Chapter 3 Pressure

Process Control & Instrumentation by Moaz Waqar Chapter 1: Pressure Measurement

Pressure: Force per unit area a fluid exerts on its surroundings. Pascal (Pa) or

Pounds per square inch (psi)

Vacuum: Vacuum is a pressure measurement made between total vacuum

and normal atmospheric pressure (14.7 psi). Atmospheric pressure is the pressure on the earth’s surface due to the

weight of the gases in the earth’s atmosphere and is normally expressed at sea level as 14.7 psi or 101.36 kPa.

Absolute pressure is the pressure measured with respect to a vacuum

and is expressed in pounds per square inch absolute (psia).

Gauge pressure is the pressure measured with respect to atmospheric

pressure and is normally expressed in pounds per square inch gauge (psig).

Differential pressure is the pressure measured with respect to another

pressure and is expressed as the difference between the two values..


Pressure measurement units: A number of measurement units are used for pressure. They are as follows: 1. Pounds per square foot (psf) or pounds per square inch (psi) 2. Atmospheres (atm) 3. Pascals (N/m2) or kilopascal (1000Pa) 4. Torr = 1 mm mercury 5. Bar (1.013 atm) = 100 kPa

Head is sometimes used as a measure of pressure. It is the pressure in terms of a column of a particular fluid, i.e., a head of 1 ft of water is the pressure that would be exerted by a 1 ft tall column of water, that is, 62.4 psfg, or the pressure exerted by 1 ft head of glycerin would be 78.6 psfg

What pressure in Pascals

corresponds to 15 psi?

What is the pressure at the base of a water tower which has 50 ft of head?


Low Range Pressure Measurement: (14.7 – 30 psi)

U-Tube Manometer

Range: 14.7 – 30 psi Advantages: Highly accurate, linear and hysteresis free Limitations: Cannot be used for higher pressures, fragile, special measurements are required, temperature variation causes error


Low Range Pressure Measurement: (14.7 – 30 psi)

a) Inclined leg b) Well Type Manometer

Advantages: Higher sensitivity than U-tube manometer.


High Range Pressure Measurement: >14.7 psi


High Range Pressure Measurement: >14.7 psi Diaphragms


High Range Pressure Measurement: >14.7 psi Capsules


High Range Pressure Measurement: >14.7 psi Bellows


High Range Pressure Measurement: >14.7 psi Bourdon tubes






Vacuum Measurement: <14.7 psi Thermal Type Principal: At normal pressures, the heat conductivity of gases is

independent of its pressure but as pressure decreases to about 1 cm of mercury and below, heat conductivity starts falling.

1. Pirani gauge 2. Thermocouple gauge


Vacuum Measurement: <14.7 psi Pirani Gauge The basic gauge probe consists of a fine wire of platinum or tungsten of 0.001 in. diameter mounted in a tube which is connected to the system whose vacuum is to be measured.


Vacuum Measurement: <14.7 psi Pirani Gauge Calibration Curve


Vacuum Measurement: <14.7 psi Thermocouple Gauge The temperature of the hot wire is now directly measured by a thermocouple which is attached to the hot wire as shown in Figure 4.34.


Vacuum Measurement: <14.7 psi Thermocouple Gauge Affect of current on sensitivity


Vacuum Measurement: <14.7 psi Thermocouple Gauge

Vacuum comparison by thermocouple gauges


Vacuum Measurement: <14.7 psi Thermocouple Gauge

Composition dependence of thermocouple gauges


Vacuum Measurement: <14.7 psi Ionization gauge

Circuit arrangement of the ionization gauge



Connections for the internal control type ionization gauge



Calibration curves for three different gases

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Chapter 3 Pressure