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.
Two-Pressure Reactor
Air with a pressure p1 consisting of dry air and a certain vapor pressure e is expanded to a pressure p2. During the expansion all components of 
the air will be expanded with the same ratio p2/p1, i.e. also the vapor pressure e is expanded.
Initial state:
total pressure p1 = pda + e
expanded state:
total pressure p2 = p1 * p2/p1 = p2/p1 * (pda + e)
After expansion the vapor pressure of the moist air is reduced to e’ = p2/p1* e
Two Pressure Humidity Generator HUMOR20
The two pressure humidity generator HUMOR20 consists of two chambers, one built within the other.
Air or nitrogen with a pressure p1 is saturated in the saturation chamber 1. The vapor partial pressure
is at the maximum, the RH is 100%. Then the saturated air is expanded to the ambient pressure p in the measuring chamber
2. The saturation and measuring chambers of HUMOR20 are built one inside the other and are
manufactured in materials with high thermal conductivity, insuring an uniform temperature in both chambers. Under these conditions the partial pressure of vapors is reduced in the same ratio as the total pressure of air and becomes:
The generated RH depends only on the ratio of two pressures, which allows for a very short stabilization time. The RH in the measurement chamber is set to the desired value by adjusting the inlet pressure p1. The saturated partial pressure depends slightly on pressure. The correction is made by a microprocessor. Practically the generated humidity is calculated from the pressures measured by two pressure sensors with excellent long term stability and reproducibility.