As I understand it, both thermistors and thermocouples are temperature sensors. So what are the advantages/disadvantages of using one over the other to measure the temperature? What are the specific applications for either of the sensors?
- wide range of temperature sensing (Type T = -200-350°C; Type J = 95-760°C; Type K = 95-1260°C; other types go to even higher temperatures)
- can be very accurate
- sensing parameter = voltage generated by junctions at different temperatures
- thermocouple voltage is relatively low (4.3mV for Type T thermocouple with one end at 0 C, other at 100 C, so that's 43uV/C tempco)
- mostly linear
- more narrow range of sensing (Quality Z thermistors spec'd at -55 to +150 C)
- sensing parameter = resistance
- usually very nonlinear
- NTC thermistors have a roughly exponential decrease in resistance with increasing temperature
- good for sensing small changes in temperature (unless you are careful in your signal conditioning, it's hard to use a thermistor accurately and with high resolution over more than a 50 C range).
- sensing circuit doesn't need amplification & is very simple (voltage divider with reference resistor tied to reference voltage usually is sufficient) – see my blog for more information about signal conditioning.
- accuracy is usually hard to get better than 1°C without calibration
Different Principles A thermistor is an electrical resistance, made of semiconducting material, that can be wired into a circuit. The semiconducting material is usually made of manganese oxides and nickel oxides. The thermistor works based on the principle that the electrical resistance of this material changes with temperature. A thermocouple, on the other hand, is made of two wires of different metals, such as copper and iron. The equal length wires are connected electrically together at one end and open at the other end. The principle is that if the open ends of the wires are at a fixed temperature and you change the temperature at the connected end, this generates a voltage between the two wires at the open end of the thermocouple.
Measuring Temperature With a thermistor, you could use an instrument that measures electrical resistance and connect it across the thermistor. You will measure a resistance change with a change in temperature. If you then refer to a table that lists temperature change versus resistance, you can find out the temperature from this table. In the case of a thermocouple, you will use an electrical circuit to measure the difference in voltage between the two wires at the open end and use this to measure the temperature difference between the two ends of the wires.
Thermistor or Thermocouple? In general, thermocouple readings are more precise than thermistor readings. However, they react more slowly to changes in temperature. Thermistors are also more expensive generally than thermocouples, due to the need for an external power source and the device's circuitry. The decision to use one or the other will depend on your specific needs.
The resistance of a thermistor varies with temperature. putting it in series with a resistor to form a voltage divider and you can measure temperature with a voltmeter. For even more accurate measurements it can be part of a Wheatstone bridge. They are often used as an over temperature sensor.
A thermocouple is a pair of dissimilar wires connected together. When heat is applied a small current is generated. The amount of current is roughly proportional to temperature. They work at higher temperatures than thermistors. A common use for a thermocouple is a detector on a gas pilot light. If the pilot light goes out, the thermocouple cools down and the gas supply is turned off.