Dewpoint Meters – Guide to Typical Applications

Shaw dew point meter


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Rather than metal-to-metal contact as in traditional bearings, some applications – particularly very high end applications, use compressed air to provide a ‘cushion’ on which to support a rotating shaft. Any possibility of condensation must be avoided, as this would cause failure of the bearing: hence the need for dry air, and the need for a moisture meter. Usual moisture:<-10°C.

(Or Dry Boxes) are enclosures with – usually – two access holes fitted with Neoprene gloves so that operations can be carried out in the protected environment inside the box. These operations vary widely and include the handling of dangerous or hygroscopic chemicals, assembly of transistors and other semiconductor devices, and numerous research applications. In many of these applications it is essential that the atmosphere inside the glove box is dry. The atmosphere is often air – but it may be nitrogen or any one of several other gases depending upon the application. The best way of installing the moisture monitor is to fit the sensor in the top of the box (where the moisture level will be at its highest – as water vapor rises), with a loose length of cable inside so that the sensor can be moved around to look for wet areas – caused by uneven flow of the purge air or gas. The alternative is to fit the sensor in the exhaust line, when it will give an average reading of the box moisture. Usual moisture: generally drier than -60°C.

waveguides.jpegWave guides are tubes that are used in place of cables for the power connection to radar and microwave telecommunication dishes. The tube is tuned to the frequency of the power and, because of the high voltages involved the tube must be continuously purged with dry air. All the European early-warning radar stations are equipped with Shaw Meters. Usual moisture: <-60°C.

fizzydrink.jpeg(Polyethyleneterapthalate) P.E.T. is a plastic developed specifically for making bottles for ‘fizzy’ drinks such as Coca Cola. With an external coating to improve its porosity to oxygen it is also used for beer. It has become very popular as it reduces weight and hence transport costs, and the bottles are regarded as disposable. Its only real problem is that it is extremely hygroscopic – so hygroscopic that it is inevitably too wet to use (producing opaque or porous bottles) when it reached the drink manufacturer. The first part of the bottle making process is therefore to dry the P.E.T. by passing warm dry air through it prior to the actual molding process. There is no equipment which will measure the moisture content of the plastic, but our instruments are used in hundreds world-wide to check that the dryer producing the dry air is working correctly. Usual moisture: <-40°C.

shopping.jpegDate marking (sell-by or consume-by) of foods is increasingly important. There are many ways of making such marks on food packs, and one of the latest is with a laser beam. A high-power beam of laser light shines through a stencil and ‘burns’ the date onto the surface of the packet. The advantage is the speed – up to 25 packs per second. The laser is switched on and off at high speed by a special device which relies on a flow of dry purge air to operate correctly. The switch costs about £600 and is damaged if the air is too wet: hence the Shaw Moisture Meter. Usual moisture: <-50°C.

As can be seen from the ‘Black list’ , Chlorine is one of the few gases which attack the sensor, even at very low concentration. However, many hundreds – if not thousands – of Shaw instruments are used to measure the moisture content of the compressed air used to pressurize and purge tanks and piping used for the storage and transporting of chlorine liquid. This air must be dry, or it will become highly corrosive when mixed with the chlorine vapor, and attack the materials of the pipes and tanks. Usual moisture: <-50°C.

Dry compressed air is used in many systems for the transport of powdered or granular materials, such as soap or detergent powder, hops and grain. The air must be reasonably dry or the material will absorb moisture from it, and then will not flow properly, with the system becoming blocked. Usual moisture: <-40°C.

vessel.jpegLarge vessels – for boilers, transformers, storage tanks, etc. – are usually tested hydraulically for leaks and pressure security. Following this they must be dried to prevent rusting, and this is usually carried out by passing warm dry air through them (or, sometimes, nitrogen). During the first part of the drying process the exhaust air from the vessel is, of course, very wet, and no measurement is needed. Towards the end of the drying, which may take many days, it is necessary to measure the moisture in the exhaust. This may be done with an in-line instrument with remote sensor located in the outlet port of the vessel, or with the Shaw Automatic Dewpoint Meter by simply putting a length of flexible pipe into the outlet, so that some of the air passes through the instrument head assembly. Usual moisture: <-20°C.

pool.jpegThis is another application – see ‘Chlorine Padding Air’ – where our instruments are used in an application which is potentially hazardous to the sensor. Ozone is increasingly being used instead of chlorine to purify the water in swimming pools: the pool water is much more pleasant, and does not irritate the eyes. The ozone is simply bubbled into the re-circulating water. Because the ozone is a very powerful oxidizing agent, and contact with moisture before it actually enters the water will result in severe corrosion of the gas generating plant, the ozone is blown into the water with dry air. Usual moisture: <-50°C.

