I have noticed (my truck included) that the two-wire cable that connects at the back of the pyrometer and then to the K-type probe-leads is always much longer than the distance, with the excess either folded or coiled.

Is there a reason for not cutting that wire to length ??

Is that two-wire cable something special, or will any old two wires work ??

Thanks.

 

the length of the wire is used to calibrate the gauge at the factory. I beleive through resistance.
Cutting or adding to teh wiring harness will result in faulty readings.

 

I don't think that's true in all cases.

 

it may not be, I am far from an expert, but that is what I have heard...

 

Anyone familar with what these do and how they work ?? :


http://www.isspro.com/proddetail.php?prod=R600


And, it appears that lead-wires can be had in lengths from 6' to 50', or in infinite length if bought by the foot :


http://www.isspro.com/products.php?cat=60

: which leads me to wonder if what I had always thought about the length of the leads having a bearing on the reading of the gauge is true.

Thanks.

 

In this case length does not matter, no matter what she says!

Thermocouples use voltage not resistance to work and unless the line resistance is over 15 or 20 ohms then you will have very little effect on the performance of the thermocouple.

The wire and connectors are special in that the yellow wire is made of one material and the red wire is made of a different type. The biggest problem with these thermocouples is that people cut them short then use copper or some other common metal connectors and this creates a cold junction that puts a bias on the reading.

It is easier to just leave the full length of wire coiled up rather then mess it up when you shorten them

 

The first link it to a millivolt generator which is used to simulate a thermocouple. You can use this to verify your temperature readings on your gauge.

 

In other words, that could be used to verify the gauge and see whether or not it is accurate ??

I can see how that could be worth having on occassion.


Thanks.

 

Expensive for the home user but there are other multi function devices for around the same price and does a lot more. But as you say it is a nice tool to have.

 

like camperandy said the gauge works off of voltage and not off of current. However, if the probe produces a current and not a voltage the length and gauge of wire matters. Voltage is the current multiplied by the resistance and resistance is dependent upon wire length and cross sectional area. However, if the probe produces a voltage, which it probably does, then the resistance is entirely negligible. Therefore, if the probe produces a voltage then any length will work but if it produces a current your gauge may be off slightly or hugely depending on how much you cut off.

 

"Thermocouple theory allows the extension of the thermocouple without affecting its emf output when extension wire and connectors have the same thermoelectric characteristics. For example, when a type "K" thermocouple is being used, the wires and connectors used to extend it should also be type "K". The different types have color codes; for instance "K" type is yellow, assigned to them for easy identification so as to help prevent mismatching of extension wire connectors, and thermocouples."

 

I assume you are meaning the color of the outer sheathing of the two-wire cable.

On my truck, that cable is bright yellow; while, on the wife's truck, it is a brown color; the probe in her truck looks just like the one in mine (which I know for a fact is a K); so, it is possible that her's is not a "K", or the cable is wrong, right ??


Thanks.

 

I have brown sheathing on the one that came with my TST but the wire itself is yellow.

 

Well, I stand corrected and am thankful.. this thread has good timing.
I went to install my "new to me " pyro the other day and found the previous owner had cut the wiring
Just got off the phone with Autometers tech support( they are sending me a new harness for free, even tho i bought a used gauge) and he told me that it is NOT the length, it is the material, and Andy is correct, cutting or splicing it can be done, but must be done correctly.
I learn more on this site every time I log on!!!

 

Any thermocouple supplied for EGT use will be Type K - alumel/chromel - due to it's high temp rating (~ 1400-1800*F depending on wire guage) - probably of the sheathed grounded-junction type. If you don't see a witness mark from the weld on the sheath's tip, it should be an ungrounded junction, which doesn't respond as quickly - not a significant difference in light of the high temps being sampled. Even though exposed junction TCs respond (virtually) instantaneously, the thermoplastic coating tends to char - but it is rather simple to TIG-weld the junction as needed to keep it in service.

Type J TCs - iron/constantan - are for medium temp use, usually around half that of Type K, but the sheathed grounded-junction Js we use are good to 1400*F.

Most of the really useful data we collect is from our exposed-junction Type Ks - they have essentially no hysteresis and are easy to insert in most mediums that you'd be interested in - whether liquid (such as turbo oil drain), gas (multi-point charge-air) or solid (VP44 case temp).

As posted above, wire length is basically irrelevant, assuming secure terminal connections are made. I do think that shielded TC cable is better, as I believe it's cheap insurance against inaccurate/erratic guage readings resulting from EMF issues.

Obviously, you can get away with murder when you're not bound by NIST standard requirements!