Measurements have got to be one of the most important equipment in any processing plant. Any decision made on what the plant should do is based on what the measurements tell us. In the context of process control, all controller decisions are similarly based on measurements.
With the advent of computers, it is now possible to do inferential measurements, meaning telling the value of a parameter without actually measuring it physically. It should however, be remembered that inferential measurement algorithms are also based on physical measurements. Therefore, rather than rendering measurements redundant, they have made measurements all the more important.
The measurement of pressure is considered the basic process variable in that it is utilized for measurement of flow (difference of two pressures), level (head or back pressure), and even temperature (fluid pressure in a filled thermal system).
All pressure measurement systems consist of two basic parts: a primary element, which is in contact, directly or indirectly, with the pressure medium and interacts with pressure changes; and a secondary element, which translates this interaction into appropriate values for use in indicating, recording and/or controlling.
An electronic-type transmitter is shown in the figure above. This particular type utilizes a two-wire capacitance technique.
Process pressure is transmitted through isolating diaphragms and silicone oil fill fluid to a sensing diaphragm in the center of the cell. The sensing diaphragm is a stretched spring element that deflects in response to differential pressure across it. The displacement of the sensing diaphragm is proportional to the differential pressure. The position of the sensing diaphragm is detected by capacitor plates on both sides of the sensing diaphragm. The differential capacitance between the sensing diaphragm and the capacitor plates is converted electronically to a 4-20 mA dc signal.
Numerous types of flowmeters are available for closed-piping systems. In general, the equipment can be classified as differential pressure, positive displacement, velocity and mass meters.
Differential pressure devices include orifices, venturi tubes, flow tubes, flow nozzles, pitot tubes, elbow-tap meters, target meters, and variable-area meters.
Positive displacement meters include piston, oval-gear, nutating-disk, and rotary-vane types. Velocity meters consist of turbine, vortex shedding, electromagnetic, and sonic designs.
Mass meters include Coriolis
and thermal types. The measurement of liquid flows in open channels generally involves weirs and flumes.
How can I measure temperature?
Temperature can be measured via a diverse array of sensors. All of them infer temperature by sensing some change in a physical characteristic. Six types with which the engineer is likely to come into contact are: thermocouples, resistive temperature devices (RTDs and thermistors), infrared radiators, bimetallic devices, liquid expansion devices, and change-of-state devices.