TC wire question..

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dacflyer
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TC wire question..

Post by dacflyer » Wed Jun 03, 2015 9:43 am

i need to make a TC (thermocouple) extension cable, how do i go about it with minimal reading errors ?

right now i have a TC sensor connected to copper cable, and direct to my meter (analog) and my reading are lower than they should be..

do i need to fine TC wire the same as the material of the TC itself ? and if so how should i make the connections ? wire nuts, solider, crimp ???
the TC is 2 different wires itself ( different metals )

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MrAl
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Re: TC wire question..

Post by MrAl » Wed Jun 03, 2015 11:50 am

Hi there dac,

First question, does it work normally without the extra wire? You should test that first to make sure it works normally before adding anything.

The main idea here is that the meter has to know the temperature of the end of the wire that the meter has access to. That is so that it can provide the correct cold junction compensation, but the meter is assuming that the wire is of a certain type metal and so the voltages and temperature correlate the way that kind of probe should correlate.
With a different kind of wire, it will be measuring the temperature of the ends of the extension wires which are not of the same material as the normal wires, so it could easily get the wrong voltage because the voltage will be different with the different materials. The difference probably depends on how long the extension is.

However, using the same type of wires as the normal cable has, the voltage and temperature will still correlate, so it should work much better.

The main idea for the extra wire extension is that there must be normal electrical conduction and normal thermal conduction. The thermal conduction will happen no matter how big you make the joint, but the electrical conduction will go up if the joint is really big. So it would probably be best to make the joint as small as possible yet with the two wires making really good contact to each other. Soldering should be ok as long as the wires make contact before soldering (like maybe twisting them together).

They actually make extensions that plug into the meter and the original probe plugs into the extension.
Only thing is i am not sure how far you can take this...how long you can make the extension...until errors start to set in because of thermal leakage into the surrounding environment, which may not even be constant along the whole length.

You could always experiment a little too, but it is probably best to start with the same type of wires that the probe had to being with. That at least ensures that it will work eventually.

The electrical/thermal equivalent to a probe and extension of the same type metals at a constant temperature would look like four batteries, where two are lower voltage than the other two, and the four batteries form a parallel circuit where the difference at the ends of the batteries is measured, and the temperature is also measured at the ends. The two lower voltages subtract some voltage from the two higher voltages, and thus there is a small difference, and that difference correlates to the temperature at the probe tip.

The wires are represented by four batteries:
(+)---B1---B2---(-)
(+)---B3---B4---(-)

In the above, B1 and B2 are lower voltage than B3 and B4, and the two (+) terminals are connected together thermally and electrically. The two (-) terminals are connected only thermally, and so there is a voltage difference across the two but no temperature difference. The voltage difference is measured by the meter and so is the temperature at the two (-) terminals and they are both at the same temperature, and this voltage difference and temperature correlate to a certain temperature at the two (+) terminals (the probe tip).

If we use the same type wire for B2 and B4 (extension with two equal wire types), we see B1 slightly lower than B3 as before, but now B2 and B4 equal, so we only get half the difference voltage as before, yet we get about the same temperature as before. This will correlate to a different temperature at the (+) ends as interpreted by the meter, and thus will inaccurately represent the temperature at the probe tip.
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Re: TC wire question..

Post by jwax » Wed Jun 03, 2015 6:57 pm

Yes, the extension wire must be of the same materials as the TC itself.
Best way to join TC wires is by welding. No added metal- just hold the two metals together, and spot weld the junction.
Here's an excellent source of info on TC's:

http://www.omega.com/prodinfo/ThermocoupleWire.html

Enjoy!

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Re: TC wire question..

Post by dacflyer » Thu Jun 04, 2015 4:43 pm

MrAL >> thought you was EDD for a moment..lol but a good explanation... yes me meters are new and so are the probes, i just used the wrong cable ( copper ) as an extension..
the cables will be in free ambient air, not in a engine compartment. so i just need to order the exact same wire as the TC probes are made of.. i saw also that they make special TC plugs also... so i would not have to spot weld the wires, since i do not have the ability to do that anyway.. Oh. also i read that the extension cables can run as far as 40ft if needed.

thanks for the info guys

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Re: TC wire question..

Post by Lenp » Sat Jun 06, 2015 12:11 am

If the same wire as used in the thermocouple is not used for the extension, or the connectors pins, then what you wind up with is a lot of unintentional thermocouple junctions due to the dissimilar metals. Unfortunately, the the need for special connectors and wire make them a bit cumbersome for all but single point monitoring. Thermocouple rotary switches and relays for monitoring multiple probes are available, but expensive.

I once was asked to 'look at' a 20+ point DIY automatic thermocouple monitoring scheme that was centered around a salvaged telco stepper relay. It yielded absolutely useless readings!

In the past I have stripped and twisted about 3/8" of the thermocouple wires together, dipped them in brazing (not soldering) flux and passed them through the torch until a small metal ball melted on the ends. Do not use any filler rod! If you weld the the two different thermocouple wires together, you have a thermocouple junction that performs and tracks almost as well as 'factory" beaded probes at a fraction of the price. If you weld the same wires together, you have an extension!

