Design Home Sensors Sensor Home Instruments/Devices Methods/Principles Displacement Stress & Strain Pressure Fluid Flow Flowmeter Temperature Thermocouple Intro Thermocouple Theory RTD Intro RTD Theory Thermistors Intro Thermistors Theory Pyrometers Intro Pyrometers Theory Resources Bibliography
Chemical Engineering

The industry gateway for chemical engineering and plant operations.

Hydraulics & Pneumatics

For design and manufacturing engineers involved in buying or specifying fluid power components and systems.

Reinforced Plastics

Informed and impartial coverage on the global composites industry.

Offshore

International news and technology for marine/offshore operations around the world.

more free magazines       Thermistor Equation The thermistor is a resistance thermometer. The relationship between its resistance and the temperature is highly nonlinear. Furthermore, the resistance changes negatively and sharply with a positive change in temperature, as shown schematically below. Characteristics of Three Temperature Transducers

The thermistor resistance-temperature relationship can be approximated by, where: T is temperature (in kelvin), TRef is the reference temperature, usually at room temp. (25 °C; 77 °F; 298.15 K), R is the resistance of the thermistor (W), RRef is the resistance at TRef, b is a calibration constant depending on the thermistor material, usually between 3,000 and 5,000 K.

The thermistor resistance can easily be measured, but the temperature is buried inside an exponential. Since all R and T are positive real numbers, we can apply a logarithm ln to both sides of the equation. Doing so allows us to solve for the temperature T, Alternatively, some references use the negative temperature coefficient (NTC) a to describe the sensitivity of a thermistor, Typically, the value of a falls between -2% ~ -8%.

With the above equations, the temperature can be directly obtained from the measured resistance. Note that the material constant b may vary slightly with temperature and is usually provided by vendors. One can also use several well known temperature conditions as check points, e.g., ice water at 0 °C (32 °F) and boiling water at 100 °C (212 °F), or use other pre-calibrated thermometers to calibrate/curve-fit the value of b.

However, b may vary considerably across the temperature range of interest. In this case, one should resort to a calibrated curve-fit of the R-T relationship and neglect the equations presented above. A suitable curve fit is suggested by, 