The Bourdon tube is the most common type of pressure gauge, where a metal tube senses pressure and converts it to displacement. As pressure increases, the tube tries to straighten out, causing its tip to move. This movement is amplified mechanically and indicated by a pointer to indirectly show pressure level. Bourdon tubes come in different shapes like C-shaped, helical, and spiral, with the material and size depending on the desired pressure range and gauge type. C-shaped tubes are most common, where pressure inside tries to straighten the tube and moves the tip to indicate pressure.

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Bourdon's tube pressure gauge
The majority of pressure gauges in use have a Bourdon-tube
as a measuring element. The Bourdon tube is a device that
senses pressure and converts the pressure to displacement.
Since the Bourdon-tube displacement is a function of the
pressure applied, it may be mechanically amplified and
indicated by apointer.
Thus, the pointer position indirectly indicates pressure.
The Bourdon-tube gauge is available in various tube shapes:
curved or C-shaped, helical, and spiral. The size, shape, and
material of the tube depend on the pressure range and the type
of gauge desired.
Low-pressure Bourdon tubes (pressures up to 2000 psi)
are often made of phosphor bronze. High-pressure Bourdon
tubes (pressures above 2000 psi) are made of stainless steel or
other high-strength materials.
High- pressure Bourdon tubes tend to have more
circular cross sections than their lower-range counterparts,
which tend to have oval cross sections. The Bourdon tube most
commonly used is the C-shaped metal tube that is sealed at one
end and open at the other gauge.
Bourdon-tube gauges are now commonly available to
measure a wide range of gauge, absolute, sealed, and

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differential pressures, plus vacuum. They are manufactured to
an accuracy as high as 0.1% of span and in dial diameters from
1 1/2 to 16 in. A variety of accessories can extend their
performance and usefulness. Snubbers and gauge isolators can
be installed to protect the sensitive internal workings from
pressure spikes.
Three types of elastic chambers are commonly used in
gauges :
 C-shaped
Gauges using C-shaped Bourdon tubes as the elastic
chamber, as shown above left, are by far the most common.
Pressurized fluid enters the stem at the bottom (which is
sometimes entrancement instead) and passes into the
Bourdon tube. The tube has a flattened cross section and is
sealed at its tip. Any pressure in the tube in excess of the
external pressure (usually atmospheric) causes the Bourdon
tube to elastically change its shape to a more circular cross
section.
This change in shape of the cross section tends to
straighten the C-shape of the Bourdon tube. With the bottom
stem end fixed, the straightening causes the tip at the
opposite end to move a short distance — 1/16 to 1/2 in.,
depending on the size of the tube. A mechanical movement
then transmits this tip motion to a gear train that rotates an

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indicating pointer over a graduated scale to display the
applied pressure. Often, a movement is incorporated to
provide mechanical advantage to multiply the relatively
short movement of the tube tip.
Bourdon tubes also may be made in the form of a
spiral or a helix. Each uses a long length of flattened tubing
to provide increased tip travel. This does not change the
operating principle of the Bourdon tube, but produces tip
motion equal to the sum of the individual motions that
would result from each part of the spiral or helix considered
as a C-shape. Small-diameter spirals and helices can be
manufactured to provide enough motion to drive the

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indicating pointer directly through an arc up to 270° without
having to use a multiplying movement.
Alternatively, they may be manufactured to be used
in conjunction with a multiplying movement. In this case,
the required motion is distributed over several turns,
resulting in lower stress in the Bourdon material. This
improves fatigue life when compared to a C-shaped
Bourdon tube in the same pressure range.
 Helical

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 spiral
Other designs
Low-pressure applications do not generate enough
force in the Bourdon tube to operate the multiplying
mechanism, therefore, Bourdon tube gauges are not
generally used for pressure spans under 12 psi. For these
ranges, some other form of elastic chamber must be used,
ametallic bellows, for example. These bellows generally are
made by forming thin-wall tubing. However, to obtain a
reasonable fatigue life and motion that is more linear with
pressure, a coil spring supplements the inherent spring rate
of the bellows. These spring-loaded bellows gauges

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generally are used in pressure ranges having spans to 100
psi and to 1 in.Hg.