the oxygen and add some heat to the process. The following or any combination of these gases are frequently used as blanketing gases in the first category: nitrogen, carbon dioxide, argon, and helium. The scavenging function is often accomplished using hydrogen, carbon monoxide, methane, ethane, propane, butane, lithium vapour, or any combination.
These gases can be generated on-site or supplied from bulk storage. A dew point wetter than -50 °C can cause a severe corrosion concern.
All protective atmospheres primarily prevent the metal, generally steel, from oxidising or being attacked while it is hot. Oxygen, moisture, and frequently sulphur dioxide and other corrosives react with the metal and can cause significant problems if not protected. Alpha Moisture Systems dewpoint hygrometers can monitor trace moisture levels and allow precise process control for better product quality.
A gas sample must be extracted and passed across the sensors from two sample points. One sensor can monitor the gas being fed to the furnace, and the other can monitor from inside the furnace or the furnace exhaust to indicate if any unwanted changes have occurred due to leaks, diffusion, or outgassing.
Each process is different in the atmospheres used and the materials to be treated. Some materials may outgas or produce gasses that may damage our moisture sensors, so every application must be carefully studied to ensure a proper application. Sensor exposure to heat and the specific chemistry are concerns, but every heat-treating process has the potential for using Alpha Moisture Systems analysers.
As the property specifications for metals get tighter, monitoring heat treating atmospheres becomes ever more critical. Therefore, monitoring these processes is essential to ensure the product quality meets the required standards.
Compressed air is found in most manufacturing facilities. It is used in various applications; to power air tools, operate pneumatic cylinders for automation, cool components, operate valves and other mechanical elements, purge enclosures, clean and blow-off, and pressure testing. The quality of filtration and dryness required of the air supply differs for each application. Generally, compressed air can be classified in terms of dryness and cleanliness: Shop Air, Plant Air, and Instrument Air. (Not standard industry terms.)
Shop Air is the dirtiest and contains the most moisture; it has had only rough moisture separation, usually a condensate drain on the air tank. Plant Air has often been filtered and typically passes through a refrigeration dryer. Instrument Air has been carefully filtered and dried using desiccant dryers.
Compressed air is dried in three ways — in refrigeration, regenerative desiccant, or deliquescent dryers.
The air is chilled in the refrigeration dryers to condense the water and drain it away. The incoming air is cooled by the lower temperature air leaving the dryer in the air-to-air heat exchanger. The pre-cooled air also warms the outgoing air, so the piping doesn’t “sweat.” The incoming air then passes over the cold section of the refrigeration coils, where the water condenses and is drained away. The controls are set to keep the coils above 3 °C to avoid the unit freezing. The DS1200 or DS4000 dewpoint hygrometers are ideal for monitoring this type of dryer combined with an AMT Transmitter.
Regenerative dryers use desiccant towers, which are packed with a chemical desiccant that absorbs the moisture from the air.
One of the two types of desiccant dryers, ‘heatless’ or ‘pressure swing’ dryers, has two towers or beds of desiccant alternating their duty cycles between drying the air and regenerating. The airflow at pressure is dried in one of the towers and sent to the plant. In addition, a small purge flow of the dry air is sent through the regenerating tower at reduced pressure to dry the desiccant. Drying during regeneration occurs due to the reduced pressure dropping the dew point below the dryer’s, thus removing the water from the off-line drying tower.
This cycle is often controlled by a timer, which can switch at a rate ranging from minutes to several days. More sophisticated control systems use a moisture sensor to determine the switch point on dryer demand rather than a fixed time.
The heat-regenerated dryer operates in the same fashion as the heatless model, except the towers or beds are heated by steam, gas burners or electricity during regeneration, accelerating the regeneration process. This dryer must have a cool-down cycle built into the control sequence to prevent the air from being overheated for plant use. Installation of our moisture probes in air systems using this type of dryer should have a cooling coil ahead of the flow cell.
Both regeneration dryers are best monitored by the trace moisture analyzers from Alpha Moisture Systems.
Deliquescent dryers are single-tower units charged with an absorbent salt that attracts moisture and gradually dissolves. The water is drained periodically, and the chemical is recharged every couple of months.
As businesses expand, their need for dry air also increases. Eventually, this demand may exceed the capacity of the dryer. At that point, the system begins to pump water and air. If the temperature drops in the piping, it is possible the lines will freeze. This possibility is increased if the piping is exposed to cold weather. The most likely time for freeze-ups is when usage is low, e.g., during holidays and weekends.
Preventing a single freeze-up would more than return the investment in dewpoint monitoring equipment. Additional benefits of purchasing an Alpha Moisture Systems moisture instrument include: –
- Being able to schedule maintenance on a demand basis
- Predicting the proper time to invest in other dryers
- Monitoring the start-up of new dryer installations
Typical installations of continuous monitoring consist of an AMT transmitter and a single-channel Model DS1200 or DS4000 dewpoint analyzer. If multiple dryers require dewpoint to be monitored, then a transmitter and sensor holder/sample cell should be installed at each dryer. Transmitters used in this application must be recalibrated annually unless exceptional circumstances require more frequent calibration.
Compressed Air Application example PDF.
