InstrumentationPressure Measurement

How does a Bourdon tube gauge work?


 Bourdon tube uses a coiled tube which as it expands due to pressure increase causes a rotation of an arm connected to the tube

Working principle

The pressure sensing element is a closed coiled tube connected to the chamber or pipe in which pressure is to be sensed. As the gauge pressure increases the tube will tend to uncoil, while a reduced gauge pressure will cause the tube to coil more tightly.
This motion is transferred through a linkage to a gear train connected to an indicating needle. The needle is presented in front of a card face inscribed with the pressure indications associated with particular needle deflections. In a barometer, the Bourdon tube is sealed at both ends and the absolute pressure of the ambient atmosphere is sensed.
Differential Bourdon gauges use two Bourdon tubes and a mechanical linkage that compares the readings.In the following pictures the transparent cover face has been removed and the mechanism removed from the case. This particular gauge is a combination vacuum and pressure gauge used for automotive diagnosis:
  • The left side of the face, used for measuring manifold vacuum,is calibrated in centimetres of mercury on its inner scale and inches of mercury on its outer scale.
  •  The right portion of the face is used to measure fuel pump pressure and is calibrated infractions of 1kgf  /cm²on its inner scale and pounds per square inch on its outer scale

Stationary parts:

  • A: Receiver block. This joins the inlet pipe to the fixed end of the Bourdon tube (1) and secures the chassis plate (B). The two holes receive screws that secure the case.
  • B: Chassis Plate. The face card is attached to this. It contains bearing holes for the axles.
  • C: Secondary Chassis Plate. It supports the outer ends of the axles.
  • D: Posts to join and space the two chassis plates.

Moving Parts:  

1.Stationary end of Bourdon tube. This communicates with the inlet pipe through the receiver block.

2.Moving end of Bourdon tube. This end is sealed.

3.Pivot and pivot pin

4.Link joining pivot pin to lever (5) with pins to allow joint rotation.

5.Lever. This an extension of the sector gear (7).

6.Sector gear axle pin.

7.Sector gear.

8.Indicator needle axle. This has a spur gear that engages the sector gear (7) and extends through the face to drive the indicator needle. Due to the short distance between the lever arm link boss and the pivot pin and the difference between the effective radius of the sector gear and that of the spur gear, any motion of the Bourdon tube is greatly amplified.A small motion of the tube results in a large motion of the indicator needle.

9.Hair spring to preload the gear train to eliminate gear lash and hysteresis. 

Advantages of Bourdon Gauges 

1. Inexpensive
2.Simple Design
3.Good accuracy except at low pressure.
4.High-pressure range.

Disadvantages of Bourdon Gauges.

1.Susceptible to shock and vibration
2.Gauges are subjected to hysteresis

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