Thermocouple interchangeability accuracy, within a family like J or K, is pretty good and for high temp monitoring, like a kiln, they are still great performers. Not much is simpler than a thermistor / meter movement pyrometer or the thermistor powered gas safety valve!

It looks like the PT100 sensors are becoming pretty common now with PID controllers
Look at http://www.auberins.com for an interesting line of temp controllers, especially noting the Model: SYL-1512A. I've used several of these with PT100 sensors for incubator controls and have had great success!
Len

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"I must be on the way to success since I already have the junk". (Me)

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Re: TC wire question..

Post by dyarker » Sat Jun 06, 2015 5:25 am

Close to the thermo-couple convert to 4 to 20 mA current loop. It is less sensitive to noise than RAW thermo-couple voltage and doesnot require special wire. Starter link:
http://en.wikipedia.org/wiki/4-20_mA

Cheers,
Dale Y

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Re: TC wire question..

Post by Lenp » Sat Jun 06, 2015 8:30 am

Yes, the 20 ma loop is a good choice for long distances. A local fire district put a lot of serial printers on 20 ma loops so they could 'broadcast' printed messages to all their stations. I think there were several loops using repeaters to cover the distance and to provide better performance than miles long loops. It went out of operation with the coming of the internet and better networking protocols.
Len

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Re: TC wire question..

Post by Lenp » Sat Jun 06, 2015 8:33 am

Additional note!
The 20Ma loop scheme is not limited to thermocouples. There are transmitters and receivers available for almost any control need!
Len

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"I must be on the way to success since I already have the junk". (Me)

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Re: TC wire question..

Post by dyarker » Sat Jun 06, 2015 12:25 pm

Don't confuse the 4mA to 20mA analog instrument loop with the +- 20mA digital loop used by teleprinters (commonly with 5 bit baudot code).

The 4-20mA loop is a convenient way to extend analog signals; like thermo-couples, photo cells, etc, a few feet to miles.
Dale Y

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Re: TC wire question..

Post by dacflyer » Wed Jun 10, 2015 11:13 am

i found a ready made cable from the manufacturer, but the cable did not look like it was anything special .
so i called them up and asked about the cable and the cable is copper, the tech explained to me that the gauges they use are analog ( having copper windings ) and that the copper cable i made will work just fine.. the tech told me that my low reading may be because of the engine running rich..he told me to lean it out some and then see if that helps any..

i did also explain that i had brass plated tin ( molex pin connectors ) he said that is the same that they use..

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Re: TC wire question..

Post by jwax » Thu Jun 11, 2015 9:32 am

Something isn't right with that. Which manufacturer told you the extension wires could be copper?

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Re: TC wire question..

Post by dacflyer » Thu Jun 11, 2015 12:21 pm

www.westach.com gauges

Image ( I use 2 of these 1 for each CYL.)

I had talked to one of the techs there directly..
not a desk jockey.


Model: 410-SS-SS is the cable they offer ( 8-conductor )
Cable 4 conductor, 10' long, sockets & sleeves both ends.

Model: 804-SS-SS is the cable they offer ( 8-conductor )
Cable 8 conductor, 4' long with socket and sleeves on both ends

Technical Info: [email protected] ( I had called )

this is all i can say...

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Lenp
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Re: TC wire question..

Post by Lenp » Sat Jun 13, 2015 6:43 am

Well here's a bit of a fresh take on all this. These gauges are really 'crude' by instrumentation standards. The CHT is calibrated in 25° degree increments and the EGT is in 200° increments! What we often work with is sometimes down to tenths of a degree! With all that room for small errors, maybe the real clue here, is that using copper wire has a negligible effect on the overall accuracy.

There is no question that the use of special TC wire, like type J and K is used for extensions. The big difference between the extension and the sensor wire itself is the temperature rating of the insulation jacket. The extension cable is rated lower since it is away from the heat, but, sensor wire is perfectly acceptable to use as an extension.

So why not do this. Connect the gauge, directly to the TC probe, then with the factory extension then with a DIY copper wire cable. Compare the results of all the conditions then let us know. I would be very interested in the outcome.

Once said of the helicopter when it was invented....
"From an aeronautical design standpoint, a helicopter should not fly. It just does not know that"

PS: Still there is no email notification of reply!
Len

“To invent, you need a good imagination and a big pile of junk.” (T. Edison)
"I must be on the way to success since I already have the junk". (Me)

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Re: TC wire question..

Post by MrAl » Sun Jun 14, 2015 6:01 am

Hi,

It is starting to sound like this meter has some specifics that might allow it to work to some degree without the right kind of wire. For one, the accuracy doesnt sound that good. It sounds like there is also no cold junction compensation, which would make it more inaccurate than a good meter but it will still give readings.

I suggested before that you try it without the extension and see what readings you get. This will give us a comparison point to check with after the extension is added.
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Re: TC wire question..

Post by dacflyer » Mon Jun 15, 2015 10:46 am

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..

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