Thermistor
Definition and meaning of Thermistor in chemistry.
A thermistor is a resistor whose electrical resistance changes sharply and predictably with temperature. This makes it a very useful and reliable temperature sensor. Chemistry labs rely on thermistors for precise temperature monitoring and control.
In more detail
Thermistors are usually made from semiconductor materials, such as specific ceramic metal oxides or certain polymers. There are two main types. A negative temperature coefficient, or NTC, thermistor loses resistance sharply as its temperature rises.
A positive temperature coefficient, or PTC, thermistor gains resistance as its temperature rises. NTC thermistors are far more common in everyday temperature sensing. Thermistors are extremely sensitive to small temperature changes and respond very quickly.
This makes them ideal for tasks that need exact thermal control over a fairly narrow temperature range. Gas chromatography ovens, for example, rely on thermistors to hold a precise, stable temperature while separating chemical compounds. Because of their small size, thermistors also fit easily into probes and tight instrument spaces.
Standard thermocouples, in contrast, handle much wider and more extreme temperature ranges. They are usually less precise, though, at everyday lab temperatures. Many lab thermometers, ovens, and incubators use thermistors because they are inexpensive and easy to wire into a circuit.
Because thermistor behavior is nonlinear, most instruments use a lookup table to convert resistance into an exact temperature reading. Thermistors also appear in car engines and household appliances, where they help monitor and control temperature automatically. Their small size and fast response also make them useful for tracking rapid temperature changes during exothermic reactions.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Material | Semiconductor, often a ceramic metal oxide |
| Function | Temperature sensing and thermal control |
| Types | Negative temperature coefficient (NTC) and positive temperature coefficient (PTC) |
| Best suited for | Narrow temperature ranges needing high precision |
An NTC thermistor is often built into a digital thermometer probe. The probe sits in a water (H2O) bath during a kinetics experiment. As the water warms, the thermistor's resistance drops. The instrument converts this resistance change into a fast, accurate temperature reading.
Frequently asked questions
Why use a thermistor instead of a thermocouple?
Thermistors usually give higher accuracy and sensitivity, but only over a narrower temperature range. Thermocouples measure a much wider range, including extremely high or low temperatures, but with somewhat less precision.
What is the difference between NTC and PTC thermistors?
An NTC thermistor's resistance drops as temperature rises. A PTC thermistor's resistance rises as temperature rises. NTC types are more commonly used for general temperature sensing.
What materials are thermistors usually made from?
Most thermistors are made from semiconductor materials, especially ceramic metal oxides. Some are made from certain polymers instead, though ceramic types are far more common in lab equipment.