Pressure gauges with Bourdon tube - one of the most common instruments for measuring pressure in automotive systems: from fuel lines to brake circuits. Despite the simplicity of the design, this mechanism provides high accuracy and reliability, which makes it indispensable in car diagnostics and repair. But how exactly does this curved metal tube work? Why does it react to pressure, and what malfunctions may occur during operation?

In this article we will look at Bourdon tube device, its physical principles, as well as typical problems faced by owners of pressure gauges. You will learn how to independently check the operation of the device, repair it or replace damaged elements - without contacting a service center. We will pay special attention automotive applications, where accurate pressure readings are critical to safety.

The material will be useful for both beginners and experienced car enthusiasts: from theory to practice with step-by-step instructions and tips for choosing quality pressure gauges.

What is a Bourdon tube and how does it work?

A Bourdon tube is pressure gauge sensor, invented by a French engineer Eugene Bourdon in 1849. Structurally, it is a hollow metal tube with an oval or elliptical cross-section, bent in an arc (usually 270°). One end of the tube is sealed and connected to the arrow of the device, and the second is open for connection to the system where the pressure needs to be measured.

The operating principle is based on Hooke's law and the effect of elastic deformation: when pressure is applied inside the tube, its cross-section tends to take a round shape, which causes the arc to unbend. This movement through a mechanical transmission (gears, levers) is converted into a rotation of the arrow on the pressure gauge scale. The higher the pressure, the stronger the deformation and the greater the angle of rotation.

Basic materials for making Bourdon tubes:

  • 🔹 Brass - a budget option for low and medium pressures (up to 40 bar).
  • 🔹 Stainless steel — resistant to corrosion, used in aggressive environments (for example, to measure engine oil pressure).
  • 🔹 Bronze - used in marine and chemical applications.
  • 🔹 Special alloys (for example, Monel) - for high temperatures and pressures (up to 1000 bar).

Interestingly, the shape of the tube can vary: in addition to the classic C-shaped exist spiral and screw modifications. The latter allow you to achieve a larger angle of rotation of the arrow at the same pressure, which increases the accuracy of measurements.

📊 Where do you most often use a Bourdon tube pressure gauge?
In the fuel system
In the braking system
To check oil pressure
In air suspension
Other

Bourdon tube pressure gauge device

The design of a typical Bourdon tube pressure gauge includes several key components, each with a different function:

  1. Bourdon tube - the main sensitive element that converts pressure into mechanical movement.
  2. Transmission mechanism - a system of levers and gears that enhances and transmits movement from the tube to the arrow.
  3. Pointer and scale - a visual pressure indicator, usually graduated in bar, kPa or psi.
  4. Housing — protects internal parts from external influences (dust, moisture, shock).
  5. fitting — threaded or quick-release connector for connection to the system.

The work flow looks like this:

  1. Pressure enters through the fitting into the Bourdon tube.
  2. The tube is deformed, its free end moves.
  3. The displacement is transmitted through the rod to the sector gear.
  4. The gear turns the axis with the arrow.
  5. The arrow indicates the pressure value on the scale.

Additional elements are often used in automobile pressure gauges:

  • 🔧 Damper — dampens needle vibrations during pressure pulsations (for example, in a fuel rail).
  • 🔧 Relief valve — protects the device from water hammer.
  • 🔧 Thermal compensator — minimizes the influence of temperature on readings.
Component Material Purpose
Bourdon tube Stainless steel, brass Converting pressure into movement
Gear mechanism Steel, aluminum Strengthening and transmitting motion
Arrow Aluminum, plastic Visual indication
fitting Brass, steel Connection to the system
⚠️ Attention: In pressure gauges for aggressive media (for example, for measuring pressure in systems with antifreeze), the Bourdon tube must be made of AISI 316 stainless steel or Hastelloy. The use of brass tubes in such conditions will lead to corrosion and failure of the device.

Advantages and disadvantages of a Bourdon tube

Despite its more than 170-year history, the Bourdon tube remains relevant due to a number of advantages:

  • ✅ High accuracy — the error of high-quality devices does not exceed ±0.5% of the scale.
  • ✅ Simplicity of design - minimum moving parts, which increases reliability.
  • ✅ Wide measuring range - from 0.1 to 1000 bar.
  • ✅ Linear characteristic — a uniform scale makes it easier to read the readings.
  • ✅ Vibration resistance - critical for automotive applications.

However, there are also limitations:

  • ❌ Sensitivity to water hammer - sudden pressure surges can deform the tube.
  • ❌ Limited temperature range — standard tubes operate at −40…+80°C.
  • ❌ Mechanism wear — over time, gears and axles can play, reducing accuracy.
  • ❌ Unsuitable for vacuum — the Bourdon tube is not intended for measuring vacuum.

