Gas Monitoring Sensor Technology – A Detailed Guide for Gas Detection Users

Gas Monitor Sensors

Gas Monitor SensorsPersonal gas monitors are essential for monitoring Hazardous gases in various environments. They utilise different sensor technologies, each with its own advantages, disadvantages, and potential errors. This guide sets out to highlight the important factors to consider when selecting and using gas detectors.

Selecting the appropriate sensor type depends on the specific application, target gases, environmental conditions, and required sensitivity. Understanding the strengths and limitations of each sensor technology is crucial for effective gas detection and ensuring safety in various settings.

A full range of gas monitors can be found on The Castle Shop at https://www.castleshop.co.uk/air-gas-and-hvac/gas-detection/portable-gas-detectors

Detailed Insight into gas sensor types

Electrochemical Gas Sensors

How Electrochemical Gas Sensors Work:

1. Electrochemical Gas SensorsElectrochemical sensors are widely used for detecting gases in various industrial and environmental applications due to their sensitivity, selectivity, and low cost. These sensors work based on a chemical reaction that occurs when a target gas interacts with an electrode. The process involves the reduction or oxidation of the gas at an electrode surface, generating a measurable current. The current is proportional to the concentration of the gas, allowing for precise monitoring of gas levels. It’s a bit like a battery with one of the chemicals missing. That chemical is the gas or vapour you’re looking to detect.

Working Principle:
  • The sensor consists of three key components: the working electrode, counter electrode, and reference electrode.
  • The target gas diffuses through a membrane and reaches the electrode surface.
  • The gas is either oxidised or reduced, depending on the electrochemical reaction, and this reaction generates an electrical current.
  • The current is measured and converted into a gas concentration reading.
Examples of Gases Detected:
  • Carbon Monoxide (CO): Commonly detected by electrochemical sensors due to their sensitivity to CO. It is a colourless, odourless gas produced by the incomplete combustion of carbon-containing fuels, such as natural gas, wood, or gasoline.
  • Nitrogen Dioxide (NO₂): A toxic gas commonly found in vehicle exhaust and industrial emissions. NO₂ can cause respiratory problems and is a major contributor to air pollution.
  • Ozone (O₃): Electrochemical sensors can detect ozone, a highly reactive gas that can cause respiratory irritation and contribute to smog formation.
  • Hydrogen Sulphide (H₂S): Often found in natural gas, sewage, and industrial processes. It is a highly toxic and foul-smelling gas that can cause severe health issues even at low concentrations.
Hazardous Gases:
  • Carbon Monoxide (CO): CO is highly Hazardous in confined spaces and can cause poisoning. High levels of CO exposure can lead to dizziness, confusion, unconsciousness, and death.
  • Nitrogen Dioxide (NO₂): Chronic exposure to NO₂ can cause lung damage, asthma, and other respiratory illnesses.
  • Hydrogen Sulphide (H₂S): This gas is extremely toxic at higher concentrations and can cause immediate health problems such as difficulty breathing, nausea, and loss of consciousness.

Metal Oxide Semiconductor (MOS) Gas Sensors

How Metal Oxide Semiconductor (MOS) Gas Sensors Work:

2. Metal Oxide Semiconductor (MOS) Gas SensorsMOS gas sensors operate by using a metal oxide material, typically tin dioxide (SnO₂), that is sensitive to specific gases. When a gas molecule encounters the surface of the metal oxide, it reacts with the surface, causing a change in the resistance of the sensor. The change in resistance is then measured and used to calculate the concentration of the target gas.

Working Principle:
  • The sensor operates by heating the metal oxide layer, which allows gases to interact with the material.
  • When a target gas is present, it either donates or accepts electrons from the metal oxide, altering its electrical resistance.
  • The change in resistance is directly related to the gas concentration.
Examples of Gases Detected:
  • Carbon Dioxide (CO₂): MOS sensors can detect elevated levels of CO₂ in confined or poorly ventilated spaces, where the gas may accumulate due to human respiration or combustion processes.
  • Methane (CH₄): A common component of natural gas, methane is a potent greenhouse gas that can accumulate in mining, oil, and gas extraction sites. Detecting methane is crucial for avoiding explosions.
  • Ammonia (NH₃): MOS sensors can detect ammonia, commonly used in fertilisers, refrigeration, and industrial processes. Ammonia exposure can irritate the eyes, throat, and lungs.
Hazardous Gases:
  • Methane (CH₄): Methane is highly flammable and can cause explosions in confined spaces when mixed with air in certain concentrations. It is a significant risk in natural gas pipelines, mining operations, and wastewater treatment plants.
  • Ammonia (NH₃): Ammonia is a highly toxic gas that can cause severe respiratory problems, eye irritation, and in extreme cases, asphyxiation.

