Sampling is needed when direct measurement of the air or gas is not possible or desirable. This may be related to, for example, a high process temperature, protecting the sensor from water spikes, the need to easily detach the instrument from a pressurized process without running the whole process down, or wanting to make the measurement at a more convenient location. To get a representative sample of the process gas and avoid error sources caused by the sampling system, the following aspects should be considered:
Pressure
It is important to keep in mind that changing the pressure of a gas changes the dew point temperature of the gas. If the sensor is at a different pressure than the process itself, there may be an error of several tens of degrees in the dew point measurement. You can use the Vaisala Humidity Calculator to simulate the effect of pressure change to dew point. The Humidity Calculator can be found at www.vaisala.com/humiditycalculator.
Leak tightness and materials
Due to the typically very low level of humidity, dew point measurements are sensitive to ambient moisture leaks. It is therefore important to have a leak tight sampling system construction.
The system is likewise sensitive to diffusion of water molecules through the materials, as well as the absorption of moisture into the materials. The piping should preferably be made of metal, for example, stainless steel with a good surface finish. Hygroscopic materials such as plastics are undesirable and should be avoided.
Sample tubing should be as short in length as possible. "Dead ends" should be avoided. Also minimizing the number of connections helps in avoiding leaks. If possible, prepare for measurements by flushing the sampling lines with dry gas.
Flow
Vaisala DRYCAP sensor is not flow-dependent as such. For this reason, the flow of the system is not critical to the measurement, although a higher flow improves the response time. Typically, a flow of 1 ... 2 l/min (0.035 ... 0.7 ft3/min) is enough.