Ultrasonic detector: testing steam traps and leaks

In a commercial boiler room or an industrial plant across Greater Montréal, some losses are neither visible nor audible: a steam trap blowing through continuously, a compressed-air leak behind a compressor, a pressurized fitting whistling in a frequency band the human ear no longer registers. The ultrasonic detector exists to bring those signals back into the actionable range. The catch is knowing how to read it. Here is a field review of the tool: what it is for, when to pull it out of the case, how to interpret the measurement, and the traps that turn a good instrument into a false-positive machine.

Role: hearing turbulence

Any turbulent flow — a leak, a trap stuck open, gas passing across a valve seat — generates broadband sound that extends well above the audible range. The ultrasonic detector captures that energy in the ultrasound band (typically between 20 and 100 kHz depending on the device) and converts it, through heterodyning, into a sound the technician hears in the headset, alongside a relative amplitude reading.

Working in that band is a matter of physics: machinery noise dominates mostly below 10 kHz, while leak energy peaks higher, often around 20 to 30 kHz. Listening high in the spectrum moves you away from boiler-room din and isolates the leak signature. That is what makes ultrasound effective where plain audible inspection fails.

Two families of tools coexist. Pocket heterodyne detectors, such as the UE Systems Ultraprobe line or SDT’s trap-checking instruments (TRAPChecker), with an airborne probe and a contact probe. And multi-microphone acoustic imagers, such as the Fluke ii900 (roughly a 2 to 52 kHz range) or FLIR’s Si-series cameras, which overlay a sound map onto a visual image to point at the leak from a distance. Both measure the same thing; they do not serve the same job.

When to use it

• Steam trap testing. This is the headline use. A contact probe on the trap body reveals whether it modulates normally, blows through continuously (stuck open, dead loss), or is silent (stuck closed, condensate held back). It is the direct complement to a documented inspection program.
• Compressed-air leaks. On a plant network, every leak is energy paid to the compressor. Ultrasound locates them fast, including several metres away with an imager.
• Pressurized leak finding (gas, steam, vacuum), blow-offs, check valves and relief valves that don’t hold, heat exchangers.

When not to rely on it alone: to validate the tightness of a combustible-gas installation, ultrasound helps flag a suspect point, but confirmation goes through the methods recognized under CSA B149.1 (leak-detection solution or an approved gas detector). The instrument points you there; it does not sign off compliance.

How to read the measurement

An ultrasonic reading is comparative, not absolute. The decibels on screen depend on the gain you set, the distance, the angle and the device; a raw number means nothing on its own. The field method that holds up:

1. Establish a healthy reference. Measure an identical component in good condition (a healthy trap of the same model, a leak-free section of pipe). That is your zero.
2. Compare the suspect point at the same gain and distance. A clear, stable amplitude gap — or a continuous sound signature where you expect modulation — flags the fault.
3. Listen, don’t just watch the number. A healthy trap “breathes” in cycles; a trap stuck open produces a continuous hiss or rush. Sound quality often tells you more than amplitude.
4. Lower the gain to localize. Reducing sensitivity and using the focusing cone or contact probe narrows in on the exact source instead of picking up reflections.

Interpretation errors

• Mistaking background noise for a leak. Too much gain makes the whole environment sing. Reflections off hard surfaces (sheet metal, concrete) create phantom hot spots. Always cross-check by lowering sensitivity and changing the angle.
• Reading amplitude as an absolute value. With no healthy reference and no controlled distance, comparing two readings is meaningless. This is the error that drives the most unjustified replacements.
• Skipping the contact probe on traps. In airborne mode in a noisy boiler room, you capture the surroundings. Direct contact on the trap body isolates the component.
• Confusing normal flow with a defect. On a thermodynamic or float trap, a certain cyclic noise is expected. Diagnosing without knowing the trap type and its normal behavior leads to false positives.
• Overestimating range. Ultrasound attenuates quickly with distance and loses precision as you move away; at short range the usable band can run higher, but reliability comes from getting close and aiming well.

Field verdict

The ultrasonic detector is one of the best value-for-effort tools in steam and compressed-air maintenance: compact, fast, it turns an invisible loss into a prioritized repair decision. Its limit is not the hardware but the method. Without a healthy reference, controlled gain and knowledge of the component under inspection, it tells you what you want to hear — and gets healthy parts replaced. For a large asset base or overhead leaks, the acoustic imager speeds up the search; for targeted trap testing, the pocket detector with a contact probe is hard to beat. In every case, the instrument replaces neither the code-required gas tightness test nor the judgment of a technician who knows what they are listening to.

This is the level of rigour the Montréal Combustion team brings to every intervention.