Compressed air can be a preferred option for tool operation in hazardous areas. Pneumatic tools are safer in atmospheres where there is a risk of explosion. In common with non-hazardous areas, the air must be dry to reduce the risk of corrosion and equipment malfunction. A desiccant or membrane dryer is often the preferred option to produce the dry air, as these have no moving parts or electrical controls that might generate sparks.
Desiccant or regenerative dryers are composed of two dryer towers. The towers are filled with a desiccant material such as silica gel, alumina, or molecular sieve. The air to be dried is passed through the active tower at operating pressure. The desiccant adsorbs the moisture content before it becomes fully saturated—the flow of air switches to the other tower, leaving the first tower to be regenerated.
Membrane dryers operate on the principle of migration. The compressed air to be dried is passed over a membrane, typically a bundle of small tubes, with a high affinity for water vapour. The water vapour accumulates on the membrane surface and migrates to the low-pressure side. A dry cover gas is flowed across the low-pressure side and absorbs the water on the membrane. After absorbing the water, the cover gas is discharged into the atmosphere.
Accurate dewpoint measurement is vital for the above processes. Regenerative dryers typically switch from active to regeneration on a timer basis. In the case of heat regenerated driers, a more energy and cost-efficient option would be to use a Dewpoint Transmitter to monitor the moisture content of compressed air as it leaves the dryer. Once this reaches a certain point, the active tower switches to regeneration. Even if this option is not used, accurate dewpoint measurement is still required to check the correct working of the system.
The Model AMT-Ex Intrinsically Safe 4-20 mA Dewpoint Transmitter is ideal for monitoring the moisture content of dry air in a Hazardous Area. Powered from the Safe Area via an approved safety barrier, the 4-20 mA signal can be connected to a Dewpoint Hygrometer such as the DS4000 in the Safe Area. The Hygrometer displays moisture content in a choice of units, indicates when alarm setpoints are reached and can be re-transmitted to an external control system.
Compressed Breathing Air
Compressed breathing air, as used in scuba diving and emergency services in firefighting, is governed by international quality standards limiting chemical components and potential contaminants. These are set out in the table below: –
|Europe||USA||Australia & New Zealand|
|BS EN 12021: 2014-07||CGA G – 7.1-2011
|AA-NZS 1715: 2009|
|Oxygen||(21 ± 1)%||19.5% – 23.5%||19.5% – 22%|
|Carbon Dioxide||≤ 500 ppm||≤ 1000 ppm||≤ 800 ppm|
|Carbon Monoxide||≤ 5 ppm||≤ 10 ppm||≤ 10 ppm|
|Oil||≤ 0.5 mg/m³||≤ 5 mg/m³||≤ 1 mg/m³|
|Water Airline < 40bar||Where the apparatus is used and stored at a known temperature the pressure dewpoint must be at least 5°C below the lowest likely temperature. Where storage and usage conditions are not known, maximum dewpoint -11°C.||Dewpoint ≤50°F for SCBA use in extreme cold, dewpoint not to exceed -65°F, or dewpoint to be at least 10°F lower than the coldest temperature where the respirator is worn.|
|Water High Pressure||40-200 bar: ≤50 mg/m³
>200 bar ≤35 mg/m³
HP Charging Comp ≤25 mg/m³
|Maximum content of 100mg/m³ doe cylinders initially filled to pressure of at least 120 bar.|
NB – the above is an extract: please refer to the relevant standard for full details.
Failure to adhere to the above standards can, in extreme circumstances, threaten health and life. Excess moisture in compressed breathing air can promote the growth of moulds and harmful bacteria. In high-demand situations, air cooling due to adiabatic expansion through valves and regulators can lead to ice formation, blocking off the air supply; this can also be a problem for operations in extremely low temperatures, for example, divers or fighter pilots. Corrosion caused by moisture is dangerous if inhaled and can cause equipment malfunction; verifying dry cylinder internals can extend the period between hydraulic tests.
The air supply used to charge breathing air cylinders should be continuously monitored to confirm that it does not exceed the required moisture content. The AMT online Dewpoint Transmitter and DS1200 Dewpoint Analyser are ideal for this requirement; a low airflow is fed to the smart sensor, giving a live display of Dewpoint on the Analyser. Two alarms with relay connections can be set to warn of any developing problems.
Representative samples of filled cylinders should also be checked routinely for moisture content. The SADPmini2 Portable Hand Held Dew Point Hygrometer is the perfect instrument for this requirement allowing quick spot checks.
Moisture in welding shield gases can give rise to porosity in the weld, making the welded joint weak. Welding of specialist materials such as tungsten and single-crystal alloys is widely involved in manufacturing components for safety-critical industries such as aerospace and aeronautical.
MIG (Metal Inert Gas) and orbital welding techniques use an inert gas shield to protect the weld pool from moisture and oxygen ingress. Various gases such as Ar, Ar mixtures and He are used for shielding, depending on the materials.
These gas shields must be themselves free of moisture. The gases at the source will be dry but can pick up moisture while travelling to the weld point. Aeronautical companies specify the maximum moisture concentration of the shield gas at the MIG torch or the orbital welding head. These specifications vary between -60 °C and -40 °C dewpoint.
