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Thermal wire

g zero

Well Known Member
What is the difference between thermal coupler wire Type-k , Type-J and regular aircraft grade 22ga or 20ga ?
 
Thermocouple Wire

G-

I will make an attempt at answering this question. Here goes:

Thermocouple calibration types have been established by the ASTM according to their temperature profiles. In general a "K" type thermocouple is ranged 200 - 2300 degrees F. And, a "J" type thermocouple is ranged 200 to 1400 degrees F.

Wire type becomes important when dealing with the thermocouple (sensor) calibration. You want as close to a straight line voltage curve inside of the type thermocouple range as possible.

In short the correct wire type (resistance) and calculated length are important to maintain the calibration of the measurement point.

Hope this helps.

Chuck
 
The type K and type J wire is for thermocouples. When 2 disimilar metals are placed in contact with each (a couple) other they generate a voltage. The voltage is proportional to temperature. Type K thermocouples have one wire made from Alumel and one from Chromel. The sensor is actually a junction where the 2 metals are fused. It is this junction that is temperature sensitive.

Type J uses iron and constantan as the 2 metals. As pointed out in the previous post, the 2 types of thermocouples have different usable temperature ranges for various reasons.

A circuit sensing a thermocouple is calibrated for the temp/voltage output of the specific type materials used. You have to use the proper thermocouple wire type for the sensing circuit (or vs. versa). For example, if you use type J in a circuit calibrated for type K it will be in error.

Practically speaking, all aircraft hookup wire is copper, tinned or plated and is not used to sense temperatures.
The 20 or 22 is the wire gage and applies to all wire. The larger the number the smaller the wire diameter.

There are also special consideration for the insulation used for each type of wire, but that is another chapter.
 
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Thermocouple Wire Differences

What is the difference between thermal coupler wire Type-k , Type-J and regular aircraft grade 22ga or 20ga ?

Simply put, thermocouple wiring is composed of different alloys than regular copper wiring. The differing alloys in the positive and negative wires are what generate a small voltage (microvolts per degree) depending on the temperature of the junction (within the temp probe).

Type K (chromel ? alumel) is the most common general purpose thermocouple with a sensitivity of approximately 41 ?V/?C (chromel positive relative to alumel when the junction temperature is higher than the reference temperature). It is inexpensive, and a wide variety of probes are available in its −200 ?C to +1350 ?C / -330 ?F to +2460 ?F range. For aircraft, Type K thermocouples are generally used for EGT.

Type J (iron ? constantan) has a more restricted range than type K (−40 ?C to +750 ?C), but higher sensitivity of about 50 ?V/?C. For aircraft, Type J thermocouples are generally used for CHT.

Hope this helps...
 
Nice consistent simultaneous posts.

But they don't say what would happen if other than thermocouple wires are used. So if two dissimilar metals create a voltage, then having many junctions of various metals between the actual thermocouple and the instrument to measure the voltage will create a measurement error with additional junctions.

ref on thermocouples: http://www.omega.com/techref/themointro.html

There is more if you want it.

Better?
 
So when you add junctions of differing materials you can create errors. Lets say you add a connector with gold plated contacts. So you go from alumel to gold on one side of the connector. On the other side of the connector you go gold to alumel. The 2 junctions are opposite to each other and generate opposing voltages and assuming they are the same temperature, cancel each other out exactly. In this case they don't introduce an error.

If the junctions are at different temps, you create errors. Let's take the case of a firewall connector where the firewall side of a connector might be 160 degrees F and the cabin side 70 degrees F. There is a temperature difference for the 2 junctions and the voltage generated by the opposing junctions won't be equal and an error will be introduced.

If you splice in copper and never go back to alumel, then you only have one junction and an error is introduced.

If you are running thermocouple wires and want to splice them or run through a connector, it is fine of both sides are the same temperature. If not, you must use splices or connectors made of the same material as the wire.

In most cases we run thermocouple wire from the sensor to the indicator or data acquisition system to avoid unknown temperature gradients in the connections creating errors. Where the thermocouples come off the EGT or CHT, the wires are typically spliced in the engine compartment so both sides of the splices or connections are the same temperature.

When you connect to the sensing system, either an indicator or data acquisition system like the Dynon EMS, use the pins or connection method they call for as they have compensated for this dissimilar junction in their calibration.
 
Thanks for all the info , I now have a better understanding of how it works . Based on how sensitive the system is to resistance how important is it to have all the EGT wires the same length (same for the CHT wires ) ?
 
In modern high impedance systems like the Dynon EMS, it doesn't make a difference. If the indicator is an older analog pyrometer which typically is 8 ohm impedance, then it matters and the thermocouple circuit needs to be impedance matched.
 
Received the wire I ordered and cut the leads to length .Type K EGT (red/yellow)and Type J CHT (red/White ) . My Alcor Probes are both ( EGT /CHT ) color coded Red/Yellow . It appears they both use Type K wire ?
Any Thoughts ? Or Advice )
 
It seems that Alcor EGT gauges are type K and CHT are type J from their web site. Perhaps do you have a dual EGT gauge? The temperature ranges for the thermocouples are very different -- 500 deg for J and 1600 for K. Check the probe PNs with the guage(s) to be sure they are compatible.

As a reference, Omega (www.omega.com) is a great source of info for all types of thermocouples. I don't think they make aircraft HW, but the physics is the same and has a boat-load of good info.
 
Both Probes are Type K , no Alcor gauges being used as they plug into the Garmin System (GEA 24 / GDU 370)
I failed to look at the probes before I ordered the wire , took for granted the EGT was type K and CHT was type J . It is sold on their web site as EGT and CHT wire.
 
Reply from Stein Avionics , Garmin uses Type K for both EGT and CHT .

Anyone want some Type J wire , 5'-8" 6'-8" 9'-8 10'-8". (32'-8")
$20 shipped !!!
 
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