Photoionisation Detectors (PID)

How Photoionisation Detectors (PID) Work:

3. Photoionisation Detectors (PID)PID sensors are designed to detect Volatile Organic Compounds (VOC’s) and other gases by ionising the gas molecules using ultraviolet (UV) light. When the UV light passes through a gas sample, it causes the gas molecules to ionise, creating positively charged ions and electrons. These ions are then collected on electrodes, generating a current that is proportional to the concentration of the gas.

Working Principle:
  • The sensor uses a UV lamp as the ionising source.
  • When a gas molecule absorbs the UV light, it is ionised, releasing electrons.
  • The resulting ions are collected by electrodes, and the current generated is used to calculate the concentration of the gas.
Examples of Gases Detected:
  • Benzene (C₆H₆): A highly volatile and carcinogenic compound found in vehicle emissions, industrial processes, and tobacco smoke. PID sensors are used to monitor its levels in workplaces and urban environments.
  • Toluene (C₆H₅CH₃): Another VOC that can be detected by PID sensors. Toluene is commonly found in paint thinners, adhesives, and industrial solvents. Long-term exposure can lead to neurological damage.
  • Hydrogen Cyanide (HCN): A toxic gas often used in chemical manufacturing and released from combustion processes. It can be detected by PID sensors and is dangerous even at low concentrations.
Hazardous Gases:
  • Benzene (C₆H₆): Benzene is a known carcinogen that can cause leukaemia and other health problems with long-term exposure.
  • Hydrogen Cyanide (HCN): Even at low concentrations, hydrogen cyanide can cause headaches, dizziness, nausea, and at higher levels, unconsciousness and death.

Infrared (IR) Gas Sensors

How Infrared (IR) Gas Sensors Work:

4. Infrared (IR) Gas SensorsIR sensors detect gases by measuring the absorption of infrared light by gas molecules. Different gases absorb light at specific wavelengths, and by shining infrared light through a sample and detecting the amount of light absorbed, the concentration of the gas can be determined.

Working Principle:
  • The sensor emits infrared light across a range of wavelengths.
  • Gas molecules in the sample absorb specific wavelengths, and the amount of absorbed light correlates with the gas concentration.
  • Detectors on the opposite side of the sensor measure the transmitted light, which is used to calculate the gas concentration.
Examples of Gases Detected:
  • Carbon Dioxide (CO₂): IR sensors are widely used to monitor CO₂ levels in industrial applications, such as fermentation processes, and in buildings for ventilation management.
  • Methane (CH₄): Methane detection is a key application for IR sensors in natural gas industries and mining operations. The gas is highly flammable, making accurate monitoring crucial for safety.
  • Sulphur Dioxide (SO₂): IR sensors are used to detect sulphur dioxide, a toxic gas released during the burning of fossil fuels and industrial processes. SO₂ exposure can cause respiratory problems.
Hazardous Gases:
  • Carbon Dioxide (CO₂): High CO₂ concentrations in confined spaces can lead to asphyxiation due to displacement of oxygen.
  • Sulphur Dioxide (SO₂): SO₂ is an irritant to the respiratory system, causing coughing, shortness of breath, and even long-term lung damage with prolonged exposure.

Catalytic Bead Sensors (Pellistor)

How Catalytic Bead Sensors Work:

Catalytic Bead Sensors (Pellistor)Catalytic bead sensors are based on the principle of combustion. The sensor consists of a pair of heated beads, typically made of platinum, one of which is exposed to the target gas and the other to a reference gas. When the target gas combusts in the presence of oxygen, it produces heat, causing a change in the resistance of the bead. The change in resistance is proportional to the concentration of the gas.