The shield gas may be distributed around the factory from a bulk supply in a piped system. Where this is the case, the system’s integrity should be confirmed by installing online hygrometers at regular intervals. The ideal instrument for this requirement is the AMT Dewpoint Transmitter and DS1200 or DS4000 Analyser, providing a continuous live display of dewpoint, twin alarm relays and a 4-20 mA retransmission facility.
Whether the shield gas is from a piped system or individual cylinders, there is a requirement to validate the moisture content of the gas at the MIG torch or the orbital welding head. The most common reason for failure is the deterioration of the welding set pipes.
The SADPmini2 Portable Dewpoint Hygrometer is the perfect instrument for this requirement due to its swift and accurate response.
The fundamental principle for producing pure Nitrogen (N2), Argon (Ar), Oxygen (O2), and other industrial gases is that the components of air condense from gas to liquid at different cryogenic temperatures. These extremely low temperatures usually range from -180 °C to -270 °C.
Liquefying air and separating its component gases begins with removing all contaminants. These contaminants include dirt, dust, water vapour, CO2, and hydrates. The extraction and removal of these contaminants are vital to the success of the process.
The process begins when outside air enters a compressor and is discharged to inlet separators. The air then runs through a molecular dehydration sieve absorber, often configured as a pressure swing regenerative system. As one tower is drying the process stream, the other is regenerating its desiccant to prepare for the next process gas (air) drying cycle. The moisture is monitored by installing a Dewpoint Transmitter in the dry air stream immediately after the dryers. Upstream of the cryogenic process unit (distillation tower), the dewpoint must not exceed -60 °C. Any moisture must be detected at this point in the system. Alpha Moisture Systems can advise on the appropriate Dewpoint Transmitter and Hygrometer suitable for your installation.
In the cryogenic processing unit, the gas (air) is compressed to 60 to 80 psig, then refrigerated in an expansion turbine and a series of heat exchangers. The gas undergoes a pressure drop. As the pressure is reduced, the gas cools due to the effect known as the Joule-Thompson cooling effect. The temperatures will reach as low as -195°C. If any trace moisture enters this section, numerous problems can result. The hydrates, a combination of water vapour and hydrocarbon molecules, will freeze and cause partial or even total gas flow restriction in regulators, meters, control valves, and other areas, resulting in the loss of product and system downtime.
What is CVD?
Chemical Vapor Deposition (CVD) epitaxial reactors are used to manufacture semiconductors. The wafers are processed through these reactors, where a stable solid film of single crystal doped silicon is deposited. This process results from the decomposition of gaseous silicon compounds in the presence of the heated wafer. Any water vapour or oxygen in the reactor will contaminate the film and reduce the yield.
The reactors are operated at pressures ranging from high vacuums to atmospheric pressure and can be pre-purged at pressures to 50 psig. Vacuum systems are very susceptible to contamination from the atmosphere. Today there are atmospheric systems that are not as contaminant prone. There are three popular types of reactor geometry: horizontal reactors, pancake or vertical reactors, and barrel or cylinder reactors. These are all batch processes. The process gases include HCl, diborane, arsine, phosphine, hydrogen, nitrogen, and argon. Silicon tetrachloride and trichlorosilane gases are sources of silicon, with hydrogen as a carrier gas. HCl is used to clean the wafer. Diborane, arsine and phosphine are dopant gases used in concentrations of only a few parts per trillion in hydrogen.
Why is moisture important?
The process of depositing these films is controlled by mixing and controlling the mass flow of these gases. The yield is high if the gas exposure and temperature control can be accomplished with a high degree of precision. The measurement of moisture and oxygen will become more important as the engineers devise tighter specifications for the gas mixtures.
How do we monitor this Process?
Although we cannot monitor the reactive gases, it is essential to monitor the purge gases for both moisture and oxygen. The wafers are loaded into the reactor and purged with N2 to expel moisture and oxygen introduced during loading the wafers. Monitoring the supply gases (N2, Ar, H2) will indicate the overall quality of the house gas and warn of a bad gas load. Monitoring the exhaust of the purge gas will tell when the process can move into the reaction phase. In addition, monitoring the exhaust will track the health of the reactor by alerting the operator to a leaking system.
Furnace manufacturers often place responsibility for supplying the furnace with gases of certain purity on the customer; Alpha Moisture Systems can assist the semiconductor manufacturer with the moisture measurement instruments and sampling systems. Other CVD reactor manufacturers are incorporating gas monitoring equipment into their control systems to enhance quality control and productivity.
For decades, the electrostatic powder application process has been used to finish metal parts. High transfer efficiency and the resulting cost savings from reduced waste plus the durability of the finish make powder coating the first choice for household appliances, automotive components, and furniture.
One of the benefits of powder coating systems is the ability to reclaim nearly all the overspray powder. The ratio of coating material applied to an object divided by the amount of coating used is called transfer efficiency. The electrostatic powder application process achieves transfer efficiencies near 99% through a reclamation system. Using liquid paint, a conventional “spray gun” can have transfer efficiencies as low as 30%, depending on the operator’s skill.
Another advantage of the electrostatic powder coating process is the durability of the finish. Although the texture is not as smooth as many liquid paints, powder coatings provide an extremely durable, scratch-resistant finish.
