Disclaimer: This page is a summary for field reference only. Always consult the full IGEM/UP/1B Edition 4 standard and any current Gas Safe Register guidance. The engineer on site remains responsible for correct application of the standard.
Key changes in Edition 4
- Installation Volume (IV) drives the permissible drop. Earlier editions tied the limit primarily to meter size; Edition 4 uses the total installation volume (pipework + meter + fittings) to look up the allowable pressure loss.
- Perceptible gauge movement is not attributable to the gas installation pipework. Where any perceptible movement is seen, the installation fails the tightness test regardless of the tabulated limit.
- The standard distinguishes between gauge types and their resolution when defining what counts as "perceptible".
Test pressure by fuel gas
| Fuel gas | Test pressure |
|---|---|
| Natural Gas (NG) | 20 mbar |
| Liquefied Petroleum Gas (LPG) | 45 mbar |
| LPG/Air | 45 mbar |
Permissible pressure-drop limits
Tables 2-4 of IGEM/UP/1B Edition 4. Values are the maximum permissible drop during the tightness test period.
Natural Gas (NG)
| Installation volume | Limit |
|---|---|
| IV <= 0.005 m³ | ≤ 8 mbar |
| 0.005 < IV <= 0.010 m³ | ≤ 4 mbar |
| 0.010 < IV <= 0.015 m³ | ≤ 2.5 mbar |
| 0.015 < IV <= 0.035 m³ | ≤ 1 mbar |
| IV > 0.035 m³Out of scope for IGEM/UP/1B Edition 4 | No perceptible movement |
Liquefied Petroleum Gas (LPG)
| Installation volume | Limit |
|---|---|
| IV <= 0.0025 m³ | ≤ 2 mbar |
| 0.0025 < IV <= 0.005 m³ | ≤ 1 mbar |
| 0.005 < IV <= 0.010 m³ | ≤ 0.5 mbar |
| IV > 0.010 m³ | No perceptible movement |
LPG/Air
| Installation volume | Limit |
|---|---|
| IV <= 0.025 m³ | ≤ 1.5 mbar |
| 0.025 < IV <= 0.035 m³ | ≤ 0.5 mbar |
| IV > 0.035 m³Out of scope for IGEM/UP/1B Edition 4 | No perceptible movement |
Perceptible movement thresholds
Any visible movement of the gauge above these thresholds during the test is a fail, because it indicates a leak that is not attributable to the installation pipework.
| Gauge type | Perceptible movement threshold |
|---|---|
| Water gauge | 0.25 mbar |
| Electronic gauge (2 decimal places) | 0.25 mbar |
| Electronic gauge (1 decimal place) | 0.2 mbar |
Installation Volume (IV)
The Installation Volume is the total internal volume of the installation being tested, including:
- All gas installation pipework downstream of the emergency control valve (ECV)
- The meter body and any associated meter-control volume
- Any additional volumes such as boosters, governors or other components
IV = sum of pipework volumes + meter volume + additional volumes
Pipework volume is calculated from the internal diameter and length of each run:
Volume = pi * (ID / 2)^2 * length
Purge volume
After any interruption of gas supply, or before commissioning, the installation must be purged with the appropriate fuel gas to remove air or inert gas. The required purge volume is:
PV = 1.5 x IV
Where PV is purge volume and IV is the installation volume.
Purge time can be estimated if the meter gas rate is known:
Time (minutes) = (PV / gas rate) * 60
Step-by-step procedure summary
- Select fuel gas and gauge type (Clause 5.6)
Confirm the test pressure and the perceptible-movement threshold that apply to the installation. - Determine Installation Volume (Clause 6.3)
Measure or calculate the volume of all pipework, the meter and any additional components. - Apply test pressure and stabilise (Clause 7.3)
Raise the installation to the correct test pressure and allow time for temperature stabilisation before reading the gauge. - Record observed pressure drop (Clause 7.3, Tables 2-4)
Note the drop over the required test period. Check for any perceptible movement of the gauge. - Interpret the result (Clause 7.3)
If there is any perceptible movement, the test fails. If not, compare the observed drop to the permissible limit for the installation volume. - Retest pipework only if required (Clause 7.3)
Where a meter or regulator is isolated and suspected of contributing to a drop, repeat the test on the pipework alone. No perceptible movement is permitted on the retest. - Purge before commissioning or after supply interruption (Clause 8)
Pass at least 1.5 times the installation volume of gas through the system and verify appliances are operating correctly.
Common pipe sizes (internal diameters)
| Size | Internal diameter |
|---|---|
| 8 mm copper | 6.4 mm |
| 10 mm copper | 8.4 mm |
| 15 mm copper | 13.2 mm |
| 22 mm copper | 20.0 mm |
| 28 mm copper | 26.2 mm |
| 35 mm copper | 32.8 mm |
| 42 mm copper | 39.8 mm |
| 1/2" steel (15 mm) | 16.1 mm |
| 3/4" steel (20 mm) | 21.6 mm |
| 1" steel (25 mm) | 27.3 mm |
| 1 1/4" steel (32 mm) | 35.9 mm |
| 1 1/2" steel (40 mm) | 41.4 mm |
| 2" steel (50 mm) | 52.5 mm |
Typical meter volumes
Approximate internal volumes for common domestic and small commercial meters. Always verify against the manufacturer's badged data plate.
| Meter | Approx. volume |
|---|---|
| U6 diaphragm (domestic) | 0.0071 m³ |
| U16 diaphragm | 0.023 m³ |
| U25 diaphragm | 0.036 m³ |
| U40 diaphragm | 0.049 m³ |
| U65 diaphragm | 0.082 m³ |
| E6 (ultrasonic/rotary domestic) | 0.003 m³ |
| E16 ultrasonic | 0.008 m³ |
Remember: This calculator and reference guide do not replace the full standard, competent training, or sound engineering judgement. If in doubt, consult IGEM/UP/1B Edition 4 in full.