Acoustic and Tracer Gas Leak Location
Purpose
This procedure ends a location visit with a mark measured to two fixed references, agreed by two independent methods, and a measured loss rate attached to it. The expensive failure is not a missed leak, it is a confident mark that is wrong: somebody opens 3 ft of driveway, finds dry pipe, and the shop is guessing with a hole already cut. The second failure it prevents is a tech running one instrument for two hours on pipe that instrument cannot hear, then writing "no leak found" on a service losing water throughout the visit.
Scope
Covers locating a leak on a pressurized potable supply line in a house, small retail space, office suite or multifamily unit, using ground microphone, correlator and tracer gas, under a slab, in a wall or buried between meter and building.
Does not cover the access decision or the post-tension scan, owned by the slab leak detection and access decision SOP, which sends work here at its step 6. Does not cover drain and vent leaks, found with a camera under the sewer camera inspection SOP, since an unpressurized line makes no leak sound. Does not cover how far the water has travelled, which is the concealed leak moisture mapping and handoff SOP, or an active flood, which starts at the shutoff.
Roles and the handoff
| Role | What they own | The handoff |
|---|---|---|
| Dispatcher | Pipe material, era, foundation type | Puts them on the ticket; that pair decides which case leaves the shop |
| Technician | Isolation, both methods, the mark | Returns a rate, a segment, distances in inches, and each method's figures |
| Lead or owner | Any call to open ground or structure | Reads the agreement figure before scheduling demolition |
| Office | Filing sketch and photos against the address | Holds them for a crew who cannot re-find a paint dot on a lawn |
What each method can actually prove
| Method | What it gives you | Where it goes quiet |
|---|---|---|
| Ground microphone | The loudest point at the surface | Plastic pipe, deep cover, carpet, appliances running |
| Correlator | A computed distance from one sensor | Wrong material entry, a fitting between sensors, long plastic runs |
| Tracer gas | Where gas surfaces, on plastic as readily as metal | Saturated soil, a sealed membrane, a cleanout venting elsewhere |
Run the method the pipe can answer, not the cheapest one. Thermal imaging belongs on the same list but reads a plume rather than the hole, and the slab SOP owns it.
Procedure
Answer the electrical question before an instrument leaves the case. Water standing near a receptacle, panel or appliance cord means nobody enters and the customer calls an electrician. Acceptance: a yes or no on standing water near electrical equipment, logged with a time. Stop rule: work resumes only after a qualified person under 29 CFR 1910.332 and 1910.399 isolates circuits per 29 CFR 1910.333(b)(2). Hazard: this step is the hazard, which is why it runs first.
Prove there is a live loss and bring a rate. Run the meter and timed-interval method the high water bill investigation SOP owns. Acceptance: a gallons-per-minute figure with both register readings and elapsed minutes printed. Stop rule: a still register proves flow below what the meter resolves, not zero, so switch to a pressure hold; wrong is starting on "the bill was high". Hazard: never enter a meter pit, a permit-required confined space under 29 CFR 1910.146; read from grade.
Shrink the segment before you shrink the search area. Close fixture stops, the heater cold inlet, irrigation zone valves and branch valves, watching the register after each. Acceptance: one named segment carries the whole step 2 rate within that meter's resolution. Stop rule: a rate that falls only partly means two paths are open, so keep closing or record two leaks. Hazard: hot-side piping sits at 120 to 140 F and 140 F water scalds in a couple of seconds, so glove up and open no fitting here.
Sketch the route and set two fixed references now, not after you find it. Trace from the meter using visible stubs, hose bibs and fixture groups. Acceptance: a sketch with run length in feet and two permanent references named, never a planter or an island slated for replacement. Stop rule: with no two permanent references, mark the wall base and record the offset. Hazard: none here, a sketch on dry ground; record it as checked rather than blank.
Sweep with the ground microphone on a pressurized, quiet system. Kill the ice maker, recirculation pump and blower, then walk the route in overlapping passes at one fixed gain. Acceptance: one amplitude maximum at least twice the amplitude 3 ft either side at that same gain, all three numbers written down. Stop rule: nothing clears the two-times gate, so go to step 7 rather than picking the loudest of several similar numbers, and never raise gain over the spot you suspect. Hazard: an amplified headset turns a valve slam into a spike at your ear, so set gain low before anyone touches piping.
Correlate only where the segment supports it. Sensors on two contacts of one segment with no fitting between, then enter material, diameter and measured length. Acceptance: a computed position within 18 in of the step 5 maximum, tape to tape. Stop rule: more than 18 in apart, do not average and do not pick a favourite; shorten the spacing or move to tracer gas. Hazard: reaching behind fixtures puts hands on razor-edged escutcheons and live cords, so use a light and look first.
Charge with tracer gas where acoustic could not resolve it. Drain and purge the segment, then introduce 5 percent hydrogen in 95 percent nitrogen at or below normal static and never above the tube's rated working pressure at the temperature the line is at, which for PEX falls as temperature rises per ASTM F876. Acceptance: one probe point at least ten times the background recorded before charging, back within twice background 3 ft either side, all four numbers logged. Stop rule: readings elevated everywhere mean gas found a route up through fill and the result is void, so vent and recharge; never substitute a richer blend or a fuel gas. Hazard: hydrogen at 5 percent cannot reach its own 4 percent lower flammable limit once mixed with air, but the nitrogen still displaces oxygen, so no charging or purging in a crawlspace or pit without forced ventilation, and a space that could fall below 19.5 percent oxygen is the confined space 29 CFR 1910.146 says it is.
