From Maintenance of Instruments & Systems, 2nd Edition


A primary element is a sensor or detector that responds quantitatively to the measured variable and performs the initial measurement operation. A primary element performs the initial conversion of measurement energy. The final control device is the element that directly controls the value of the manipulated variable of a control loop. Often the final control element is a control valve. It is an instrument that takes action to adjust the manipulated variable in a process. Final control elements can also be solenoids and servomotors.


Temperature measuring instruments include liquidor gas-filled thermal systems, electrical measurements using thermocouples (TCs), resistance bulbs (RTDs) and thermistors. Noncontacting sensors are not covered here as they are usually factory calibrated and need only an emissivity check or adjustment to read properly.

In today's technology, the thermocouple and the RTD are used extensively for industrial temperature measurement and will be discussed in more detail than other temperature-measuring devices. Many local field controllers, however, still exist and use the older filled-tube sensors that mechanically move a pointer in the local instrument. These devices require mechanical adjustments that increase or decrease the arc that the Bourdon spring takeoff moves in order to provide the correct match of the process pointer to the measured temperature. In the case of gasfilled temperature systems, the measurement is nonlinear. Calibration should be made using the scale that came with the instrument. The vendor's instrument instruction book will usually explain, in detail, how to make these adjustments.

The intent of this chapter...

Products & Services
Pressure Gauges
Pressure gauges are used for a variety of industrial and application-specific pressure monitoring applications.  Uses include visual monitoring of air and gas pressure for compressors, vacuum equipment, process lines and specialty tank applications such as medical gas cylinders and fire extinguishers.
Mechanical Pressure Gauges
Analog pressure gauges are mechanical devices that include bellows, Bourdon tubes, capsule elements and diaphragm element gauges.
Pressure Sensors
Pressure sensors include all sensors, transducers and elements that produce an electrical signal proportional to pressure or changes in pressure.
Linear Velocity Sensors
Linear velocity sensors measure the linear velocity of an object using either contact or non-contact techniques.

Topics of Interest

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