All applications requiring a supply of dry air obviously need a compressor and dryer. There are three types of dryer in common use:

1. Refrigeration dryers. These simply cool the air, separating the resulting condensation. They are only capable of producing a dewpoint temperature of about +3°C at a typical operating pressure of 7 bars (equivalent to about -20°C dewpoint at atmospheric pressure) and so are only suitable for installations where no part of the system will ever be below freezing point.

2. Pressure-Swing dryers. These have two chambers filled with desiccant which are alternately in-line, and drying the air flow, or being regenerated. The changeover cycle is quite short – usually no more than 5 minutes or so – so that the desiccant is only absorbing a small amount of water vapor at the operating pressure which is usually about 7 bars. About 10 to 15% of the dried air is reduced to atmospheric pressure and passes through the chamber to be regenerated: because the pressure is now 7 times lower, and the purge air is very dry, the desiccant is regenerated. Depending on the design, these dryers may produce air with a pressure dewpoint of about -40°C or better than -70°C. The ‘hidden’ running cost is the purge air, which means that the compressor must be 10 to 15% larger than would otherwise be needed.

3. Heat regenerated dryers. Like the pressure-swing type, these have two chambers filled with desiccant. In this design the cycle time is more usually 4 to 8 hours, resulting in the desiccant being heavily loaded with moisture. A small proportion of the dried air, or atmospheric air is used to purge the chamber under regeneration, while heaters raise the temperature of the desiccant to 250°C or more to drive out the moisture.

All types of air (or gas) dryer have one thing in common: they need a moisture meter! A dryer without a moisture meter is like a car without a speedometer – there is simply no way for the user to know if it is operating correctly, and it is usually far too late when problems show up in the factory. A simple monitor and alarm is sufficient for the refrigeration dryer: the pressure swing and heat regenerated types can both have their operating cycle controlled by a dewpoint monitor with substantial cost savings being made – often saving the cost of the instrument in a few months. Usual moisture: depends on dryer type.

tower.jpegUsed for operation of pneumatic instruments in control rooms and similar situations. Excess moisture can result in condensation or ice formation, and the loss of control of the instrument readings leading, at worst to shut-down of the complete plant. Usual moisture: <-40°C Dewpoint.

bottles.jpegPneumatic tools are widely used in assembly work. They rely on a supply of clean dry air. Excess moisture can lead to a high rate of tool wear and failure. Usual moisture: <-40°C.


High quality paint spraying, particularly of motor vehicles, relies on clean dry air. Excess moisture will result in a poor surface finish, leading to expensive re-spraying. Usual moisture: <-40°C.

simulator.jpegThe simulators used for training aircrew have oxygen systems as in the real aircraft they represent. Because they are operating on the ground, and for safety reasons, air is used instead of oxygen but this makes no difference to the application. To keep the storage cylinders small, the gas is stored at high pressure: if the moisture content is too high, ice may form in the small orifice of the pressure regulator and cut off the supply. Usual moisture: <-50°C.

firefighters.jpegThis application is very similar to the aircraft simulator application previously described: to avoid the risk of ice blocking the regulator orifice, the moisture content must be low. Usual moisture -50°C or drier. (N.B. This does not apply to breathing air for diving – the water in which the system is immersed acts as a huge heat-sink, and prevents ice forming.)

Dry, sterile air is used widely in hospital operating theatres. The main uses are in pneumatically driven operating tools – where they are preferred to electrically operated tools because they are easily sterilized. Usual moisture: <-40°C. * Separate report available.

sorting.jpegYes, the Shaw Moisture Meters are used in banks! Many types of sorting and handling machinery are operated by compressed air, and check sorting machines in the major clearing banks are just one example of a system which relies on a continuous supply of clean dry air. Moisture: <-40°C.

Explosive materials are subject to batch testing to ensure efficiency and safety. The moisture in the atmosphere surrounding the explosive can have a profound effect on the resulting explosion – the charge either failing completely, or the force of the explosion being much greater than expected. In the quality control testing therefore the atmosphere’s moisture must be controlled. Usual moisture: -10 to +20°C.


