Ohio Pen - How Long Can My Thermocouple Wire Be?

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CLEVELAND - OhioPen -- It seems like at least once a week we get a customer asking, "how long of a wire can I run between my thermocouple and the data logger?" and our standard answer is, "it depends!" There are several important points to understand when answering this question:

First, a thermocouple measures temperature by generating a very small voltage based on the temperature where the 2 wires are fused together at the tip or junction. For example, at a room temperature of 72°F, a type K thermocouple will output .000888 volts.
Second, the operation of the thermocouple is based on the Seebeck effect, where a conductor will generate a voltage differential across two points when there is a temperature difference between these points.

Is There Any Interference Nearby?

Probably the most important thing to consider is the effect of electrical and magnetic interference (EMI) in the environment when combined with the small output voltage of the thermocouple.

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What Material Is Your Wire Made Of?

It's also important to consider potential voltage drops across the length of the wire. Thermocouple wire is typically in the range of 18 to 24 gauge. For 24 gauge K-type wire, the resistance is about 1.5 ohms per foot for the pair.

Wrapping It All Up

So, what does this all really mean? Practically speaking, the longest you want to go with a standard thermocouple run is about 100 feet without having to start jumping through hoops to get an accurate, low noise measurement. Even in this case, you will probably want to use shielded, SLE wire at least 18 gauge or larger. Just to be safe, we normally recommend a maximum of about 50 feet using regular thermocouples.

Using a signal conditioner like this has multiple benefits:

A high-level signal like a 4-20 mA current loop will be much more immune to the effects of EMI.
The effect of cable resistance will be eliminated as long as the power supply can drive the loop.
You can use lower-cost standard 2 conductor wire for the amplified signal instead of the more expensive shielded thermocouple wire.

As a comparison, 20 gauge, twisted pair, shielded K type thermocouple wire is about $1.41 per foot for a 500-foot roll while 2 conductor cable is $.14 per foot in the same size spool. The signal conditioner cost is a little over $50, so for anything over 45 feet, you will be ahead cost-wise using the regular wire and the TxBlock. Not to mention, you will have more accurate and lower noise measurements!

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For more information, or to find the ideal solution for your application-specific needs, contact a CAS Data Logger Applications Specialist at (800) 956-4437 or request more information (https://www.dataloggerinc.com/need-more-information/).