Normalized relative humidity calibration

Abstract

A humidity measurement process and humidity measurement scale are derived from the physics of relative humidity. Absolute moisture content is determined independent of any measurement or knowledge of the source temperature. The humidity measurement is equivalent to the volumetric fraction of water vapor in the measured gas mixture, expressed as a percentage in a fixed range from 0 to 100%. The operating principles of the measurement process and measurement scale are described for use with an instrument suitable for humidity measurements in process ovens, product dryers, exhaust stacks, and similar environments operating at temperatures to 1250.degree. F. and near atmospheric pressure. The instrument is a point-source sampling system. A continuous sample of process air is induced to flow through the instrument for humidity measurement. The temperature of the sample is regulated to a constant measurement temperature above the dew point temperature of the source, and below the operational temperature limit of a solid state relative humidity sensor. Moisture content is calculated from the sensed relative humidity at constant temperature and the absolute measurement pressure. Temperature is not a parameter in the humidity calculation. The instrument is calibrated in a humidity chamber using the fundamental relationship of saturated water vapor pressure at the dew point temperature.

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