A Bourdon tube is a mechanical pressure-measuring device consisting of a C-shaped, helical, or spiral hollow metal tube that straightens a bit if internal fluid pressure rises.
Invented in 1852 by French engineer Eugène Bourdon, it moves a pointer on a dial via gears without needing electricity.
Its used in Bourdon pressure gauges, being installed behind the gauge's dial face.
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A Bourdon tube receives pressure/fluid inside it, which straighthens it and moves a mechanism at it end of it.
it receives pressure from the socket. [2.2] The tube is often bent at 270°.
The pinion is the gear that drives/moves the pointer on the dial, by having its teeth being turned by the Bourdon tube being bent due to its receiving pressure.
The fixed (sealed) end/inlet, the input, connects to the pressure source or fluid's pressure to measure. "Fixed" refers to how this end does not move unlike the free end.
The free (closed) end, the output, is the sealed end. If pressure is applied, curves in response to the pressure change. "Free" refers to how this end is able to move.
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[2] A bourdon tube exposed after the removing the gauge's housing.
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[2] The gauge's gear being pointed by the person in the photo.
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[4] Tube material: Bourdon tubes are often made of many materials, like stainless steel, brass, bronze, or even specialized alloys, depending on application requirements. The material's properties is used to withstand the pressure and keep its shape.
Note: Althought "sealed" and "closed" mean the same thing in general, for a Bourdon tube, the 2 words depict different ends because:
The fixed end is sealed to the socket with a permanent joint (like solder or brazing) to the socket so that no pressure leaks out at the connection point. The focus is on the watertight/airtight joint.
The free end is manufactured with a solid closed tip — not open like a pipe. The focus is on the tube's physical shape (it has a dead-end tip).
[4] Levers and linkages: To magnify and transfer the small angular motion resulting from the Bourdon tube's deformation, a lever system or linkages are often used. This converts the mechanical deformation into linear motion, which can then be used to drive a pointer or an electronic sensor.
Pivot and gear mechanism: In pressure gauges, there is usually a pivot point or a gear mechanism that translates the linear motion into the rotational movement of a pointer on the gauge dial.
[4] A Bourdon tube's cross-section is oval flattened, often [4] C-shape or helical coil shape.
Its fixed end inlet connects to the pressure source. The free end, the tip, is sealed and connected to a mechanical linkage. As the fluid pressure rises, it exerts a force in the tube, causing it to straighten or uncoil a bit, a deformation proportional to the pressure applied and used to drive the pointer/electronic screen to indicate the pressure reading on a gauge/display.
[5] A C-type Bourdon tube's pointer turns CW for positive pressure and CCW for vacuum pressure.
[D1] Many cheap mechanical gauges can't be calibrated but more costly ones can be calibrated.
Calibrating a mechanical Bourdon tube pressure gauge involves adjusting 3 adjustements like many instruments: The zero, span, and linearity.
Zero: The needle doesn't point at zero without pressure applied. To correct this, remove the needle pointer and reposition to zero.
Span: At max pressure the needle points to a value greater than or less than the applied pressure. To correct this, adjust the gauge linkage inwards or CCW (of the gauge) to increase and outwards or CW to decrease the span. This may also involves turning a screw to loosen it.
Linearity: If the needle movement changes from 0 to 50% is different than the change from 50 – 100%.
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[2] What is inside a pressure gauge? How does a Bourdon gauge work? - HydroMohsen (YouTube)
[4] Dubai Sensor (Warning: This page contains incorrect informations; Infos of the fixed and free end are wrong.)
[5] BCST