Innovate Motorsports TC-4 - Manual (Page 4)

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3. Thermocouple Basics
Thermocouples are used to measure temperatures by relying on the phenomenon where a
junction of any two different metals ( Copper and Iron, for example ) will generate a small
voltage. This voltage is dependant upon which two metal are used, and the temperature of the
junction. This phenomena is known, formally, as the "Seebeck Effect". Because every junction
of different metals contributes its own voltage into the measurement, it is important to have as
few junctions between dissimilar metals as possible in order to record an accurate
measurement. This is why thermocouple wire is made completely of two different metals. The
"Type K" thermocouple to be used with the TC-4 kit is composed of Cromel and Alumel; one lead
being made of each ( the red and yellow leads ).
To make a thermocouple, strip approximately 3/4" of insulation form one end of the thermocouple
wire. Twist the two exposed metal ends together. You may optionally solder them, also. But
twist them first. Do not solder them in parallel. This will form what is called the "Hot junction".
This "Hot junction" is what you will connect to the surface that you want to measure. This is
usually either: a) under the copper gasket of a sparkplug for cylinder head temperature
(CHT) or, b) clamped to a primary header tube for exhaust gas temperature (EGT). Note that
clamping to the primary tube will NOT measure real EGT, but the surface temperature of the
header pipe. For real EGT measurements use an EGT probe that extends into the header itself.
There is also the "Cold junction." This is where the 2 leads of the thermocouple come together
again at the TC-4 terminals. The TC-4 has an internal temperature sensor at the T/C input
terminals. It uses this sensor to "offset" the effect of the "Cold junction" in the measurement.
This is called "Cold junction compensation". Once the effects of the cold junction are neutralized,
the TC-4 can accurately read the temperature of the "Hot junction" which is the twisted lead pair
at the opposite end of the thermocouple wire.