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The trick to using portable hygrometers

Portable hygrometers should offer a handy and simple-to-operate means to measure relative humidity. But there are a few tricks of the trade that make these instruments even more useful. Frank Dutton writes.

There are a number of portable hygrometers on the market today which can be used for a variety of moisture measurement applications. The most common uses for portable hygrometers are: as a means of spot-checking the performance of on-line moisture measurement devices; checking samples from gas bottles; and checking the moisture content of systems where no on-line devices are being used.

In order to get the best performance from portable hygrometers, there are a number of precautions to be taken in order to get a meaningful reading.

Connections and materials

Of course, a form of tubing always needs to be used to connect a portable hygrometer to the systems being tested. You should use a tubing that offers good flexibility, but be careful of a common problem — many flexible forms of tubing are actually potential sources of moisture, particularly rubber.

Rubber should never be present in any system that is going to be used when measuring moisture, as it will actually form a moisture source. That includes the pressure regulators to be used on bottled gas.

Even a low cost rubber regulator used on a bottled test gas of -75°C dewpoint can result in a sample measurement of about -35 to 30°C. As can be seen in Image 1, rubber does not even appear as a suitable material!

The best materials for tubing are PTFE on low pressure systems (up to 100 KPa) which measure down to -60°C and stainless steel on systems below -60°C.

Keep the number of connections to a minimum and also to ensure they are leak tight. In the normal world, a small leak on a system might not seem a problem, but when measuring moisture we have to think about partial pressures.

For example, on a system with an expected dew point of -80°C, it has an equivalent partial pressure for water vapour of 0.055 Pascals at a system pressure of 600Kpa absolute. The ambient conditions outside the system would be roughly 103Kpa absolute. If the external dew point is +10°C, then the partial pressure of water vapour is 1228 Pascals.

So, the bulk pressure is 6:1 in one direction, but the water vapour pressure ratio in the other direction is over 22000:1! That is why gas may leak out but water vapour enters the system and dilutes or ‘wets up’ the gas sample at that point.

Filtration and flow

Before using a portable device, it is wise to blow through the sample connection on the system to be checked. This helps remove any accumulated dust and moisture. However, it is advisable to have some form of filtration, either particulate or coalescing. Coalescing filters remove any entrained liquids which could damage the sensor inside the portable. This is particularly important if sampling from natural gas pipe lines where there could be glycol carryover.

Check the flow rate on the manufacturer’s data sheet and use a simple flow meter and valve to set the flow rate. Too low a flow rate results in slow readings; the sensing element in most hygrometers relies on the flow being correct and allowing the sensing element to reach a state of equilibrium with the gas being sampled, neither adsorbing nor desorbing water vapour.

The temptation to have a high flow rate is problematic too. With narrow bore pipes it is easy to build up a back pressure; pressure affects the dewpoint when read in °C. It should also be remembered that hygrometers measure dew point first, and then convert to values such as ppmV. If pressure affects the dew point reading, then the ppmV reading will be in error.

Pressure or atmospheric?

Dew points when measured in °C are affected by pressure. This is because you are in effect reading the partial pressure of water vapour. Check the pressure rating on your device. Some portable hygrometers can measure dew points from 0-30MPa (300 Bar), while other devices might not have this range and high pressures would need to be reduced via a pressure regulator.

Choose your portable hygrometer carefully. Certain models can also allow the connection of a pressure sensor via a 4-20mA output, or for a manual input of pressure in order to compensate for pressure effects, which is extremely handy.

A good sampling system will allow the connection of a correctly rated portable to measure at pressure or at atmospheric conditions. So what are the effects?

Let’s say you wanted to check the dew point on an instrument air system which operates at 1000 KPa. In general, instrument air should be -45°C or better at pressure. But -45°C at pressure would equate to about -63.64°C at atmospheric, while a reading of -45 at atmospheric would be -22.73 at pressure.

How long to wait?

Some portable hygrometers use dessicant chambers to keep the sensor in a dry condition. When new, this is a reasonable method, but desiccants age with time and get wetter, thus lowering their speed enhancement when measuring low dew points.

Moreover, few manufacturers state on their data sheets the time required to dry a sensor back down to the ambient created by the dessicant chamber.

There are some hygrometers on the market that use heated sensor tiles and intelligent firmware so that a measurement can be taken quickly, at -70°C within 10 minutes. Because of the heating cycle employed, this process is extremely repeatable and the instrument can be used several times in an hour; the time factor being the walk between the various test points and connecting the device.

Further, good quality models display a message when the instrument has reached a stable condition, telling the technician they can safely take a reading.

Moisture measurements are a complex subject and require a little more thought and procedure in order to get valid results. Establishing a standard test method will also assist.

Above all, be patient — hygrometers do not have the relatively instantaneous response that you see with pressure sensors.

[Frank Dutton is the Asia sales manager at Michell Instruments. He can be contacted on the below mobile phone number.]

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