Pressure Measurement

Pressure Measurement

-A major concern in process control applications is the measurement of fluid pressure. The term fluid means a substance that can flow; hence, the term applies to both liquids and gases. Both will occupy the container in which they are placed.

-The pressure is equal to the force applied to the walls divided by the area that is perpendicular to the force. For a liquid at rest, the pressure exerted by the fluid at any point is perpendicular to the boundary of the liquid. Pressure is defined as a force applied to, or distributed over, a surface area as:-

P=F/A

Gauge and Absolute Pressure

-Absolute pressure is the pressure measured above total vacuum or zero absolute, where zero absolute represents a total lack of pressure.

-Gauge pressure is the pressure measured above atmospheric or barometric pressure. It represents the positive difference between measured pressure and existing atmospheric pressure.

 

Pressure Gauges

-Pressure gauges are used for local indication and are the most common type of pressure-measurement instrument used in process industries. Pressure gauges consist of a dial or indicator and a pressure element. A pressure element converts pressure into a mechanical motion.

-Most mechanical pressure elements rely on the pressure that acts on a surface area inside the element to produce a force that causes a mechanical deflection. The common elements used are Bourdon tubes diaphragms and bellows elements.

-The following figure shows one of the most common and least expensive pressure gauges used in the process industries. This pressure gauge uses a “C” type Bourdon tube. In this device, a section of tubing that is closed at one end is partially flattened and coiled. When a pressure is applied to the open end, the tube uncoils. This movement provides a displacement that is proportional to the applied pressure. The tube is mechanically linked to a pointer on a pressure dial to give a calibrated reading.

- A Diaphragm is another device that is commonly used to convert pressure into a physical movement. A diaphragm is a flexible membrane. When two are fastened together they form a container called a capsule. In pressure- measuring instruments, the diaphragms are normally metallic. Pressure applied inside the diaphragm capsule causes it to expand and produce motion along its axis.

- A diaphragm acts like a spring and will extend or contract until a force is developed that balances the pressure difference force. The amount of movement depends on how much spring there is in the type of metal used.

- To amplify the motion that a diaphragm capsule produces, several capsules are connected end to end, as shown in the following figure. You can use diaphragm- type pressure gauges to measure gauge, absolute, or differential pressure.

- The Bellows pressure element is very similar to a diaphragm-type gauge in that it converts a pressure into a physical displacement. The difference is that typically the movement in a bellows is much more of a straight-line expansion. A typical bellows-type pressure gauge is manufactured by forming many accordion-like pleats into a cylindrical tube

-You can use the motion that the pressure input signal produces to position a pointer, recorder pen, or the wiper of a potentiometer.