Fcal Thermocouple Tables

• About Thermocouples
• Thermocouple Types
• Custom Thermocouple Applications With Daqarta

About Thermocouples:

Thermocouples are nonlinear temperature sensors, but they are popular nevertheless because they can handle high temperatures, and can be made with great uniformity such that two units of the same type will produce nearly identical readings. Such consistency allows fixed calibration tables to be used for all thermocouples of the same type.

A thermocouple consists of a pair ("couple") of wires made from dissimilar metals that are joined together at one end to form the temperature sensor or probe, known as the "hot junction". The other ends are connected to a sensitive voltage measurement system. There are separate positive and negative junctions here, but they are kept at the same temperature and together are known as the "cold junction".

The voltage difference between these wires (in the 0-50 mV range or less) is approximately proportional to the temperature difference between the hot junction and the cold junction. The proportionality varies slightly over the temperature range, but for any given pair of metals this can be known with high precision.

Standard thermocouple types use specific metal alloys, and for each type a table is available giving the mV output for each degree Celsius of temperature difference, assuming that the cold junction is at 0 C. In earlier times, this could be arranged by placing the cold junction in a bath of ice and water... certainly not very convenient!

Modern thermocouple systems typically have "cold junction compensation" using a non-thermocouple temperature sensor to apply a correction to the measured voltage so that it approximates that which would be measured with a real ice bath. For example, suppose the cold junction is actually at 20 C (68 F, a typical room temperature). With a type K thermocouple, the table shows 0.79812 mV at 20 C. That's the voltage that would be read when the hot junction is at 20 C and the cold junction is at 0 C, and it's also the voltage that the cold junction compensator must add to the actual voltage when the cold junction is really at 20 C.

The K-type mV table values increase at about 0.040 mV per degree C (that's 40 microvolts) in this range, so the compensator voltage needs to change to match. This can be arranged with a precision temperature sensor chip like the LM335, or even a carefully adjusted thermistor circuit. Note that different thermocouple types require different compensation adjustments. See Thermocouple To Frequency for a circuit that includes cold junction compensation using an LM335.

Note that if you do not use cold junction compensation (or an ice bath), you can toggle the Low button to Reference and calibrate to the current cold junction temperature as an approximation. See the discussion under the Fcal Low topic.

Thermocouple Types:

Daqarta includes tables (.TBL files) for all of the standard thermocouple types defined in the ITS-90 (International Temperature Scale of 1990) tables provided by the US National Institute of Science and Technology (NIST). The mV values in Daqarta's tables are given with 5 decimal places, obtained from equations provided in the NIST ITS-90 document. (The NIST ITS-90 tables themselves only use 3 places.)

The names of the .TBL files are of the form B_TypeTC.TBL, where the first letter gives the thermocouple type.

Type     File Name       Degrees C           mV
  B    B_TypeTC.TBL      0 to 1820       0 to 13.82
  E    E_TypeTC.TBL    -270 to 1000    -9.83 to 76.37
  J    J_TypeTC.TBL    -210 to 1200    -8.10 to 69.55
  K    K_TypeTC.TBL    -270 to 1372    -6.46 to 54.89
  N    N_TypeTC.TBL    -270 to 1300    -4.35 to 47.51
  R    R_TypeTC.TBL    -50 to 1768     -0.23 to 21.10
  S    S_TypeTC.TBL    -50 to 1768     -0.24 to 18.69
  T    T_TypeTC.TBL    -270 to 400     -6.26 to 20.87

Custom Thermocouple Applications With Daqarta:

The above .TBL files are intended for use with Fcal mode of the Frequency Counter to provide a large temperature readout. But that only supports a single channel, and does not show the temperature as a chart recorder trace.

You can create custom macros to support both sound card input channels, typically using a "chopper" circuit such as the External DC-to-AC Modulator to convert DC voltages (like thermocouple outputs) into AC so they can pass the sound card's AC-only input stage. You'd then use the Demodulate option in Decimate mode to convert back to DC values.

Thermocouple macros are provided with Daqarta to convert these values to temperature, which you can then use for readouts via Custom Meters, for plotting via a Chart Recorder, for data logging to a file, or for use in a temperature controller.

Alternatively, you can use an inexpensive Arduino or Numato board to read up to 6 input channels, but note that these only have 10-bit resolution. Additionally, you will need to provide external gain. The DC_Chart_Recorder macro mini-app can be modified to use the thermocouple macros to allow direct temperature recording.

See also Fcal Calibration Table, Fcal Dialog, Frequency Counter