See also – Air. Glove Boxes (or Dry Boxes) are enclosures with – usually – two access holes fitted with Neoprene gloves so that operations can be carried out in the protected environment inside the box. These operations vary widely and include the handling of dangerous or hygroscopic chemicals, assembly of transistors and other semiconductor devices, and numerous research applications. In many of these applications it is essential that the atmosphere inside the glove box is dry. The atmosphere is often air – but it may be nitrogen or any one of several other gases depending upon the application. The best way of installing the moisture monitor is to fit the sensor in the top of the box (where the moisture level will be at its highest – as water vapor rises), with a loose length of cable inside so that the sensor can be moved around to look for wet areas – caused by uneven flow of the purge air or gas. The alternative is to fit the sensor in the exhaust line, when it will give an average reading of the box moisture. Usual moisture: generally drier than -60°C.

High quality welding, especially of stainless steel, is carried out in a shielding atmosphere of Argon. The Argon must be dry to avoid oxidation of the weld. Usual moisture: <-60°C.


heliumbottles.jpegHelium in liquid form is used to freeze tissue samples, electronic components, etc. It is an expensive gas, and the cooling system re-cycles the gas. It is liquefied in a small very high speed turbine, which is easily damaged if the moisture content is too high: and it is very expensive to replace.


Many industrial processes need a supply of hydrogen gas, and users who need very high purity and large quantity may decide to make their own gas, rather than buying from a gas manufacturer. There are two common ways of producing the gas:

1. Electrolysis. Air is broken down by electrolysis, and the resulting hydrogen is used by the application. There is a risk of mercury contamination, which will damage the Shaw sensor.

2. Palladium diffusion. Methanol is the raw material in this case – the hydrogen being separated at high temperature through a diffusion process. On start-up of a new plant there can be severe contamination of the gas with many impurities, including ammonia, although this process is usually sold because of the high purity hydrogen it produces when working properly.

manintube.jpegSome years ago someone had a good idea: use hydrogen instead of air to cool an electrical generator and, because its a better conductor of heat, the power output of the generator can be increased without making it any bigger! This is now almost standard practice with the stator being water cooled, and the rotor being hydrogen cooled. The gas needs to be dry because of the high voltages and it can also act as a leak detector for the water cooled sections. (N.B. the system must be intrinsically safe. We have a specially designed sample system which is part of the recirculation system, and monitors the inlet and outlet of the dryer.) Usual moisture: <-60°C


Glycol is a hygroscopic liquid, and it is often used to dry Natural Gas simply by passing the gas over its surface in a ‘Glycol contactor’. In a continuous process the Glycol is recirculated through a distillation column, which removed the absorbed water. To check that the process is working efficiently the moisture content of the Glycol liquid may be measured immediately after it leaves the distillation column. The installation is exactly the same as for a gas application, except that the liquid flow rate should be kept at about 100ml per minute to avoid abrasion damage to the sensor. The liquid should be at about 20 to 30°C temperature when it reaches the sensor. Usual moisture: <-50°C water vapor dewpoint.


The Advanced Gas-cooled Reactor (ARG) uses carbon dioxide gas to transfer heat from the nuclear fission core to the water boiler for steam generation. The moisture content is kept low to avoid corrosion damage, and also so that the moisture monitoring will act as a leak detector. Usual moisture: <-70°C.

beerglasses.jpegSee also – Nitrogen. Although some breweries do not bother to recover the carbon dioxide gas produced as a by-product of the fermentation process, the more efficient operations do. The gas is then dried to prevent damage to the compressors, and used to carbonate the beer after fermentation. The excess gas is usually sold as an extra ‘cash crop’. Usual moisture: <-50°C.


plane.jpegOxygen is used in aircraft for breathing: in military aircraft most of the time, in civil aircraft only in an emergency. In both cases the gas must be dry, or there is a risk of freezing in the pressure regulator (see Air: Breathing air for firefighters). Usual moisture: <-50°C.

hospital.jpegGas manufacturers produce oxygen for use in medical applications. The moisture content is not important to the patient who breathes it, but it is essential that the gas is dry to avoid condensation in the cylinders in which it is supplied. Condensation would carry a risk of bacteria growth in the cylinder, and the risk of weakening of the cylinder through corrosion.


gasrig.jpegNatural Gas is used world-wide as a fuel. There are many applications for measuring its moisture content:

Offshore: The gas is usually dried offshore, before being compressed for transmission by pipe-line or tanker to the shore base. The excess moisture must be removed to prevent damaging condensation in the compressor.

