By Wu Peng, Senior Process Instrumentation Engineer · Last reviewed July 28, 2026
A bourdon tube pressure gauge measures pressure with a curved metal tube of flattened, oval cross section. Pressure inside the tube pushes the cross section toward a circle, and the hoop stress this creates makes the curved tube straighten slightly. The free end travels only 2 to 3 mm, and a small gear mechanism turns that travel into pointer rotation on the dial. No power supply is needed at any point.
That simple elastic element covers spans from 0.6 bar up to 4,000 bar in standard catalogs. It is why the bourdon gauge is still the most common pressure instrument in any plant. This guide covers the working principle, the three tube shapes, EN 837-1 accuracy classes, range selection with worked numbers, temperature error, and when to specify a transmitter instead.
Contents
- Working principle
- Tube shapes and ranges
- Accuracy classes
- Selecting the range
- Vacuum and compound ranges
- Materials and media
- Temperature and case filling
- Calibration
- Gauge or transmitter
- FAQ
Working principle
The pressure element is a metal tube, flattened into an oval and bent through roughly 270 degrees into a C shape. One end is fixed to the socket and open to the process. The other end is sealed and free to move.
When process pressure enters the tube, it acts on the flat inner walls and pushes the oval cross section toward a circular shape. The hoop stress that builds in the tube wall increases the bend radius, so the C arc opens up and the sealed tip moves outward. At full scale the tip has moved about 2 to 3 mm.
A connecting link transfers that travel to a toothed sector, and the sector drives a small pinion carrying the pointer. A hairspring keeps the gear flanks loaded so the pointer does not rattle between teeth. The result is around 270 degrees of pointer rotation from a few millimeters of tip travel.
The case around the element sits at atmospheric pressure, so the deflection depends on the difference between process pressure and the atmosphere. A bourdon gauge therefore reads gauge pressure, not absolute. If the distinction matters for your process, see our guide to absolute vs gauge pressure.
Tube shapes and ranges
The tube geometry sets the span. Three shapes cover the whole catalog, and the split points are worth knowing before you write a specification.
| Tube shape | Geometry | Typical spans |
|---|---|---|
| C-type | Single arc of about 270 degrees | Up to about 60 bar; the standard shape for most gauges |
| Spiral | Several turns wound in one plane | Low to medium spans; extra turns give more tip travel per bar |
| Helical | Turns stacked as a helix | High spans, from about 60 bar up to 1,000 bar and beyond |
Standard catalogs run to about 4,000 bar; WIKA states the element technology itself can reach 7,000 bar with suitable geometry, material and wall thickness.
Below about 0.6 bar span the tube becomes too stiff to give useful travel, so capsule and diaphragm elements take over. For low ranges or dirty media, a diaphragm pressure gauge is the usual replacement.
Accuracy classes
Accuracy is specified as a class: the permissible error as a percentage of span, constant over the whole scale. EN 837-1 defines classes 0.1, 0.25, 0.6, 1.0, 1.6, 2.5 and 4.0. The class you can buy is tied to dial size in practice, because a small dial cannot resolve a fine class.
| Dial size | Typical class (EN 837-1) |
|---|---|
| 40 and 50 mm | 2.5 (1.6 to 4.0 available) |
| 63 mm | 1.6 (1.0 to 4.0 available) |
| 100 mm | 1.0 (0.6 to 2.5 available) |
| 150 and 160 mm | 1.0 or 0.6 (0.25 to 1.6 available) |
| 250 mm test gauges | 0.25 or 0.1 |
The pairing in brackets is what the standard permits; the first figure is the common commercial build. Confirm the class on the datasheet.
ASME B40.100, the US standard, uses letter grades instead. Grades 4A, 3A, 2A and 1A are flat 0.1, 0.25, 0.5 and 1.0 percent of span. Grades A, B and C are split by scale position: 2-1-2, 3-2-3 and 4-3-4 percent over the first quarter, middle half and last quarter. The middle of the scale is held tighter than the ends, which is one more reason to size the range so your operating point sits mid-scale.
A class is a span figure, so the error as a share of what you actually read grows toward the bottom of the dial. A class 1.0 gauge with a 0 to 25 bar scale is permitted ±0.25 bar anywhere. At a 10 bar reading that is already 2.5 percent of reading. The arithmetic behind that conversion is covered in percent of full scale vs percent of reading, and the difference between accuracy and repeatability in accuracy vs precision.
Selecting the range
Do not buy a scale that matches your operating pressure. EN 837-2 practice, printed on most manufacturer datasheets, is to keep steady pressure below 3/4 of full scale value and fluctuating pressure below 2/3 of full scale. Full scale is reserved for short excursions. Running a bourdon tube at the top of its scale all day works the element near its stress limit and the gauge drifts early.
Worked example: a pump discharge runs at 10 bar steady and pulses to 12 bar on changeover. Steady rule: 10 / 0.75 = 13.3 bar minimum scale. Fluctuating rule: 12 / (2/3) = 18 bar minimum scale. The next standard ranges are 0 to 16 bar and 0 to 25 bar; the pulsating service takes the 0 to 25 bar dial.
On that 25 bar dial a class 1.0 gauge is permitted ±0.25 bar, which is 2.5 percent of the 10 bar reading. That is normal and acceptable for local indication. If the number surprises you, the answer is a larger dial in a finer class, not a smaller scale. Scales are available in bar, psi, kPa and MPa; the conversions are in our pressure units reference.