Working Principle:
  • The target gas is oxidised on the catalytic bead, generating heat.
  • The heat causes a change in resistance, which is measured by the sensor.
  • The change in resistance is used to calculate the concentration of the gas.
Examples of Gases Detected:
  • Methane (CH₄): Catalytic bead sensors are widely used for methane detection in natural gas pipelines and industrial environments.
  • Propane (C₃H₈): These sensors are also used for detecting propane leaks in homes and businesses, as propane is a common fuel for heating and cooking.
  • Hydrogen (H₂): Catalytic bead sensors can detect hydrogen gas, which is flammable and explosive in the right concentrations.
Hazardous Gases:
  • Methane (CH₄): Methane is highly explosive and poses significant fire risks when concentrations exceed lower explosive limits (LEL).
  • Hydrogen (H₂): Hydrogen is extremely flammable, and its explosive range is broader than many other gases, making it a significant hazard in industrial environments.

The above is a detailed explanation of how each type of gas sensor works, the gases they detect, and the associated hazards. Understanding these sensors and their applications is crucial for ensuring safety in various industries, particularly in confined or high-risk environments.

Gas Sensor Poisons and Inhibitors

Poisons

Gas monitor sensors are designed to operate in harsh environments, but this does not mean they are immune to everything around them. Some common cleaning and lubricating products can seriously damage a gas sensor, so it is important to know what these are, and what to do about it.

The most common type of poisoning happens with Catalytic Bead sensors (Pellistors), which can be damaged by silicon-based products such as oils, cleaning products and even cosmetics. Other lubricants, lead and sulphur compounds can also cause problems. Poisons tend to melt onto the surface of the Pellistor and prevent it from detecting the flammable gasses

Examples of catalytic bead sensor poisons.

  • Silicones: Silicone based grease, lubricants, floor wax
  • Mercaptans: Used as odorant for natural gas
  • Sulphur containing gas compounds: ex. Hydrogen Sulphide and Sulphur Dioxide
  • Lead containing compounds: ex. Tetraethyl Lead

Inhibitors

These are not usually as destructive as poisons but can still prevent your gas monitor from functioning correctly. Typical inhibitors include halogenated compounds as well as anything containing astatine, bromine, fluorine, chlorine, and iodine. If combustible gases and inhibitors are present at the same time, the catalytic bead sensor may not detect the combustible gas.

Repeated or continual exposure to inhibitors may cause irreversible damage to the sensor, so should be avoided.

Over Exposure

Excessive exposure to combustible gasses can also cause damage to a catalytic bead sensor by burning through the sensor. If you note continued exposure at high concentrations, then you should have your gas monitor checked

Toxic Gas Sensor Factors

Electrochemical or toxic gas sensors can be influenced by substances other than the target gases they are designed to detect. Below are some examples:

  • Ammonia (NH3): Background ammonia levels may reduce the sensor’s lifespan. High, sudden concentrations can poison the sensor, rendering it inoperable.
  • Chlorine Dioxide (ClO2): Negative cross-sensitivities can cause lower readings than the actual gas concentration in the air. For instance, the presence of hydrogen sulphide may lead to an underestimation of ClO2 levels.
  • Ethylene Oxide (C2H4O): This sensor should be zeroed on-site if the ambient temperature exceeds 22°C (71.6°F). Temperature increases to 25°C (77°F) can cause a drift of up to 1 ppm.
  • Formaldehyde (CH2O): Rapid changes in relative humidity (RH) can create short-term transient signals. This sensor also exhibits moderate to high cross-sensitivity to gases such as hydrogen sulphide, isobutylene, phosphine, sulphur dioxide, and hydrogen cyanide.
  • Hydrogen Chloride (HCl): High humidity levels can cause HCl absorption. To prevent degradation, store the sensor with the filter side facing downward, and avoid storage periods exceeding four weeks.
  • Oxygen (O2): Prolonged exposure to high levels of sulphide compounds, such as hydrogen sulphide, can poison the sensor.
  • Ozone (O3): Highly sensitive to changes in temperature and humidity, resulting in sensor drift.

