EGT Problem

Two possible causes, Newps...
One is a broken wire(s) going to the instrument panel where moving the wire
around to work on the lugs causes the broken ends to abrade and make better
contact for a while, and then they reoxidize... This is high on my
suspicion list since it is the same two wire runs all the time.. Don't take
the lugs apart this time, simply give the wires a good workout and moving
around and see if that changes it..

If not, then the surface of your terminal lugs (stake on rings) are
oxidizing... If your hardware for the lugs does not have those little
stainless, star washers, get them... Now, they do not go under the nut
(which we are all conditioned to do), rather they go between the lugs of the
two wires so that the sharp edges of the stars bite into the metal of both
lugs as you tighten the screw, maintaining a gas/oxide free contact between
the two lugs.. We are working with a few millivolts of signal, and it takes
almost no oxide at all to act as a resistor and block current flow...

Not sure what you mean't by 80 and 250 for EGT, so I suspect a typo...

Denny

"Newps" wrote in message
news:x7L2b.200219$cF.66312@rwcrnsc53...
I have the Electronics US8A, which is a 14 channel EGT/CHT. My problem
is with two of the EGT temps. What happens is my number 2 and number 4
cylinder report EGT's of between approx 80F and 250F. They dance all
over the place. Very rarely will the EGT be right. Now, if I take apart
the two connections for each probe and spray them with carb or brake
cleaner they will work properly for about 2 hours, then I will go back
to the same problem. So it seems to me the electrical connection in one
or both of the connectors is intermittant. Any ideas? I did buy a new
probe and installed that but that doesn't change the problem.

Dennis O'Connor wrote:
Two possible causes, Newps...
One is a broken wire(s) going to the instrument panel where moving the wire
around to work on the lugs causes the broken ends to abrade and make better
contact for a while, and then they reoxidize... This is high on my
suspicion list since it is the same two wire runs all the time.. Don't take
the lugs apart this time, simply give the wires a good workout and moving
around and see if that changes it..

All the wires are securely fastened within 2-3 inches of the
connections. So when I pull them apart there is no slack, they cannot
be moved any farther apart. Because of the plastic around the terminals
I can't get in there effectively to clean the terminals with say sandpaper.

If not, then the surface of your terminal lugs (stake on rings) are
oxidizing... If your hardware for the lugs does not have those little
stainless, star washers, get them...

Actually the terminals are male female slide in connectors. There are
no nuts, bolts and star washers.


Now, they do not go under the nut
(which we are all conditioned to do), rather they go between the lugs of the
two wires so that the sharp edges of the stars bite into the metal of both
lugs as you tighten the screw, maintaining a gas/oxide free contact between
the two lugs.. We are working with a few millivolts of signal, and it takes
almost no oxide at all to act as a resistor and block current flow...

Not sure what you mean't by 80 and 250 for EGT, so I suspect a typo...

No, that's the reading I get. 80 degrees F up to about 250F. And it
constantly jumps around between those numbers. Every once in a while
I'll get a normal reading of between 1250-1450F.

Newps wrote:

: No, that's the reading I get. 80 degrees F up to about 250F. And it
: constantly jumps around between those numbers. Every once in a while
: I'll get a normal reading of between 1250-1450F.


This could also be caused by a short in the thermocouple wire at any point,
because the two dissimilar metals in contact are the actual thermocouple
junctions. If the thermocouple insulation is broken allowing the two wires
to short out, you've got an extra thermocouple junction in the loop, and
that could cause readings like you've seen. The short could also be inside
the plastic connector plugs!

You could check this by unplugging the problem thermocouples. You should get
the smallest possible temperature reading in this configuration, no matter
what. If the reading is still jumping around, the problem would have to be
in the wiring from the analyzer to the plug.

Also, on the Omega thermocouples I've worked with, those plugs are two piece
riveted together, so perhaps you could remove the rivets. Otherwise I'd try
carefully squeezing the female plug with pliers to make the female quick-
disconnect a little tighter. This would help bite through any oxide
or dirt.

Good Luck.
--
Aaron Coolidge (N9376J)

Knife connectors are not really appropriate for thermocouples. The resistance,
especially after some oxides have formed will cause the type problems you are
seeing. You might be able to temporarily fix the problem by crimping the
connector a little so it bites harder. You should however consider replacing the
kinfe connectors with ring lugs bolted together with a star washer between the
lugs as was suggested. If you do, you should also double the wire back to get
twice the thickness when inserting the wire and crimping it.

Newps wrote:

Dennis O'Connor wrote:
Two possible causes, Newps...
One is a broken wire(s) going to the instrument panel where moving the wire
around to work on the lugs causes the broken ends to abrade and make better
contact for a while, and then they reoxidize... This is high on my
suspicion list since it is the same two wire runs all the time.. Don't take
the lugs apart this time, simply give the wires a good workout and moving
around and see if that changes it..

All the wires are securely fastened within 2-3 inches of the
connections. So when I pull them apart there is no slack, they cannot
be moved any farther apart. Because of the plastic around the terminals
I can't get in there effectively to clean the terminals with say sandpaper.

If not, then the surface of your terminal lugs (stake on rings) are
oxidizing... If your hardware for the lugs does not have those little
stainless, star washers, get them...

Actually the terminals are male female slide in connectors. There are
no nuts, bolts and star washers.

Now, they do not go under the nut
(which we are all conditioned to do), rather they go between the lugs of the
two wires so that the sharp edges of the stars bite into the metal of both
lugs as you tighten the screw, maintaining a gas/oxide free contact between
the two lugs.. We are working with a few millivolts of signal, and it takes
almost no oxide at all to act as a resistor and block current flow...

Not sure what you mean't by 80 and 250 for EGT, so I suspect a typo...

No, that's the reading I get. 80 degrees F up to about 250F. And it
constantly jumps around between those numbers. Every once in a while
I'll get a normal reading of between 1250-1450F.

--
--Ray Andraka, P.E.
President, the Andraka Consulting Group, Inc.
401/884-7930 Fax 401/884-7950
email
http://www.andraka.com

"They that give up essential liberty to obtain a little
temporary safety deserve neither liberty nor safety."
-Benjamin Franklin, 1759

AHHHAAAAAaAaaAaaaaa....
Some dork put jap shack, stake-on's with tongue and blade connectors onto
the wires, using his $4.95 Kmart special, wire crimper from the automobile
aisle... Why dintcha say so in da foist place? (even if the dork is the
factory, the allegation stands)
Get some "good" like AMP aerospace quality, ring lugs and the proper die
set, and stainless star washers like I said, and replace those oxidized
beauties and life will be good again... (btw, the oxidation is between the
collar of the stake-on and the wire strands because it was not a gas tight
crimp)

Cheers ... Denny
off to fight a war, or save the universe, or something...

ps: you could still have broken strands in the wires at the tie wrap, 2
inches or not...

pps: tie wraps touching wires without padding underneath are verboten on any
aircraft I take responsibility for for good reasons...

"Newps" wrote in message
newsbN2b.201071$cF.66266@rwcrnsc53...
Actually the terminals are male female slide in connectors. There are
no nuts, bolts and star washers.