The study of the effect of water vapour in methane and helium on humidity sensing

Abstract

The calibration of humidity sensors is usually carried out in air and at near ambient pressure. In many industries, humidity sensors are sometimes used to make measurements in carrier gases other than air and this can give rise to errors. It is presumed that humidity sensors have responses that differ depending upon gas species–i.e. they are gas species dependent. The number of atoms in a gas molecule is called its Atomicity. A gas with a high atomicity can remove more heat out of a sensor than a gas with a small atomicity. The gases chosen in this research were methane and helium. The Flow Mixing Generator (FMG) has been developed to generate humid gas. The operation relies on the basis of mixing wet gas and dry gas. The major component was dry gas, >95% by volume. The experimental dew point temperature range was around -30 °C at a pressure of 1.5 bar A with this wet gas and dry gas ratio. The FMG was controlled to generate humid gases with the same dew point, although the dry gas‟ dew points were not equal. Therefore, the gas flow rates were adjusted. The mass flow controller that was a part of the FMG used to measure and control the gas flow rate was calibrated in air. When the other gases were used, the gas correction factor had to be applied in order to obtain the actual flow rate. The correction factor‟s uncertainty was not reported and presumably, this quantity may be included with the instrument‟s uncertainty (0.25% of value). Humidity quantities may be stated in various units. Water vapour is directly sensed by a humidity sensor. This quantity is thus important and usually reported in terms of water vapour pressure. Based on Dalton‟s partial pressure, total pressure is the sum of the pressures of gases in the mixture including water vapour pressure. However, this is only valid for gas at low pressure with no molecular interaction. The ideal condition does not exist – even in ambient air. The actual water vapour pressures need to be corrected by the enhancement factor. The present equations for the enhancement factor are valid for air at pressures up to 20 bar. The equations of states with respect to molecular interaction were chosen to calculate the enhancement factor. Nevertheless, the equations of state were insufficient. The overall uncertainty evaluation for the standard hygrometers, the FMG and the test humidity sensors have been presented. The FMG gives an acceptable uncertainty of ± 0.3 °C at the 95% confidence level (k = 2). Weak reproducibility gave the largest source of uncertainty. The polymer sensor showed a difference of 1.2 °C between air and methane, but the difference was insignificant in the aluminium oxide sensors. It can be concluded that gas species has an effect on a polymer sensor.