We focused on the measurement and control of a commercial tin-oxide based ozone sensor. We previously showed that the sensor has more than adequate sensitivity as can be seen in Figure 1. However, the response was not repeatable especially at higher ozone (>100 ppb) concentrations with the first generation, constant voltage control software. The problem was traced to heater aging resulting in a gradual decrease in operating temperature of the sensor from the 430oC recommended by the manufacturer.
We spent several months pursuing an approach to use the heater resistance as a resistance temperature detection device with some success, but characterization of heater aging proved elusive. Figure 1 shows the sensor response at two different humidity values on several different days. The 20%RH data was reasonably repeatable. The lack of repeatability of the 40% data was correlated to a gradual decrease in the power to the heater from a flaw in the temperature correction algorithm that cannot be corrected.
Figure 1 – Sensor resistance vs. ozone repeatability for 20 %RH and 40%RH
We will pursue a constant power approach that corrects for differences in ambient temperature and relative humidity. Figure 2 shows that the power to keep the heater at constant temperature is a function of humidity due to the effect of the heat capacity of the gas on the heat transfer coefficient. We will determine the dependence of the heat transfer coefficient on humidity using heater resistance as the temperature sensor over short time periods to avoid the complicating effect of heater aging. The temperature of the device will then be controlled using the on-board temperature and humidity sensors according to the equation where is the heater power and is the effective heat transfer coefficient of the sensor. We expect that the repeatability issues will be solved with this approach.
Figure 2 Power to keep sensor at constant temperature vs. humidity
We will place a set of three of these sensors at Thompson Farm and collect data for several months and compare the data against a commercial, ozone sensor. We will also prepare the sensor system for balloon launch.
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