Gas detectors can detect combustible gases, oxygen and toxic gases. These gases are typically detected using electrochemical sensors, catalytic sensors, photoionisation (PID) sensors, infrared sensors, and semiconductor sensors.
The purpose of a gas detector is to detect the presence of gas in an area. When gas concentration exceeds the set alarm value of the gas sensor, the gas detector activates an alarm. This is in the form of; a sound alarm, a visible alarm or a vibrating alarm.
So what types of gas sensor technology are available?
Electrochemical sensors are most commonly used to identify and measure specific toxic gases at the ppm level. Oxygen is measured at percent of volume (% vol).
Electrochemical sensors are available for monitoring for a number of toxic gases. The most common gases being:
- Hydrogen Sulphide (H2S)
- Carbon Monoxide (CO)
- Sulphur Dioxide (SO2)
- Chlorine (Cl2)
- Ammonia (NH3)
- Hydrogen Cyanide (HCN)
- Ethylene Oxide (C2H4O)
- Oxygen (O2)
- Nitrogen Dioxide (NO2)
- Nitric Oxide (NO)
- Ozone (O3)
Advantages – Electrochemical sensors can be used over a wide temperature range (-20° to +50°C is common). Generally, electrochemical sensors are compact, have a long lifespan (1-3 years), require little power and response is good.
Disadvantages – Although the sensors are designed to be specific to each gas, there can be cross interferences.
Overall, electrochemical sensors offer very good performance for the routine monitoring of toxic gases.
A non-dispersive infrared sensor is more commonly referred to as the infrared sensor (IR). IR works in a wide range of temperatures, humid conditions and successfully operates in inert atmospheres.
The operating principle of IR works whereby the infrared light pulses on and off through the sample gas at two wavelengths. The two light sources (reference and sample beams) are guided along an optical path and then through the sample gas. One wavelength is set to absorb the sample gas to be detected whilst the other isn’t. The two beams of light are then reflected back and subsequently back through the gas sample and into the unit. It’s here where the detector compares the light signal beams and can measure the gas concentration.
IR detectors can be either point or open-path and are used mainly for hydrocarbon vapours from 0-100% vol, Carbon Dioxide, Methane and Nitric Oxides. IR is used in both portable and fixed gas detection instrumentation.
For point detectors, the beam length is short (centimetres). For open-path sensors, the source of infrared light is a powerful narrow beam that illuminates the space between source and detector.
An added benefit of using the infrared sensors for combustible gas detection described above is that each of the infrared sensors is also designed to detect Carbon Dioxide (CO2) from 0–5% of volume.
Advantages – Can detect gases in inert atmospheres (little or no oxygen present). They are not susceptible to poisons and can be made very specific to a particular target gas. IR sensor has a number of advantages over catalytic type sensors because of the speed of response, low maintenance and unaffected by known poisons.
Disadvantages – Caution should be taken in applications where Hydrogen or Acetylene may be encountered as these gases will not be detected by an infrared sensor.
Photoionisation (PID) Sensors
PID sensors are used for the detection of Volatile Organic Compounds (VOC’s) such as benzene/toluene/xylene, vinyl chloride and hexane at very low levels / high sensitivity (sub-ppm levels)
A PID is suitable for detecting entire groups of hazardous substances. The usage and range of this detector are dependent on the energy of the UV lamp.
A PID can measure VOC’s in very low concentrations from ppb (parts per billion) up to 10,000ppm (parts per million/ 1% Volume). The most common lamp ratings are 9.8eV, 10.6eV and 11.7eV.
10.6eV is the most commonly used lamp. It has a 2-3 year lifespan and offers a wide detection of VOC gases.
The 9.8eV PID lamp requires the lowest power and has the longest lifespan. However, it is limited to the number of VOCs it can detect.
Therefore, 11.7eV lamps should only be used when it is absolutely necessary to detect VOCs above the 10.6eV spectrum.
The detector is fast and sensitive but humidity may affect the readings. These detectors are small and can be handheld. Life expectancy of these sensors is 1-3 years.
Advantages of PID technology include fast response time and excellent shelf life.
Disadvantages are that PIDs suffer from sensor drift and humidity effects. This makes calibration requirements more demanding than other common gas detectors. Sensor life is poor too.
Flame Ionisation Detector (FID)
Flame ionisation detectors are analytical devices that are used to detect hydrocarbons, and other flammable compounds. The FID is very sensitive and provides a linear response across a wide variety of combustible gases. The ionisation energies of a flame ionisation detector are lower and have a large spread that results in response for all gaseous hydrocarbons such as methane and ethane, up to and including the heaviest fuel oils.
A FID reading has a better representation of the actual gas concentrations. The technical side of a FID is somewhat more complex than a PID. Hydrogen is needed for ionizing the sample and it must be of high purity.
Changing or filling the hydrogen cartridges is a high-pressure operation that requires training. However, calibration is required less frequently. FIDs tend to be considered for specialised applications and are more costly than the other ionisation devices.
Colorimetric (colour change)
Colorimetric technology differs from other gas detecting technology in the way that it doesn’t use a sensor but it’s worth mentioning.
Colorimetry has the capability to detect gas down to very low levels (including ppb) for a specific gas.
Also known as papertape technology, it is used to measure a wide range of toxic substances. These include Carbon Monoxide, Chlorine, Di-isocyantes, Fluorine, Hydrogen Sulphide and Phosgene.
Applications include semiconductor manufacturing, aerospace, speciality chemicals and industrial research laboratories.
Gas detector tubes are a totally different measuring principle than any of the above sensors. The cross-sensitivities of other interfering gases are different meaning that this is the ultimate way to verify the concentration showed by an electronic detector. In this way, you can exclude most cross-sensitivities to be sure of a non-hazardous working atmosphere. For most gas detector tubes, there is no Oxygen needed for doing a measurement and detector tubes can often detect gases in extreme low and high measuring ranges, where other gas sensors cannot reach due to over-range or insensitivity.