NSEU

Hi, folks.

I haven't done much work on this system and was asked to take measurements of each EGT probe on a CF6. The AMM asks you to take a resistance reading across the terminals of each probe, then swap over the pos/neg leads of the meter and measure again (then work out the average).

Some of the measurements were fairly standard, but then others were showing a higher than normal resistance in one direction and a negative resistance(???) in the other.

Is this normal?

Does it have something to do with the Chromel /Alumel (disimilar metals) junctions ... or was I doing something wrong?

Thanks.
Rgds.
NSEU

cockney steve

Well, no-one else has replied, so here goes......NOTE! I'm not an electronics expert nor an aircraft engineer.

A resistance-meter normally works by massing a battery's voltage through it's meter and calibrated resistances,,,,the current through the circuit is thus proportional to the unknown resistance placed on series in the "circuit" the Ua 's of current through the meter are read off the appropriate Ohms scale.

A thermistor alters it's resistance with temperature....I would expect to see the same resistance irrespective of test-meter polarity, though I suppose it's possible that meter-current could heat the thermistor, thus altering it's value dynamically whilst measuring.

Now the interesting bit!.....you talk of a bi-metallic junction device......well, from my limited knowledge, this is akin to a thermocouple or even a semiconductor (transistor, diode? )

One effect of these junctions is that electricity is produced when some are heated (exactly how a thermocouple works)....so , logically, your multimeter's internal battery would ADD with the generated voltage in one connection, and in the opposite connection, they would oppose, therefore, less current would flow in the latter circumstance and the meter would show a high resistance compared with the former situation, where a properly zeroed meter could be expected to show "negative resistance..

explanation....when you join the probes and zero a meter, the current is set by the trimming resistor to read "0 Ohms) (full scale current) when the item tested has voltage, and it's added to the internal battery, the meter is overdriven and "pegs"


Of course, i'm talking proper analogue meters here, but AFAIK, it's similar with these newfangled digital ones.

Incidentally, the above effect is used to do a quick, rough check on electrolytic capacitors connect,-needle kicks and slowly sinks as cap charges......reverse connection and meter kicks violently and slowly sinks as charge in cap dissipates.

Now wait whilst the professionals tear me to shreds and flay me alive

Jetdoc

Which CF6 engine is that? I have an AMM for a CF6-80C series engine on a B747-400 and a CF6-80E series on an A330.
The -400 mentions an optional test using a specific tester (Barfield TT1200A) for checking the probes and gives specific values for the probes.
The Airbus AMM only mentions using an ohmeter and looking for a positive movement on the meter.
I used to work on RB211 engines and the AMM specifies using a wheatsone bridge and gives specific values although we used to do a quick check with a fluke meter just to get a ballpark figure.
What kind of meter were you using? I never heard of swapping the leads and rechecking.
The main thing when doing the check with the proper equipment is that you need a really good meter connection because the voltage generated at the thermocouple junction is very small.

MrFixer!!!

NSEU,I also want to ask the same question???? Which CF6 are you on about??? I have also looked at AMMs for our CF6-80C2 on 747-400, it doesnt say anything similar to what you experienced.

the Description of EGT ADJUSTMENT TEST says:

Type K thermocouple resistance values are obtained by measuring and recording the circuit resistance test value when it becomes stable. Reverse the test lead polarity and measure and record the circuit resistance test value again when it becomes stable. Average these two readings, and then subtract the test lead resistance from the averaged reading to obtain the circuit resistance value.

AND I THINK YOU THEN MOVED ON TO DO THE OPTIONAL TEST!!!!!!!

and what kind of Multimeter were you using???????

, the AMM says you need a Adaptor Unit/ Special tester, which is SPL-5666 :

HERES THE OPTIONAL TEST :

Disconnect the adapter cables from the junction box.(b) Connect the adapter unit to the upper thermocouple cable connector P3 as follows:

1) Connect the adapter cable 101-00905 found in the SPL-5666 EGT test kit, to the upper thermocouple cable connector P3.

(c) Do the steps that follow to measure the resistance of the upper harness/probe circuits:
NOTE: This EGT indicating system can use either type 1 (Ametek) EGT probes or type 2 (Weston) EGT probes. The resistance values will be slightly different.

1) Set the adapter unit switch to each position shown in the table below and push the PUSH TO MEASURE switch.




and goes on and on...........so mate dont know what engine you are working on and what kind of Tester were you using????

You sure your start was not an Adjustment Test and then moved on to the Optional Test???? (using the multimeter and not the approved Kit).

Doesnt matter whether you worked on this system or not......but if thats how it was done the results Should not have been what you got!!!!

NSEU

Thanks for all the great responses, guys.


It was a CF6 fitted to a 767-300 (non-FADEC).

Ch 77-21-00 Standard Test (the intro says we can use common test equipment for the first part). I was using a Fluke meter my boss gave to me (I didn't get the chance to read the manual properly until today). The manual just says "ohmmeter (Range 0-2Megohms)"

I don't know how close the different airline manuals are to each other, but it's Part F (4) and involves removing all the leads from the thermocouples and measuring the individual resistances. The engine was fairly cool, but perhaps not cold. Some readings went up to 7ohms, but with a negative value in the opposite direction (so the average was ok).

That negative resistance really threw us :P

Thanks, again.
Cheers.
NSEU

Fargoo

A Fluke will generally only read a negative resistance when there is a potential across whatever you're measuring.
I'd guess the heat at those probes was just enough to generate the required voltage in the hot junction to cause this to happen. Absolutely no idea why this is but I guess that's why they ask you to swap the leads around and measure again.

wombferret

Reversing the probes is standard practice. As already explained, the thermocouple is going to be producing a voltage so you won't get an accurate reading measuring only one way.

Blacksheep

The EMF generated at the junction will drive the reading off-scale in one direction on most ordinary ohmmeters or multi-meters. The EMF is proportional to the temperature with respect to absolute temperature, so whether the engine is stone cold in the arctic or just too hot to touch in the desert, the variation of 230°K to 300°K doesn't make that much difference to the effect.

To get an accurate reading for thermocouples you need to use a bridge. The best results I've had were with a Wayne-Kerr capacitive bridge - which has a massive impedance. You balance the bridge for the dip position and note the first reading, then reverse the leads and repeat. The nominal value is then, just as you describe, the average of the two readings.

NSEU

Nice to know, thanks : )

Perhaps Boeing's comment about ensuring the engine is cold prior to measurement is so you don't burn your hands? : P

Thanks to all.
Cheers.
NSEU