Set the mark only where two methods agree. Acceptance: the two results within 18 in of each other tape to tape, recorded as two distances in inches to the step 4 references plus a photo showing mark and reference in one frame. Stop rule: one method only, or two more than 18 in apart, means no mark leaves this visit; the record reads "not located" and names what was ruled out. Hazard: marking paint is flammable and its label ventilation instruction applies indoors as written, so mark interior floors with tape and an offset.
Put the water back and prove the building is as safe as you found it. Reopen slowly with a high fixture running, never snapping a valve open, which slams the system and drives scale into fill valves. Acceptance: static within about 5 psi of the arrival reading at the same gauge point, every fixture running solid, and the detector at background in the work area. Stop rule: a valve you closed that will not reseal is replaced today, because you introduced that failure. Hazard: repressurizing puts a tech's face at fittings, so crack the valve a quarter turn and stand off the plane of any joint; then verify what you disturbed still protects, meaning any segment that carried tracer gas is flushed at every fixture until the nearest tap reads background, and where a metallic line was separated the bonding path is broken, so jumper the gap and restore the bond per NEC Article 250 in the edition your jurisdiction has adopted.
The record this produces
Nine fields before the tech leaves: the standing-water answer with its time; the loss rate with both register readings and elapsed minutes; the isolated segment; the sketch with run length and two named references; the three step 5 amplitudes at their stated gain; the correlator position and its distance from the acoustic maximum, or why correlation was not attempted; the four tracer readings including background; the mark as two distances in inches, or "not located" with what was ruled out; and post-restore static against arrival static. They land on the address, not the invoice line, because the repair crew re-finds the point from those two distances weeks later.
One worked pass, including a step that fails
A 1998 single-story house, 3/4 in polyethylene service from a curb meter to the garage wall, no irrigation. Complaint: a wet strip in the lawn through two dry weeks.
Step 1: no standing water near electrical equipment, logged 09:04. Step 2: register at 4471.0 cu ft, then 4474.5 cu ft after a timed 30 minutes. That is 3.5 cu ft in 30 minutes; at 7.48 gal per cu ft, 3.5 times 7.48 is 26.2 gal, and 26.2 divided by 30 is 0.87 gpm. Carried out, 0.87 times 1440 is about 1,260 gal per day.
Step 3: the house-side valve in the garage closes and the register keeps turning at the same rate, so the loss is upstream of it, in the buried service. Step 4: route sketched meter lid to garage wall, 62 ft, referenced to the meter lid and the outside corner of the garage slab.
Step 5 FAILS. At one fixed gain the highest reading is 41 on the instrument scale, with 34 and 37 recorded 3 ft either side. Forty-one against thirty-seven is nowhere near the two-times gate. Stop rule taken: no acoustic mark, and the gain stays where it was. That is the expected result on polyethylene. Step 6: correlation is attempted because the meter yoke and a garage stub are both metallic contacts, and the computed position lands 11 ft from the step 5 maximum, far outside the 18 in gate, so both are logged as attempted and failed.
Step 7: the service is drained, purged, then charged with the 5/95 blend at 55 psi, below the 60 psi static recorded on arrival. Background before charging read 2 on the detector's ppm scale. On a 3 ft grid one hole reads 180, with 3 and 4 at holes 3 ft either side: 90 times background against a ten-times gate, and inside twice background either side.
Step 8: the tracer peak and the correlator position sit 9 ft apart. The grid is rerun at 18 in spacing, holding the peak within 12 in of the first hole, and the correlator is rerun with a probe rod at the tracer peak as its second contact, landing 14 in from it. Fourteen inches clears the 18 in gate. Mark set at 38 ft 2 in from the meter lid and 14 ft 6 in from the garage slab corner.
Step 9: water back on with the laundry bib cracked first. Static reads 58 psi at that bib against 60 psi on arrival, a 2 psi difference and inside the 5 psi acceptance. Every cold fixture runs until the kitchen tap detector reads 2, matching pre-charge background. Nothing metallic was separated, so no bonding jumper was required, recorded as checked.
When the site does not match this procedure
The building cannot be taken off water, or has no shutoff that holds. Tracer gas needs an empty line, so either schedule the charge for a stated off-hours window or run acoustic only and write that the second method was unavailable. A building you cannot isolate at all goes to the shutoff location and labeling visit first.
Shared meter serving several units. Step 2 cannot produce a rate for one address. Substitute a pressure hold on the isolated unit supply, record the drop over a stated interval, and write plainly that no gallons-per-minute figure exists here.
Asbestos-cement service pipe or pre-1980 pipe insulation on the route. Cutting or breaking it releases fibres no glove addresses, so nothing is cut until the material is identified, and presumed asbestos-containing material goes to a licensed abatement contractor.
References
- 29 CFR 1910.333(b)(2) for electrical work practices, with qualified-person scope at 29 CFR 1910.332 and 1910.399; 29 CFR 1910.146 for permit-required confined spaces, covering meter pits and any space that may fall below 19.5 percent oxygen during a nitrogen purge.
- 29 CFR 1910.1001 and 29 CFR 1926.1101 for asbestos, split by the activity performed rather than by building type.
- ASTM F876 for PEX pressure ratings and their reduction with temperature.
- NEC Article 250 for bonding of metal water piping where a metallic line is separated, in the edition your jurisdiction has adopted.
- See related: the slab leak detection and access decision SOP and the high water bill investigation SOP.