THC4 (4-channel Thermocouple Module)

(c) 1998 EME Systems

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EME Systems offers a thermocouple conditioner module that takes as it input up to four type T thermocouples,
and at its output produces an amplified and compensated voltage that covers the range from -25°C up to +225°C.

°C = volts / 0.02 - 25. 

Other ranges and calibration for type K or type J couples are available on request. This is compatable with data loggers such a our OWL2c, or with any data logger that accepts a voltage input. The module is calibrated at 0.0 and 50.0°C, and the accuracy is ±0.5°C within that range. Additional compensation for the curvature of the thermocouple response curve is possible in software for greater accuracy or to cover a wider range of temperatures. The four different thermocouples are addressed with two voltage output lines from the controller, with binary addresses of 00, 01, 10 and 11. The THC4 comes with an 11 page operation guide.

The THC4 enclosure is polycarbonate, measuring 2.5"l x 2"w x 1.4"h plus gland nut for cable entry. Weight, 4 ounces. The thermcouples are soldered to the circuit board, and the thermocouple wires project through a lip on one side of the case. There are no connectors on the side of the case, and this contribes to the economy of the module, and it can be used as is when the desired lengths of thermocouple wire are known in advance. If connectors for extension wire are desireable, they can be fitted on the pigtail thermocouple cables outside the case. The calibration constants are noted on a calibration label on the side of the case.

open THC4


There is a five-conductor cable from the THC4 to the logger:

  1. +6-18 volts dc,
  2. address A (low=5 volts, high=3 to 18 volts)
  3. address B (low=5 volts, high=3 to 18 volts)
  4. temperature output signal
  5. common.

We calibrate the THC4 at the factory with type T thermocouples to produce 0.500 volts at 0.0°C (32°F), and 1.500 volts at +50°C (122°F). The sensitivity is 20 millivolts per °C. This calibration is chosen to cover a useful biological temperature range. Please consult the factory for other calibration options.

Using our standard calibration, the formula to convert from volts to temperature is:

°C = volts / 0.02 - 25      or the inverse: 
volts = 0.02*(°C+25)

The minimum temperature measureable is about -25°C (-13°F) At the high end, with a 5 volts A/D converter, the response extends to about +225°C (437°F). The accuracy of the formula is ±0.5°C over the -10 to +60°C range. The THC4 does not correct for the nonlinearity of the type T voltage vs temperature curve, which is concave downward. Better accuracy can be had by correcting for the curvature in software. This formula and the errors in the straight line approximation are described in the THC4 operation manual.

There are many situations where thermocouples are the best choice as temperature sensors. Their calibration is determined by the dissimilar metal alloys that form the thermocouple, and not by the size or shape of the junction. The junction can be formed into many different sizes or shapes suited to different applications. Prefabricated thermocouples can be purchased from several sources in many forms, from welded beads made from wire the size of a hair, that can be passed thru a syringe, to heavy junctions built into steel cases or washers suitable for attachment to pipes or heavy machinery. In its simplest form, the temperature sensor is simply the ends of the pair of thermocouple wires, twisted together, and braised or soldered. The distance to the point of measurement can be extended simply by splicing on additional lengths of thermocouple wire. Thermocouples can cover a very wide range of temperatures, up to nearly the melting point of the constituent metals.

In biology thermocouples are used to construct tiny probes that can be used for example to measure temperatures inside flowers or on leaf surfaces. Tiny probes also respond rapidly, so they are useful for example in sudies of the temperature of inhaled and exhaled air. Type T (copper-constantan) couples are often used in biology, because of their ease of construction, and resistance to mildly reducing and mildly oxidizing aqueous environments. Type K thermocouples (chromel-constantan) are used when higher temperatures need to be measured. The temperature ranges of the two types of thermocouple are as follows: Type T has a range from -200°C to +350°C and is very accurate in this range. Type K has a range from -200°C to +1250°C, but it is not as accurate as the Type T.

The difficulty in using thermocouples comes from two considerations. First, the signal they produce is small and has to be amplified and may have to be shielded from sources of electrical noise. Second, the signal is not produced as an absolute value, but is relative to a reference temperature which must be known or measured independently. Our THC4 module both amplifies the tiny signal and provides the known reference signal. It also provides a multiplexer so that the relatively expensive amplifier and reference circuitry can be shared between four type T thermocouple probes.


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