For automobile pressure gauges, two factors are critical: vibration resistance (especially in the engine compartment) and corrosion resistance (when working with fuel, oil, brake fluid). Therefore, models with a tube made of stainless steel and filled with silicone damper.

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When choosing a pressure gauge for the fuel system, pay attention to models with rear eyeliner - they are less susceptible to clogging and leaks than devices with a bottom fitting.

Typical faults and their symptoms

Even the most reliable Bourdon tube pressure gauge can fail over time. Let's look at the main symptoms of malfunctions and their possible causes:

Sign Possible reason Consequences
The arrow doesn't move Clogged fitting, broken tube or mechanism Unable to measure pressure
The arrow shakes or “sticks” Gear wear, axle contamination, water hammer Inaccurate readings, risk of breakdown
Indications are “overestimated” or “underestimated” Deformation of the tube, play in the mechanism Errors in diagnosing car systems
Leaking from under glass or fitting Damage to seals, corrosion of housing Pressure loss, system contamination

The most vulnerable nodes:

  1. Bourdon tube - may burst if the maximum pressure is exceeded or corrode upon contact with aggressive liquids.
  2. Gear mechanism - wears out from friction, especially during vibrations.
  3. Seals — rubber gaskets become tanned over time and begin to leak fluid.
  4. Glass - may crack from impacts or temperature changes.
⚠️ Attention: If a pressure gauge was used to measure pressure compressed air (for example, in air suspension) and then connected to hydraulic system (brakes, power steering), it needs to be washed! Remaining oil in the Bourdon tube can cause it to clog and fail.

Fault diagnosis: step-by-step instructions

Before repairing or replacing the pressure gauge, you need to accurately determine the cause of the malfunction. Follow this algorithm:

Check the tightness of the connections (there are no leaks)

Take readings at a known pressure (eg from a compressor)

Inspect the glass for cracks and the body for deformation.

Rotate the pointer by hand (if possible) to check for play

Compare readings with reference pressure gauge -->

Step 1. External inspection

  • 🔍 Check it out glass integrity - cracks or chips can allow moisture to enter the housing.
  • 🔍 Take a look fitting for corrosion or mechanical damage.
  • 🔍 Make sure that arrow not bent or touching the scale.

Step 2. Checking the tightness

Connect the pressure gauge to a pressure source (for example, compressor or pump) and watch:

  • 💨 If the pressure drops without visible leaks, it is possible microcrack in the Bourdon tube.
  • 💨 If air comes out from under the glass, it is required replacing the o-ring.

Step 3. Accuracy test

Compare your pressure gauge readings with a known-good gauge at several pressure levels (for example, 1, 3, and 5 bar). Differences of more than 5% indicate:

  • 📉 Bourdon tube deformation (usually due to water hammer).
  • 📉 Backlash in the transmission mechanism.
  • 📉 Pointer axis dirty.

Step 4: Checking the mechanism

Carefully remove the back cover of the pressure gauge (if the design is collapsible) and:

  • 🔧 Rate the condition gears and levers - they should move smoothly, without jamming.
  • 🔧 Check it out Bourdon tube mounting - it should not be weakened.
  • 🔧 Make sure that return spring not stretched.
How to check a pressure gauge without a reference device?

If you don't have a second gauge for comparison, you can use bicycle pump with pressure gauge (error ~10%) or car compressor with digital display. Connect the pressure gauge being tested in parallel and compare the readings when the pressure is increased to 2-3 atmospheres. A difference of more than 0.3 bar indicates a malfunction.

Do-it-yourself repair of a pressure gauge with a Bourdon tube

Many pressure gauge malfunctions can be repaired yourself with a minimal set of tools. Let's consider typical cases:

1. Cleaning the mechanism

If the needle moves jerkily or “sticks,” the reason often lies in contamination of the axle or gears. To clean:

  1. Remove the back cover of the pressure gauge.
  2. Carefully remove the mechanism (remember the order of the parts!).
  3. Clean the gears and axle alcohol solution or WD-40.
  4. Dry the parts and apply a drop watch oil on rubbing surfaces.
  5. Reassemble the mechanism in reverse order.

2. Replacing the O-ring

If there is a leak from under the glass:

  1. Unscrew the screws securing the glass (usually 3-4 pieces).
  2. Remove the glass and replace the rubber ring with a new one (you can use rings from hydraulic systems suitable diameter).
  3. Reinstall the glass, tightening the screws evenly criss-crossto avoid skew.

3. Elimination of Bourdon tube deformation

If the tube is bent due to water hammer, you can try to straighten it:

  • 🔧 Remove the tube from the pressure gauge by unsoldering or unscrewing the mount.
  • 🔧 Gently straighten the arc using plastic or wooden block (metal tools may damage the surface!).
  • 🔧 Check the tightness of the tube by immersing it in water and supplying air - bubbles will indicate cracks.
⚠️ Attention: If the Bourdon tube has visible cracks or severe corrosion, its repair is impossible - replacement is required! Using a damaged tube will result in leaks and inaccurate readings.

