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Mercury Pressure Sensor

Monday, November 8, 2010

Pressure measurement history was primarily based on the pioneering work of Evangelista Torricelli,who, for a short time,was a student of Galileo. During his experiments with mercury-filled dishes, in 1643, he realized that the atmosphere exerts pressure on Earth. Another great experimenter, Blaise Pascal, in 1647, conducted an experiment, with the help of his brother-in-law Perier, on the top of the mountain Puyde Dome and at its base. He observed that pressure exerted on the column of mercury depends on elevation. He named the mercury-in-vacuum instrument they used in the experiment a barometer.

In 1660, Robert Boyle stated his famous relationship: The product of the measures of  pressure and volume is constant for a given mass of air at fixed temperature. In 1738, Daniel Bernoulli developed an impact theory of gas pressure to the point where Boyle’s law could be deducted analytically. Bernoulli also anticipated the Charles–Gay–Lussac law by stating that pressure is increased by heating gas at a constant volume.

In general terms, matter can be classified into solids and fluids. The word fluid describes something which can flow. That includes liquids and gases. The distinction between liquids and gases are not quite definite. By varying pressure, it is possible to change liquid into gas and vice versa.

Concepts of Pressure

For a fluid at rest, pressure can be defined as the force F exerted perpendicularly on a unit area A of a boundary surface.

       p = dF/dA

Pressure vary with elevation as:

       dp = − w.dh

where w is the specific weight of the medium and h represents the vertical height.

The kinetic theory of gases states that pressure can be viewed as a measure of the total kinetic energy of the molecules:

       p = (2/3).KE/V = (1/3) ρ.C² = NRT

where KE is the kinetic energy, V is the volume, C² is an average value of the square of the molecular velocities, ρ is the density, N is the number of molecules per unit volume, R is a specific gas constant, and T is the absolute temperature.

Units of Pressure

The SI unit of pressure is the pascal:

       1 Pa = 1 N/m²

that is, one pascal is equal to one newton of force uniformly distributed over 1 squaremeter of surface.

Sometimes, in technical systems, atmosphere is used, which is denoted 1 atm. One atmosphere is the pressure exerted on 1 square centimeter by a column of water having a height of 1 meter at a temperature of +4oC and normal gravitational acceleration. 

A pascal can be converted into other units by the use of  the following relationships:

       1 Pa = 1.45 × 10-4  lb/in² = 9.869 × 10-6 atm = 7.5 × 10-4 cmHg

For practical estimation, it is useful to remember that 0.1 mm H2O 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 0oC 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 × 103 Pa = 0.0703 atm

A simple yet efficient sensor is based on the communicating vessels principle. 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 ΔR of the wire arms which are not shunted by mercury:

       Vout = V.(ΔR/R) = V.β.Δp

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.

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