Basic Thermistors Design Notes in Process Industry

This article establishes guidelines for the use of thermistors in process plants. Main keywords for this article are Basic Thermistors Design Notes in Process Industry, Thermistors References, Thermistors Definitions.

Thermistors References

Reference is made in this article to the following document.

American Petroleum Institute (API)
RP 550 Manual on Installation of Refinery Instruments and Control Systems
RP 551 Process Measurement Instrumentation

Thermistors Definitions

For the purpose of understanding this standard the following definitions apply:
Span. The algebraic difference between the upper and lower range values.
Temperature coefficient. The rate of change of some physical property-electrical resistance instance-with temperature; the coefficient may be constant or nearly constant.
Thermistor. A temperature transducer constructed from semiconductor material and for which temperature is converted into a resistance, usually with negative slope and highly nonlinear.

Basic Thermistors Design Notes in Process Industry

• Thermistors are not encouraged for temperature sensing in an industrial environment because they do not offer a significant advantage over thermocouples or RTDs.
• The installation of thermistors shall conform to API-RP 550/551.
• Only thermistors with negative temperature coefficients shall be used.
• Thermistors shall be used where extreme accuracy is required.
• The use of thermistors shall be limited to alarm applications.
• Thermistors have low mass and limited heat dissipation capabilities. Therefore, the manufacturers data shall be used in calculations of heat dissipation to ensure self heating does not occur.
• Thermistors shall not be used for wide span measurement because the resistance versus temperature relationship is nonlinear.
• The application of thermistors shall be at the discretion of Company. 

For Your information Please check following article related to Temperature Measurements.

General Requirements of Temperature Instruments in Process Industry

Resistance Temperature Detector RTD Design Requirement in Process Plant

SAES-J-400 Temperature Instrumentation

TEMPERATURE TRANSMITTER EXPLANATION – COMPARISON BETWEEN RTD AND THERMOCOUPLE

CATEGORIES OF TEMPERATURE MEASUREMENT DEVICES

THERMODYNAMIC CONSIDERATIONS – TEMPERATURE MESUREMENT SCALES

TEMPERATURE TRANSMITTER CALIBRATION – CHECKING OF TEMPERATURE SENSORS