Bourdon Tube Pressure Gauges Explained: How They Work and How to Choose

A Bourdon tube pressure gauge measures pressure with a curved, hollow metal tube that tries to straighten when pressure is applied inside it. That small movement is amplified by a linkage and a geared sector to turn a pointer across a dial. The design was patented by the French engineer Eugène Bourdon in 1849 and is still the most widely used mechanical pressure gauge in the world, because it is simple, needs no power and works across a huge range of pressures. This guide explains how a Bourdon tube gauge works, the main tube shapes, the materials, the sources of error and how to choose and maintain one.

How a Bourdon tube gauge works

The heart of the gauge is a thin-walled tube with an oval or flattened cross-section, bent into a C shape and sealed at one end. The open end is fixed to the connection block, which is attached to the process. When fluid enters, pressure tries to turn the flat cross-section into a rounder one. Because the outer wall of the bend is longer than the inner wall, the tube uncurls slightly. The sealed free end moves a short distance, and that motion is the measurement.

The movement then passes through four parts:

1. Connecting link: a small rod joining the tube tip to the sector arm.
2. Geared sector: a toothed segment that multiplies the small tube movement into a larger rotation.
3. Pinion: a tiny gear on the pointer shaft that engages the sector.
4. Hairspring: a fine spring that removes backlash in the gears so the pointer reads steadily.

The pointer then shows the pressure on the dial. When pressure falls, the tube springs back and the pointer returns to zero.

Gauge pressure, not absolute

A standard Bourdon gauge reads gauge pressure. The outside of the tube is exposed to atmospheric pressure inside the case, so the gauge reads the difference between the process pressure and the surrounding air. At atmospheric pressure the pointer sits at zero. Gauges can also be made to read vacuum, or both vacuum and positive pressure in a compound range. Our range selection guide explains how to choose between them.

Types of Bourdon tube

C-type tube

The C-type is the most common shape and the one found in most general-purpose industrial gauges. It is economical and suits a very wide range of pressures.

Spiral tube

A spiral tube is wound in a flat coil of several turns. The longer length produces a bigger movement for the same pressure, which can make a gauge more sensitive at low pressure or remove the need for as much gearing.

Helical tube

A helical tube is coiled like a spring. It gives a large rotation and is often used in recording instruments and in gauges for higher pressures.

Capsule and diaphragm elements

For very low pressures, a Bourdon tube is not sensitive enough. Capsule gauges use two thin metal diaphragms joined at the edge, which flex when pressure enters. The KPCh low pressure gauge in our range is a capsule design. It reads from 0 to 2.5 kPa up to 0 to 60 kPa for air and gas, but it is not suitable for liquids. Diaphragm gauges are also used where the process media is corrosive or viscous, because the diaphragm can isolate the sensing element.

Materials of construction

The tube and connection are the wetted parts. They must withstand the pressure and resist the media.

• Copper alloy and brass: good for air, water, oil and non-corrosive fluids at moderate pressures. The L63 and L100 liquid filled gauges use a brass Bourdon tube and copper alloy connection.
• 316 stainless steel: a strong choice for higher pressures and corrosive media. The AD-D-2, AL-F-2 and AL-H-2 gauges use a 316 stainless steel tube and a 316L stainless steel connection.
• Other alloys: specialist materials exist for aggressive chemicals, oxygen and extreme duty.

The case and window matter too. Our gauges use 304 stainless steel cases with polycarbonate, acrylic or shatterproof safety glass windows. A laminated or shatterproof window is a good choice where fragments could hurt people.

Dry and liquid filled cases

The movement can sit in air or in a liquid. A dry case is lighter and cheaper, while a case filled with glycerine or silicone oil damps vibration and lubricates the movement. Read our detailed comparison in liquid filled vs dry pressure gauges.

