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How Gas Leak Detectors Work

By MSF MSF in  
August 21, 2019

Gas Leak detectors, often called sniffers, are a class of instruments specially designed to find small leaks in enclosed gas systems.

The use of pressurized gases in pipelines and vessels is very common in industrial or commercial settings. Pipes carry LPG or natural gas to furnaces; any number of refrigerant gases to cooling systems; and caustic or toxic gases to and from the production floor as an ingredient or byproduct of an industrial process.

Long, complex piping runs with an assortment of valves, adapters and fittings provide a multitude of places leaks can develop. When those leaks do develop, they can lead to decreased performance or the failure of equipment and the release of dangerous gases.

The need for leak detectors is well established. Refrigerant leaks are the number one cause of poor HVAC cooling performance. Leaks in natural gas or LPG lines can create immediate fire/explosion hazards for workers and facilities. Many gases used in commercial or industrial processes have been identified as harmful to the environment and are therefore highly regulated and leaks can result in heavy fines. Having and knowing how to effectively use a leak detector is an important role for plant technicians.

Using a Gas Leak Detector

There was a time before gas leak detectors came into the market that finding a leak involved a lot of detective work. If the leak was large enough a technician could perhaps hear it, provided background noise didn’t drown out the sound. Sometimes pressure gauges would identify a leak and shut-offs could be used to isolate the area of the leak. Sometimes technicians, armed with spray bottles filled with soap and water, had to work their way down a pipe looking for bubbles to announce the location of the leak.

While all these methods are still used today, none of them provide the reliable means to identify and locate gas leaks in a closed pipeline.

Though leak detectors are closely related to personal gas detectors, they are designed to do different things. Personal gas detectors identify the presence of a gas in the environment while leak detectors are used to check for leaks and identify their source.

Aside from the sensor, gas leak detectors are generally equipped with features allowing them to easier pinpoint leaking pipelines. Flexible probes, for example, permit operators to closely trace pipeline routes checking for leaks along the way. These probes often extend several inches from the meter making it easier to extend into hard-to-reach areas.

Most gas leak detectors also include sensitivity adjustment. When gas leak detectors find evidence of an escaped gas and alarm is triggered. Many detectors do not include a display to show concentration readings. To find the location of the leak once the alarm has sounded involves lowering the sensitivity of the instrument and directing it until the alarm sounds again. This indicates increasingly high areas of concentration and point the direction towards the leak.

Having the right gas leak detector and checking its operation technique could mean the difference between finding a leak and missing it.

Leak Detector Technology

Since detecting gases leaking from closed systems is fundamentally different from identifying the presence of gases in the environment, leak detectors use different sensing technology than gas detectors.

Though there are some new technologies for detecting leaking gas, most leak detectors use one of the following sensing technologies:

Heated Diode

Heated diode gas leak detectors operate on the principle that a small electrical current is produced when halogenated gases—commonly used as refrigerants—come into contact with a heated ceramic diode. That current, in turn, triggers an alarm.

Heated diode sensors are very sensitive. They are capable of finding leaks as small as 0.1 ounce per year with the refrigerant R-134a. Also, because they only react with halogenated gases, are less prone to false alarms caused by the presence of other gases.

Heated diode sensors are susceptible to contamination via moisture and oils. They have a short service life, typically just 2-3 years though replacement sensors are inexpensive.

Solid State

Solid state sensors use a metal oxide (often tin oxide or aluminum oxide) formed into a bead-type sensor. A heating element is used to keep the sensor at an optimal temperature for the gas to be detected.

Since solid state sensors identify leaks by sensing changes in conductivity, a pair of biased electrodes are embedded into the sensor to measure the conductivity change. When the sensor comes in contact with gas escaping from a leak, that gas is adsorbed onto the sensor surface, changing the resistance of the sensor material. When the gas disappears, the sensor returns to its original condition.

What makes solid state sensors especially good for finding gas leaks is the strong signal they produce, which increases in the presence of high gas concentrations. This makes it easier to “point” the detector towards the area of highest concentration– the leak.

Solid state sensors are very versatile. They detect both low and high concentrations of gas and can be customized to detect many different gases by varying the sensor’s materials, construction and operating temperature.

The primary strength of solid state sensors is their long life expectancy. In clean conditions, the sensor can last up to 10 years, considerably longer than other sensor types. This is a big advantage for an instrument that is used only intermittently (unless you have a lot of gas leaks!).

Solid state sensors are, however, more susceptible to interference from background gases than other sensor types. In applications where background gases are present, solid state sensors can trigger false alarms. Sometimes, these background gases can be filtered out minimizing their adverse effect.


Ultrasonic leak detectors operate on a very different principle than other leak detectors. Whereas solid state and heated diode sensors look for traces of specific gases seeping from leaks, ultrasonic leak detectors identify the sound waves emitted when gas escapes through a leak.

When gas escapes a pressurized line, it generates a sound in the range of 25 kHz to 10 MHz, well above the frequencies the human ear is sensitive to but in a range easily identifiable to ultrasonic sensors. When the detector senses ultrasonic frequencies they are isolated from normal background noise, amplified, and converted to a frequency audible to humans.

Since ultrasonic gas detectors search for the sounds of leaks rather than escaping gases, they are able to detect leaks of any gas type. Though they are unable to measure gas concentration, the device is able to determine the leak rate of an escaping gas because the ultrasonic sound level depends on the gas pressure and size of the leak.

Ultrasonic gas detectors work in just about any environment but they are especially useful for remote sensing in outdoor environments where gases can quickly dissipate before reaching solid state or heated diode sensors which require contact with the gas escaping from a leak to identify the leak.

If you have any questions regarding gas leak detectors or would like purchase one please don’t hesitate to contact us.

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