The coating applied during the electrostatic process is typically an organic powder. The powder is sprayed out of the applicator; it travels through an electrostatic field. Parts targeted for coating are usually grounded by hanging them from an overhead conveyor. The static charge in the airborne powder is attracted to the component, and enough residual charge remains to prevent the powder from falling off. The part continues along the conveyor line into an oven. The powder coating melts, flowing into a resilient, colour-fast coating. A large volume of components can be coated if the conditions within the system are appropriately monitored and controlled. Large quantities of powder must course through the entire system smoothly and without interruption in any continuous coating application. Large hoppers store bulk powder for transfer into the system. They typically use compressed air to fluidize the powder, simplifying its transfer. Powered by compressed air, a venturi pump usually transfers the fluidized powder into the system.
Fluidized bed hoppers, venturi pumps, and other pneumatic components of a powder coating system demand a dry compressed air supply. If the compressed air becomes saturated with water, the wet powder will clog the entire system in minutes, forcing the whole coating system to grind to a halt.
Powder coating systems using either refrigerated air dryers or desiccant air dryers would greatly benefit from Alpha Moisture Systems Online Dewpoint Hygrometers and Sampling Systems that continuously monitor compressed airline moisture content. The continuous monitoring offered by the AMT Transmitter and DS1200 Hygrometer provides advanced warning of problematic moisture levels, avoiding disruptive blockages and allowing time to take preventative action.
Moisture analysis is critical when filling industrial gas cylinders to ensure that the product delivered to customers is within specification and to confirm that the cylinders are maintained in a dry condition, thus extending the period between routine cylinder testing.
The main industrial gases, nitrogen, oxygen, argon, helium, mixed gases for welding, etc., are generally filled by connecting the cylinders, typically 15 in a pallet, to a manifold. The various gases are supplied to the manifold from bulk storage and can be selected as required. A sample pipe connects to the manifold close to the cylinders. A gas sample is extracted for analysis at an instrument panel incorporating several analysers, including a dewpoint hygrometer. The hygrometer is typically configured for concentration in ppm(v). The gas sampled from the manifold will give a combined moisture level for the group of cylinders. If this is below a predetermined level, the filling of the cylinders can proceed.
The AMT Dewpoint Transmitter and DS1200 Analyser are ideal for incorporation in the instrument panel; the 4-20 mA retransmission signal can be connected to a central DCS.
When the sampled gases show too high a moisture level, the group of cylinders will be removed for an investigation to identify the wet cylinder using a portable dewpoint hygrometer, such as the SADPmini2 Handheld Portable Dewpoint Hygrometer.
After filling, the gas from the cylinders is again sampled to confirm it is within specification.
Random cylinders are sampled using a portable hygrometer as a QA check. The Model SADPmini2 is ideal for this job due to its rapid speed of response and excellent portability.
Medical gas cylinders are filled the same way but with special dedicated facilities and are subject to more stringent testing regimes. The European Pharmacopoeia specifies that an Electrolytic Hygrometer, such as the Model EDM Dewpoint Hygrometer, should be used for medical gases. It is common to use a portable hygrometer such as the SADPmini2 and periodically standardise it against an Electrolytic Hygrometer.
The largest freeze dryer market for process moisture analysers is the pharmaceutical industry. Its products are expensive and have the highest potential for loss if a process is not entirely under control. A specific formulation can be tested and approved by the compliance agencies but marketing the product can be problematic if it has a short shelf life. Freeze drying addresses this problem in many products, such as injectable drugs, allowing these medications to be more readily available.
Freeze-drying is a low-temperature dehydration process involving freezing the product, lowering the pressure, and removing the ice by sublimation. For over 50 years, manufacturers have benefitted from freeze-drying; however, until recently, the process has mainly been controlled manually. As manufacturing becomes increasingly automated, the process requires a new approach to achieve the desired product specification. The most critical of these is determining when the product is dry, which is essential for maximising the output of a freeze dryer. A dewpoint hygrometer shows when the desired moisture level is achieved, signifying the process is complete.
When the product is a water-based material, Alpha Moisture Systems trace moisture analysers can provide the data necessary to give endpoint determination. Even if the material requires the removal of a mixture of organic solvents and water, our analysers may provide a relative process reference point to indicate when the solvent is removed. In this Application Note, we’ll assume the solvent removed is water.
Extending the shelf life of a product often involves maintaining a product’s critical physical or chemical property while removing its moisture. The first step is to freeze the product to stabilise its structure. The material is loaded on shelves in a vacuum chamber. Coolant is pumped through coils in these shelves to freeze the material. Because the various chemical components in the product may have different freezing or eutectic points, the product temperature must be driven lower than the lowest freezing point before the drying phase can begin. Once that point is reached, the vacuum pump is started.
The ice begins to sublime as the vacuum is drawn down to roughly 0.1 torr. The water vapour is drawn off through the vacuum pump and collected on a condensing surface near the product at -40 °C. This phase, called primary drying, can last several hours to several days. Throughout this phase, moisture evolves from the material at a steady rate, indicated on our dewpoint hygrometers or moisture analysers as a stable dew point temperature, usually between -35 °C and -65 °C dew point.
