Innovate Motorsports LogWorks 3 - Manual (Page 142)

LogWorks3_Manual_1.01.doc - 142 -
15.4 Using Thermocouples
Thermocouples are nothing but two wires of different materials joined together at one end. The
materials used determine the thermocouple type. The most common is Type K.
When two different metals are joined, a voltage is created at the junction. This voltage is
determined by the temperature gradient along the wires and by the metals used. That’s why only
wires of the same materials as the two thermocouple sides should be used. The voltage is very
small (in the microVolt range). Therefore it has to be amplified. Also the voltage is dependent on
the temperature gradient, meaning the temperature difference between the ends of the wire. So
the temperature of the device measuring (the cold junction) has to be known. This is called cold-
junction compensation. The LMA-3, TC-4 and DL-32 have thermocouple inputs and also provide
the cold-junction compensation.
Inserting different metals in between thermocouple and measuring device (for example to extend
the wires) should be avoided. The junction between the thermocouple wire and for example a
copper wire would also create a voltage and change the readings. That’s why thermocouple wires
of the right type for each side of the thermocouple must be used.
Sometimes it is necessary that multiple thermocouples have to be logged.
The following schematic uses the AD595 integrated circuit from Analog Devices. This device is a
single chip thermocouple amplifier with cold-junction compensation. The device can be ordered
from
www.digikey.com .
The circuit can measure between 0 degC (-32 degF) at 0V output and 1100 degC (2012 degF) at
5V output.