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Dave Saylor
Joined: 11 Jan 2015
Posts: 207
Location: GILROY, CA
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 Posted: Tue Mar 14, 2023 11:14 am Post subject: Thermocouple test |
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I got into a little debate at work the other day about testing thermocouples. The task is to check for a millivolt rise while heating each probe. My coworker was checking for resistance and got a little defensive when I pointed out that it calls to check for millivolts. The justification was "well, I use resistance". He was seeing a change in resistance with heat applied. I was seeing a change too, but I was looking at millivolts.
Now I'm wondering. Does checking for resistance give a less valid result than checking for voltage? The instructions just say to look for a rise in voltage, not any specific value. Does a "rise" in millivolts give any better information than a change in resistance? FWIW, the resistance was rising when heat was applied as well.
The system consists of four thermocouples in parallel. Application is a helicopter engine.
Thanks for your help,
Dave
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nuckolls.bob(at)aeroelect
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| Posted: Tue Mar 14, 2023 12:55 pm Post subject: Thermocouple test |
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At 02:13 PM 3/14/2023, you wrote:
| Quote: | I got into a little debate at work the other day about testing thermocouples. The task is to check for a millivolt rise while heating each probe. My coworker was checking for resistance and got a little defensive when I pointed out that it calls to check for millivolts. The justification was "well, I use resistance". He was seeing a change in resistance with heat applied. I was seeing a change too, but I was looking at millivolts.
Now I'm wondering. Does checking for resistance give a less valid result than checking for voltage? The instructions just say to look for a rise in voltage, not any specific value. Does a "rise" in millivolts give any better information than a change in resistance? FWIW, the resistance was rising when heat was applied as well.
The system consists of four thermocouples in parallel. Application is a helicopter engine. |
The procedure(s) you've described are gross
functionality tests to (1) show continuity across
the thermocouple and (2) verify that the thing
on the other end is indeed a thermocouple. I.e.
marked change of voltage with temperature.
These tests say nothing about calibration or accuracy
although if there is continuity, then there is little doubt
as to accuracy. Thermoelectric effects are kinda
molecular and not subject to 'drift'.
4 thermocouples in parallel, assuming similar
environments within the engine, will yield an
AVERAGE temperature measurement for the 4
items of interest. At the same time, testing
one of the 4 while still parallel connected
to the other 3 greatly reduces the sensitivity
of effects attributable to the one.
But you are correct . . . Thermocouples are
VOLTAGE generators with known scale factors
(millivolts per degree C change). An ohmmeter
deduces resistance of a conductor path by measuring
the voltage drop through that path when it's
'excited' by some test bias. Since the ohmmeter
is translating voltage values to displayed
resistance, it stands to reason that heating
a thermocouple connected to an ohmmeter would
manifest in a change of resistance reading.
But again, while this test says, "yeah, there
may be a thermocouple out there I'm seeing,"
it's no better or worse test of function than
simply looking for the generated voltage. It's
VOLTAGE that offers knowledge about what's
happening in the engine.
Bob . . .
////
(o o)
===========o00o=(_)=o00o=========
< Go ahead, make my day . . . >
< show me where I'm wrong. >
=================================
In the interest of creative evolution
of the-best-we-know-how-to-do based
on physics and good practice.
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Dave Saylor
Joined: 11 Jan 2015
Posts: 207
Location: GILROY, CA
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| Posted: Tue Mar 14, 2023 1:31 pm Post subject: Thermocouple test |
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Thanks Bob, that's just the kind of answer I was looking for. We'll all talk about this and maybe each learn a bit. --Dave
On Tue, Mar 14, 2023 at 1:57 PM Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:
| Quote: | At 02:13 PM 3/14/2023, you wrote:
| Quote: | I got into a little debate at work the other day about testing thermocouples. The task is to check for a millivolt rise while heating each probe. My coworker was checking for resistance and got a little defensive when I pointed out that it calls to check for millivolts. The justification was "well, I use resistance". He was seeing a change in resistance with heat applied. I was seeing a change too, but I was looking at millivolts.
Now I'm wondering. Does checking for resistance give a less valid result than checking for voltage? The instructions just say to look for a rise in voltage, not any specific value. Does a "rise" in millivolts give any better information than a change in resistance? FWIW, the resistance was rising when heat was applied as well.
The system consists of four thermocouples in parallel. Application is a helicopter engine. |
 The procedure(s) you've described are gross
 functionality tests to (1) show continuity across
 the thermocouple and (2) verify that the thing
 on the other end is indeed a thermocouple. I.e.
 marked change of voltage with temperature.
 These tests say nothing about calibration or accuracy
 although if there is continuity, then there is little doubt
 as to accuracy. Thermoelectric effects are kinda
 molecular and not subject to 'drift'.
 4 thermocouples in parallel, assuming similar
 environments within the engine, will yield an
 AVERAGE temperature measurement for the 4
 items of interest. At the same time, testing
 one of the 4 while still parallel connected
 to the other 3 greatly reduces the sensitivity
 of effects attributable to the one.
 But you are correct . . . Thermocouples are
 VOLTAGE generators with known scale factors
 (millivolts per degree C change). An ohmmeter
 deduces resistance of a conductor path by measuring
 the voltage drop through that path when it's
 'excited' by some test bias. Since the ohmmeter
 is translating voltage values to displayed
 resistance, it stands to reason that heating
 a thermocouple connected to an ohmmeter would
 manifest in a change of resistance reading.
 But again, while this test says, "yeah, there
 may be a thermocouple out there I'm seeing,"
 it's no better or worse test of function than
 simply looking for the generated voltage. It's
 VOLTAGE that offers knowledge about what's
 happening in the engine.
 Bob . . .
                  ////
                 (o o)
  ===========o00o=(_)=o00o=========
  < Go ahead, make my day . . .  >
  < show me where I'm wrong.     >
  =================================
Â
  In the interest of creative evolution
  of the-best-we-know-how-to-do based
  on physics and good practice.
|
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_________________
--Dave
KWVI |
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