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An ultrasonic detector listens for the broadband hiss of turbulent gas escaping through a small orifice. It works on any pressurised gas, including air and nitrogen, and it can find a leak in a noisy plant room from several metres away. A gas sniffer responds only to the specific gas its sensor is built for, and it must be brought close to the leak. Neither is a substitute for the other. For refrigerant work, a heated diode sniffer such as the Elitech WJL-6000S or the RALBIL heated diode unit is the instrument that confirms which gas is escaping. For compressed air, vacuum and inert gas systems, ultrasound is the only one of the two that will find anything at all. For confined space entry, neither is sufficient: you need a multi gas monitor such as the NOYAFA JMS19 that measures oxygen and the specific toxics.
Two instruments listening for different signatures
A gas sniffer is a chemical instrument. Its sensor, whether heated diode, infrared, catalytic bead or electrochemical, produces a signal when molecules of a particular class reach it. The output is a concentration, usually in parts per million or percent by volume, and it is specific to the gas the sensor was chosen for. A refrigerant sniffer responds to halogenated compounds. A combustible gas sniffer responds to gases that burn. An electrochemical cell responds to one toxic gas. The instrument knows what it is smelling.
An ultrasonic leak detector is an acoustic instrument. It does not know or care what gas is escaping. When gas at pressure passes through a small hole, the flow becomes turbulent and generates broadband noise with a strong ultrasonic component, typically centred in the 38 to 42 kHz region. The detector converts that ultrasound down to an audible range and presents it as sound or a level reading. Any gas that leaks under pressure makes that noise. Air, nitrogen, argon, steam, refrigerant and compressed hydrogen all produce it.
That difference decides the job. If you need to prove that the gas in the pipe is the gas escaping, you need the chemical instrument. If you need to find where a pressurised system is losing pressure and you do not care what is inside it, the acoustic instrument is faster and works on gases no sniffer is built for.
What the refrigerant sniffer adds that ultrasound cannot
Refrigerant circuits are usually small, sealed and charged with a known gas. The question is not whether gas is escaping but which joint, valve or coil is passing it. A heated diode sniffer is built for exactly this. The listing for the Elitech WJL-6000S states a maximum sensitivity of 0.21 oz per year and coverage of R22, R134A, R404A, R410A, R1234YF and other halogenated refrigerants. The listing for the RALBIL unit states 0.05 oz per year in the stationary state, a heated diode sensor, a 20 inch probe and nine leak levels on the display. Those figures are the manufacturer’s claims, not measurements made here.
The reason a sniffer wins on refrigerant is not only sensitivity. It is selectivity. A plant room contains brazing fumes, cleaning solvents, vehicle exhaust and sometimes other refrigerants. An ultrasonic detector will hear a leaking compressed air line, a steam trap or a vibrating bearing as readily as a refrigerant leak. The sniffer ignores all of that and responds to the halogenated molecule. On a system where you must distinguish a refrigerant leak from a pneumatic one, the sniffer is the instrument that can tell you.
The trade is reach. A sniffer must be brought to the leak, and the probe tip must be moved slowly along every joint. A leak inside a duct, behind a panel or at the top of a high wall may be unreachable. Ultrasound can hear a leak from a distance, but it cannot tell you the gas. The two instruments are complementary, not competing.
Where ultrasound is the only option
Compressed air is the obvious case. A factory may have hundreds of metres of air line, and no gas sniffer responds to air. An ultrasonic detector finds the leak because the escaping air makes the same turbulent noise as any other pressurised gas. The same applies to nitrogen purging, argon shielding gas, vacuum systems and steam. None of these has a chemical sniffer that will find them, and all of them are audible to ultrasound.
Ultrasound also has an advantage in noisy environments. Because the detector listens in a narrow ultrasonic band, audible plant noise, conversation and machinery rumble are largely rejected. The operator can hear the leak through headphones even when the room is loud. A gas sniffer in the same room is limited by its sensor response time and by the need to place the probe in the plume, which in a draughty plant room may never happen.
The limitation is equally clear. Ultrasound cannot tell you whether the leak is refrigerant, air or nitrogen. On a mixed site, that matters. It also cannot quantify a leak in the units a refrigerant engineer needs. A sniffer reading a rising concentration on a calibrated scale gives some indication of severity. An ultrasonic detector gives a relative signal strength, useful for comparing one leak with another but not a leak rate in ounces per year. The listing for the RALBIL detector publishes a leak rate figure for its sniffer mode; ultrasonic instruments generally do not publish an equivalent, and where a listing is silent the honest answer is that the figure is not stated in the listing bullets.
The arithmetic of sensitivity claims
Sensitivity figures on refrigerant sniffers are usually quoted in ounces per year or grams per year. They describe the smallest leak the sensor can detect under ideal conditions, with the probe stationary and the gas presented directly. A figure of 0.05 oz per year is roughly 1.5 grams per year. A figure of 0.21 oz per year is roughly 6 grams per year. The conversion is standard: one ounce is about 28.35 grams.
Those numbers are useful for comparing instruments, but they are not a leak rate measurement in the field. Air movement, probe speed, sensor contamination and the distance from the leak all change what the sensor sees. A sniffer rated at 0.05 oz per year will not reliably find a 0.05 oz per year leak in a draughty plant room. It will find larger leaks more easily. The same caution applies to ultrasonic detectors, which are usually specified by frequency and by a detection distance rather than a leak rate. Where a listing does not publish a detection distance, the figure is not stated in the listing bullets.
