Dew point instruments:  Selection matters

What do you need the instrument to do? 

Depending on your application, you’ll either need a fixed-mount transmitter or a portable handheld device. Portable units are battery-powered and 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 range in complexity from basic analog devices to highly sophisticated units with many additional functions. Consider whether you need continuous monitoring on a local display, signals sent for control or other purposes, an alarm if certain extremes are reached, or data logging.

What sort of dryer are you using? 

In most compressed air systems, the type of dryer used – desiccant or refrigerant – will dictate the operating range required from the dew point instrument. There are instruments for all dew point conditions, but there is no single product that excels at all levels – so in order to ensure accuracy and stability your instrument choice must be based on your individual application’s operating range. Desiccant dryers, for example, often require a sensor that is specifically designed for low dew point conditions. Refrigerant dryers, on the other hand, operate at higher dew point temperatures, making a different type of sensor more beneficial. 

What is the pressure and temperature at the measurement location?

While the temperature of the compressed air will not have any effect on the dew point value, it may negatively affect the performance of the sensor. If the air is too hot at the intended measurement point, a cooler downstream location or sampling system should be used. Pressure should also be carefully managed and recorded at each measurement location in order to calculate a single pressure value.  

How will the measurement be made? 

Deciding how to install a dew point probe can be a difficult choice. Two options are usually considered – in-situ measurement or a sample line and cell outside the system. There are advantages and disadvantages to each. In-situ measurements usually have a faster response time and a simpler and cheaper installation process, but they can result in fluctuating dew point values caused by changing line pressures. Service and calibration is also more difficult. The external approach uses a sample cell with stainless steel tubing connected to the main line at the point of interest. The advantages of this approach are minimised fluctuations, the ability to isolate the sensor from the system, and the option to cool and condition the air sample if necessary. Disadvantages include possible inaccurate readings if the flow rate through the cell is not carefully controlled.

The complete application note can be downloaded at www.vaisala.com/4keyquestions.