Image Credit: Rosemount Analytical | Mettler-Toledo Process Analytics

Gas sensors interact with a gas to initiate the measurement of its concentration. The gas sensor then provides output to a gas instrument to display the measurements.  Common gases measured by gas sensors include ammonia, aerosols, arsine, bromine, carbon dioxide, carbon monoxide, chlorine, chlorine dioxide, diborane, dust, fluorine, germane, halocarbons or refrigerants, hydrocarbons, hydrogen, hydrogen chloride, hydrogen cyanide, hydrogen fluoride, hydrogen selenide, hydrogen sulfide, mercury vapor, nitrogen dioxide, nitrogen oxides, nitric oxide, organic solvents, oxygen, ozone, phosphine, silane, sulfur dioxide, and water vapor.

Measurement Specifications

Important measurement specifications to consider when looking for gas sensors include the response time, the distance, and the flow rate. 

• The response time is the amount of time required from the initial contact with the gas to the sensors processing of the signal. 
• Distance is the maximum distance from the leak or gas source that the sensor can detect gases. 

• The flow rate is the necessary flow rate of air or gas across the gas sensor to produce a signal.

Sensor Output

Gas sensors can output a measurement of the gases detected in a number of ways.  These include percent LEL, percent volume, trace, leakage, consumption, density, and signature or spectra. 

• The lower explosive limit (LEL) or lower flammable limit (LFL) of a combustible gas is defined as the smallest amount of the gas that will support a self-propagating flame when mixed with air (or oxygen) and ignited. In gas-detection systems, the amount of gas present is specified in terms of % LEL: 0% LEL being a combustible gas-free atmosphere and 100% LEL being an atmosphere in which the gas is at its lower flammable limit. The relationship between % LEL and % by volume differs from gas to gas. 
• Also called volume percent or percent by volume, percent volume is typically only used for mixtures of liquids. Percent by volume is simply the volume of the solute divided by the sum of the volumes of the other components multiplied by 100%. 
• Trace gas sensors given in units of concentration: ppm. 
• Leakage is given as a flow rate like ml/min. 
• Consumption may also be called respiration. Given in units of ml/L/hr. 
• Density measurements are given in units of density: mg/m^3. 
• A signature or spectra measurement is a spectral signature of the gases present; the output is often a chromatogram.

Common outputs from gas sensors include analog voltage, pulse signals, analog currents and switch or relays.  Operating parameters to consider for gas sensors include operating temperature and operating humidity.

Standards

The International Society of Automation (ISA) specifies standard ISA 12.13.01 which describes test methods that apply to portable, transportable, and fixed devices for the detection and measurement of flammable gas or vapor concentrations in air.

Resources

Operating Principle of Semiconductor Type Gas Sensors

Principles of Gas Detection