Precise calibration of capacitive hygrometers
The measuring accuracy of hygrometers should be regularly checked in order to rule out any drift effect of the sensor, for example, due to hazardous chemicals present in the measuring area. Recalibration and adjustment at regular intervals is also recommended.
The humidity reference cells are the ideal standard for stable humidity values for the purpose of experimentation or calibration of measuring instruments. Despite simple application, an accuracy of 1 % relative humidity can be achieved in the laboratory. These are available with eight different salt fillings which cover the entire humidity range of 11 % rH to 97 % rH.
The operating principle of the cells is based on a saturated salt solution, over which a particular value of relative humidity gets adjusted depending upon the salt filling. The measuring chamber in the inner part of the cell is separated from the salt solution by a semi-permeable membrane (diaphragm) which is permeable to water molecules in vapour form, but however, is not porous for the salt solution or water in liquid form. The reference cell can also be used in overhead application, without any danger of salt solution spilling out. The vapour pressure, and hence, the relative humidity in the measuring chamber corresponds to the relative humidity above the salt solution.
The mechanical construction is optimised in terms of essential requirements, specially the inner volume, to provide a large membrane surface so that humidity value stabilises in a short time after start of the checking process. Through the transparent construction, the liquid condition and reduction in salt solution can be monitored from outside. With proper usage, the cells can be used for years together. The cells can be refilled through an inspection cover at the base. The scope of supply covers detailed documentation and many application tips.
Press information No. 8 January2006
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