The dewpoint falls at the end of primary drying, indicating the last of the water has sublimed and been drawn off by the vacuum pump. The operator can now raise the shelf temperature to allow the product to warm up without being spoiled. If the freeze-drying is complete, the container can be sealed and removed from the chamber. In some instances, a secondary drying process is required; dewpoint hygrometers may also be used at this stage, depending on the circumstances. Our experienced Application Engineers and authorised representatives are always available to assist you with your project.
Many manufacturing processes and laboratory procedures are conducted in modified atmospheres to achieve the required quality. Creating a modified atmosphere requires maintaining a controlled environment that differs from ambient conditions.
Some processes are sensitive to oxygen and water vapour. For example, various stages of lithium battery production are susceptible to water vapour. Unless the production phase is carried out in an extremely dry environment, a significant decrease in productivity will result. Other processes require a physical barrier to protect operators from direct contact with hazardous materials, vapours, or dust associated with a process.
Glove Boxes are sealed enclosures incorporating a pair of gloves that enable the operator to handle components and perform a process while maintaining the modified atmosphere inside the chamber.
The assembly and encapsulation of semiconductor devices and integrated circuits often utilise Glove Boxes. Encapsulation involves sealing an element or circuit for mechanical and environmental protection. A Glove Box provides a means of performing these operations while maintaining the dry environment essential for a tight seal.
Glove Boxes vary in size and shape, but they are all characterised by apertures that allow people to insert their hands into the box. Gloves with extended cuffs are tightly sealed around both apertures on the front panel. When the operator inserts their hands and arms into the box, the gloves allow the product to be handled while the integrity of the internal conditions remains intact. When designing or selecting a Glove Box, consideration must be given to the space needed to perform the procedure, which is always greater than required for the finished product. Manipulating the product must not compromise the integrity of the modified atmosphere.
For processes that require a dry environment, Glove Boxes will often be purged with dry nitrogen, referred to as a blanket gas. Speciality gas suppliers produce blanket gases for such applications.
The gas is purged through the Glove Box until enough of the water vapour has been removed. Alpha Moisture Systems Dewpoint Hygrometers can be used to determine when the glove box is sufficiently dry to begin the process. The Dewpoint Hygrometers continuously monitor the conditions inside the Glove Box to warn the operator if the moisture level becomes too high during the procedure. A high moisture level would indicate a leak or the loss of purge gas flow into the Glove Box
Even with all the plastic and polymer advances, metal products still dominate our lives with their usefulness, strength, and durability. Today’s metal products used in everyday life are superior to those just a few years ago. This advancement is due mainly to the improved methods of heat treating that add strength and durability to those metal products.
Many of the new properties of common metals like steel, aluminium and copper, or more exotic metals such as magnesium, nickel, tin, lead, titanium, uranium, precious metals, and new alloys are produced by heating them in controlled atmospheres. For example, a moist nitrogen atmosphere is used in “nitriding” to add strength and toughness to the outer layer of tool steel for longer-lasting cutting surfaces. Other atmospheres can remove impurities from or change the physical behaviour of the formed metal parts.
Often the atmosphere is also used to prevent the metal from corroding when hot since the speed of the corrosive reaction is significantly increased at the very high temperatures inside a furnace. This atmosphere must be dry and oxygen-free to do its job. The gas, or gas mixture, must also be dense enough to prevent the intrusion of oxygen and water vapour into the openings of the furnace as the product is inserted and removed from the furnace. Every aluminium drink can is made from metal that has been heat-treated dozens of times. The aluminium ingot is repeatedly rolled. After every pass between the rollers, the metal is heated to relieve stress, so the next time it is worked, it won’t crack from being too brittle.
Precise control of the atmosphere in heat treatment processes helps maximise productivity and minimise energy and gas consumption.
Gases used in protective atmospheres are classified as non-reactive blanketing or scavenging gases.
The non-reactive blanketing gases do not react with the heat-treated component and serve to displace moisture, oxygen, and corrosive gases. Examples include Argon, Carbon dioxide, Nitrogen and Helium, used singly or in combination.
Scavenging gases scavenge or combine with oxygen preventing it from contacting the component. These are frequently fuel gasses, burning with oxygen and adding heat to the process. Examples include Hydrogen, Ethane, Methane, Carbon monoxide, Propane, Butane, and Lithium vapour.
The primary purpose of a protective atmosphere is to prevent the metal, commonly steel, from oxidising or being attacked while hot. Oxygen, moisture, sulphur dioxide and other corrosives will ruin the metal component if these two techniques are not deployed correctly and monitored diligently.
Alpha Moisture Systems Dewpoint Hygrometers and Transmitters can continuously monitor trace moisture levels allowing precise process control for optimum product quality. The gas must be extracted from the furnace and passed across the sensor, and the Dewpoint or ppm(v) displayed. Monitoring the inlet and exhaust gas is recommended to detect unwanted changes due to leaks, diffusion, or out-gassing.
Each heat treatment process is different regarding the atmosphere used and the material and format of the heat-treated component. Some materials may out-gas or produce gases that damage our sensors, so every application must be explored in advance to avoid instrument damage. Sensor exposure to heat and the specific chemistry of your process are considerations that our experienced applications engineers will review and aim to mitigate during the discovery stage of your enquiry.
