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 430^oC recommended by the manufacturer.

Figure 1 – Sensor resistance vs. ozone repeatability for 20 %RH and 40%RH

Current Work

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

Future Work

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.

>> Solid state CO Sensor