TC wire question..

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MicroRem
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Re: TC wire question..

Post by MicroRem » Tue Jun 16, 2015 2:33 pm

Dac,
Is this for your ultralight?
Tom

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dacflyer
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Re: TC wire question..

Post by dacflyer » Wed Jun 17, 2015 4:20 am

yes it is...

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MrAl
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Re: TC wire question..

Post by MrAl » Wed Jun 17, 2015 5:06 am

dacflyer wrote:well i am not able to do the check directly at the sensor with the meter, because with the engine running and a swinging prop is out of my comfort zone, and the CHT probes are very difficult to get to, they are under engine shrouds..
the EGT probe is not too bad to get to, but still not easy.
to do what i could would require 2 people to do it..one sitting in the seat controlling the engine and the outside..

Hi again,

Ok well usually when something like this happens a test is in order in order to determine if something might be strange about the circuit, like an open wire or poor connection, or something else that we dont know about yet might be wrong. By doing a test, we establish a ground floor to work from, which we can then compare to other tests and try to determine exactly what it is that is causing the error.

In this case we dont know if it is the meter itself or the wires, because there is a question of the resolution and precision of the meter itself. If the precision isnt that great anyway then the wires might not be doing anything too significant.

The test is what tells us what is wrong, and without that we can never know for sure. If it is too hard to test in place then sometimes we can come up with other ways to test. For example, get a regular probe and try it with that, then try it with the wires, measuring some source temperature that is close to the temperature that will be measured in the real life application. So at this point it is just about testing the meter and ANY probe with and without wires.
If you dont do something like this you will never know for sure what is wrong. This might mean the same thing could happen with wires that are of the same material.
LEDs vs Bulbs, LEDs are winning.

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dacflyer
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Re: TC wire question..

Post by dacflyer » Wed Jun 17, 2015 12:58 pm

i have 2 gauges, 4 probes..

1 gauge has 1 EGT and 1 CHT - both set have the same readings..so any defect can
be ruled out I.M.O.

i'll get it figured out..

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MrAl
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Re: TC wire question..

Post by MrAl » Thu Jun 18, 2015 5:57 am

Hi again,

Ok great. We could go through the math, but because of the accuracy limits here it's probably not worth the effort.

Good luck with it...
LEDs vs Bulbs, LEDs are winning.

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dacflyer
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Re: TC wire question..

Post by dacflyer » Thu Jun 18, 2015 9:08 am

ya, i think anything more accurate will require electronics...

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Re: TC wire question..

Post by UhClem » Sun Jun 21, 2015 11:54 am

Looking at the web page for the gauge manufacturer, it appears that the device is designed assuming a constant cold junction temperature of 24C. Variations from that result in errors. So you do not need to use TC extension wire unless the temperature at the connection to the TC wire is at a location with uncontrolled temperature.

If you wanted to get really carried away you could put the cold junctions into an isothermal block (something thermally conductive like aluminium with some thermal insulation around it) along with an absolute temperature sensor. Then use the temperature of that block to adjust the readings.


Extension wire is thermocouple wire with less well controlled material properties. TC wires are made from combinations of metals that usually do not like to mix well. Because the voltage generated in the wire is a function of the temperature gradient and the Seebeck voltage, you need to have well controlled material properties in areas with high temperature gradients. The highest gradient is typically at the point where you want to make the measurement.
David Schultz

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Lenp
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Re: TC wire question..

Post by Lenp » Mon Jul 27, 2015 7:27 am

Side note,
I just got through repairing a batch of laboratory hot plates. Most used a thermocouple to read the plate temperature and they all used a generic 2 pin board connector for the TC wire, So I guess the devil is in the details. For instrumentation down to parts of a degree it is critical, otherwise dissimilar junctions must be insignificant errors.

Len
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