FAQ - Fixed Gas Detection Systems

How far apart should the sensors be?

This depends on the application, but for most commercial applications, each sensor should 'cover' 5,000 - 7,000 ft2. This translates to about 80 - 94 ft apart, or a 40 - 47 ft radius. In more hazardous areas, the sensor coverage area should be reduced.

What should the mounting height of the sensors be?

This depends on the density of the gas relative to air. Heavier than air gases should be detected 6 inches from the floor, lighter than air gas sensors should be placed on or near the ceiling, and gases which have a density close to that of air should have sensors installed in the "breathing zone" 4 - 6 feet from the floor. Consider accessibility for calibration when locating sensors.

Are there other considerations for locating sensors?

Sensors should not be placed near ventilation fans or openings to outside. They should be placed in areas where there is good air circulation, but not in the path of rapidly moving air. Pay particular attention to "dead air spots" where there is little or no air movement.

What is the breathing zone?

The breathing zone refers to the area 4 - 6 feet from the floor, where most human breathing takes place. This is a good default location for sensors.

What gases can be detected with CETCI's equipment?

Ammonia NH3
Arsine AsH3
Carbon Dioxide CO2
Carbon Monoxide CO
Chlorine Cl2
Combustible Gases n/a
Ethylene C2H4
Ethylene Oxide C2H4O
Fluorine F2
Formaldehyde CH2O
Hydrogen H2
Hydrogen Chloride HCl
Hydrogen Cyanide HCN
Hydrogen Fluoride HF
Hydrogen Sulphide H2S
Methane CH4
Oxygen O2
Ozone O3
Nitric Oxide NO
Nitrogen Dioxide NO2
Phosphine PH3
Propane C3H8
Refrigerants R11, R12, R22, R114, R123, R134A, R401A, R401B, R402A, R404A, R407C, R408A, R409A, R410A, R422A, R422D, R438A, R500, R502, R507, R507A, R717
Silane SiH4
Sulphur Dioxide SO2

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Are some gases heavier or lighter than others?

All substances have a molecular weight determined by the number and type of atoms in the molecule. In a gaseous state, the larger and heavier the molecule, the heavier the gas.

What are the molecular weights of gases?

Molecular weight of some common gases:

Air   29
Ammonia NH3 17
Carbon Monoxide CO 28
Carbon Dioxide CO2 44
Chlorine Cl2 71
Hydrogen H2 2
Hydrogen Sulphide H2S 34
Methane CH4 16
Oxygen O2 32
Ozone O3 48
Nitric Oxide NO 30
Nitrogen Dioxide NO2 46
Propane C3H8 44

Are there many types of sensors used for gas detection?

There are many sensors used to detect gases. Some gases can be detected by some sensors and not by others, and some sensors are more accurate or gas-specific than others. Types of sensors include Solid State (also known as Metal Oxide Semiconductor), Electrochemical, Catalytic, Infrared, Galvanic, Photoaccoustic, PID and others.

Why are different kinds of sensors used?

Most gases can only be detected by one or two sensor types. Then consideration is given to required accuracy and specificity, sensor life expectancy, and finally cost.

What are the advantages and disadvantages of Solid State (MOS), Electrochemical, Catalytic, Infrared, & Galvanic sensors?

Solid State (MOS) Low cost, long life, quite resistant to poisoning. Broad spectrum, non-specific, not as accurate as other types.
Electrochemical Accurate, repeatable, more gas specific, defined cross sensitivities. Relatively short life, moderately expensive.
Catalytic Range 0 to 100% LEL for many combustible gases, accurate, long life. Moderately expensive, can be poisoned.
Infrared Very gas specific, best way to detect CO2, very accurate & stable, long life. Expensive.
Galvanic Range 0 to 100% volume Oxygen, long life. Expensive, poor low-end accuracy.

Does gas detection equipment need maintenance and calibration?

Yes. Maintenance on a properly installed, quality system is minimal and need only consist of a visual inspection and verification of operation. Calibration frequency depends on sensor type and application. Typical frequency for commercial applications is 1 to 2 times per year. For areas where health hazards may exist, 3 to 4 times per year. Industrial applications 4 to 6 times per year or even monthly.