4. Pointer calibration

If the pressure gauge shows incorrect values, but the mechanism is working properly, you can calibrate the pointer:

  1. Connect the pressure gauge to the source precise pressure (for example 2 bar).
  2. Remove the glass and carefully loosen the screw arrow.
  3. Rotate the arrow so that it points to the correct value.
  4. Tighten the screw and check the reading at a different pressure level.
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When disassembling the pressure gauge, always take photographs of the location of the parts - this will help to avoid errors during assembly. Pay special attention to the position return spring and sector gear!

Choosing a new pressure gauge: what to look for

If repair is impossible or impractical, you will have to buy a new pressure gauge. When choosing, consider the following parameters:

1. Measuring range

  • 📊 For fuel system — 0–7 bar (gasoline) or 0–10 bar (diesel).
  • 📊 For brake system - 0–10 bar.
  • 📊 For air suspension - 0–15 bar.
  • 📊 For oil system - 0–10 bar.

2. Accuracy class

Indicated on the pressure gauge scale (for example, 1.6 or 2.5). The lower the number, the more accurate the device. Enough class for automotive needs 1.6–2.5.

3. Bourdon tube material

Wednesday Recommended Material
Air, nitrogen Brass, stainless steel
Fuel, oil Stainless steel AISI 316
Brake fluid Stainless steel or bronze
Aggressive gases Monel metal, Hastelloy

4. Design features

  • 🔧 Rear eyeliner — convenient for installation in hard-to-reach places.
  • 🔧 Liquid filling (glycerin) - dampens needle vibrations.
  • 🔧 Stainless steel housing — for work in aggressive conditions.
  • 🔧 Digital scale - makes reading readings easier.

5. Brands and prices

Among the trusted manufacturers:

  • 🏆 WIKA (Germany) - premium segment, high accuracy.
  • 🏆 Ashcroft (USA) - reliable devices for industry.
  • 🏆 KIMO (France) - optimal price/quality ratio.
  • 🏆 Domestic brands (for example, "Manotron") - budget solutions.

Cost of pressure gauges with Bourdon tube:

  • 💰 Budget (brass, class 2.5) - from 500 to 1500 rubles.
  • 💰 Middle segment (stainless steel, class 1.6) - 1500–4000 rub.
  • 💰 Premium (special alloys, class 0.6) - from 4000 rub.
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When purchasing a pressure gauge for the fuel system, give preference to models with built-in damper - it will smooth out pressure pulsations from the operation of the fuel pump, and the needle will not “bounce”.

FAQ: Frequently asked questions about the Bourdon tube

Can a Bourdon tube pressure gauge be used to measure vacuum?

No, a standard Bourdon tube is only designed to positive pressure. To measure vacuum (rarefaction) are used vacuum gauges with a membrane or spring mechanism. Attempt to use a Bourdon tube pressure gauge in a vacuum system will lead to its irreversible deformation.

Why does the pressure gauge needle not return to zero immediately after the pressure is released?

This is a normal phenomenon associated with hysteresis — delay in returning the Bourdon tube to its original position. In high-quality pressure gauges, hysteresis is minimal (no more than 0.5% of the scale). If the arrow “freezes” for a long time, this indicates:

  • Wear return spring.
  • Pollution arrow axis.
  • Deformation Bourdon tubes.

Solution: Disassemble the pressure gauge, clean the mechanism and check the spring.

How to check a pressure gauge without special equipment?

You can use available tools:

  1. Connect the pressure gauge to bicycle pump with built-in pressure gauge.
  2. Pump up the pressure to 2-3 atmospheres and compare the readings.
  3. Repeat the test at 1 and 4 atmospheres.

If the discrepancy exceeds 0.3 bar, the pressure gauge requires repair or replacement.

What is the difference between C-tube and spiral Bourdon tube pressure gauges?

Main differences:

Parameter C-tube Spiral tube
Sensitivity Below Higher (higher rotation angle)
Accuracy good Excellent
Cost Below Higher
Application Household and automobile pressure gauges Precision instruments, laboratories

For automotive needs it is usually sufficient C-tube - it is cheaper and more reliable in vibration conditions.

How often should the pressure gauge be checked for accuracy?

Recommended frequency:

  • 📅 Household pressure gauges (for example, to check tire pressure) - once every 2-3 years.
  • 📅 Automotive pressure gauges (fuel, oil, brakes) - once a year or after 50–100 hours of operation.
  • 📅 Professional devices (service stations, diagnostic centers) - every 6 months with verification.

Signs that the pressure gauge requires checking:

  • The needle shakes or moves jerkily.
  • The readings are different from what was expected (for example, the engine oil pressure suddenly dropped).
  • After releasing the pressure, the needle does not return to zero.