Calibration of hygrometers at non-static conditions

Abstract

The current practice of calibrating humidity sensors at discrete measurement points under static conditions is time-consuming, i.e. expensive, due to the long stabilization times. In order to reduce the calibration time, we introduce a new calibration method based on humidity ramp measurements, i.e. measurements are performed at first while humidity is increased with constant speed and then decreasing with the same speed. A calibration system based on the mixing flow humidity generation principle was developed for generating linear humidity ramps inside a measurement chamber. Two different calibration approaches were investigated. In the first one, high accuracy was targeted with moderate ramp speeds (2 h to 12 h) and a large volume measurement chamber using a chilled mirror hygrometer as a reference. In the second approach, a shorter calibration time was achieved with fast ramp speeds (0.5 h to 2 h) and a small volume chamber using a fast capacitive humidity sensor as a reference. The developed calibration procedure was validated by comparing results of non-static and static calibrations to each other for five different capacitive humidity sensors from two manufacturers. The non-static calibration was found to provide equivalent results compared to the static calibration with a potential of reducing the overall calibration time by up to 50%. Preconditions of the non-static calibration related to the ramp speed and sensors response times are discussed, and an estimation of the calibration uncertainty is given. The main advantage of the developed non-static calibration method is that calibrations can be performed faster and more data on the sensor behaviour is obtained than with the conventional point-wise calibration without any significant increase in uncertainty.

Defining relative humidity in terms of water activity: III. Relations to dew-point and frost-point temperatures

“…Relative humidity (RH) is an important thermodynamic quantity in various branches of science and technology, such as meteorology, climatology, air-conditioning, wood drying and ceramics industries, that is subject to advancing metrology [1][2][3][4][5][6][7][8]. However, conventional RH is not uniformly defined, and the definitions available typically do not cover the full range of potential applicability [9,10].…”

“…In its latest strategy document, the Consultative Committee for Thermometry of the CIPM [11]: pp 2-17 has described the unsatisfactory present metrological situation by declaring that 'the expression of RH is not yet standardised' and its engagement to 'continue development of relative fugacity (RF) as a real-gas alternative for the conventional RH definition, together with options for realization within the SI'. Related work is ongoing in cooperation between the BIPM 7 and IAPWS 8 under the umbrellas of the CCT 9 and JCS 10 .…”

Special Section on TEMPMEKO 2019: a feature on the XIV International Symposium on Temperature and Thermal Measurements in Industry and Science & IV International Temperature Conference, Beijing (TEMPMEKO & TEMPBEIJING 2019) and Metrology for Meteorology and Climate (MMC 2019)

“…A new primary humidity measurement generator is described in FitzGerald et al [10], focusing on the conditions for sufficient mixing to take place. Högström et al [11] adopt a humidity ramp method to reduce the calibration time for hygrometers by, potentially, up to 50% without a significant increase in uncertainty. For space application, Pearce et al [12] describe miniature gallium phase-change cells for in situ recalibration of contact thermometers monitoring the temperature of blackbodies for on-board calibration of satellite-borne radiometric infrared sensors.…”