SiteWalk the site on a Sunday afternoon when the lines are off. The compressor is still loading and unloading. Nothing's using air, but it keeps running.
Stand still in the packing hall and you'll hear why. A hiss from a push-in fitting. Another from a cracked hose on a cylinder. A steady whistle from a drain that's stuck open.
On Monday morning the lines start, and every one of those leaks disappears under the noise. They're still there. You just can't hear them.
That's where an ultrasound detector earns its money. It listens above the range of the human ear, where leaks, failing bearings and arcing all make noise, and turns it into something you can hear through headphones over the din of a running plant.
What it actually finds
Air leaks are the obvious one. Gas escaping through a small hole makes a high-pitched noise, mostly above what you can hear. The detector picks it up from a few metres away, through the noise of the line. Walk the pipework with it and you'll find leaks you'd never find by ear on a weekday.
Bearings come next. A healthy bearing makes a smooth, steady rush. A dry one gets louder and rougher. A damaged one adds clicks and crackles. With a contact probe you can hear the change long before a bearing gets hot enough to show on an IR gun.
Valves and traps. A valve that should be shut but is passing makes a steady flow noise. So does a steam trap that's blowing through. You can check them without breaking into the line.
Electrical faults. Tracking, corona and arcing make ultrasonic noise too. A scan of a panel's vents can point to a problem before it burns. More on that below.
Grease by ear
Here's one of the best uses, and one of the cheapest.
Most sites grease on a calendar. So many shots, every so many weeks, whatever the bearing needs. Some bearings run dry. Others get packed so full they run hot, push out the seals, and fail early.
With a contact probe on the housing, you can hear the bearing while you grease it. Add a shot, wait, listen. When the noise drops to its quietest, stop. If it starts to rise again, you've gone too far.
Lubrication isn't only about reliability. HSE's PUWER code of practice notes that "failure to lubricate bearings or replace clogged filters might lead to danger because of seized parts or overheating" (L22, paragraph 70). Getting it right is part of keeping the kit safe. For the signs of a bearing that's already going, see our guide to bearing noise and heat.
Leak surveys that stick
The first survey on most sites turns up a long list. Tag every leak with a numbered label and log the location. Then fix them.
That last step is where most surveys die. The tags fade on the pipework for a year, and the next survey finds the same leaks plus new ones. So treat the list as a set of jobs with owners and dates, not a report. A lot of leaks are five-minute fixes: a fitting, a hose, a seal. Some need the line stopped. Plan those into the next stop.
Then survey again, on a schedule. Leaks come back. Fittings get knocked, hoses chafe, drains stick.
Compressed air is a pressure system as well as a utility. HSE lists "compressed air systems (fixed and portable)" among the systems the pressure regulations cover. Its leaflet also says to "look for tell-tale signs of problems with the system" and gives the example of a safety valve that keeps lifting (INDG261). A survey with an ultrasound detector is a good time to listen to those valves too. Our post on air receivers and the pressure regulations covers what the law asks of the compressor itself.
Want to know what the leaks are costing you? Our post on compressed air leaks covers how to put a figure on it with your own compressor data.
Panels: listen, don't open
An ultrasound scan of a panel is done from outside, through the vents and door seals, with the doors shut. That's the point of it. You find the crackle of tracking or arcing without exposing anyone to live parts.
If you find something, don't open the door to have a look. Plan the work dead. The Electricity at Work Regulations only allow live work when it's unreasonable for the kit to be dead, reasonable to work on it live, and suitable precautions are in place (regulation 14). "We heard a noise" doesn't meet that test.
Ultrasound works well alongside thermal imaging for electrical panels. Heat finds loose terminals under load. Sound can find tracking that isn't hot yet.
Mind the noise you're working in
Your surveyor will spend hours in the noisiest parts of the site. The employer must provide hearing protection at 85 dB(A)*, daily or weekly average (HSE, noise regulations). The detector's headphones are for listening. Unless the maker says they're rated as hearing protection, wear the site's protection as normal.
Who does the survey
The detector is easy to use. Reading it well takes practice. Bearing sounds, valve sounds and electrical noise all need a trained ear, and a reading means nothing without a baseline from the same point.
Here's our view. Buy or hire a detector and train one person properly. Then give them the time. Too many sites buy the kit and hand it to a fitter who is already covering two lines. It goes in a drawer within a month.
If you can't spare anyone, bring in an engineer for the first survey and the fixes that follow. The leaks and dry bearings are costing you every shift they're left. A short burst of extra hands to clear the list often pays for itself before the next quarter's air bill.
For where ultrasound fits with the cheaper checks, see condition monitoring on a small budget.
What to do on Monday
- Walk the site on the next quiet shift with the lines off, and listen. Tag every leak you can hear.
- Pick one person to own ultrasound and book them on proper training.
- Turn every tagged leak into a job with an owner and a date.
- Pick ten critical bearings and take a baseline reading on each.
* Noise action value correct as of 16 October 2026. Check HSE's noise regulations page for the latest. Paragraph numbers refer to L22, fourth edition (2014, amended 2018).