Flammable Gas Sensors: Which is the Best?

flammable gasFlammable gas sensors monitor ambient air for flammable gases like propane, methane, butane, isopropanol, hydrogen or any of over a dozen gases that can easily ignite in air.

Even though you don’t see them, flammable gas sensors are everywhere. They are used to check for gas leaks in home gas heaters and furnaces, at gas stations, in oil and fuel refineries, or in factories that stores or uses flammable gases. If you’ve ever had your home gas furnace inspected, the technician tested it using a hand-held gas detector with a flammable gas sensor inside it.

What makes a gas flammable?

While gases like propane are obviously flammable, scientists rate all gases on their ability to burn in the presence of oxygen and ignition.

Flammable gases at the right concentration catch fire immediately upon exposure to a spark or flame. This is different from combustible gases are those that can burn in air. For example, gasoline vapors are flammable, while diesel fumes are combustible.

To determine the relative flammability of any gas, the term “flash point” is used. The flash point is a description of how easy it is to ignite a gas in air. Gases that will ignite below 100 °F (38 °C) are all considered flammable, while those above that temperature are considered only combustible. This table illustrates different flammable and combustible gases.

The flash point of a gas assumes the gas is at a specific concentration or volume of molecules in the air. Too much or too little of a gas concentration in air will not burn. Experiments have confirmed the minimum and maximum concentration of every flammable gas. These are known as the lower explosive limit (LEL) and the upper explosive limit.

Of these, the LEL is the most important, because it describes the minimum amount of a gas by volume that is required to be flammable. The LEL for most flammable gases is between 1% and 5%. Therefore, any sensor used for monitoring flammable gases is rated by its ability to measure LEL.

This is important to know because while oxygen sensors are rated as measuring percent of oxygen in a volume of gas and CO2 sensors measure parts-per-million, flammable gas sensors are rated at 0-100% LEL. A measurement of 0% LEL means the air is free of flammable gas, while 100% LEL means there is enough gas in the air to reach the lower explosive limit and be dangerous.

What are some common flammable gases?

Flammable gases can come from many sources: from pressurized tank leaks to vapors given off by liquid gases to natural chemical reactions. For example, the most common flammable gas methane is produced naturally by the breakdown or decay of organic material. Below are some of the most common flammable gases:

  • Acetylene
  • Ammonia
  • Butane
  • Hydrogen
  • Methane
  • Propane

Types of flammable gas sensors

Catalytic Gas Sensors - Pellistor

The oldest type of flammable gas sensor is the catalytic calorimetric or catalytic type, also known as a “pellistor.” Designed in the early 1960s it brought about major improvements in safety in the field of flammable gas detection.

methane gas sensor

Pellistor sensors work by burning the target gas to produce heat. This changes the resistance inside the sensor which is proportional to the amount of gas in the air. To stop the sensor from becoming a flame source, the heater is enclosed in a metal housing with a gas-permeable membrane of steel mesh or pellets.

Pellistor sensors are generalists: they monitor LEL for several flammable and combust gases. This makes them useful for general safety devices where the exact gas composition is not required.

While pellistor sensors are relatively inexpensive, they have a short lifespan as the internal heat slowly degrades the components. They cannot work in a non-oxygen environment. In addition, they are subject to “poisoning” from silicon, lead Sulphur and other molecules in the air.

Non-Dispersive Infrared Gas Sensors

Non-Dispersive Infrared (NDIR) sensors work by the absorption of infrared radiation at certain wavelengths as it passes through a volume of gas. These sensors have a light source and a detector tuned to a specific wavelength. As air passes between them, the amount of light is reduced proportionately to the amount of gas in the air.

This graphic shows the gas spectra for many common gases.

infrared gas spectra

A variation of an NDIR gas sensor has been developed using a laser light source and a detector mounted several feet from each other where they could monitor a “cloud” of flammable gas between them. However, most NDIR flammable gas sensors are available in small form factors capable of being mounted inside hand-held gas detectors.

The benefit of NDIR sensor technology is that it is reliable, immune to poisoning and has a long lifespan. The challenge is that they require periodic calibration and are built to target one specific flammable gas like methane.

MPS – Molecular Property Spectrometer Sensors

The latest generation of flammable gas sensors has been developed by NevadaNano. They have developed a single chip sensor that can accurately report 0-100% LEL across over a dozen different gases with a single factory calibration.

flammable gas sensor MPS

The MPS sensor chip incorporates a patented array of micro-cantilevers with integrated piezoelectric sensing elements that provide electrical actuation and sensing of resonance frequency. Monitoring resonance is a highly sensitive way to measure very small masses of adsorbed analyte.

The advantage of the MPS gas sensor is its ability to detect over a dozen unique flammable gases including hydrogen with a single sensor and to report each LEL level individually. Like pellistor or NDIR sensors, they are immune to drift, decay or poisoning and require no field calibration or maintenance.

Which flammable sensor is best?

All flammable sensors have their advantages depending on the application. At GasLab we have years of experience testing different types and can help find the sensor that is right for your application.

Photo Credit: Honye Sanges from Pexels

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