Compressed Air Leak Detection With the SMC PLD Pocket Ultrasonic Detector

A compressor that hardly ever unloads, or line pressure that collapses soon after the shift ends, usually points to compressed air leaks. Most plants already accept that leaks waste energy. The practical problem is different: where exactly are the leaks, and how do you find them quickly and repeatedly? This guide compares three common leak-finding methods, then uses SMC’s PLD pocket ultrasonic leak detector as a worked example. It covers how the detector works, how to use it step by step, and where it cannot help, based on SMC’s own catalogue and operation manual. It ends with a checklist for routine leak surveys.

The short answer

Large leaks (around 500 cc/min or more in SMC’s documentation) can usually be heard or felt. Small and medium leaks can be confirmed at fittings with soapy water, or traced along the piping with an ultrasonic leak detector. A leak emits a strong ultrasonic signal around 40 kHz, and the detector uses that signal to home in.

SMC rates the PLD for small to medium leaks of several tens of cc/min or more, with 100–300 cc/min given as a reference range. Tiny leaks of a few cc/min, pressures of 0.1 MPa or less, large leaks above 500 cc/min, and narrow or obstructed spots can be hard to detect or pinpoint. Most importantly, the PLD’s LEDs show relative ultrasonic intensity only. They cannot be converted into a leak flow rate. It is a tool for finding leaks, not a flow meter.

Where leaks usually are

SMC’s PLD catalogue lists these typical target locations:

The catalogue also uses a one-touch fitting with tube as an example: an uneven tube cut or a tube that is not fully inserted will leak at the fitting. A logical survey route runs from the compressor room through the main header and branch lines to the fittings and components on each machine.

Three leak-finding methods compared

SMC’s catalogue lists touch-and-listen, soapy water, commercial leak detectors and “no inspection” as conventional approaches and notes the problems with each. The table combines that material with some general shop-floor judgement, marked as such:

In practice the methods work together. Sweep the lines with an ultrasonic detector to flag suspects, then confirm at the fitting with soapy water.

How ultrasonic leak detection works

Air escaping at high speed through a gap generates a strong ultrasonic signal around 40 kHz. The PLD picks this up with a microphone, and SMC’s own processing circuits and signal processing reduce the effect of other frequencies such as ambient noise (SMC catalogue).

Ultrasound is directional and fades quickly with distance, so the signal grows as you approach the leak and you can follow it in. The flip side is that air blowers, other noise sources or ultrasonic equipment producing sound around 40 kHz can trigger false readings. SMC’s manual advises working as far from air-blow points as possible and at quiet times.

SMC PLD: official specifications

Sources: SMC PLD catalogue (CAT.ES100-182A), SMC PLD operation manual (DOC1159676), SMC new product page.

Finding leaks with the PLD, step by step

1. Keep the system pressurised. Leaks only “sound” under pressure; at 0.1 MPa or less they are hard to detect.
2. Start on Hi or Mid sensitivity. Higher sensitivity catches weak leaks but picks up more ambient noise. Lower sensitivity reduces noise but only shows strong leaks.
3. Pull the trigger and move slowly, close to the piping. The PLD is powered only while the trigger is held. SMC recommends keeping the unit as close to the piping as possible and moving it gradually around suspect areas.
4. Follow the signal. Rising LED levels mean you are getting closer. Step down to Lo to narrow the detection range and pinpoint the spot.
5. Choose the right nozzle and angle. The 100 mm short nozzle suits narrow spaces at close range. The 300 mm long nozzle reaches high locations and areas behind equipment; always fit the protective cover when using it. Detection is easiest from the front within about 90°; if the reading is unstable, change the angle within that range.
6. Back off if the meter is pinned. A large leak can keep every LED lit, which makes it hard to locate. Lower the sensitivity or increase the distance.
7. Tag, record, recheck. (General practice.) Tag each leak, note its location and symptoms, schedule the repair, and re-survey after the fix.

SMC’s manual adds that if there is no response at high sensitivity, apart from very small leaks, the chance of a leak is extremely low.

Reading the display: it is not a flow meter

This is the point most often misunderstood. SMC’s manual states that the LEDs are a simple display of the detected ultrasonic level, useful as a rough level meter for how big a leak is. Because flow depends on the size of the leak and the pressure, flow values cannot be displayed or converted. The product is meant to shorten the time it takes to find whether and where air is leaking, not to measure leak volume precisely.

SMC’s catalogue includes test curves relating detection distance, sensitivity and number of LEDs lit. For example, at 0.1 MPa a ø0.1 mm leak hole is equivalent to about 150 cc/min. These curves help you understand how distance affects the reading. They are not a way to back-calculate leak flow on site.

To quantify total system leakage and verify savings, you need flow measurement (general practice). Our article on SMC digital compressed air management covers that side. For why leaks matter financially, see the hidden costs of compressed air and how much compressed air savings are really worth.

Usage notes

• Low battery still detects, but without vibration. The blue LED flashes and the vibration alert turns off; leak detection still works. Replace the batteries promptly.
• Use the matching nozzles. For any nozzle other than the short or long nozzle sets, ask SMC first. Material or shape can prevent detection.
• Environment. Operating range is 5–45 °C with IP40 protection, and the manual says to avoid exposure to corrosive gases, chemicals, sea water or water.
• Not for flammable gases, and do not use it in atmospheres containing flammable or explosive gases.

Turning it into a routine leak survey

General recommendations; adapt them to your plant:

Once found, most leaks are fixed by replacing a fitting, recutting a tube, tightening a thread or replacing a seal. Design and air quality matter too: see reducing packaging machine air consumption through design changes, FRLs for compressed air preparation and moisture in compressed air. For audit examples, read when a remote compressed air audit reveals big savings, a 40 kW energy audit wake-up call and five low-cost machine adjustments.

Talk to us

Planning a leak survey, choosing a PLD model and nozzle, or not sure what to replace once you have found a leak? Email us for technical help.

FAQ

Can the PLD show how much air is leaking?

No. SMC’s manual states that the LEDs only indicate ultrasonic intensity and cannot display or convert flow.

Does it work in a noisy plant?

Signal processing reduces the effect of other frequencies, and the unit vibrates at full scale. Lowering sensitivity reduces ambient noise pickup. Air blowers or other sources around 40 kHz can cause false readings, so SMC advises keeping away from air-blow points and working at quiet times.

Can it detect nitrogen leaks?

The manual lists compressed air and nitrogen. Ask SMC about other gases; it must not be used with flammable gases.

How long do the batteries last?

SMC’s guideline is about 75 days at 100 uses a day with standard 2000 mAh alkaline AA batteries, depending on conditions.

Can I fit a different nozzle?

The manual recommends the short or long nozzle sets. Ask SMC before using any other nozzle, as material or shape may prevent detection.

Can it be used outdoors in the rain?

It is rated IP40 and the manual says to avoid exposure to water, so it is not suited to wet or condensing conditions.