Long-term stability in dewpoint measurement of compressed air
July 27th 2007

Compressed air is widely used in production facilities for purposes such as the operation of process machinery and the actuation and control of pneumatic valves, cylinders and controllers. It can also be used as a transport medium or purge gas. High humidity can adversely affect the performance of the compressed air distribution system and the processes in which it is used. But the use of compressed air dryers and reliable dewpoint monitoring ensures that the wheels of industry keep turning.

In a compressed air system, the aim is to produce dry, oilfree and dust-free compressed air at low cost. Oil and dust can be removed with filter systems, but humidity must be removed with dryers. Exactly how dry the air should be depends on the specific process using the compressed air. For most applications dewpoint transmitters are the best choice for measuring humidity.

HUMIDITY CAUSES PROBLEMS IN COMPRESSED AIR

A typical pressure of a compressed air system is 7 bar (101.5 psia). The dewpoint temperature is a pressuredependent parameter, so that, as the compressor compresses the air, the dewpoint of the air increases. At a certain point, the air becomes saturated and water vapour condenses out as a liquid.

Moisture in compressed air can cause many problems both to the end product and to the components of the air distribution system itself. Humidity may affect the quality of the end product by, for example, causing adherence of hygroscopic products and leading to bubble formation in coating processes. As the majority of compressed air systems are made of steel or non-galvanized steel, they corrode when they are in contact with moist compressed air. Rust and scale forms in the piping and is blown into sensitive equipment. This in turn leads to faults in control systems and widespread operating problems. If the system is partly installed outside, iceformation can also be a problem.

VAISALA’S AUTO-CALIBRATION ENSURES THE PROCESS KEEPS GOING

Long-term stability is an important consideration when talking about accuracy in industrial applications. This means the accuracy of the instrument not only at the time of manufacture, but in operation for months and years to come. A reasonable drift is acceptable in most applications, and calibration at routine intervals is good manufacturing practice. However, in many cases the drift between calibration checks can be several times the specified dewpoint accuracy, which may result in reduced plant efficiency or affect end product quality.

With Vaisala DRYCAP® the measurement technology takes care of the accuracy, so that the drift is kept to a minimum. Measurement accuracy with long-term stability is the basic requirement for all instruments Vaisala develops and manufactures. The autocalibration feature automatically checks calibration and compensates for drift. This ensures good long-term stability and reduces maintenance costs.

Vaisala DRYCAP® dewpoint transmitters DMT340, DMT142, DMT242 and DM70 are all suitable for monitoring the dryness of compressed air. The DMT340 with a display is a high-end product for versatile use.

The DMT142 and DMT242, compact and rugged transmitters, are especially well suited to OEM applications such as compressed air dryers. The DM70 complements the product range. It is a light weight portable meter with separate probe that can be used for spot measurements at the compressed air line or for field checking installed dewpoint transmitters.

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