What is calibration?

Instrument calibration refers to exposing the instrument to a known quantity of the measured substance (real or simulated) and resetting the instrument if required. This verifies that the instrument is operating properly and adjusts for any sensor drift. For gas detection equipment this involves exposing the sensor to a known concentration of the target gas, usually from a calibration gas cylinder, and resetting the instrument to adjust for sensor drift.

Can gas detection equipment produce energy savings?

Yes. A properly installed, maintained and calibrated gas detection system can produce significant energy savings by running ventilation fans only when required, minimizing energy usage and expense.

What is cross-sensitivity?

Cross-sensitivity refers to the response of a sensor to a gas other than the target gas (also called an interference gas).

What are combustible gases?

Combustible gases are those that are monitored because they present a risk of explosion or fire.

Where does CO come from?

Usually occurs as a product of combustion of an organic compound. Most commonly found in vehicle exhaust.

I get confused between CO and CO2 - what's the difference?

CO is the chemical symbol for Carbon Monoxide which usually comes from vehicle exhaust in commercial applications.

CO2 is the chemical symbol for Carbon Dioxide which usually comes from human respiration in commercial applications and is used as an indicator for Indoor Air Quality.

Where do NO and NO2 come from?

Nitric Oxide (NO) and Nitrogen Dioxide (NO2) usually occur as a byproduct of combustion, often from vehicle engines. (see Nitrogen Oxides).

What is Nox?

Nox is an abbreviation for Nitrogen Oxides. (see Nitrogen Oxides).

What are Nitrogen Oxides?

A group of compounds most commonly found in vehicle exhaust, where Nitrogen Oxides are a byproduct of combustion formed from the Nitrogen and Oxygen in the air. In exhaust, the most common is Nitric Oxide (NO) followed by Nitrogen Dioxide (NO2). Other Nitrogen Oxides found in much smaller quantities are N2O, N2O3, N2O4, N2O5, N3O4, and NO3.

Where does CO2 come from?

Carbon Dioxide is a naturally occurring gas in air (less than 1%) and since it is exhaled during respiration, it is often used as an indicator of Indoor Air Quality. CO2 is produced in the combustion of organic compounds and it is also formed during fermentation. CO2 is used in many industrial and food & beverage applications and may present a hazard if it leaks from cylinders.

Where does H2S come from?

Hydrogen Sulphide is commonly produced from decay of organic matter and in industrial processes such as Pulp & Paper and Oil & Gas. It is often found in 'Sewer Gas'.

What does % LEL mean?

Combustible gases form flammable mixtures with air. For each gas there is an explosive range within which the fuel to air mixture will support combustion. LEL is an abbreviation for Lower Explosive Limit, which is the minimum concentration of each gas in air that must be present for combustion or explosion to occur.

% LEL refers to a method of measuring the concentration of a combustible gas where the range of the sensor is set to correspond with the concentration of gas that is below the explosive range. In this case the Lower Explosive Limit equals 100 % LEL.

What does ppm mean?

Parts Per Million. 100% volume equals 1,000,000 'parts' , 1% volume equals 10,000 'parts'.

What does % volume mean?

The percentage of a gas in the total volume of air.

Where are the different transmitters used?

Click here for transmitter matrix.

What gases are present in parking garages?

Vehicle exhaust gases: CO, Nitrogen Oxides
Combustible gases from fuel leakage: Propane, Gasoline, Methane
Interference gases: Solvent fumes from paint or sealer

What gases are present in arenas?

Ice resurfacing equipment exhaust gases: CO, Nitrogen Oxides
Combustible gases from ice resurfacing machinei fuel leakage: Propane, Gasoline, Deisel, Methane

What gases are present in chiller rooms?

Chiller room gases: Ammonia, Refrigerants

What gases are present in swimming pools?

Water purification chemicals: Chlorine, Ozone

What gases are present in warehouses?

Forklift or truck exhaust gases: CO, Nitrogen oxides
Interference gases: Solvent fumes from warehoused products

What gases are present in repair garages?

Vehicle exhaust gases: CO, Nitrogen Oxides
Combustible gases from fuel leakage: Propane, Gasoline, Methane
Interference gases: Solvent fumes