Gas modules use sensor technology that’s best suited to their target gas. For example, the O3 module uses gas sensitive semiconductor (GSS) sensor, and the VOC module uses a photo ionization detector (PID).
Gas modules come in 3 sizes. The size depends on the sensor, the measurement method, and the size of the other components it contains.
Common components
All gas modules have a gas inlet port (white ring), a gas exhaust port (blue ring), two 12 V power connectors to allow the module to be connected to a 12 V power bus, two RJ45 connections to allow the module to be connected to the RS-485 communications bus, and a module status LED.
Measurement methods
Gas modules use one of 2 measurement methods:
- Direct: The gas is passed directly to the sensor continuously.
- Zero switching: The module uses a solenoid to switch between a zero path which removes the target gas and creates a “zero” or baseline measurement, and a measurement path which measures the ambient air. The final measurement in ppm is an arithmetic combination of the zero and sample measurement. This technique is referred to as Automatic Baseline Correction.
In both designs, the gas flow rate is controlled using a flow control orifice.
