Pressure measurement is one of the most used process variables in many industrial applications. It is widely used on HVAC and refrigeration systems, mechanical pumps and compressors, hydraulic and pneumatic testing stations, and within testing, chemical and petrochemical laboratories.
Pressure is defined as the amount of force applied to an object over an area in which the force is distributed and is equal to the force divided by the area.
In the international system, pressure is measured in pascals (Pa). In the United States, pounds per square inch (PSI) is the most common unit of measurement.
Pressure gauges are reliable instruments designed to measure equipment or system pressures. A fluid, typically a gas or liquid, such as oil or fuel, is used as the medium for the force that is exerted on the gauge. Mechanical pressure gauges are the primary instruments used for pressure measurement however, new electronic pressure sensors are able to offer higher accuracies and various computer readout and connectivity options that make them more suitable for automated processes.
The pressure measurement theories for each type of equipment below are used for different applications and the appropriate equipment should be chosen based on the specific process requirements.
Mechanical Pressure Gauges
The Manometer is one of the simplest devices for measuring pressure. A Manometer consists of a liquid filled reservoir and tube or a U-shaped tube in which one side of the system is open to the atmosphere and the other side is subjected to the pressure.
The difference in the level of the liquid in the tubes indicates the pressure. Manometers are mainly used in low-pressure environments. The diameter of the tubes and the density of the liquid determines the range of the pressures available in the systems. Manometers filled with water would require much less pressure to move the liquid column than a Manometer filled with mercury.
Bourdon Tube Gauges
Bourdon Tube pressure gauges contain an inner C-shaped tube where one end is fixed to the gauge pressure opening and the other end is connected to a small gear, shaft and linkage assembly that is attached to a pointer. When the pressure within the tube increases, the free end of the tube expands in an arc shape and the gear system moves the pointer on the gauge dial. The degree of movement is proportional to the applied pressure and is magnified by the gear ratio attached to the pointer.
Electronic Pressure Sensors
Piezo-Resistive Sensors (Strain Gauges)
Piezo-Resistive sensors, commonly referred to as Strain Gauges, work by the change in resistance of the semiconductor materials when subjected to a slight deformation by an applied pressure. Strain gages are usually connected in a bridge pattern to reduce the temperature effects inherent with semiconductors. They can be connected to other circuitry to provide zero adjustment, sensitivity and temperature compensation.
Piezo -Electric Sensors
Piezo-Electric sensors contain a crystal that produces an electrical charge proportional to the pressure applied on its surface. Quartz and tourmaline crystals are widely used as sensor materials. Piezoelectric sensors are useful for the measurement of changing pressures, as the crystals respond quickly to pressure fluctuations.
Capacitive sensors are one of the most reliable sensor types and have been used on a wide variety of applications. Capacitive sensors work by measuring the change in capacitance as the distance between the capacitor plates is changed after pressure has been applied to the sensor. The change in capacitance is compared to a reference capacitance within the sensor and the pressure measurement can be obtained.