Two safety details belong in the same specification line. For gas service above 25 bar, EN 837-2 calls for the S3 safety pattern: a solid baffle wall between the element and the dial plus a blow-out back. A burst tube then vents to the rear instead of through the window. An S in a circle on the dial marks the build.
For steam or any media above 100 °C, mount the gauge behind a siphon or a diaphragm seal so the element stays cool.
Vacuum and compound ranges
The same element works below atmosphere. Pull a vacuum inside the tube and the cross section flattens further, so the tip moves the opposite way and the pointer reads negative gauge pressure. A bourdon tube vacuum gauge is scaled −1 to 0 bar. Compound gauges combine both sides, for example −1 to +3 bar for a pump suction line that swings across atmospheric.
The limit is resolution, not principle. The deepest scale division on a −1 to 0 bar dial is far coarser than what vacuum processes call for. Rough vacuum indication is bourdon territory; anything finer needs a different technology.
Where a falling vacuum has to trip an alarm or interlock rather than just show on a dial, a vacuum pressure switch does the job with a relay output. For the units, range bands and instruments across the whole vacuum span, see our vacuum pressure measurement guide.
Materials and media
The wetted parts are the tube and the socket, and the media list decides the metal. Copper alloy is the standard economy build for air, water, oil and other neutral media. 316L stainless steel covers process and chemical service; in WIKA catalog practice the stainless C-type carries scales below 100 bar and helical tubes take over at 100 bar and above. Monel handles media that attack stainless, such as seawater and some acid streams.
Material solves corrosion, not clogging. A crystallizing, viscous or particle-laden medium will block the tube bore whatever the metal, because the tube only works if pressure can reach its inner walls. That service takes a diaphragm seal in front of the element, or a diaphragm gauge where the whole element is a flexible plate. The typical failure of a bare tube on crystallizing media is a gauge that reads low and responds slowly, then not at all.
Temperature and case filling
The tube stiffness changes with temperature because the modulus of elasticity of the metal changes. EN 837-1 gauges state an additional error of up to ±0.4 percent of span per 10 K deviation from the 20 °C reference.
On the 0 to 25 bar gauge above, a 40 °C summer installation adds up to ±0.2 bar on top of the ±0.25 bar class error. Worst case, the 10 bar reading now carries ±0.45 bar, or 4.5 percent of reading. If that matters, keep the gauge cool or measure electronically.
Case filling is the other temperature decision. A glycerin fill damps pointer flutter on vibrating or pulsating service and lubricates the movement. It stays workable to about −20 °C ambient, though some makers derate it to about −5 °C for full accuracy. Silicone oil extends the fill to −40 °C.
A dry gauge is fine on a calm panel, and a snubber in the process line is the cheaper first answer to pulsation before you specify a filled case.
Calibration
A bourdon gauge is calibrated by comparison. The gauge under test and a reference share one pressure source. That source is a dead-weight tester for the finest work, or a hand-operated comparator pump with a digital reference gauge for bench checks.
Take readings at 0, 25, 50, 75 and 100 percent of scale, rising then falling. The gap between the rising and falling curves is hysteresis, and a gauge that has been overpressured shows it as a pointer that no longer returns to zero. Most industrial gauges have an adjustable pointer or movement for span and zero correction; a bent sector from overpressure is a replacement, not an adjustment.
Application example
Instrumentation buyer, Middle East. The customer asked for a hydraulic pressure comparator pump to 700 bar by a specific part number for bench-checking high pressure instruments. The exact model was not in our line, so we proposed an equivalent 700 bar comparator and passed the substitution to the end user for approval. Naming the pressure standard you check against is as much a part of the gauge specification as the gauge itself.
Gauge or transmitter
A gauge gives a local reading with no wiring and no power, and that is the whole decision. The moment a reading has to reach a control system, log a trend or close a loop, specify a pressure transmitter instead, or alongside the gauge on the same tap. Where the pressure only has to trip something at a set point, a pressure switch is the simpler instrument; bourdon elements drive many mechanical switch designs too.
For filter monitoring across two taps, the mechanical answer is a differential pressure gauge. The full mechanical line is on our pressure gauges page.
FAQ
Why is it called a Bourdon tube?
It carries the name of Eugene Bourdon, the French engineer who patented the curved-tube gauge on June 18, 1849. Edward Ashcroft bought the US patent rights in 1852. The element replaced tall mercury columns such as the U-tube manometer for high pressures, and the same tube also drives filled-system dial thermometers.
How accurate is a Bourdon pressure gauge?
Accuracy is the class marked on the dial: 0.1 to 4.0 percent of span under EN 837-1. Common industrial builds are class 1.0 on a 100 mm dial and 1.6 on 63 mm. Because the class is a span figure, error as a share of reading roughly doubles at half scale.
What are the disadvantages of bourdon tubes?
Vibration and pulsation wear the movement, and overpressure sets the tube permanently. Temperature adds up to 0.4 percent of span per 10 K, and hysteresis limits fine work. Spans below about 0.6 bar are impractical, and there is no signal output unless you add contacts or pick a transmitter.
What is the use of Bourdon tube pressure gauge?
Local pressure indication wherever no power is available or wanted: pump discharges, compressors and receivers, boilers, hydraulic power packs, gas cylinder regulators and fire extinguishers all carry one. The same element also works inside pressure recorders and mechanical pressure switches.
Request a quote
Tell us the media, the operating pressure and the ambient conditions, and we will size the range, class and fill for the service. Reach our application engineers or use the form below.
Written and technically reviewed by Wu Peng and the Instranova engineering team.