One of the first challenges for choosing a gas detecting product is to determine which gas sensor type you need. Gas sensor technologies have their limitations and are not suitable across all gas types or applications.
Understanding the properties of the various types of sensor technologies that are used in detecting gas will support your product selection decision making. Choosing the right sensor type for gas monitoring involves an assessment of many factors: Target Gas, Costs, Sensor Placement, Environmental Conditions (Temperature/Humidity), Oxygen Content, Power Consumption and Cross Interference.
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. For oxygen, it’s measured in levels of 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)
- 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
Electrochemical sensors can be used over a wide temperature range (-20° to +50°C is common). Generally, electrochemical sensors are compact, have a long life span (1-3 years), require little power and response is good.
Although the sensors are designed to be specific to each gas, there are often some cross interferences with other gases present.
Routine calibration against a known concentration of the target gas, commonly available in disposable cylinders, is necessary for stability and accuracy of the sensor response.
Overall, electrochemical sensors offer very good performance for the routine monitoring of toxic gases and percent of volume oxygen presence and are available for both portable and fixed gas detectors.
Non-dispersive infrared sensors is commonly referred to simply as IR. They operate in a wide range of temperatures, humid conditions and successfully operate in inert atmospheres.
How it works
The operating principle of IR sensors works whereby the infrared light pulses on and off through the sample gas at two wavelengths. Firstly, the two light sources (reference and sample beams) are guided along an optical path and then through the sample gas. Then, one wavelength is set to absorb the sample gas to be detected whilst the other isn’t. From this, the two beams of light and then reflected back and subsequently back through the gas sample and into the unit. As a result, the detector compares the light signal beams and can measure the gas concentration.
Infrared (IR) detectors can be either point or open-path. They are used mainly for detecting hydrocarbon vapours from 0-100% vol, Carbon Dioxide, Methane and Nitric Oxides 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.
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.
An added benefit of using IR for combustible gas detection is that it’s able to detect Carbon Dioxide (CO2)
Caution should be taken in applications where Hydrogen or Acetylene may be encountered as these gases will not be detected by an infrared sensor.
PID sensors detect 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).
What’s the difference between lamps?
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.
PID technology has fast response time and excellent shelf life
PIDs suffer from sensor drift and humidity effects, making calibration requirements more demanding than other common gas detectors. Sensor life is poor.
The sensor consists of two elements, both comprised of a wire coil. The elements are heated to an operating temperature of approximately 250°C. Heat is transferred from the element to the surrounding gas. The amount of heat transferred depends on the thermal conductivity of the gas.
For many years, the thermal conductivity sensor has been used in instruments for measuring combustible gases above the % LEL range and for leak detection. Thermal conductivity sensors are used primarily in portable gas leak detectors.
Advantages – The thermal conductivity sensor does not require oxygen to operate, and it is not susceptible to poisons.
Disadvantages – One drawback is that it cannot measure gases with thermal conductivities similar to the reference gas (i.e. Nitrogen).
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. They have a large spread that results in a response for all gaseous hydrocarbons such as methane and ethane, up to and including the heaviest fuel oils.
Advantages of FID vs PID
As the response factors are limited, a FID reading has a better representation of the actual gas concentrations, while a PID does not.
The FID is somewhat more complex than a PID as hydrogen is needed for ionising the sample and it must be of high purity.
Calibration is less frequently required, while a PID requires calibration and a zero adjustment before taking the measurements in the field to compensate for background conditions.
Due to the nature of the technology, flame ionisation detectors tend to be considered for specialised applications. However, given the superior technological advantages, FIDs are more costly than PIDs.
Metal Oxide Sensors (MOS)
A variety of MOS sensors are available for the detection of combustible gases, chlorinated solvents and some toxic gases, such as carbon monoxide and hydrogen sulphide. MOS sensors, also referred to as solid-state, are inherently non-specific, and as a result are quite useful in applications where the atmospheric hazards are unknown. The output of the MOS sensors varies logarithmically with the gas concentration. This limits the accuracy of the sensor and the overall measuring range of the sensor.
Changes in the oxygen concentration, humidity and temperature also affect the sensor performance. Although MOS sensors are relatively low cost, the stability and repeatability of the sensor are poor. Power consumption is high due to the heating of the element, which restricts the use of this sensor in portable devices. MOS sensors are commonly used in low cost, hard-wired fixed gas detection systems.
Colorimetric (colour change)
Colorimetric technology allows gas detection down to very low levels (including ppb) for a specific gas.
Most colorimetric detectors are ideal for spot checking due to the access of a large gas library within one device. Spot checking is a measurement technique carried out to determine on-the-spot gas levels in applications usually where an alarm is not needed for immediate personal safety.
Also known as papertape technology, it is used to measure a wide range of toxic substances including Carbon Monoxide, Chlorine, Fluorine, Hydrogen Sulphide and Phosgene. It is the only recognised technology for measuring isocyanates.
Applications include semiconductor manufacturing, aerospace, speciality chemicals and industrial research laboratories.
The cross-sensitivities of other interfering gases are different meaning that this is the ultimate way to verify the concentration showed by an electronic gas detector. In this way, you can exclude most cross-sensitivities to be sure of a non-hazardous working atmosphere.
For most colorimetric based gas detector tubes, oxygen doesn’t need to be present for measurement (benefit over electrochemical sensors). 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.
Sensors and Calibration
Until now, there is no electronic method for self-calibration of sensors that will correct the effects of drops, shocks, or extreme exposures to gas or temperatures.
When you think about a typical industrial environment and the multiple workers that carry the gas detectors into confined spaces, you realise that there is a good chance that they will be bumped or dropped, or hit with a strong blast of gas. These factors affect the sensor’s ability to react to gas at the maximum accuracy possible.
The importance of regular instrument calibration is critical to prevent inaccurate readings. Using a known concentration of test gas, the instrument reading is compared to the actual concentration of the gas and then adjustments are made to the readings if they do not match.
Today, most direct-reading instruments require regular calibration as part of a procedure that includes a schedule for bump testing and full calibration for all gas detectors in a company’s fleet.
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.
Standard CO sensor
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.
CO high or CO high range sensor
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.
Dual CO/H2S sensor
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 when detecting for several gases and there’s not enough sensor slots available. The dual sensor comes in handy as it frees up a sensor slot. While this is extremely convenient and helpful 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;
- Your application
- Your CO gas levels
- 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.