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Tightness test reference (IGEM/UP/1B Edition 4)

A plain-English checklist of the Edition 4 tightness test, built only from public guidance by IGEM and Gas Safe Register. Each step names its source. Checked 21 Oct 2026.

"No perceptible movement": the gauge rules

Source: Gas Safe Register ISU 133 (Edition 2, 15 April 2026), "Perceptible Gauge Movement".

The test, stage by stage

1. Visual inspection and set-up. Check the installation and make sure appliances are set up correctly, for example cooker lids upright and permanent pilots extinguished. (ISU 133)
2. Turn off the ECV (or appropriate valve). Isolate at the emergency control valve or the appropriate valve. (ISU 133)
3. Let-by test: 7–10 mbar for 1 minute. Connect the gauge and carry out the let-by test. (ISU 133)
4. Raise to test pressure, stabilise for 1 minute. If let-by passes, raise the pressure to the tightness test pressure for the fuel gas and allow 1 minute for temperature stabilisation. If the minimum test pressure can't be achieved, isolate and investigate the low pressure before going on. For natural gas, if 20 mbar can't be achieved the test can be done at a minimum of 18 mbar, with operating pressure checks to IGEM/G/13 after a pass. (ISU 133; Registered Gas Engineer)
5. Test for 2 minutes. Adjust the pressure if needed, then test for a further 2 minutes. (ISU 133)
6. Read the result. No perceptible movement (and no smell of gas): the installation can be considered gas tight. Pipework only, or only new appliances connected: there can be no perceptible movement. Existing appliances connected and perceptible movement: go to the next step. (ISU 133; IGEM FAQ)
7. Assess against the permissible limits. Compare the drop with the permissible limits table for the fuel gas, using the installation volume (IV). If the drop is below the smallest permissible limit for that gas, it is within limits and you don't need IV for this comparison. If it is above, calculate IV and check the table. Outside the limits: trace and repair the escape, or make safe. (ISU 133; IGEM FAQ)
8. Retest the pipework only. Where a drop is within limits, isolate all appliances and repeat the test on pipework only (including the meter, unless testing from an AECV). There must be no perceptible movement, or the test fails. If it isn't reasonably practicable to isolate an appliance, test with it connected and apply "no perceptible movement". (ISU 133; IGEM FAQ)
9. Finish and LDF. Remove the gauge, refit the test nipple, reintroduce appliances and gas. Test all disturbed joints with leak detection fluid, plus any installation pipework between an isolation valve and the appliance it serves. There must be no smell of gas. (ISU 133)

Permissible limits: where to find the tables

Edition 4 sets the maximum permissible drop by fuel gas (natural gas, LPG/air, LPG) and installation volume. IGEM says the limits are based on a leakage rate of 19 ml per minute for the installation volume, which is why a larger installation has a smaller allowance. The limits only apply with existing appliances connected and no reported smell of gas.

We don't reproduce the tables here. Use the official versions:

• Gas Safe Register ISU 133 (Edition 2): the permissible limits for LPG/air (Table 2), natural gas (Table 3) and LPG (Table 5).
• IGEM's Edition 4 FAQ: worked scenarios and when IV is needed.
• IGEM/UP/1B Edition 4 itself, including Appendix 7 for calculating IV.

Quick scenarios (from IGEM's FAQ)

Summarised from IGEM, "Frequently Asked Questions on IGEM/UP/1B Edition 4", example scenarios.

Purging

Calculate IV for every purge and use purge volume = 1.5 × IV (ISU 133; IGEM FAQ). The procedure itself hasn't changed. For natural gas, Registered Gas Engineer says to purge to air up to 0.02 m³ and to ignition above that. All LPG and LPG/air installations are purged to ignition.

Why this isn't a calculator

We looked at building a permissible-drop calculator and decided not to. To be right, it needs the installation volume, and IV depends on pipe and meter volume figures published in the standard (Appendix 7), which isn't freely available. A calculator built on guessed volumes could pass an installation that should fail. Gas Safe Register's ISU also asks that its tables aren't copied. So this page sticks to what the public sources say, and points you to the official tables.

Record it

Registered Gas Engineer says Gas Safe Register recommends recording which version of the tightness test you applied on your work records. Most certificate apps have a tightness test form. Check it's been updated for Edition 4.

Choosing a gauge? See the digital manometer guide. More background: what changed in Edition 4.

What counts as "no perceptible movement" under Edition 4?

Gas Safe Register ISU 133 (Edition 2) says up to 0.25 mbar on a fluid (water) gauge or an electronic gauge, except electronic gauges that read to one decimal place, where the maximum is 0.2 mbar.

Do I have to calculate installation volume on every tightness test?

Not for every test. IGEM's FAQ says that if the observed loss is below the smallest permissible limit for the fuel gas, IV isn't needed to compare it with the limits. Above that, IV must be calculated. IV is always needed for purging (PV = 1.5 × IV).

What is the smallest permissible drop for natural gas?

Registered Gas Engineer says that on a natural gas installation with existing appliances connected, any drop over 1 mbar means you must calculate IV and compare it with Table 3. Below 1 mbar is automatically within permissible limits, but you must still retest the pipework only and there must be no smell of gas.

Why doesn't GasStack have a tightness test calculator?

Working out IV needs pipe and meter volume data from the standard itself, which isn't publicly available, and a mistake would be a safety risk. Gas Safe Register also restricts copying of its ISU tables. So we explain the stages and link to the official tables.