Other Detrimental Conditions

There are also other factors that may affect an electrochemical gas sensor, which include the environment they are used in as well as storage conditions and general care of the equipment.

  • Extreme temperatures can cause issues. At high levels, the electrolyte may evaporate, and low temperatures can cause a drop in sensitivity
  • Prolonged exposure to low relative humidity can also dry out the sensor causing incorrect readings
  • High concentrations of organic solvents can cause damage over time, especially where sensors are stored in these environments.
  • Some exotic gas sensors, such as chlorine and ozone can have shorter lifespans than more common sensor types
  • Dust, sand and direct water ingress can also cause issues for these sensor types
  • Beware how long your gas monitor has been in stock at the seller before you purchase it as oxygen sensors typically only have a 2-year lifespan, so storage time may eat into that.

How to mitigate for the effects of poisons and inhibitors

Bump testing and calibration are crucial to maintaining any gas monitor and should be carried out regularly.

Bump testing is a daily check that uses a calibration gas bottle to pass a known quantity of gas over the sensor to check that it is reading the right levels. Most modern equipment can be bought with bump testing stations that house the gasses and allow users to clip in the gas monitor and carry out a test automatically.

Calibration is usually carried out every 6 months and it a more in-depth check to make sure everything is functioning correctly. This can be carried out in-house with the right training or you can use a calibration house. It is at these calibration checks that sensors will be replaced if necessary.

What safety measures should be considered when using these sensors?

When using gas sensors, it is crucial to implement safety measures to ensure accurate readings, protect the operators, and prevent accidents. Each type of sensor and the gases they detect present different risks, and safety protocols must be tailored accordingly. Below are general safety measures that should be considered when using the sensors mentioned previously:

Electrochemical Gas Sensors

Safety Measures:

  • Calibration: Electrochemical sensors should be regularly calibrated to ensure their accuracy and reliability. Calibration should be performed according to the manufacturer’s instructions, using known concentrations of the target gas.
  • Proper Ventilation: Since electrochemical sensors detect gases like carbon monoxide (CO) and hydrogen sulphide (H₂S), it’s important to use them in well-ventilated areas. Poor ventilation can lead to dangerous gas accumulation, even if the sensor is functioning properly.
  • Regular Maintenance: Electrochemical sensors have limited lifespans. Be sure to monitor their performance and replace sensors or components as needed, especially if they fail to respond correctly or if readings become erratic. Many modern gas monitors will tell you when your sensor should be replaced.
  • Personal Protective Equipment (PPE): For gases like H₂S and CO, where exposure can lead to serious health effects, workers should wear appropriate PPE such as gas masks, gloves, and protective clothing.

Hazardous Gases:

  • Carbon Monoxide (CO): CO is highly Hazardous in confined spaces and can cause poisoning. High levels of CO exposure can lead to dizziness, confusion, unconsciousness, and death.
  • Nitrogen Dioxide (NO₂): Chronic exposure to NO₂ can cause lung damage, asthma, and other respiratory illnesses.
  • Hydrogen Sulphide (H₂S): This gas is extremely toxic at higher concentrations and can cause immediate health problems such as difficulty breathing, nausea, and loss of consciousness.

Metal Oxide Semiconductor (MOS) Gas Sensors

Safety Measures:

  • Avoid Contamination: MOS sensors are sensitive to environmental conditions like humidity, temperature, and particulate matter. Ensure that the sensor is protected from extreme environmental conditions or interference.
  • Calibration and Testing: Regular calibration is necessary to maintain accuracy, especially when detecting gases like methane and ammonia, which may be present in low concentrations.
  • Proper Housing: MOS sensors should be installed in protective enclosures to avoid exposure to corrosive gases or extreme environmental factors, which could degrade their performance.
  • Explosion-Proof Equipment: When using MOS sensors in environments with flammable gases like methane or propane, ensure that the equipment is explosion-proof and complies with relevant safety standards (e.g., ATEX or IECEx certification).

