By Bob Hardy
The two-pressure humidity generation technique is a well-known, highly documented method used for precise control of humidity values. This straight-forward method is in practice world-wide in a large number of standards and calibration laboratories, and is most common in the generation of %RH in the range of 10% to 98%, and for realization of the dew point temperature. While there are systems commercially available, it is commonplace in National Metrology Institutes (NMI) to design and construct the system locally with a goal to build a generator better than those used by their foreign colleagues. Generators that are designed, constructed, and offered commercially are also commonly used in NMIs and other calibration laboratories. While the humidity generating technique remains the same, it is often size, cost, and user-interface that distinguish commercially produced systems from their locally constructed cousins. Regardless of whether a system is commercially available or locally constructed, some areas of two-pressure generator design warrant consideration. Some common problems and potential solutions will be discussed.