handbook of modern sensors

10.3 Mercury Pressure Sensor 341
1 meter at a temperature of +4 ◦ C and normal gravitational acceleration. A pascal can
be converted into other units by the use of the following relationships (see also Table
A.4 in the Appendix):
1Pa= 1.45 × 10 −4 lb/in 2 = 9.869 × 10 −6 atm = 7.5 × 10 −4 cm Hg.
For practical estimation, it is useful to remember that 0.1 mm H 2 O is roughly equal
to 1 Pa. In industry, another unit of pressure is often used. It is defined as pressure
exerted by a 1-mm column of mercury at 0 ◦ C at normal atmospheric pressure and
normal gravity. This unit is named after Torricelli and is called the torr. The ideal
pressure of the Earth’s atmosphere is 760 torr and is called the physical atmosphere:
1 atm = 760 torr = 101,325 Pa.
The U.S. Customary System of units defines pressure as a pound per square inch
(lb/sq in.) or psi. Conversion into SI systems is the following:
1 psi = 6.89 × 10 3 Pa = 0.0703 atm.
A pressure sensor operating principle is based on the conversion of a result of the
pressure exertion on a sensitive element into an electrical signal. Virtually in all
cases, pressure results in the displacement or deformation of an element, having a
defined surface area. Thus, a pressure measurement may be reduced to a measurement
of a displacement or force, which results from a displacement. Thus, we recommend
that the reader also becomes familiar with displacement sensors covered in Chapter
7 and force sensors of Chapter 9.
10.3 Mercury Pressure Sensor
A simple yet efficient sensor is based on the communicating vessels principle (Fig.
10.1). Its prime use is for the measurement of gas pressure. A U-shaped wire is
immersed into mercury, which shorts its resistance in proportion with the height
of mercury in each column. The resistors are connected into a Wheatstone bridge
circuit, which remains in balance as long as the differential pressure in the tube is
zero. Pressure is applied to one of the arms of the tube and disbalances the bridge,
which results in the output signal. The higher the pressure in the left tube, the higher
the resistance of the corresponding arm is and the lower the resistance of the opposite
arm is. The output voltage is proportional to a difference in resistances R of the
wire arms which are not shunted by mercury:
V out = V R = Vβp. (10.4)
R
The sensor can be directly calibrated in units of torr. Although simple, this sensor
suffers from several drawbacks, such as necessity of precision leveling, susceptibility
to shocks and vibration, large size, and contamination of gas by mercury vapors. 1
1 Note that this sensor can be used as an inclination sensor when pressures at both sides of
the tube are equal.