This is just a brief overview of how to do thermocouple measurements correctly and simply.
The principles of measuring with thermocouples are described by a lot of web pages. However, one can easily misinterpret the informations obtained form the internet.
The first confusion usually comes from the use of “ice bath” as cold junction compensation, the other is when someone tries to use the thermocouples’ mV – temperature tables.
Although the thermocouples’ physical principles are thoroughly described, many people simply loose themselves in the details.
So, just to be short: a thermocouple is a very simple device for measuring temperature. It consists of two wires made from different metal alloys.
At one end these two metals are welded together, and they are separated till the other end of the thermocouple.
The welded end is the hot junction point, the other end is the cold junction point.
Let’s put a digital voltmeter to the cold juntion point.
We are in a room, which has a temperature of 25 °C, the thermocouple is in this room too, thus it at this temperature too.
This way the digital voltmeter shows 0 Volts. (The + and – signs on the picture correspond to the “red” and “back” flying leads of the digital multimeter).
Thermocouple with both ends at room temperature
By principle the thermocouple produces voltage proportional to the temperature difference between the hot and the cold junction points.
So let’s put the hot junction point in boiling water. This way the hot junction point will be at 100 °C, while the cold junction point stays at 25 °C.
According to the following table (considering a K type thermocouple) the digital voltmeter shows 3.059 mV (the value seen at the 75 °C point), because the temperature difference between the hot and cold junction point is 75 °C.
Thermocouple measurement 1
This is very important because thermocouples only sense to the temperature DIFFERENCE between their hot and cold junction point, so if we measure 3.059 mV with our digital voltmeter, we can observe a 75 °C difference, but we measure this value even if the cold junction is at 300 °C and the hot junction is at 375 °C!
Thermocouple measurement 2
This way we must measure the temperature at the cold junction point as well.
The other thing about thermocouples is that they can produce negative voltages too, in case their hot junction point is colder than their cold junction point.
For example: on a hot summer day when the temperature is 38 °C, we put the thermocouple’s hot junction point to a fridge.
In this case we measure -2.45 mV with the digital voltmeter. This voltage corresponds to -56 °C. We must add the environmental +38 °C to this value and we get that the fridge’s internal temperature is -18 °C.
Termocouple measurement 3
Measuring so little voltages can be challenging, but there are several cheap and high resolution analogue to digital (A/D) converters available on the market, which can do the job. On the other hand, if one would like to use the A/D converter of a cheap microcontroller, this small signal has to be amplified. This amplification factor is in the range of 50x – 500x.
In a single supply system (which is common in practice) the negative terminal of the cold junction point cannot be connected to the supply GND, because the thermocouple can produce negative voltages as mentioned above. This way an offset is needed to rise the potential of the negative terminal (the 1M – 100k voltage divider in the picture). This offset causes the need of using differential input A/D converter or differential input amplifier.
Thermocouple measurement with A/D and MCU
The temperature at the cold junction is measured by a high accuracy integrated sensor, while the calculations and the linearisation is done by the microcontroller unit (MCU).
The following picture shows a full analogue solution. Here a +12/-5 V power supply unit (PSU) is needed.
Thermocouple measurement with only analog components
The output voltage is proportional to the measured temperature. The INA128 instrumentation amplifier’s gain is set by R, whose value depends on the applied thermocouple type, for eg. a K type thermocouple (in case the measuring range is 800 °C) needs a 208,83 ohm resistor
In addition if the thermocouple’s hot junction point is to be connected to the protective earth (PE), the amplification / A/D section’s power supply has to be floating/isolated. This isolation is done by the PSU.
To maintain accuracy when measuring with thermocouples an additional linearisation element should be accompanied, since thermocouples aren’t linear elements at their full measuring range.
When using an A/D converter and a MCU the linearisation is done in software, but in case a full analogue solution, linearisation is a hard task.
In practice linearisation is usually omitted, and a “simple” amplification is used. This is the price for the toughness.
Both solutions can be supplemented with a comparator and relay section which in this way can act as a temperature switch (trip amplifier).
This can serve an overtemperature protection.
One can easily realize that there are some completely integrated thermocouple interfaces on the market, which do all the connection, cold junction compensation and linearisation in one package.
These circuits can be added to a 4-20 mA loop powered transmitter electronics.
These examples are schemas, the EMI filters, fuses, supply filtering capacitors and other “little things” are left out, to be as clear as possible.