As energy costs and material specifications continuously increase, monitoring heat-treating atmospheres becomes increasingly necessary.
The two measuring points of interest are shown below: –
Compressed air used in medical applications for patient care must be kept clean to the point of being almost sterile. There are also concerns regarding contaminants such as water vapour. The presence of water in medical breathing air can interfere with the operation of critical medical equipment and, over more extended periods, lead to corrosion. Excess humidity can also promote the growth of harmful fungi and bacteria and lead to disease.
The control of medical gases in the UK is regulated by the Health Technical Memorandum HTM02, which states a maximum allowable dewpoint in medical gas of -46 °C, or 67 ppm(v). Regular testing should be carried out at various points around the distribution system to ensure these limits are met, and HTM02 clearly states that an electronic dewpoint meter should be used for such measurement.
15.146 The plant test point and a representative sample of terminal units distributed throughout the pipeline systems should be tested for total water content.
15.117 An electronic dewpoint meter should be used in preference to water content measurements.
HTM 02 also states:
7.41 The dryer control system should include a dewpoint hygrometer and display with a minimum accuracy of ±3 °C in a range from -20 °C to -60 °C atmospheric dewpoint, with an alarm set point of -46 °C. The Dryer output should be monitored continuously to warn of potential dryer failure.
The AMT Dewpoint Transmitter and DS1200 Analyser are ideally suited to this use, giving a digital display, twin alarm relays and a 4-20 mA retransmission output.
The ideal instrument for routine testing of terminal units is the SADPmini2 Hand Held Portable Dewpoint Hygrometer.
In electricity generation at power plants, the turbine generators produce a large amount of heat as a by-product. Cooling the generators allows for a more efficient generation of power and prolongs the lifetime of the generators and associated equipment.
Hydrogen is frequently chosen to remove heat from high-power generators. Hydrogen has a high heat capacity and removes excess heat efficiently. Hydrogen also has a very low viscosity (or windage), thus allowing higher capacity operation of the generators while maintaining efficient cooling.
The hydrogen must be kept free from moisture for several reasons. Moisture increases the viscosity of the hydrogen and decreases its ability to carry away excess heat. In addition, moisture deteriorates the seals on the rotating shaft, causing leakage of the explosive hydrogen. Leaking seals require a costly generator rebuild to replace. Finally, moisture increases the danger of arcing the high voltage (up to 12,000 V or more) high current generators. Such arcing would not only seriously damage the generators but could also ignite the explosive hydrogen in the generator enclosure, causing severe damage and life-threatening injury to plant operators.
Typically, the hydrogen gas circulates in a closed loop through a water-cooled heat exchanger and a molecular sieve or desiccant dryer before returning to the generator enclosure. To effectively detect any leakage in the water cooling system or the gas system, an Alpha Moisture Systems moisture probe should be installed in the stream between the cooler and the dryer.
A second sensor should be installed after the dryer to monitor its performance. The probe should be mounted after the dryer in the recirculating loop for single-channel installations. Typically, the moisture content after the dryer ranges from -30°C to -100°C Dewpoint. If the moisture content exceeds a critical point, an alarm relay can be closed, prompting corrective action or triggering a safety control device.
Alpha Moisture Systems intrinsically safe aluminium oxide transmitters may be used to monitor the moisture in hydrogen. However, precautions are necessary for hydrogen applications; please consult our technical team for guidance.
Generator manufacturers usually acknowledge the need for installing moisture sensors in this critical application and typically supply them already fitted. However, the maintenance procedures of these OEM moisture systems are not well documented, and inaccurate readings can give the operator a false sense of security. Therefore it is essential to have the appropriate equipment for the dew point range and maintain it with regular calibration. These issues can often be better addressed by fitting specialist instruments supplied by Alpha Moisture Systems.
Measuring Moisture (dewpoint) in Hydrogen – Application Example PDF.
When first extracted from underground wells, natural gas is usually saturated with water and heavy hydrocarbon components. There are several processing stages before a standard is reached for dry gas fuel which can be stored, transmitted through the pipeline network, and burned by end-users. During transmission and storage, any excess moisture in the natural gas can condense as liquid or, in cold climates or under Joule-Thompson cooling due to gas decompression, form as ice. When liquid water is condensed in the presence of methane at high pressure, solid methane hydrates can be formed. Once formed, hydrates can block pipelines and processing equipment.
Moisture content in natural gas also reduces the product’s heating value (BTU) and its value. Water can react with carbon dioxide and hydrogen sulphide to form acids that corrode the pipeline and reduce its economic life.
Local tariff sets maximum allowable moisture levels at points of custody transfer of natural gas between existing and future owners. Typical specifications are 7 lbs/mmscf in the USA or -8 °C dewpoint at 70 bar in Europe. Other countries have similar specifications, and there are revenue loss penalties known as ‘shut-ins’ for suppliers who fail to comply.
Any increase in moisture content above the required levels must be detected as soon as possible before any pipeline damage or financial loss due to tariff noncompliance.
Where gas is transported in long pipelines, covering even trans-continental distances, monitoring stations are located at intervals along the pipeline to warn of any moisture ingress.
