Sampling is needed when direct measurement of the air is undesirable or is simply not feasible. This may be due to a high process temperature, the need to protect the sensor from water spikes, or just to have the flexibility to isolate the measurement and remove the dew probe.
To get a representative sample of the process gas and to avoid potential sources of error caused by incorrect sampling practices, the following factors should be considered.
1. Changing the pressure of the gas changes the dew point
It is important to remember that changing the gas pressure changes the dew point temperature of the gas. If the sensor is at a different pressure than the process itself, an error of several tens of degrees dew point may be apparent.
Generally one should make the measurement at the actual system pressure in order to avoid pressure drops in the sampling system. Exceptions can sometimes be made when standards require reporting the dew point at ambient pressure thus excluding the effect of pressure fluctuations.
2. To avoid condensation the dew point must be below the ambient
temperature in the sampling line
When the need for sampling is due to a high process gas temperature prohibiting direct measurement, it is important to ensure that the dew point level of the process is lower than the ambient temperature to which the sample gas is cooled in the sampling line. Download the full document here (pdf)