Accuracy of a Bourdon tube gauge

Bourdon gauges follow accuracy classes defined by standards such as EN 837-1. The class number is the permitted error as a percentage of the span, and classes include 0.6, 1.0, 1.6, 2.5 and 4.0. The gauges we supply show the typical spread:

• L63 and L100: ±2.5% of full scale
• AD-D-2 and KPCh: ±1.6% of full scale
• AL-F-2 and AL-H-2: ±1.0% of full scale
• Solfrunt 1981: 0.5% of full scale

Accuracy applies to the whole scale, and it is best near the middle. For high precision, a digital gauge is often a better answer. See our digital pressure gauge buying guide.

Sources of error in Bourdon gauges

• Hysteresis: the reading differs depending on whether pressure is rising or falling, because the metal does not return perfectly.
• Zero shift: after overpressure the tube takes a permanent set and the pointer no longer returns to zero.
• Temperature: heat changes the elasticity of the tube and the fill liquid. Our datasheets give a working temperature of up to 60 degrees Celsius, and recommend a pigtail siphon for hotter media.
• Vibration and pulsation: these wear the gears and make the pointer hard to read.
• Parallax: reading the dial from an angle creates an apparent error. A mirror scale or a reading from straight on avoids it.
• Mounting position: some gauges are calibrated in a vertical position and read differently when tilted.
• Wear: the geared movement ages, and a regular calibration check will spot it. See pressure gauge calibration.

Safety features

A pressure gauge contains stored energy. If the tube bursts, the case and window must protect people nearby. Safety features include:

• Blowout back: a weak panel at the rear that releases pressure away from the operator. The Solfrunt 1981 has full rear blowout protection.
• Solid front: a solid wall between the movement and the window, so a burst does not push fragments towards the front.
• Safety glass or polycarbonate: windows that resist breaking into sharp pieces.
• Overpressure protection: a stop or reinforced construction that allows a short spike above full scale. The AD-D-2 states 125% of full scale up to 10,000 kPa.

Choose a gauge with these features for hazardous duty, such as refinery or high pressure applications. For oxygen service, use only gauges cleaned and labelled for oxygen.

Our Bourdon tube gauges

Care and maintenance

• Fit an isolation valve so you can remove the gauge without depressurising the whole line.
• Release pressure slowly. Do not crack a valve fully open on a gauge line.
• Use a snubber, restrictor or pigtail where the line pulses or runs hot.
• Inspect regularly for a bent pointer, leaks, a cracked window or a pointer that rests away from zero.
• Replace a gauge that has been overpressured, because the damage is not always visible.
• Calibrate at a set interval and keep records.

Choosing between a Bourdon gauge and a digital gauge

A Bourdon gauge needs no power, tolerates rough handling and is the natural choice for permanent indication on pumps, boilers and compressors. A digital gauge adds resolution, units, memory and logging, and suits testing and calibration. Many sites use both, with Bourdon gauges as permanent indicators and a digital gauge as a portable reference.

Frequently asked questions

Why does a Bourdon tube straighten under pressure?

The flattened cross-section tries to become round. The outer wall of the curve is longer than the inner wall, so the tube uncurls as it rounds out.

Can a Bourdon gauge measure vacuum?

Yes. A vacuum or compound gauge has a range that goes below zero, and the tube moves the other way as the pressure drops below atmospheric.

Why does my gauge read above zero with no pressure?

The tube may have taken a set after overpressure, or the pointer may have slipped. Replace the gauge, because the readings are likely to be wrong across the scale.

Are Bourdon gauges accurate enough for critical use?

The best are, with accuracy of 0.5% or better. For tighter accuracy or data logging, use a digital gauge.

How is a Bourdon gauge different from a manometer?

A manometer measures pressure using a column of liquid, while a Bourdon gauge uses a mechanical tube. Read more about the first in our article on the calibration of manometers, and compare it with other devices in pressure gauge vs pressure transmitter vs manometer.

Choose your gauge

Browse the full pressure gauges category, or contact our team with your pressure range, media and connection, and we will help you choose.