Any increase in moisture content of the gas should be detected as quickly as possible to enable processing conditions to be modified to avoid the above problems and ensure that the gas is kept within specification.
The rapid speed of response of the Alpha Moisture Systems’ AMT-Ex Intrinsically Safe Dewpoint Transmitter makes it the ideal instrument for this purpose. Combined with the DS4000 Dew-point Hygrometer unit, it provides a very powerful system incorporating features such as switching between units of measurement and calculating pressure dewpoint.
Sampling natural gas for moisture analysis requires special care. Alpha Moisture Systems manufacture Dewpoint Sampling Systems specifically for this application: The SS-NGH Dewpoint Sample System is highly recommended for sampling clean sales gas.
Glycol carryover from TEG contactors is an ever-present problem. Still, if the sample handling system and measuring sensor are correctly designed and installed, their effects can be reduced to an absolute minimum. Because the buffering effects of glycol contamination apply to all components of the sample handling system, any carryover should be removed from the sample as soon as possible after the take-off point. The Alpha Moisture Systems TEG Sample System is designed to overcome these problems when installed close to the sample take-off point.
Nitrogen and Oxygen generators are increasing used as an alternative to bottled gas in high-demand applications. Onsite generation can offer greater flexibility, reliability of supply and economic advantages.
Dewpoint measurement is critical at two points in the process:-
- Measuring Dewpoint after the refrigerant dryer provides a warning if the dryer malfunctions, protecting the generator from damaging condensate.
- Measuring the Dewpoint of the Nitrogen or Oxygen generated is an essential part of quality control.
Continuous online measurement of Dewpoint is recommended in this application using a Dewpoint Transmitter such as Alpha Moisture Systems’ Model AMT and a Dewpoint Hygrometer such as Model DS1200. Ideally, the gas should be sampled at atmospheric pressure with a flow rate of 1-5 l/min; we can identify and supply ancillary items such as a sensor holder, valves and regulators.
Many Nitrogen and Oxygen Generator manufacturers now incorporate Dewpoint Measurement as part of their original equipment supply; Alpha Moisture Systems can provide the engineering support required to add this feature to your equipment.
Our latest Portable Dewpoint Hygrometer Model SADPmini2 is recommended for system functionality periodic spot checks.
Moisture Measurement (dewpoint) in Nitrogen Application PDF.
Handling and treating water for drinking, industrial processes, or other uses, as well as wastewater treatment, requires sophisticated engineering and considerable investment. Purifying freshwater for human consumption requires particular attention to removing various naturally occurring bacteria and organic substances.
Ozone (O3) is one of the most powerful oxidisers available, yet it produces only O2 when it has done its job. It kills all dangerous bacteria and decomposes many organic substances that are otherwise difficult to remove from water. Ozone treatment of water is one of the most effective methods for preparing clear freshwater. This treatment is especially critical for surface water (water obtained from lakes and rivers) that has been more likely contaminated with unwanted micro-organisms and chemicals. Other treatment methods, such as chlorination, often require more significant exposure times to remove these same toxins and bacteria.
In addition, using ozone generators on-site avoids the hazards of handling corrosive chlorine gas.
Over-chlorinating water produces a foul taste; this is not a problem in ozone purification systems. Another concern with chlorine treatment is the production of carcinogenic chlorocarbons (especially trihalomethanes (THMs)) restricted in drinking water.
There are many water treatment plants in operation using ozonation for purification. Many of these may have no moisture measurement system, potentially decreasing plant efficiency and increasing the cost of freshwater. Ozone water treatment facilities are growing in use worldwide.
Ozone is very simple to prepare continuously by electrolysis of oxygen (O2) in a chamber. Air or pure O2 is passed through a cell containing two electrodes. A high-voltage (12,000 V) discharge between the electrodes produces ozone from the oxygen.
The supply gas for ozone production (air or oxygen) has a recommended maximum dew point of -50 °C. Ozone generation is improved markedly by lowering the dew point to -70 °C. Moisture measurement is desired in water treatment plants incorporating ozonises because ozone generation is adversely affected at higher dew point temperatures.
Measuring trace moisture in compressed air is one of the most widespread applications for Alpha Moisture Systems instruments. Compressed air is referred to by many names, depending on how it is used; this article explores a compressed air system called ‘Pad Air’.
A typical pad air system is configured just like many instrument air systems. It consists of a large compressor with one or more stages of dryers. ‘Pad Air’ comes from using the air as a pad to protect a container from a corrosive substance.
One of the most common examples of padding is to protect rail car shipments of chlorine to paper mills. The chlorine used to bleach the paper reacts with water to become hydrochloric acid — a highly corrosive substance. Even the water in the air is enough to cause corrosion problems. Since the rail companies do not want their tank cars to corrode from the inside out, they require mills to pad, or backfill, the space above the chlorine with a dry gas as the liquid is drawn off; this prevents moisture from the outside air from entering the tank, significantly reducing corrosion.
Pad air is used for many different chemicals and vessels, so the process conditions can vary greatly. The air system may be a branch of the primary plant air system or a discrete system with compressors and dryers. The piping to the point of use can vary from a few feet to several hundred feet. The flow rate might be very high, as when a tank is first connected down to zero flow when nothing is being drawn from the tank. The pressure dew point at the dryer exhaust is -40 °C or lower for most systems.
