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I have thermocouples and thermistors on my job measuring temps. I accidently ran type k thermocouple wire to my thermistors. I am wondering if they will still read accurately or if I need to replace the wire with a 2 pair shielded conductor ? thank you

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  • \$\begingroup\$ You need or don't need shielding regardless of your wire composition. If you were using unshielded copper wire, using thermocouple wire should not change your noise pickup. \$\endgroup\$ – WhatRoughBeast Apr 15 '16 at 19:43
  • \$\begingroup\$ What are you worried about, what does shielding have to do with a type k thermocouple? Please be more specific. \$\endgroup\$ – Voltage Spike Apr 15 '16 at 20:22
  • \$\begingroup\$ This would make a nice homework problem for students in a sensors class. \$\endgroup\$ – Nick Alexeev Apr 15 '16 at 20:56
  • \$\begingroup\$ I am worried that my thermistors will not read correct if it is wired with thermocouple wire due to the added resistance that the thermocouple wire has. \$\endgroup\$ – m boerste Apr 16 '16 at 2:09
  • \$\begingroup\$ Will a thermistor read 75deg c if it is wired with a thermocouple wire? Thermocouple wire has higher resistance then regular copper wire. \$\endgroup\$ – m boerste Apr 16 '16 at 2:15
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There are 2 questions here.

  1. Wire is made with thermocouple materials. How much would that affect thermistor reading, compared to regular copper wire?

Here's a hypothetical example. Let's say that the temperature on the thermistor end of the wire is 100 °C, and temperature on the proximal end of the wire is 20 °C. Type K thermocouple has sensitivity of 41 µV/°C, so an 80 °C gradient will generate 3.3 mV. If we also have a common 10 kΩ NTC thermistor (for example, NTCS0603E3103*LT), it would have the resistance around 1 kΩ at 100 °C, and its sensitivity would be 2.62%, which corresponds to 26.2 Ω/°C change in resistance. If there is 0.5 mA current running through the thermistor, then the sensitivity becomes 13.1 mV/°C.

So, the voltage generated by the thermocouple wire may or may not be appreciable. The higher the temperature that you will be measuring with the the NTC thermistor, the greater the impact of the thermocouple wire.

  1. EMI and shielding (since the O.P. had mentioned shielded conductor).

EMI would affect thermocouple wire and regular copper wire equally.

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Interesting question! The K thermocouple extension wire will introduce an EMF dependent (to first order) on the temperature difference between the ends of the wire.

The effect of that thermal EMF will depend on the exact circuit used and will tend to be much, much larger at high temperatures (when the thermal EMF is also maximized) so you cannot simply test it at room temperature and be assured it will work okay at the maximum temperature.

To pick an example, if the voltage across a typical NTC thermistor that changes about 5% per degree C is 100mV then a thermal EMF of (say) 5mV (corresponding to about 150°C temperature difference between ends) would result in an error of about 1°C. So it may not have a huge effect, but it will be dependent on the circuit used in the measuring instrument.

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