Dewpoint meters: Selection matters

Dewpoint is among the most important measurements to ensure a clean, dry compressed air system. There are plenty of meter options available in the market but it pays to consider your selection process carefully, as Vaisala exp

Careful choice influences the dewpoint meter's stability and accuracy, eventually affecting process efficiency and the finished product's quality. Vaisala experts have identified four questions that they believe are helpful in narrowing down the options.

Portable or fixed? Buyers needs to decide whether they require a portable or fixed platform.

Portable battery powered devices vary in size and function but are typically used for verifying fixed mount transmitters in the field or for spot-checking various points in a compressed air system.

Fixed mount transmitters usually require an external power source and can greatly range in complexity.

The most sophisticated units have displays, alarm relays, datalogging, ethernet connections and more. These can be installed in the supply side, for example for dryer monitoring and control, or in the demand side connecting to PLCs, dataloggers and panel meter displays.

Dryer type dictates range Vaisala recommends the buyer check the type of dryers being used in the system. In most compressed air systems the type of dryer will dictate the dewpoint operating range required from the dewpoint sensor.

Sensing technologies are optimised to excel in specific ranges - very dry conditions, midrange ambient levels, or high humidity condensing environments. Choosing the instrument with the right measurement range will impact both the short and long term accuracy and stability of the dewpoint measurement.

Temperature and pressure tolerance Temperature doesn't affect the dewpoint value but may impact the sensor's performance. If the air is very hot at the intended measurement location, installation of a sampling system may be required. Pressure conditions need to be understood as well.

Changes in pressure have a dramatic effect on dewpoint. Also, the mechanical pressure rating of the probe needs to withstand the pressure demands of the process.

In-the-line or via a sample cell Last but not least, the decision on whether the measurement should be in-situ or outside the system using a sampling line narrows down the dewpoint transmitter options. According to Vaisala, in-the-line measurements can be less costly to install and have potentially faster response time.

Drawbacks can include fluctuating line pressures resulting in changing dewpoint values and difficulty in removing the probe conveniently from the compressed air system when it's time for service or calibration. The alternative approach involves the use of a sample cell and stainless steel tubing connected to the main line. The advantages are better control of air pressure minimising main line fluctuations, ability to isolate the sensor from the system and the option to cool the air sample if necessary.

Help available As dew point measurements can be made almost anywhere on the supply or demand side of a system, the application conditions can vary significantly thereby affecting instrument performance requirements.

Buyers should also make sure that sensors withstand and recover from exposure to many contaminants, such as compressor oil, water spikes, ambient humidity and chemical impurities. If you have any doubts about selection, manufacturers of dewpoint meters should be able to advise. Vaisala, for example, offers help 24/7 (www.vaisala.com).