Bourdon Tube
Measuring pressure with bourdon tube
The Bourdon tube is the third type of
elastic-element pressure transducer and is a very common industrial
measuring instrument used for measuring the pressure of both gaseous
and liquid fluids. It consists of a specially shaped piece of
oval-section flexible tube which is fixed at one end and free to move
at the other. When pressure is applied at the open, fixed end of the
tube, the oval cross-section becomes more circular. As the
cross-section of the tube tends towards a circular shape, a deflection
of the closed, free end of the tube is caused. This displacement is
measured by some form of displacement transducer, which is commonly a
potentiometer or LVDT, or less often a capacitive sensor. In yet
another version, the displacement is measured optically.
The maximum possible deflection of the
free end of the tube is proportional to the angle subtended by the arc
through which the tube is bent. For a C-type tube, the maximum value
for this arc is somewhat less than 360 degrees. Where greater
measurement sensitivity and resolution are required, spiral and
helical tubes are used, where the possible magnitude of the arc
subtended is limited only by a practical limit on how many turns it is
convenient to have in the helix or spiral. This increased measurement
performance is only gained, however, at the expense of a substantial
increase in manufacturing difficulty and cost compared with C-type
tubes, and is also associated with a large decrease in the maximum
pressure which can be measured.
C-type tubes are available for measuring pressures up to 6000 bar. A
typical C-type tube of 25 mm radius has a maximum displacement travel
of 4 mm, giving a moderate level of measurement resolution.
Measurement accuracy is typically quoted at ± 1% of full-scale
deflection. Similar accuracy is available from helical and spiral
types, but whilst the measurement resolution is higher, the maximum
pressure measurable is only 700 bar.
The existence of one potentially major source of error in Bourdon-tube
pressure measurement has not been widely documented and few
manufacturers of Bourdon tubes make any attempt to warn users of their
products. The problem concerns the relationship between the fluid
being measured and the fluid used for calibration. The pointer of
Bourdon tubes is normally set at zero during manufacture, using air as
the calibration medium. If, however, a different fluid, especially a
liquid, is subsequently used with a Bourdon tube, the fluid in the
tube will cause a non-zero deflection according to its weight compared
with air, resulting in a reading error of up to 6%. This can be
avoided by calibrating the Bourdon tube with the fluid to be measured
instead of with air, assuming of course that the user is aware of the
problem. Alternatively, correction can be made according to the
calculated weight of the fluid in the tube. Unfortunately,
difficulties arise with both of these solutions if air is trapped in
the tube, since this will prevent the tube being filled completely by
the fluid. Then, the amount of fluid actually in the tube, and its
weight, will be unknown.
In conclusion, therefore, Bourdon tubes only have guaranteed accuracy
limits when measuring gaseous pressures. Their use for accurate
measurement of liquid pressures poses great difficulty unless the
gauge can be totally filled with liquid during both calibration and
measurement, a condition which is very difficult to meet practically.
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Pressure Measurements
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