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Gas Sensors

Determining which gas sensor type you need is one of the first challenges for choosing a gas detecting product.

Understanding the properties of the various types of sensor technologies that are used in detecting gas will support your product selection decision making. You must think about the environment in which the gas sensor will be used.

So what types of gas sensor technology are available?

Electrochemical Sensors

Electrochemical sensors are most commonly used to identify and measure specific toxic gases or classes of gases. Generally, electrochemical sensors are compact, have a long life span (1-3 years), require little power and response is good.

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)
  • Chlorine Dioxide (ClO2)
  • Ammonia (NH3)
  • Phosphine (PH3)
  • Hydrogen Cyanide (HCN)
  • Hydrogen (H2)
  • Ethylene Oxide (C2H4O)
  • Oxygen (O2)
  • Nitrogen Dioxide (NO2)
  • Nitric Oxide (NO)
  • Ozone (O3)
  • Hydrogen Fluoride (HF)
  • Hydrogen Chloride (HCl)
  • Phosgene (COCl2), and others

 

Catalytic Bead Sensors

Catalytic sensors represent a large number of gas detection devices that are manufactured today. Catalytic bead technology is used to detect combustible gases such as Hydrocarbons and is a proven technology for measuring Hydrogen.

The catalytic sensor consists of two beads wrapped in platinum wire coil – the reference bead and the active bead with a thermal barrier between the two beads. The operating principle of this point detector works when a combustible gas comes into contact with the catalytic surface on the active catalytic bead. As a result, heat is generated. The resulting rise in temperature of the catalyst bead causes a change in electrical resistance. The change in resistance is related to the measure of gas concentration. 

A standard catalytic bead sensor can be used to detect combustible gases on the scale of 0–100% of the lower explosive limit (LEL). The same catalytic bead sensor can be configured to detect Methane on the scale of 0–5% of volume.

For combustible gas monitoring applications in environments that may have Oxygen concentrations below 10% of volume or a high level of contaminants that will poison a traditional catalytic bead sensor, we would recommend the use of an IR sensor.

 

Infrared Sensors

Infrared sensors or IR detectors have a number of advantages over catalytic type sensors because of the speed of response, low maintenance and unaffected by known poisons. They can also operate in a wide range of temperatures, humid conditions and successfully operate in inert atmospheres.

IR sensors work via a system of transmitters and receivers to detect combustible gases, specifically Hydrocarbon vapours and Carbon Dioxide. However, caution should be taken in applications where Hydrogen or Acetylene may be encountered as these gases will not be detected by an infrared sensor.

The operating principle of IR sensors 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 and 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.

Infrared (IR) detectors can be either point or open-path and are used mainly for hydrocarbon vapours from 0-100% vol.

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.

 

PID Sensors

PID sensors are used for the detection of Volatile Organic Compounds (VOC’s) at very low 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 because 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 but is limited to the number of VOCs it can detect.

Therefore, 11.7eV lamps should only be used when it is 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. 

 

Colorimetric (colour change)

Colorimetric technology allows gas detection 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 including 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.

 

What’s the difference between the following Carbon Monoxide (CO) sensor names; CO, CO high and dual CO/H2S?

Carbon Monoxide (CO) poses a big threat to workers carrying out their jobs in environments where toxic CO gas might be present. CO sensors are found in gas detectors used by workers to protect themselves against the dangers posed by CO. Not all CO sensors are the same. While most CO sensors are based on the same electro-chemistry, there are many different types of CO sensors.

Understanding the different types and the specific advantages and disadvantages of each sensor types are critical to selecting the right CO sensor for your application.

The standard CO sensor is the most commonly used CO sensor type. While it will measure CO and usually includes a Hydrogen Sulphide (H2S) filter to eliminate H2S cross-interference, it is vulnerable to cross-interference from other gases, most notably Hydrogen (H2). When using the standard CO sensor, consider the likelihood of other gases being present in the facility that might interfere with this sensor’s readings.

Another thing to consider is the sensor’s measuring range. A standard CO sensor measures up to 1,000 or 1,500 ppm (parts per million), which might not be high enough for mine and rescue applications or the steel industry. This leads us onto the CO high range sensors.

The CO high or CO high range sensor is more commonly used in industries such as mining or mine rescue and steel. Rather than the typical measuring range of 1,000 or 1,500 ppm, this sensor is capable of measuring Carbon Monoxide up to concentrations of 9,999 ppm.

The dual CO/H2S sensor is commonly used to detect for CO. This sensor is a combination of both a Carbon Monoxide sensor plus a Hydrogen Sulphide sensor, with both sensors being built into a single housing.

The dual CO/H2S sensors are commonly used to detect for four gases using three sensor slots or to detect for six gases using five sensor slots. While this is extremely convenient and helpful in achieving smaller size gas monitors, remember that since this sensor must allow both gases to diffuse into it, it will not include the H2S filter. In this instance, there is a trade-off between gas detector size and the sensor’s cross-sensitivity to H2S.

There are three considerations when choosing a CO sensor;

  1. Your application
  2. Your CO gas levels
  3. The potential background cross-interfering gases that could be present.

If you’re still in doubt about which CO sensor to use, talk to a1-cbiss. We can help make the best recommendation for you and your application.

 

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