Bourdon Tube Pressure Gauge Explained: How It Works and Where It Fits
TL;DR: A bourdon tube pressure gauge is a mechanical dial gauge. Inside is a flattened, C-shaped metal tube sealed at one end; when pressure enters, the tube tries to straighten, and that few-millimetre tip movement is magnified by a link, sector gear and pinion to swing a needle across a calibrated scale — no battery needed. It is a rugged general-purpose gauge for medium and higher pressures, but for the low-pressure gas and air work you meet on boilers and HVAC it is usually the wrong tool: a digital manometer such as the ManomHVAC Pro Digital Gas Manometer (£53.14) reads in mbar directly and is easier to trust on your first jobs.
If you are building your first tool kit on an apprentice's budget, this guide explains the gauge type you will see everywhere, so you can tell it apart from the low-pressure manometer your employer's tool list actually wants — and avoid paying for the wrong instrument.
How a Bourdon tube pressure gauge works
The heart of the gauge is the Bourdon tube itself — a metal tube with a flattened, oval cross-section, bent into a "C" and sealed at the free end. The process pressure is fed into the open, fixed end.
- Pressure enters the tube and pushes outwards on the oval walls.
- The oval section tries to become more circular, which makes the curved tube straighten very slightly.
- The sealed tip moves only a few millimetres, but a link connects it to a sector gear.
- The sector gear meshes with a small pinion on the pointer shaft, multiplying that tiny movement into a pointer sweep of up to about 270° across the dial.
- A hairspring takes up the slack in the gears so the needle does not jitter or read low on rising pressure.
Because it is purely mechanical, a Bourdon gauge needs no power and gives an instant local reading — which is exactly why you still see analogue dials on compressors, filling loops and hydraulic rigs.
Reading the accuracy class
For mechanical gauges, accuracy is quoted as a percentage of the full-scale span, not the reading. Under the European standard EN 837-1 the classes are 0.1, 0.25, 0.6, 1.0, 1.6, 2.5 and 4.0 — the number is the percent of span. A typical industrial dial is class 1.0 (roughly Grade A in the ASME system), meaning ±1% of the full range.
The catch for a trainee: 1% of a wide span is a big error near the bottom of the dial. A 0–10 bar gauge at class 1.0 can be ±0.1 bar anywhere on the scale, so a low reading taken near zero is barely meaningful. That is one reason low-pressure gas and air work moved to digital.
Where a Bourdon gauge fits — and where it doesn't
Honest boundary: a Bourdon tube gauge is strong at medium to high pressures and poor at very low pressures. For the millibar-range gas and air readings in domestic and light commercial heating, a Bourdon dial simply cannot resolve the numbers you need. The mistake that costs new engineers money is buying the wrong type of gauge — a hydraulic or tyre-style Bourdon dial — when the tool list means a low-pressure manometer.
| Job | Right tool |
|---|---|
| Boiler standing/working gas pressure (mbar) | Digital manometer |
| Duct or filter pressure drop (Pa/mbar) | Dual-port digital manometer |
| Sealed system / filling loop (1–3 bar) | Bourdon dial or digital |
| Hydraulic test rig (tens–hundreds of bar) | Hydraulic Bourdon or digital hydraulic gauge |
For the gas and low-pressure air side of the trade, a purpose-built manometer is the safer buy. The ManomHVAC unit reads directly in mbar with ±0.3% FSO, offers 12 selectable units and a one-button zero, and has a backlit screen for dark boiler cupboards — see our U gauge vs digital manometer comparison for the low-pressure side of the story.
Building your first tool kit without wasting money
Apprentice budgets are tight, so buy once and buy right. Keep an analogue Bourdon dial in mind for sealed-system and pressurisation checks where a rugged, power-free gauge is handy, but make the low-pressure manometer your priority purchase — it is the instrument that appears on nearly every gas and HVAC commissioning list. An affordable digital manometer at £53.14 with a 12-month warranty and UK support is far more useful on day one than a high-pressure dial you will rarely connect.
Whatever you choose, confirm it with your assessor or employer against the specific units and range your course and jobs require, and remember that factory calibration is not the same as a dated calibration certificate if your workplace demands one.
Frequently asked questions
Is a Bourdon tube gauge the same as a manometer?
Not quite. "Manometer" is used broadly for pressure meters, but in the trade it usually means a low-pressure instrument (a water column or a digital unit) reading in mbar or Pa. A Bourdon gauge is a mechanical dial better suited to higher pressures.
Can I use a Bourdon pressure gauge for boiler gas pressure?
It is not recommended. Domestic gas works around 18.5–23 mbar at the meter, which is far too low for a typical Bourdon dial to read accurately. Use a digital manometer designed for low-pressure gas instead.
Why do Bourdon gauges still exist if digital is more accurate?
They are rugged, cheap, need no power and give an instant glance-value — ideal for sealed systems, compressors and hydraulic rigs where a rough medium/high-pressure reading is all that is required.
Ready for the right low-pressure tool? Shop the Pro Digital Gas Manometer — £53.14 · Free UK delivery · CE/UKCA · 12-month warranty · 30-day returns