Examples of Gases Detected:

  • Carbon Dioxide (CO₂): MOS sensors can detect elevated levels of CO₂ in confined or poorly ventilated spaces, where the gas may accumulate due to human respiration or combustion processes.
  • Methane (CH₄): A common component of natural gas, methane is a potent greenhouse gas that can accumulate in mining, oil, and gas extraction sites. Detecting methane is crucial for avoiding explosions.
  • Ammonia (NH₃): MOS sensors can detect ammonia, commonly used in fertilisers, refrigeration, and industrial processes. Ammonia exposure can irritate the eyes, throat, and lungs.

Photoionisation Detectors (PID)

Safety Measures:

  • UV Lamp Safety: PIDs use ultraviolet (UV) light to ionise gases, so it’s important to ensure that the UV lamp is safely enclosed to prevent harmful exposure to UV radiation. Never look directly at the UV light source.
  • Ventilation: PIDs detect VOCs (volatile organic compounds) that may be flammable. Ensure that the area where the PID is used is well-ventilated to prevent gas buildup.
  • Gas-Specific Calibration: PIDs are sensitive to a wide range of gases, but they should be calibrated with the specific gases that are likely to be present in the environment. Regular calibration with proper standards is essential.
  • Protective Gear: For Hazardous gases like hydrogen cyanide, workers should wear proper respiratory protection (e.g., air-purifying respirators) and gloves to avoid direct exposure to dangerous substances.
  • Hazardous Gases:
  • Benzene (C₆H₆): Benzene is a known carcinogen that can cause leukaemia and other health problems with long-term exposure.
  • Hydrogen Cyanide (HCN): Even at low concentrations, hydrogen cyanide can cause headaches, dizziness, nausea, and at higher levels, unconsciousness and death.

Infrared (IR) Gas Sensors

Safety Measures:

  • Prevent Contamination: IR sensors detect gases by measuring the absorption of infrared light, and their accuracy can be affected by dust or moisture in the environment. Ensure the sensor is kept clean and protected from contamination.
  • Regular Calibration: To maintain accuracy, IR sensors should be calibrated regularly, especially when used to monitor gases like methane or carbon dioxide.
  • Check for Cross-Sensitivity: Some IR sensors can be cross-sensitive to other gases. Be sure to use the correct type of IR sensor designed for the target gas to avoid false readings.
  • Explosion-Proof Housing: For gases like methane, which are highly flammable, ensure that the IR sensor is housed in an explosion-proof enclosure to reduce the risk of ignition.

Examples of Gases:

  • Methane (CH₄): An explosive gas, methane can form explosive mixtures with air. Always monitor methane in potentially Hazardous areas, such as natural gas storage facilities.
  • Carbon Dioxide (CO₂): While not explosive, CO₂ can displace oxygen in confined spaces, leading to asphyxiation. Ensure the area is adequately ventilated when using IR sensors for CO₂ detection.

Catalytic Bead Sensors

Safety Measures:

  • Avoid Overheating: Catalytic bead sensors operate by heating the target gas to cause combustion, so it’s important to ensure that the sensor does not overheat or malfunction. This is particularly critical in explosive environments.
  • Regular Calibration: Catalytic bead sensors should be calibrated regularly to ensure that they are detecting the correct concentrations of gases like methane and propane. Ensure proper calibration standards are used.
  • Explosion-Proof Design: In environments where flammable gases like methane or propane are detected, the sensor housing should be explosion-proof to prevent potential sparks from igniting the gas.
  • Routine Inspections: Since catalytic bead sensors rely on combustion, they should be checked regularly for signs of degradation or contamination, as this can affect their sensitivity and response time.

Examples of Gases:

  • Methane (CH₄): Methane is extremely flammable, and catalytic bead sensors are used to detect it in environments like natural gas facilities. Explosion-proof sensors are essential to avoid accidental ignition.
  • Hydrogen (H₂): Hydrogen is highly flammable and can form explosive mixtures with air. Use sensors that meet safety standards to protect workers from potential ignition hazards.

General Safety Measures for Gas Sensors:

Training and Education:

  • Ensure that all personnel using gas sensors are properly trained on how the sensors work, how to maintain them, and how to interpret their readings. Training should also cover emergency response protocols in case of gas leaks or sensor malfunctions.