This is where buyers go wrong. They compare the sensitivity number on two sniffers and assume the lower number is the better instrument for every job. In practice, a lower sensitivity number usually means a more expensive sensor with a shorter life and a greater susceptibility to contamination. For locating a leak that is already large enough to matter, a mid range sniffer is often the more practical tool. The difference between ppm and LEL is a separate arithmetic, and it matters when the gas is combustible rather than a refrigerant.
Choosing by the gas, not by the instrument
The decision starts with the gas. If the system contains a halogenated refrigerant and you need to confirm which joint is passing it, a refrigerant sniffer is the right instrument. If the system contains compressed air, nitrogen, argon or steam, ultrasound is the only one of the two that will respond. If the system contains a combustible gas such as methane or propane, neither a refrigerant sniffer nor a generic ultrasonic detector is the correct primary instrument: you need a calibrated combustible gas detector, and the article on why a percent LEL alarm is not a ppm leak locator explains the difference between safety alarms and leak location.
If the question is whether a space is safe to enter, neither instrument answers it. A multi gas monitor such as the NOYAFA JMS19 measures oxygen, carbon monoxide, hydrogen sulphide and combustible gas, and its listing states a response time of 0.5 seconds with LED, vibration and sound alerts. That is a safety instrument. It does not locate a leak and it does not replace a sniffer. Conversely, a sniffer does not certify a space as safe. The two jobs are different and the instruments are not interchangeable.
For refrigerant work in particular, the sniffer is the instrument that confirms the gas. The article on methane detectors for professional leak location covers the equivalent decision for natural gas, where the sensor chemistry and the safety standards are different again.
A practical comparison
| Question | Gas sniffer | Ultrasonic detector |
|---|---|---|
| What it responds to | The specific gas or gas class its sensor is built for | Any pressurised gas escaping through an orifice |
| Can it identify the gas | Yes, within its designed range | No |
| Works on compressed air | No | Yes |
| Works on refrigerant | Yes, if rated for halogenated refrigerants | Yes, but cannot confirm the gas |
| Typical sensitivity figure | Published in oz per year or ppm, where the listing states it | Published as frequency and detection distance, where the listing states it |
| Effect of draughts | Large; the plume may never reach the probe | Smaller; ultrasound is directional and less dispersed |
| Effect of background noise | Chemical interference possible from other gases | Audible noise largely rejected by the ultrasonic band |
| Suits confined space entry | No | No |
The table makes the complementarity obvious. Neither column is better in every row. A technician carrying only one of these instruments will eventually meet a job the other one was built for.
The short answer for a buyer
If you work on refrigerant systems, buy a refrigerant sniffer first. It is the only one of the two that confirms the gas and gives you a sensitivity figure in the units the trade uses. If you work on compressed air, vacuum or inert gas, buy an ultrasonic detector, because no sniffer will find those leaks. If you work on both, carry both; they are not redundant. If the job is confined space safety, buy neither as your primary instrument and buy a multi gas monitor instead. The listings referenced here publish their own figures, and where a figure is absent this page has said so rather than inventing one.
Frequently Asked Questions
Can an ultrasonic leak detector find a refrigerant leak?
Yes, if the refrigerant is escaping under pressure and the leak is large enough to produce turbulent flow. It cannot tell you that the gas is refrigerant rather than air or nitrogen, and it does not give a leak rate in ounces per year. For confirmation of the gas and for small leaks, a refrigerant sniffer is the better instrument.
Can a gas sniffer find a compressed air leak?
No. A gas sniffer responds to the specific gas its sensor is designed for. Compressed air is not one of those gases, so the sensor produces no signal. Ultrasound is the correct instrument for air, nitrogen, argon and steam.
Which is more sensitive, a heated diode sniffer or an ultrasonic detector?
The two are not measured on the same scale. A heated diode sniffer such as the RALBIL unit is listed at 0.05 oz per year in the stationary state, and the Elitech WJL-6000S at 0.21 oz per year. Ultrasonic detectors are usually specified by frequency and detection distance rather than leak rate, and where a listing does not publish a distance the figure is not stated in the listing bullets. Sensitivity figures are not directly comparable across the two technologies.
Does a refrigerant sniffer detect carbon monoxide?
No. A refrigerant sniffer is built for halogenated compounds. Carbon monoxide requires an electrochemical cell tuned to that gas, as found in a multi gas monitor such as the NOYAFA JMS19. A refrigerant sniffer will not warn you of carbon monoxide, and a combustible gas sniffer will not either.
Can I use either instrument to certify a confined space as safe for entry?
No. Confined space entry requires measurement of oxygen, combustible gas and the specific toxic gases identified in the risk assessment, using a calibrated multi gas monitor. Neither an ultrasonic detector nor a refrigerant sniffer measures oxygen or toxic gas concentration, and neither is a substitute for a four gas monitor.
If I can only buy one, which should it be?
Buy the one that matches the gas you work on most. Refrigerant technicians should buy a refrigerant sniffer. Compressed air and vacuum technicians should buy an ultrasonic detector. There is no single instrument that does both jobs, and a buyer who expects one to cover the other will be disappointed on the first job that needs the missing capability.