Bourdon Gauge

Pressure Sensor, C-type

Bourdon Tube is one of the most widely used sensors for Static pressure measurement. The basic principle behind this sensor is that elastic materials tend to inflate or deform their shape when subjected to a force. The fluid whose pressure needs to be measured is made to exert force on this elastic material.
The elastic material is a thin-walled metal tube bent in the shape of the letter 'C', with one end fixed open to external pressure and the other end is closed and movable. This tube is known as the Bourdon tube. 
There are basically 3 types of Bourdon Gauges:
  • C type: Range below 7031 Kg/cm2
  • Spiral: Range below 4000 psi
  • Helical: Range below 80000 psi
The image below shows a C-type Bourdon Gauge.
Pressure Measurement            Mechanical Gear movement

The working of the Bourdon Gauge is as shown in the above image on the right side. When pressure is applied, the closed end of the tube starts deforming from is 'C' shape and tends to straighten. The closed end is attached to a lever system that magnifies the motion of the tube using gear system. The magnified output deflects a needle on a pointer. The scale is calibrated in pressure units. The deflection is proportional to applied pressure when calibrated. Its calibration can be done using Mercury Barometer.
 The motion of the bourdon tube can be alternatively attached to a strain gauge to get output in the form of electrical voltage. Thus, the output can be displayed digitally using a signal conditioning circuit.                
Now let us have a look at the different parts of the mechanical assembly attached to the Bourdon tube.

                                        Different parts of pressure sensor

Let us start with stationary parts:
A: Receiver part. This part joins the pressure inlet to the fixed end of the Bourdon tube.
B: Chassis plate: It has bearing holes for the axles.
C: Secondary Chassis plate: It supports outer ends of the axles.
D: Space for joining the two chassis plates.

The moving parts are as follows:
1: Stationery end of Bourdon tube.
2: Moving end of Bourdon tube with sealing.
3: Pivot and pivot pin.
4: Link joining pivot pin to lever, allowing rotation.
5: Lever.
6: Sector gear axle pin
7: Sector Gear.
8: Indicator needle axle. It has a spur gear that amplifies the motion of the tube, before passing it to the               needle.
9: Hair spring to preload the gear train and eliminate gear lash and hysteresis.

The Bourdon Gauge has threads below it which makes it easy for installation on pipes.

The Bourdon Gauge can be used for measuring vacuum as well. Such gauges are calibrated as 0-30 in Hg. or 0-760 mm Hg. In such case, the needle moves clockwise on increasing vacuum. Such gauges can even be calibrated as 0-1 bar, where the needle moves anti-clockwise on increasing vacuum. The latter proves beneficial when a single gauge needs to be used for measuring both, pressure and vacuum.

Material of the Bourdon tube:

The material of the Bourdon tube depends upon the fluid under measurement. The different materials are as follows:
  • Bourdon Gauges with Brass connections and Phosphor-Bronze tube are suitable for use in air, water, oil and other fluids compatible with these materials. These materials are generally preferred for non-corrosive and low pressure applications (400 psi).
  • For non-corrosive and medium pressure applications, Beryllium-Copper tubes are used (6000 psi). 
  • For high pressure applications, SS316 tubes are preferred (80,000 psi).
  • For corrosive fluids and for use in corrosive environments, SS403 tubes must be used.
  • When used for steam, a syphon should be fitted and filled with cold water prior to use.
  • When the Bourdon Gauge needs to be used in conditions subjected to severe vibrations, we need to use liquid filled Bourdon gauges. The liquid helps to damp down oscillations and also lubricates the gauge.
Applications of Bourdon Gauge:
  • Medium to very high pressure applications.
  • Can be used for air, water, oil, steam and corrosive fluids as well.
Advantages of the Bourdon Gauge:
  • High Accuracy: Generally +2% of span. They can even be manufactured for accuracies ranging to 0.1% of full scale.
  • Simple in construction.
  • Safe for high pressure measurements.
  • Can be modified to give electrical output.
  • It is a portable device and installation is easy.
  • Low cost.
Limitations of Bourdon Gauge:
  • Cannot be used for dynamic pressure measurements as its response to changing pressures is slow.
    It can even malfunction if pressure is varied rapidly.
  • Cannot be used in very high temperature applications. It also needs to be mounted away from heat sources.
  • Cannot be used for high precision measurements.
  • Sensitive to shock and vibrations.
  • They are subjected to hysteresis.

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