Proper Installation and Placement:

  • Place sensors at the appropriate height and location based on the gas density. For example, gases like methane are lighter than air, so sensors should be placed near the ceiling, while heavier gases like carbon dioxide should be placed closer to the floor.

Backup Alarm Systems:

  • Install alarm systems to alert workers when gas concentrations exceed safe levels. These alarms should be set to trigger when concentrations approach the normal alarm trigger points for gas monitors. This is often taken to be 10% of the permitted Workplace Exposure Limit (WEL)

Routine Calibration and Maintenance:

  • Gas sensors should undergo regular calibration and maintenance as recommended by the manufacturer to ensure reliable performance. This includes replacing sensors or components that have reached the end of their useful life.

Personal Protective Equipment (PPE):

  • Depending on the gas being monitored, PPE such as respirators, gloves, and eye protection may be required to ensure worker safety in case of gas leaks or exposure.

By adhering to these safety measures, the risk of accidents and health hazards can be minimised, and the effectiveness of gas monitoring systems will be enhanced.

Appendix 1: Gas Sensor Overview

Sensor TypeAdvantagesDisadvantagesPotential Errors
Electrochemical SensorsHigh selectivity for specific gases
Low power consumption
Compact size
Linear output
Limited lifespan (typically 1 to 2 years)
Susceptible to poisoning by certain gases
Sensitive to environmental conditions like temperature and humidity
Cross-sensitivity to other gases
Calibration drift over time
Reduced accuracy in extreme conditions
Catalytic Bead SensorsRobust and durable
Suitable for detecting combustible gases
Wide detection range
Requires oxygen to operate
Can be poisoned by silicones, lead, and other contaminants
Higher power consumption
Slow response time
Reduced sensitivity after exposure to certain gases
Calibration challenges
Infrared (IR) SensorsNon-consumable sensing element
Low maintenance
Immune to poisoning
Suitable for detecting gases like CO2‚ and hydrocarbons
Higher initial cost
Sensitive to temperature fluctuations
Limited to gases that absorb IR light
Interference from water vapor
Reduced accuracy in high humidity environments
Calibration required for each gas
Metal Oxide Semiconductor (MOS) SensorsLow cost
Long lifespan
Broad detection range
High sensitivity
Poor selectivity
High power consumption
Affected by humidity and temperature
Baseline drift over time
Cross-sensitivity to humidity and other gases
Non-linear response
Requires regular calibration
Photoionisation Detectors (PID)Detects a wide range of gases, including VOC’s
High sensitivity
Fast response time
High power consumption
Limited to gases with ionisation potentials below the lamp’s energy
Higher cost
Non-specific detection
Interference from ambient light
Calibration challenges
Ultrasonic Gas DetectorsDetects gas leaks without direct contact
Suitable for outdoor environments
Immune to environmental factors like temperature and humidity
Cannot measure gas concentration
Limited to detecting leaks
Higher cost
Cannot detect gas concentration
Limited to specific applications
Requires regular maintenance

Appendix 2: Gas Detection Glossary

Accuracy: The degree of agreement between a measured value and the true or expected value of the quantity concerned.

Ambient Temperature: The air temperature where computers and related equipment are kept. Ambient means the “immediate surroundings.”

Analyser: This is an instrument which can determine quantitatively and qualitatively the components within a mixture.

Bump Test: This is the process which verifies the performance of the gas detector and can ensure that the sensors are still responding to their target gas. A Bump Test Does NOT calibrate the sensors.

Calibration: A calibration will re-configure the output of a gas monitor to take account of the changing sensitivity of a sensor. If carried out by a professional calibration house, a certificate will be issued.

Catalytic Bead: This is a type of gas sensor that uses combustion to detect the presence of explosive atmospheres. Also called Pellistors.

Dew Point: This is the temperature at which a gas vapour begins to condense as a liquid.

Electrochemical Sensor: Sensors used mainly for ‘toxic’ gasses as well as Oxygen (O2). They contain 2 electrodes that generate a voltage in the presence of the target gas.

Gas Monitor: Sometimes called Gas Detectors, this is an instrument that houses one or more gas sensor and will alert the wearer when the atmosphere becomes potentially dangerous.

Infrared (IR) Sensor: Commonly used for Carbon Dioxide (CO2), this type of sensor uses the absorption of infrared light to infer the concentration of the targe gas.

