Effects of Gravity on Output of Humidity Sensors with Different Detection Mechanisms at Constant Humidity

The effects of gravity on output of sensors with different detection mechanisms under constant humidity were investigated to screen humidity sensors for small plant growth chambers under microgravity. Different levels of gravity were established by parabolic flight using aircraft in which air pressure and temperature were maintained almost constant (the variations were within 0.6kPa and 0.2°C, respectively). Four types of polymer sensors, a thermistor sensor and an electrolytic sensor were used for the experiment. Air with relative humilities of 75% and 58% was prepared using saturated salt solutions of NaCl and NaBr, respectively. Six types of sensors were fixed either in a vessel (1 L) or independently in small vessels (10mL) including the solution absorbedby polymer film. When the sensors were placed in avessel, only the output signal of the thermistor sensor varied with gravity level. It was lowestunder 2 G and highest under low gravity under both relative humidity conditions. This variation was observed in both cases when air in the vessel was and was not stirred. This might be due to a filter protecting the sensor surface to cancel the effect of wind velocity on sensor. Air diffusion inside the filter in both cases seemed to be governed by natural convection of which the strength was affected by gravity level. The same tendency in thermistor sensor output was observed in the small vessels in which each sensor was independently fixed, indicating that an interaction between sensors placed in one vessel is negligible. Thus the change in air diffusion with gravity seemed to be the reason why the output signal of thermistor sensor changed with gravity level.