Before a tightness test you need the installation volume — the meter, the pipework and the fittings added up — because under IGEM/UP/1B Edition 4 that number decides how much pressure drop you are allowed. Mucka does the sums. You say what the meter is and list the pipe runs, and it gives you the IV, the purge volume and the permissible drop, then puts the lot on the record.
You need the Gas Calculators skill installed. It sits alongside Gas & Oil Certs.
The Edition 4 change, in one line
Edition 4 (mandatory from 1 October 2026) sets the permissible pressure drop by the installation volume, not the meter size — and purge volume is now 1.5 × IV on every installation. So the IV is no longer just a box on the form; it is the number the whole test hangs off. Mucka works to Edition 4.
Where to find it
- Calculators in the menu, under Trade, then pick Installation volume off the board.
- From the record. On a Gas Testing & Purging certificate, tap Work it out next to the total installation volume or the total purge volume.
- Ask Mucka. Type or say it: "U6 meter, two metres of 22 mil and nine of 15 — what's my IV and what drop am I allowed?"
- With no signal. Open the offline screen and tap Calcs. Meters live in cellars.
Doing the sum
- Pick your gas — the permissible-drop table differs between natural gas and LPG.
- Pick the meter: U6 / G4, E6 (the ultrasonic smart meter — much smaller inside), U16 / G10, or Other with the volume off the data badge. Testing pipework only, with no meter on the section? Pick Other and enter 0.
- List the pipe runs: a size and a total length for each. Copper 15 to 35mm and steel ½″ to 1¼″ are built in; anything else goes in as a volume per metre. You do not need to add anything for fittings — the standard's 10% allowance is applied for you.
The figures update as you type:
- Total IV, broken down into meter, pipework and fittings.
- Purge volume — 1.5 × the IV.
- Permissible drop for that IV and gas, from the Edition 4 tables. On the biggest LPG installations the standard allows no perceptible movement at all, and Mucka says so rather than showing a number.
If the IV comes out over 0.035 m³ the installation is outside IGEM/UP/1B altogether — it needs testing to IGEM/UP/1A — and the calculator tells you loudly instead of quoting a limit that does not apply.
Putting it on the record
From the record, tap Work it out, do the sum, then tap Use on the record. One tap fills every field the sum feeds: the IV breakdown and the permissible drop on the tightness test, and the purge volumes on the purging section.
The working — the meter, the runs, the standard applied — is kept with the certificate, so months later you can show how the number was reached, and it prints on the PDF. The fields stay yours to edit: change a figure after the calculator filled it and the record shows the value you chose and what was calculated, rather than pretending the edit never happened.
The result of the test itself is still your call. Mucka gives you the limit; it never marks a tightness test pass or fail for you.
Saving the working to a job
From the calculators page, Save to a job keeps the whole working against the job — same as gas rating, and it works with no signal too, sitting on your phone until you are back in range.