LEL: Lower Explosive Limit. This is the lowest concentration (%) of a vapour or a gas in the air which can produce a flash of fire in presence of an ignition source.

mbar: Millibars. A measure of atmospheric air pressure.

Metal Oxide Semiconductor (MOS) Sensors: This type of sensor uses the change in resistance of a warmed metal oxide layer in the presence of a target gas. Typically used for Carbon Dioxide (CO2), Methane (CH4) and Ammonia (NH3)

Personal Protective Equipment (PPE): Anything worn to protect the wearer from workplace hazards can be PPE, although it is usually regulated by legislation. For hazardous substances, this includes masks and respirators, which are controlled under the COSHH Regulations as well as separate HSE guidance.

ppm: ‘Parts per million’ by volume.

Photo Ionization Detector (PID): This is a type of detector which measures volatile organic compounds and other gases in concentrations from sub-parts per billion to 10,000 parts per million (ppm).

Relative Humidity: This is the amount of water vapour which is present in the air and is expressed as a percentage of the amount needed for saturation at the same temperature.

Response Time: The length of time it takes a system to react to an event or a stimulus.

STEL: Short-Term Exposure Limit.

Time Weighted Average (TWA): For a measured concentration of a hazardous substance, the TWA is determined by how long the person is exposed compared to an 8-hour standardised day.

Transducer: This is a device which converts variations in a physical quantity into an electrical signal, or it could be vice versa.

Volatile Organic Compounds (VOC): They are organic chemicals which have a high vapour pressure at room temperature.

Workplace Exposure Limit (WEL): These are limits set by the UK Health and Safety Executive for exposure to workers under the Control of Substances Hazardous to health (COSHH) Regulations 2002

Appendix 3: Gasses you can Monitor

Not all gasses can be detecting using direct monitoring, and below shows most of the common gasses where electronic sensors are available. For substances not on this list, it is more usual to carry out Air Sampling using a personal air sampling pump and sampling media.

Toxic Gases

Gas detectors monitor toxic gases to ensure safety in environments where exposure can harm health. Examples include:

  • Carbon monoxide (CO)
  • Hydrogen sulphide (H2S)
  • Nitrogen dioxide (NO2)
  • Ammonia (NH3)
  • Chlorine (Cl2)
  • Formaldehyde (CH2O)
  • Sulphur dioxide (SO2)
  • Phosphine (PH3)
  • Hydrogen cyanide (HCN)

Combustible Gases

These gases are flammable and pose explosion risks. Gas detectors monitor lower explosive limits (LEL) for safety:

  • Methane (CH4)
  • Propane (C3H8)
  • Butane (C4H10)
  • Ethanol (C2H6O)
  • Hydrogen (H2)
  • Acetylene (C2H2)
  • Ethylene (C2H4)

Oxygen (O2)

Gas detectors measure oxygen levels to ensure proper air quality. Humans require oxygen to be present in a relatively narrow band of concnetrations:

  • Oxygen deficiency (levels below ~19.5%) can occur in confined spaces.
  • Oxygen enrichment (levels above ~23.5%) increases flammability risk.

Inert Gases

Monitoring inert gases ensures safety in applications like welding, cryogenics, or confined spaces. These gases are not toxic to humans, but can cause oxygen deficiency:

  • Nitrogen (N2)
  • Argon (Ar)
  • Helium (He)
  • Carbon dioxide (CO2)

Specialty or Industrial Gases

Used in specific industries, these include:

  • Chlorine dioxide (ClO2)
  • Ozone (O3)
  • Ethylene oxide (C2H4O)
  • Hydrogen chloride (HCl)
  • Nitric oxide (NO)

VOCs (Volatile Organic Compounds)

Photoionization detectors (PIDs) are often used to detect VOCs in environments like chemical plants:

  • Benzene (C6H6)H6)
  • Toluene (C6H5CH3 or PhCH3)
  • Xylenes (C8H10)
  • Hexane (C6H14)

Application

The selection of a gas detector depends on the specific gases expected in the environment and the application. Always ensure the sensor type matches the target gases to guarantee safety and accuracy.

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