User`s guide

Chapter 4 _____________________________________________________ Dewpoint Measurement
CHAPTER 4
DEWPOINT MEASUREMENT
The following recommendations should be taken into account when
measuring in very dry environments.
- A clean environment is always best for humidity measurements.
- The number of connections should be minimized to avoid leaks.
- The flow rate must be adequate.
- Dead ends must be avoided as they cannot be flushed easily.
- The tube temperature must never lie under the dewpoint of the
sample gas. This may lead to condensation and false results.
- The sample tubing should be of as short length as possible. The
surface area should be minimized using the narrowest tubing that
the flow conditions allow.
- The surface finish of the pipework is important. Polished or
electro-polished steel is recommended for best results.
- Hygroscopic materials should be avoided in the sampling lines. Use
stainless steel membranes instead of rubber membranes.
- Impermeable materials should be selected to avoid inward diffusion
of moisture through the sampling tubes and enclosures. Such
impermeable materials are, for example, high-quality stainless steel
and metals. Avoid PVC or nylon tubes.
Measuring
If you start DM70 for the first time, see section Preparations Before
Use on page 13. Otherwise, follow the instructions below.
1. Connect the probe to the MI70 indicator.
2. Press the Power On/Off button.
VAISALA _______________________________________________________________________ 17