Selection of Temperature Instrument Types
Based on their operating principles, temperature measurement and control instruments can be categorized into three types: analog (pointer-type), digital, and intelligent.
Analog instruments are primarily of the traditional moving-coil type. Characterized by simple construction and low cost, they remain in use in industry; however, they suffer from low measurement accuracy (typically Class 1.0 to 2.5) and significant reading errors. Lacking a transmission output, they are increasingly being replaced by digital instruments.
Digital instruments-such as the widely used digital display meters-offer significantly higher measurement accuracy than moving-coil types, typically around 0.5%. Digital displays provide precise, intuitive readings free from human error. Control methods are predominantly two-position or three-position control, though some models offer continuous analog PID regulation.
The integration of single-chip microprocessors has revolutionized the structure, performance, and appearance of instruments, enabling functions that were previously unimaginable with analog devices. These intelligent digital instruments not only perform detection, conversion, display, and regulation but also incorporate features such as program control, fault self-diagnosis, data communication, and remote monitoring and control to meet the requirements of computer networking.
Selection of Measurement Range and Accuracy
Similar to the range selection for temperature transmitters discussed in the previous section, the instrument's range should be determined based on the actual operating scope, allowing for some margin at both the upper and lower limits to ensure that measurements rarely exceed the range. However, an excessively wide range should be avoided to prevent a reduction in measurement accuracy and resolution.
When selecting the instrument's accuracy, the type of temperature-sensing element used must be considered. If a high-precision platinum resistance thermometer is used, the instrument's accuracy can be higher (e.g., 0.2%–0.5%). If an industrial-grade thermocouple is used, an instrument accuracy of 0.5% is sufficient.
