Sensor Principle
Additional Information • 35
It may be necessary to check the calibration if the sensor has been exposed for a long
time to a high concentration of flammable gases, vapors, or the above-mentioned
contaminating substances.
The nature of catalytic bead sensor technology means that sensor drift may typically
be up to ±5% LEL per month. Instruments using these sensors should be zeroed
regularly following the instructions in
Section 5, Care and Maintenance
6.1.3. Semiconductor Sensors
Semiconductor or metallic oxide sensors (MOSs) are among the most versatile of all
broad-range sensors. They can be used to detect a variety of gases and vapors in low
ppm or even combustible ranges. The sensor is made up of a mixture of metallic
oxides. They are heated to a temperature between 302ºF and 572ºF (150ºC and
300ºC) depending on the gas(es) to be detected. The temperature of operation as well
as the “recipe” of mixed oxides determines the sensor selectivity to various toxic
gases, vapors, and refrigerants. Electrical conductivity greatly increases as soon as a
diffusion process allows the gas or vapor molecules to come in contact with the
sensor surface. Water vapor, high ambient humidity, temperature fluctuations, and
low oxygen levels can result in higher readings.
IMPORTANT: Certain substances in the environment to be monitored may impair the
sensitivity of the sensors:
1. Materials containing silicone or silicone rubber/putty.
2. Corrosive gases such as hydrogen sulfide, sulfur oxide, chlorine, hydrogen chloride, etc.
3. Alkaline metals, salt water spray.
6.1.4. Infrared Sensors
The infrared (IR) gas sensor is designed to measure the concentration of combustible
gases and vapors in the ambient air. The sensor principle is based on the
concentration-dependent absorption of infrared radiation in measured gases.
The monitored ambient air diffuses through a sintered metal material into the
enclosure of an optical “bench.” The broadband light emitted by an IR source passes
through the gas in the optical bench and is reflected by the walls from where it is
directed towards a dual-element detector. One channel of the detector measures the
gas-dependent light transmission, while the other channel is used as a reference. The
ratio between measurement and reference signal is used to determine the gas
concentration. Internal electronics and software calculate the concentration and
produce an output signal.
