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IGEM/UP/1B Edition 4: what changed

Checked 21 Oct 2026 against IGEM's standard page and FAQ, Gas Safe Register's ISU 133 (Edition 2, 15 April 2026) and Registered Gas Engineer (11 May 2026).

Key dates

• March 2026: IGEM publishes Edition 4 (IGEM).
• 15 April 2026: Gas Safe Register issues ISU 133 Edition 2, replacing the original ISU 133 of 25 March 2026 after industry feedback about "confusion and misinterpretation" (ISU 133).
• 30 September 2026: Edition 3 withdrawn.
• 1 October 2026: Edition 4 effective "in all circumstances" (IGEM). All tightness testing must now follow Edition 4 (Registered Gas Engineer).

What changed

1. Permissible drops depend on installation volume (IV), not meter size alone. The limits are set by fuel gas and IV, and apply only where existing appliances are connected and no smell of gas has been reported (ISU 133; Registered Gas Engineer).
2. Prove it isn't the pipework. Where there's a perceptible drop within limits, isolate the appliances and retest the pipework only. There must be no perceptible movement on that test. If you can't reasonably isolate an appliance, test with it connected and apply "no perceptible movement" (ISU 133; IGEM FAQ).
3. Gauge rules for "no perceptible movement": up to 0.25 mbar on a fluid (water) gauge or an electronic gauge, except gauges that read to one decimal place, where it's a maximum of 0.2 mbar (ISU 133).
4. Purge volume = 1.5 × IV on all installations. You now calculate IV for every purge, even if you didn't need IV for the tightness test (ISU 133; IGEM FAQ). The old assumed PV of 0.01 m³ for pipe up to 28 mm and a meter up to 6 m³/h has gone (Registered Gas Engineer).
5. Low test pressure. A new note says that if the minimum test pressure for the fuel gas can't be reached, isolate and investigate the low pressure before testing. For natural gas, if 20 mbar can't be achieved, the test can be done at a minimum of 18 mbar, followed by operating pressure checks to IGEM/G/13 (Registered Gas Engineer).
6. LDF after the test. After a pass, reintroduce appliances and test all disturbed joints with leak detection fluid, plus any installation pipework between an isolation valve and the appliance it serves (ISU 133).

What didn't change

• The test itself is "broadly unchanged" (ISU 133): let-by at 7–10 mbar for 1 minute, 1 minute temperature stabilisation, then a 2-minute test.
• The purging procedure is unchanged. Only the purge volume calculation changed (ISU 133). For natural gas, Registered Gas Engineer says you purge to air up to 0.02 m³ and purge to ignition above that. All LPG and LPG/air installations are purged to ignition.
• The LPG permissible limits table is unchanged from earlier editions (Registered Gas Engineer).

Scope in brief

IGEM says Edition 4 covers installations with a nominal bore up to 35 mm (DN32, R1¼), a meter with badged capacity up to 16 m³/h, an IV up to 0.035 m³, and stated operating pressures (21 mbar for NG and LPG/air, 37 mbar for LPG). It treats natural gas as including blends with up to 20% hydrogen. For IV over 0.035 m³, IGE/UP/1 applies (IGEM).

Does your gauge still work?

The 0.25 mbar / 0.2 mbar rules depend on your gauge's resolution. Our digital manometer guide compares resolutions and built-in tightness tests, with prices checked.

Buying the standard

IGEM listed IGEM/UP/1B Edition 4 at £190.82 (hard copy or 1-year digital licence), or £127.85 for IGEM members, on 21 Oct 2026 (IGEM). Gas Safe registered engineers can read ISU 133 free.

Tightness test reference: the stages step by step →

When did IGEM/UP/1B Edition 4 come in?

IGEM published it in March 2026. It became effective from 1 October 2026, and Edition 3 was withdrawn on 30 September 2026 (IGEM).

What is the main change in IGEM/UP/1B Edition 4?

Gas Safe Register ISU 133 (Edition 2) says permissible pressure drops are now set by the installation volume (IV) rather than meter size alone, and any perceptible movement must be shown not to come from the installation pipework by retesting the pipework only.

Has the purge volume changed?

Yes. ISU 133 says the installation volume must be calculated and purge volume worked out as PV = 1.5 × IV on all installations. Registered Gas Engineer notes that the old assumed PV of 0.01 m³ (for pipe up to 28 mm and a meter up to 6 m³/h) no longer applies.

Can I retest to Edition 3 if a test fails under Edition 4?

No. Registered Gas Engineer says that if a test to the new procedure fails, you must trace and repair the escape or make safe under IGEM/G/11, and recommends recording which version of the test you applied.

Sources (checked 21 Oct 2026)

1. IGEM: IGEM/UP/1B Edition 4 (standard page and scope)
2. IGEM: Frequently Asked Questions on IGEM/UP/1B Edition 4
3. Gas Safe Register: Industry Standard Update 133 (Edition 2, issued 15 Apr 2026), published on Registered Gas Engineer
4. Registered Gas Engineer: "Updated tightness testing standard: what you need to know", 11 May 2026

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