Vaisala DM500 - Manual (Page 27)

Chapter 4 _________________________________ General Information About Sampling and Results
VAISALA _______________________________________________________________________ 25
CHAPTER 4
GENERAL INFORMATION ABOUT
SAMPLING AND RESULTS
This chapter provides you with useful information concerning gas
sampling and the effects that different parameters have on the
measurement results.
Gas Sampling
When measuring low dewpoints, all installations must comply with
the strict requirements. Clean environment is always beneficial for
humidity measurements, but this is especially critical at very low
humidities. It is important to use high quality stainless steel tubes
(electropolished), particularly clean and dry. Contamination has often
a hygroscopic effect. Attention must be paid to correct and tight gas
connections. The ambient temperature and the temperature of all parts
of the sampling pathway must be kept above the dewpoint to avoid
condensation.
The following recommendations shall be taken into account when
measuring in very dry environments.
- Minimize the number of connections to avoid leaks
- Ensure that flow rate is adequate
- Avoid dead ends as they cannot be flushed easily
- The temperature of the gas line components must never lie under
the dewpoint of the sample gas, as this leads to condensation and
false results