temperature characteristics

Some are slightly curved, while others are perfectly straight, but the measurable temperature range also varies. What they have in common is that the electromotive force is 0 mV at a temperature of 0°C. This is due to the addition of a correction called "cold junction compensation".

Correction method for "cold junction compensation"

A thermocouple is a device that measures the temperature of the contact area by contacting one side of two dissimilar metals and measuring the potential difference that appears on the other side. The part where two dissimilar metals come into contact is called the "temperature measuring junction," and the point of contact between the metal and the measuring instrument is called the "reference junction (cold junction)."

Thermocouple measurement basically converts the temperature of the measuring junction by measuring the voltage between the reference junctions when the reference junction is at 0°C. This is called "cold junction compensation".

Real problem, "cold junction compensation" is difficult

However, for "cold junction compensation", for example, it is not realistic to install a Peltier element or the like inside to keep the reference junction at 0°C all the time. For this reason, actual products are corrected using a method called "reference junction compensation." Reference junction compensation measures the temperature of the thermocouple connection part (terminal block, etc.) on the circuit board and corrects it by adding the thermoelectromotive force with 0°C as the reference.

Since a Pt100 platinum resistance temperature detector is often used for reference junction compensation, let us first explain the resistance temperature detector (RTD).

Temperature characteristics of Pt100 platinum resistance thermometer

Pt100 has very good linearity and is widely used for accurate temperature measurement. By the way, the name "Pt100" comes from "100Ω at 0℃".

Example of a thermocouple circuit using a platinum resistor

Regarding the "reference junction compensation" of thermocouples, which I explained earlier, since RTDs have extremely high accuracy, they are used for reference junction compensation of thermocouples. This can be achieved by using a high-precision resistor, a reference voltage (Vref), and a platinum resistor (Pt100) to configure the circuit as shown below.

However, Pt100 is quite expensive because the material is platinum. Therefore, thermistors are often used in inexpensive temperature measurement circuits.

temperature characteristics

The thermistor temperature characteristics are shown below.

The temperature characteristic of the thermistor is that the resistance value changes significantly in the range of 0°C to 100°C, making it easier to clearly detect temperature changes than other temperature sensors.

NTC is commonly used for temperature measurement

There is a difference in the characteristic curve between NTC and PTC. The NTC changes slowly, while the PTC changes sharply after reaching a certain temperature. NTC is commonly used for temperature measurement.

However, if you look closely, its characteristics are not linear, unlike platinum resistors. Furthermore, since it is basically a resistor, self-heating cannot be avoided, and hysteresis appears in the resistance value, which causes errors.

A typical semiconductor temperature sensor is a "Si diode". Si diode Vf (forward voltage) has a temperature coefficient of -2mV/°C, and Vf decreases as the temperature rises. The temperature can be measured by measuring the voltage across the diode.

A PNP transistor can also be substituted for the Si diode. If so, measure the voltage between emitter and base. Since the Vf of the diode depends on the current value, a highly accurate constant current source is required. This measurement method is often used for temperature measurement inside ICs such as processors and ASICs.