Sewer Testing Equipment: Air, Vacuum, and Deflection Tests Explained

Before a new sewer main or manhole goes into service, it has to prove it's watertight and built to spec. Three acceptance tests do that job - low-pressure air, vacuum, and deflection (mandrel) - and each needs its own equipment. Knowing which test applies, and what gear it takes, is the difference between a clean acceptance and a re-do. Here's the practical rundown.

1. Low-pressure air test (pipe)

The most common way to prove a pipe section is sealed. You plug both ends of the run, pressurize the isolated section with air to a specified low pressure, let it stabilize, then time the pressure drop. Hold within the allowable drop over the required interval and the line passes.

Equipment: a pair of plugs to seal each end - pneumatic test balls for a range of diameters, or mechanical test plugs for fixed sizes - plus an air source, a regulator, and a calibrated gauge. One plug typically carries the air inlet and gauge tap. Our sewer line pressure-test guide walks the procedure and the safety rules end to end.

2. Manhole vacuum test

Where an air test proves the pipe, a vacuum test proves the manhole. You seal the manhole (typically at the frame or the top), draw a specified vacuum on the sealed structure, then time how long it takes the vacuum to decay. A slow, in-spec decay means the manhole and its pipe connections are tight.

Equipment: a manhole vacuum tester - the sealing plate/head, vacuum pump, and gauge assembly sized to the manhole opening. Vacuum testing is fast, dry, and doesn't flood the structure, which is why it's become the standard manhole acceptance method on many specs.

3. Deflection / mandrel test (flexible pipe)

Flexible pipe (like PVC) is checked for deflection - how much the pipe has ovalized under load after backfill. A mandrel (a rigid, go/no-go gauge sized to the allowable deflection, usually 5%) is pulled through the line. If it passes freely, the pipe is within tolerance; if it hangs up, the pipe has deflected too far.

Equipment: a properly sized mandrel for the pipe diameter and spec, plus a pull line. No pressure or vacuum involved - it's a pure geometry check.

Which test do you need?

• New gravity pipe, watertightness → low-pressure air test (plugs + gauge).
• New or rehabbed manhole, watertightness → vacuum test (manhole vacuum tester).
• Flexible pipe, structural roundness → deflection/mandrel test.

Many jobs require more than one - an air test on the pipe and a vacuum test on the manholes, for example. Always work to the governing spec (ASTM/local) for pressures, hold times, and allowable drops.

A note on safety

Every pressurized test carries risk: a plug under head or air pressure that slips becomes a projectile. Restrain plugs, never exceed back-pressure ratings, and bleed pressure before removing anything. The pneumatic test plug guide covers safe handling.

Browse the Sewer Testing Equipment collection for plugs, test balls, and gauges, or call (954) 516-1620 and we'll spec the gear for your test.

Frequently asked questions

What equipment do I need to air-test a sewer line?

Two plugs to seal the ends of the section (pneumatic test balls or mechanical plugs), an air source with a regulator, and a calibrated gauge. One plug usually carries the air inlet and gauge connection.

What is a manhole vacuum tester?

A rig that seals a manhole, draws a specified vacuum on the sealed structure, and times the decay to confirm it's watertight. It's a fast, dry alternative to flooding the manhole, and it's the standard acceptance test on many specs.

What's the difference between an air test and a vacuum test?

An air test pressurizes an isolated pipe section and watches for pressure loss. A vacuum test pulls a vacuum on a sealed manhole and watches for vacuum decay. Pipe vs. structure.

What is a mandrel (deflection) test?

A go/no-go check for flexible pipe: a rigid mandrel sized to the allowable deflection (commonly 5%) is pulled through the line. If it passes freely, the pipe hasn't ovalized beyond tolerance.