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Muhammad kashi
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Join Date: Apr 2011
Posts: 3

thermocouple Cable

04/26/2011 1:44 PM


can any body tell me why thermocouple outputs voltage do not drop? we know that voltage drops depends on resistance? why cable ristance not effect on mV if effect on mV then how much. how much drop comes in mV and how much chance comes in TEMPERATURE?
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RAMConsult
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Engineering Fields - Power Engineering - New Member

Join Date: May 2007
Location: NYC metropolitan area.
Posts: 3230
Good Answers: 444
#1

Re: thermocouple Cable

04/26/2011 3:28 PM

Ohm's law is why. Thermocouple circuits are low voltage, low current measured with high impedance circuits. A high impedance circuit puts a very low burden on the thermocouple so there is very low current which when squared gives even lower voltage drop. We're interested in the mV output of the thermocouple, not its current generating capability, if you draw too much current you will affect the accuracy of your readings.

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67model
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Join Date: Dec 2010
Posts: 1686
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#2

Re: thermocouple Cable

04/29/2011 5:37 PM
  1. Practical modern thermocouple amplifiers have very low input current. Even the 741 operational amplifier, available since 1967, has a maximum input current of 0. 5 microamp at 25 Celsius.
  2. Modern amplifiers have input currents of 10 nanoamps, but often an "upscale/downscale" drive current of a fraction of a microamp is applied, so that an open - circuit thermocouple is made obvious, by zero or full-scale indication.
  3. A typical thermocouple circuit with 10 metres run from hot end to amplifier has a loop resistance of about 50 ohms (including the thermocouple itself).
  4. So the voltage drop in the resistance is 50 ohms x 0.5 microamp = 25 microvolts or less.
  5. The microvolts per degree Celsius for the common Chromel-Alumel and Copper-Constantan thermocouples is about 40.
  6. So 25 microvolts causes an error of about 0.5 Celsius maximum. Which means that, if you have a long cable run or use thin wire, you must not forget the possible error!
  7. Note that thermocouple materials typically have resistance many times that of copper of the same cross-section. For example a Chromel-Alumel cable run has a loop resistance 60 times that of a cable of the same conductor cross-section area using copper wire.
  8. Do NOT use copper wire for thermocouple connection, in place of the matching thermocouple materials! It introduces errors equal to the difference between the variable and usually unknown temperatures at the start of the copper cable and at its end at the amplifier!
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