Thermocouples as active analog transducers

This lab activity will make use of a type T thermocouple cable, which has two conductors composed of copper (typically blue insulation) and constantan (typically red insulation). Constantan is a copper-nickel alloy (6:05 min).

A thermocouple wire can generate a small amount of electrical power from any thermal potential differences created by a temperature gradient along the cable. The potential to draw a small current associated with that power is indicated by an electrical potential difference across the two conductors of the cable. Any volt meter with sufficient sensitivity can be used to measure this voltage (2:07 min).

The Campbell PC400 software provides a graphical user interface (GUI) tool called Shortcut to program the data logger to collect measurements from various types of transducers. Shortcut also provides several tools for measuring basic electrical properties, like differential voltages across two leads. Before we can estimate a temperature from a thermocouple wire, we first have to be able to measure a precise voltage across the two conductors of the cable (18:02 min).

Note that the "AutoRangeC" option will not be available for voltage measurements with a CR310. You should use the more sensitive setting, or the 34 mV setting, if working through these exercises with a CR310.

Once programmed to measure a voltage, you can check the real time voltage measurement using the PC400 software. If the data logger has an independent keypad and screen, you can also look at the real time voltage measurement on that screen (5:01 min).

The Shortcut tool creates a program in the CRBasic programming language that can be compiled and run by the operating system on the data logger. You can customize that program using the CRBasic editor included with PC400, which is often required for transducers not sold directly by Campbell or for more advanced data logging configurations. The following program should resemble the program created in the Shortcut creation of a program for measuring voltage across the conductors of a thermocouple wire. You should have a copy of this program if you want to follow along with customizations developed in later materials.

Note that the "AutoRangeC" option will not be available for voltage measurements with a CR310. You should use the more sensitive setting, or the 34 mV setting, if working through these exercises with a CR310.

Let's use the CRBasic editor to carefully review the program for measuring differential voltage that was created by Shortcut (16:21 min).

Note that the "AutoRangeC" option will not be available for voltage measurements with a CR310. You should use the more sensitive setting, or the 34 mV setting, if working through these exercises with a CR310.

Eventually, we will be editing this program to estimate the temperature from the thermocouple wire based on more than one calibration technique. To get started, we can review how the TCDiff function in CRBasic estimates the temperature from the thermocouple using Campbell's choice of calibration scheme, which is likely based on the National Institute of Standards and Technology (NIST) polynomials (11:35 min).

Note that the "AutoRangeC" option will not be available for either the voltage measurements or TCDiff measurements with a CR310. You should use the more sensitive setting, or the 34 mV setting, if working through these exercises with a CR310.

Video errata: In these videos I reference that you cannot get CRBasic without paying for it. As of this update to the web page, that is no longer true. The PC400 software is now available free of charge from Campbell Scientific and that includes the CRBasic editor.

After adding the code for estimating the temperature from Campbell's TCDiff function, the CRBasic code should look something like the following.

Note that the "AutoRangeC" option will not be available for either the voltage measurements or TCDiff measurements with a CR310. You should use the more sensitive setting, or the 34 mV setting, if working through these exercises with a CR310.

Boudry (1976 in Journal of Physics E: Scientific Instruments) provides a more computationally efficient calibration equation (relative to high-order NIST polynomials, see reference materials) for the relationship between voltage and temperature for type T thermocouples. Let's program our own function for performing a Boudry type calibration calculation from the differential voltage measured on the thermocouple wire (18:52 min).

Click to go to the digital object identifier for Boudry (1976 in Journal of Physics E: Scientific Instruments)

The following is the CRBasic program incorporates the Boudry-based estimate of temperature added in the previous activity. This program provides two different estimates of temperature from two separate voltage measurement across the thermocouple conductors: TempCampbellTC and TempBoudryTC.

Note that the "AutoRangeC" option will not be available for either the voltage measurements or TCDiff measurements with a CR310. You should use the more sensitive setting, or the 34 mV setting, if working through these exercises with a CR310.

Click this link to download the MS PowerPoint file

CRBasic has subtle differences in measurement functions depending on the features of the data logger used. If using the CRBasic editor, extensive documentation is available directly through the software, and there is typically little need for external documentation.

CRBasic has no specific online documentation specific to the CR800 Series data logger. The CR300 documentation will be close, but may have some subtly different arguments for some of the measurement functions. 

Link to the online CRBasic documentation for CR300 Series data logger

The CRBasic documentation for a CR1000X may also be closer to the same as the CR800 Series. 

Link to the online CRBasic documentation for a CR1000X

This is a US National Institute of Standards and Technology database including the experimental data and optimized high-order polynomial models used to relate differential voltages to temperature gradients in various types of thermocouple wire.

Link to NIST calibration database for thermocouples

Use of this database is a bit cryptic, but you can navigate to the following raw download of all the data and calibration equations defined for a type T thermocouple.

Link to NIST raw data and calibration equations defined for a type T thermocouple