5 Sensor Theory of Operation

The ammonia sensor on the Ammonia Click is a heated, resistive sensor. As the is non-linear and the atmosphere increases, the sensor resistance decreases. This sensor is non-linear and highly dependent on environmental conditions such as humidity and temperature. It also requires a warm-up period for the sensor to become stable.

Tip: A multi-hour warm-up period is required for the gas sensor used. 24 hours is the default in this application, but the manufacturer uses 48 hours for their characterization data. From observations during development, only a few hours seem to be required for stability.

To convert the sensor output to a gas concentration, a best-fit function was computed in Microsoft Excel. This function assumes the reference resistance of the sensor is equal to 100Ω, the minimum sensitivity of the Click board. The sensitivity adjusment isn’t set to the highest level as this could cause the sensor to output 5V, while the Curiosity Nano runs on 3.3V. Intermediate gain values aren’t usable, as the potentiometer can’t be reliably set to a known resistance without removing it from the circuit.