Installing a Vaisala dew point instrument into a compressed air line

Isolating the Sensor
The most common installation for a dew point sensor isolates the sensor from the compressed air line. This is accomplished by installing the sensor in a sample cell and connecting the cell to a T-piece in the compressed air line at the point of interest. A small amount of compressed air is then bled past the sensor. The cell should be made of stainless steel and connected to the T-piece with tubing (1/4" or 6 mm). It is useful to install an isolation valve between the cell and the air line. This enables easy installation and removal of the sensor.

Regulating the flow
A flow-regulating device is necessary to control to airflow past the sensor. The desired flow rate is only 1 L/min (2 scfh). The regulating device can be a leak screw or a valve. To measure pressure dew point, the regulating device is installed downstream of the sensor, so that when the isolation valve is opened, the sensor is at the process pressure. To measure dew point at atmospheric pressure, the regulating device should be installed upstream of the dew point sensor.

Avoiding a local pressure drop
Do not exceed the recommended flow rate. When measuring pressure dew point, an excessive flow rate will create a local pressure drop at the sensor. Because dew point temperature is pressure sensitive, this will create an error in the measurement.

Selecting the tubing material
The best tubing material is stainless steel. Nonmetallic tubing can absorb and desorb water vapor, creating a lag in measurement response. If stainless steel tubing is not available, consider using PTFE or other materials that do not absorb water. Avoid the use of clear plastic tubing or yellow rubber tubing.

Reducing installation costs
It is possible to reduce installation costs for permanent dew point instruments by installing the sensor directly in the compressed air line. In these cases, it is important to choose a location where the sensor has adequate airflow and where the temperature of the compressed air is at or near ambient. Not only does this save on installation costs, but it is also a good choice if the pressurized dew point reading is the desired measurement parameter.

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