Humidity Calibration Basics
It is known and accepted that relative humidity is one of the physical quantities most difficult to calibrate. The main problem is to generate humidity with high
stability for calibration outside a
special humidity lab. There are different methods to generate
humidity, whereby all classical methods require either
temperature stability and uniformity or accurate measurement of the temperature.
Saturated Salt Solutions
A closed box partly filled with saturated salt solutions generates relative humidity in the free room above the salt with good accuracy. The value of the relative humidity depends on the type of salt used. It is mainly independent of temperature, but strongly dependent on temperature uniformity. For an accuracy of ±2 %RH a temperature uniformity better than 0.5
°C is necessary.
Non Salt Saturated Solutions
Instead of saturated salts non concentrated LiCl-solutions can be used. The obtained values of the
relative humidity depend on the salt concentration.
Mixing Reactor
A stream of dry air (0 %RH) is divided
into two separated flows. One air flow is
saturated with water vapor in a saturation chamber (100 %RH), the other one remains dry. The RH in the measuring chamber is set by adjusting the mixing ratio of the two air streams with a mass flow controller.
Two-Temperature Reactor
Air or nitrogen is saturated with vapor in a saturation chamber and cooled down to the dew point
temperature Td, corresponding with the requested relative humidity RH at temperature T. Excess
vapor condenses and the vapor partial pressure equals to the saturation partial pressure.
The saturated air warms up to temperature T, the
vapor partial pressure corresponds to the required RH. (Principle of reverse dew point mirror)
In an ideally designed two-temperature-reactor the accuracy depends only on the measurement of two
temperatures (T and Td).
Main disadvantage is a long stabilization time when changing the humidity. |
|
is
at the maximum, the RH is 100%.
