Commercial Gas Tightness Testing Calculator

What the tightness test calculator works out

A tightness test proves that a commercial gas installation is not leaking at a rate that could create danger. Under IGEM/UP/1 the length of the test depends on how much gas the installation holds and how finely the gauge can be read: a larger installation needs a longer test for the same leak to show as a pressure drop, and a more sensitive gauge can shorten it.

The calculator first works out installation volume from your pipe runs, adding ten per cent for fittings and the internal volume of the meter. It then applies the tightness test duration formula for natural gas: test duration in minutes equals the gauge readable movement (GRM) multiplied by the installation volume multiplied by a factor of 42. The result is rounded up and held within the usual limits of a minimum of two minutes and a maximum of thirty minutes.

The stabilisation period is shown as at least the test duration and never less than one minute. Pressure in a freshly pressurised installation moves as the gas temperature settles, so the engineer lets it stabilise before starting the timed test. Where pipework runs through areas with changing temperature, such as roof voids or sunlit external runs, a longer stabilisation period is sensible.

Choosing the gauge

A water gauge (U-gauge) reads to about 0.5 mbar, while a suitable electronic gauge reads to 0.1 mbar. Because GRM sits directly in the formula, switching to an electronic gauge cuts the test duration to a fifth for the same installation, which is why digital gauges are normal on larger commercial jobs. Whatever gauge is used must be in calibration and suitable for the test pressure.

For new installations, IGEM/UP/1 expects no perceptible pressure drop over the test period. For existing installations, a small permissible drop may be acceptable depending on the size and ventilation of the areas the pipework passes through and whether there is a smell of gas; that permitted drop is worked out from the standard's tables and is not calculated here. Any smell of gas means the leak must be found and repaired regardless of the gauge reading.

Worked example

An office plant room has 30 m of 40 mm steel, 15 m of 25 mm steel and a U40 meter. Pipe volume is 30 x 0.0015 plus 15 x 0.00064, which is 0.0546 m³. With ten per cent for fittings and 0.067 m³ for the meter, installation volume is about 0.127 m³.

With a water gauge, the duration is 0.5 x 0.127 x 42, about 2.7 minutes, rounded up to 3 minutes. With an electronic gauge reading to 0.1 mbar, the raw result is about 0.5 minutes, so the two-minute minimum applies. The engineer stabilises the pressure, then watches the gauge for the full test duration with no perceptible drop for a new installation.

Limits of this calculator

This calculator covers natural gas installations at low pressure using the general IGEM/UP/1 method. It does not set test pressure, calculate permissible pressure drop for existing installations, cover strength testing, medium pressure, LPG, or installations where the duration would exceed thirty minutes and a different approach is needed. Small installations up to 35 mm and 0.035 m³ may be tested under IGEM/UP/1B instead.

Tightness testing on commercial installations must be carried out by a Gas Safe registered engineer with the relevant testing and purging competence. Our London engineers carry calibrated electronic gauges, calculate the test from a site survey, test valves for let-by and issue a CP16 certificate on completion.