On-shore: The measurement is needed for exactly the same reason, as the gas is compressed further for pipe-line transmission/distribution, or bulk storage in the liquid phase.

Re-gassing: When stored in bulk as a liquid, the refrigerated liquid passes through heat-exchangers in the evaporation stage. These heat-exchangers are prone to leakage – introducing unwanted moisture into the very dry gas, so the moisture content is monitored immediately after the heat-exchanger for leak-detection purposes.

The measurement in Natural Gas is not difficult, but there are several special considerations:

Offshore drying is often achieved by passing the gas over Glycol liquid (in a ‘Glycol contactor’). The gas at the outlet will be dry, but may well contain some Glycol liquid residue, or carryover. This will not damage the sensor, but if the sensor is coated with a film of Glycol then its response will become very slow – it is therefore to be avoided by using a knock-out pot or coalescing filter in the sample line. In exactly the same way, hydrocarbon condensate liquid in the gas will contaminate the sensor and result in a very slow response.

Obviously an installation on Natural Gas must be safe because of the fire/explosion risk. The Automatic Dewpoint Meter is Intrinsically Safe as standard. In-line instruments may be made intrinsically safe by using the Zener Barrier Unit and, if there is no safe area for the instrument location, then the explosion-proof model SDAEXD makes a ‘local safe area’ for the instrument. The model SWM-SSNG has two-stage pressure regulation with condensate drain, and will deal with hydrocarbon condensate or Glycol contamination, while the model SWM-SSNGH has heated two-stage pressure regulation and is intended for installations where there may be heavy condensate/Glycol contamination or for light hydrocarbon liquids which will vaporize.


See also Air. Nitrogen is often used in place of air, for an oxygen-free atmosphere.

food.jpegSome foil-wrapped foods such as butter have been found to have extended storage life if the small amount of air trapped within the foil is replaced by nitrogen with a very low oxygen and moisture content. Usual moisture: <-60°C.

beer.jpegThe ‘fizz’ in bottled and cask beer is usually produced by Carbon Dioxide gas (see also Carbon Dioxide). This may be the result of fermentation – in naturally conditioned beer – or the gas may be injected after fermentation in cask or draught beer. Some beers – notably a well-known Irish brew are now using nitrogen gas instead of the CO2. It has the same effect of giving life to beer, but it helps to give the product a longer shelf life. While the moisture is not at all important in the beer (it’s nearly all water anyway) the gas compressors will be damaged by moisture in the gas before it is injected into the beer, hence the need for measurement. Usual moisture: <-50°C.

boat.jpegBecause of the risk of explosion, the tanks of oil and LPG carrying ships must be purged with inert gas. Nitrogen is most commonly used, and the gas is frequently produced by plant on the ship itself. The gas must be dry to avoid corrosion, and in the case of the LPG carriers, to avoid condensation in the refrigerated tanks and void spaces. Usual moisture: <-40°C.

Although a rather limited application, the general principle is worth remembering: if the dewpoint temperature of the atmosphere within the periscope tube is above the temperature of the sea, condensation will form, misting the prisms, so the captain cannot see where he is going! Our instruments are used to automatically purge the tube with nitrogen if the moisture is too high. Usual moisture: <-10°C.

furnace.jpegNitrogen gas is often used, frequently mixed with Hydrogen, for the atmosphere in heat treatment furnaces. The moisture content varies with the application and is usually drier than -50°C dewpoint to avoid oxidation of the metal surface.


Sulphurhexafluoride is used in transformers and high-voltage switches as an insulator. If it is wet it no longer insulates, old SF6 may become contaminated with corrosive impurities if there has been much arcing at the switch contacts. This can degrade the sensor. Another application is when the gas is used as an insulator in particle accelerators. Usual moisture: <-60°C.


The Shaw Sensor is suitable for use with most industrial gases, and usually has a life of several years continuous operation.

There are, however, a few gases which cause problems, and these are listed below, with comments:

– Chlorine even in small quantities as an impurity in other gases, chlorine must be avoided. It will attack the sensor rapidly, and destroy it completely.

– Ammonia is similar to chlorine in its effect on the sensor, and rapid failure will occur. It is useful to note that ammonia causes a blue coloration of the sintered bronze filter.

-Acidic and caustic gases and any known to corrode any of the following sensor materials: high purity aluminum, PTFE, gold, nylon and modified molecular sieve.