Electronic Design

Thermocouple Thermometer Features A 1-mV/°F Output

An inexpensive, battery-powered, cold-junction-compensated, thermocouple thermometer with an output of 1 mV/°F can be constructed using type-K thermocouple wire. Because type-K thermocouple wire has a relatively linear output characteristic, only an ice-point reference and a scaling factor are necessary to produce an output that can be measured with a digital meter.

By plotting the thermoelectric voltage versus temperature, it can be seen that the slope of the type-K thermocouple approaches a constant over a range of −0° to 500°F. The best straight-line fit between −40° to 500°F yields the equation:

Y= 0.226X − 0.707

where Y is the thermoelectric voltage in millivolts and X is the temperature in degrees Fahrenheit.

Solving the equation for X and multiplying by 101 for the gain of the inverting amplifier, the following equation can be derived:

X= (101Y+71.4) / 22.8

Therefore, by adding 71.4 mV to the thermoelectric voltage (101Y) and dividing the summed voltages by 22.8, 1-mV/°F output can be obtained.

The circuit shown performs this function by adding a cold-junctioncompensated voltage to the thermocouple (LT1025) and amplifying the output by 101 (MAX430) (see the figure). Then 71.4 mV is added (via the LM336 and associated circuitry) to the amplified signal and the voltages are summed together in the summing amplifier (LF356). Finally, the output voltage is divided using a 15-turn, 10k potentiometer.

To calibrate the circuit, use a digital voltmeter and adjust the 200-Ω 15-turn potentiometer until the meter reads 71.4 mV. Then obtain high-quality thermometer and connect a digital meter to the 10k, 15-turn output potentiometer and adjust it until it reads the same temperature as the thermometer.

A more accurate way to calibrate the circuit is to immerse the thermocouple in a foam cup filled with ice water (32°F) and measure the output at pin 6 of the inverting amplifier (MAX430). Then repeat the procedure using boiling water (212°F). After that, plot the thermoelectric voltage versus temperature. When this is completed, a straight-line equation between these two points can be derived. This will give a more accurate value of b, the y intercept. Finally, adjust the 200-Ω, 15-turn potentiometer until the meter reads the same value as b. The adjustment of the 10k potentiometer is performed as described above.

This method takes into account the small thermocouple voltages generated wherever dissimilar materials are joined together. They can be generated where the thermocouples are soldered to the circuit board, or can even occur at the junction between the IC package and sockets.

The circuit was optimized to measure temperatures from −40° to 500°F, although higher temperatures up to 1800°F can be measured with lower accuracy. The circuit is ideal for many HVAC, automotive, or laboratory applications.

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