These variables could make the installation different for each application, but the recommended installation is the same for all pad air systems. Because the air will contact a potentially corrosive material, certain precautions must be followed to protect a probe installed. First, we must have a sample system that prohibits measuring air contaminated with corrosive gas(es) through diffusion. While the system diagrammed below is virtually the same as an instrument air application, it must be duplicated precisely if any of the air in the system is used for padding. This configuration is designed to minimize the possibility of corrosive gas diffusion, which means no in-line Dewpoint Transmitter installations. The sample point should be close to the dryer and far from the point of use as possible.
Another factor in the installation is the location of the intake compressor itself. If corrosive gases are in the air being compressed, these vapours can contaminate the air stream with a corrosive effect. So, check the prevailing wind direction if outside air is used for the inlet. It should be upstream of any corrosive gas exhaust for best results. Do not hesitate to contact an Alpha Moisture Systems Application Engineer if you have questions.
The need to efficiently dry polymers before processing increases as polymer processing advances.
Before processing, the maximum allowable moisture content is typically 0.005% for a moisture-sensitive polymer such as PET. The usual way of achieving such low moisture levels is to use a dehumidifying dryer system. These systems heat the dried air, pass it through the polymer granules in the feed hopper and recirculate the wet air through a cooling unit back to the desiccant unit. There are many suppliers of dehumidifying dryers, and their features and mechanical function differ significantly from one supplier to the next. Despite these differences, the drying process is fundamentally the same.
Polymer granules are contained in a hopper with a material inlet at the top and an outlet at the bottom. The extrusion or moulding process is fed from the hopper outlet, the level in the hopper being maintained by the addition of ‘wet’ granules at the hopper inlet. Hot, dry air is blown into the drying hopper through its dry air inlet, directed up through the granules. As the granules are heated, they shed moisture, and since the airflow is hot and dry, it has a high capacity to absorb this moisture, carrying it out of the hopper through the return air outlet. The return air is cooled (typically to less than 40°C) before entering the circulation fan that blows the air around the system. After leaving the fan, the air passes through a desiccant column that absorbs the moisture carried over from the granules. It then passes through a heater before re-entry to the hopper dry air inlet. The system must be sealed from the ingress of ambient air, which would significantly reduce the effectiveness of the drying process with its high moisture content.
Moisture from the polymer is ultimately absorbed into a desiccant column. The desiccant column would eventually become saturated, so, periodically, regeneration occurs by blowing very hot ambient air through the column, which is vented off to the atmosphere. It is mainly in this area where the various types of dryers differ. Some smaller dryers use a single desiccant column, and thus drying is interrupted during the regeneration phase. Most larger systems use two or more desiccant columns and achieve continuous drying by alternating columns from absorption to regeneration.
To efficiently remove moisture from the material being dried, it is essential to achieve very low moisture content in the drying air. A dewpoint temperature lower than -40°C is usually considered acceptable. The only accurate performance measure for the dehumidifying unit within the drying system is to directly measure the moisture content of its dry air output. Alpha Moisture Systems offer Dewpoint Hygrometers for this purpose.
Use of Dewpoint Hygrometers With Dehumidifying Dryer Systems
Different installation practices may apply depending on the drying system’s configuration.
Continuous Online measurement
Where a drying system is used continuously, online measurement is recommended. Several instruments are available for this task. Features such as configurable moisture alarms with digital or analogue output signals are available, together with digital or analogue instrument displays. Instruments are available in single-channel or multi-channel configurations for installations where several dryers are monitored. The dewpoint hygrometer should be installed with the sample inlet taken after the dryer’s desiccant column and its outlet connected before the circulation fan. The pressure difference between these points will ensure flow through the sample pipework (See schematic diagram).
Continuous measurement can also offer significant benefits in optimising the energy efficiency of dehumidifying dryer systems. The instrument’s output signal can be linked to the dryer control system to initiate desiccant column changeover. The adsorption column remains on process until all its adsorption capacity has been used. This point is detected as the outlet dewpoint rises above a predetermined level. This facility minimises the frequency of desiccant column changeovers, thus saving the energy used during the unnecessary regeneration cycles associated with a fixed cycle time changeover.
Portable and spot check measurement
Where a dryer is not being used continuously, a portable instrument can provide a more economical alternative, enabling the user to make dewpoint measurements on many dryers. Portable equipment is also ideal for commissioning, field service, and verifying readings from other dewpoint hygrometer installations. The sampling points should be located at the outlet of the desiccant column and the inlet of the circulation fan (See schematic diagram).
The moisture content, commonly expressed as Dewpoint, in the compressed air must not exceed a specified level to avoid the formation of defects known as ‘fisheyes’, minor circular defects with a tiny crater in the centre, which can ruin the finish.
Line pressure can be reduced to atmospheric pressure using a pressure regulator, enabling continuous measurement of atmospheric Dewpoint with the AMT Dewpoint Transmitter fitted in the sensor holder/sampling cell.
Monitoring dryer performance and moisture ingress by sampling the compressed air supply after the dryer and at the point-of-use are strongly recommended.