Eating JPI exhaust probes for lunch

[Joe Larussa]

Just had my #2 cylinder exhaust probe fail for the third time in about 400 hours. It’s the only one that ever goes bad. Thoughts?

[carusoam]

Joe,

Post the data, and we can have a look...

Post a pic of the probe tip... we might see what is going on...

Or... what are they dying from? (If you know)

-a-

[Bolter]

I have never seen anyone else on this board lose multiple probes on the same cylinder in a short time. 

Random idea 1) I would wonder if the diagnosis is correct?  Maybe the failure is on the other side of the connector?  Maybe a bad connection that gets closed again with each repair, then vibrates open.  Was the probe swapped to another cylinder to verify it was bad?

Random idea 2) I cannot imagine that the thermocouple is not rated for ample margin over the predictable range of exhaust temperature.  But the temperature capability may just be the tip.  It may be that it is getting hot in the wrong place because of an exhaust leak around the tip, and hot gas is overheating the part that is intended to be outside of the exhaust flow.  Measure the hole, and see if it was overdrilled a small amount in the past.  

[carusoam]

EGT sensors typically last thousands of hours....

Unless they are in the combined exhaust streams like a TIT sensor or downstream of a 3in1 exhaust collector...

Where the average EGTs are very high over time....

The jacket of the probe erodes at a higher rate in these environments...

ROP has more O2 molecules available for corroding the sensor’s jacket...

LOP is an O2 free environment...

PP thoughts only, not a mechanic...

-a-

[PilotX]

Doubtful this is your problem but I had a pinched wire once. Replaced the probe and got the same fault, somewhat intermittently. The wire from the JPI that connects to the probe was pinched by the cowl (not a Mooney) coming out of the firewall. Splice the wire and wrapped it with that plastic protective stuff and it was good as new.

[tmo]

8 hours ago, PilotX said:

Splice the wire

This. Do NOT solder the thermocouple wire, always splice it.

[Joe Larussa]

My 830 was flashing bad probe when I landed. With the engine still running I recycled the avionics master and

it restarted the 830. Warning went away, so I did a run up. All okay till I brought it back to idle, bad probe popped

back up.

[Joe Larussa]

Can a probe be tested without running the engine?

[Bunti]

I just learned about a case of a twin airplane  which lost parts of an EGT sensor during flight. This was (as far as I remember) not a JPI-system or sensor.

For a non turbo airplane this would be no problem: Buy a new sensor and fly again.  But for this turbo airplane it was very different: The broken EGT sensor moved through the exhaust into the turbo and destroyed parts of the turbo system. 

The owner first saw  on the display that the probe was going bad. Then he checked the probe and found out that most of it was missing. He was worried about possible damage to the turbo. So he got the turbo checked and found the damage and the missing part. 

Really bad and expensive day.

I have an EDM930 in my Mooney. I never had a probe mechanically broken. Over the years, we had to replace some because they stopped working right. I even bought a new one which did not display right. It was good at outside temps but as soon as the engine started and temps got higher, the probe showed wrong values. JPI checked the probe and gave me a new one.

For the JPI it is really important that the  unit is grounded to the engine. At initial installation this was not done right at my plane. After that had been corrected, the reliability improved. 

Nevertheless, I always have a spare EGT and CHT probe in the plane. It looks to me that the CHT probes last longer than the EGTs. 

[N201MKTurbo]

8 minutes ago, Joe Larussa said:

Can a probe be tested without running the engine?

Yes, use a barbecue lighter. You can use a cigarette lighter, but your thumb will get hot after a while.

[PT20J]

First, I’d check all connections. Then, I’d swap probes with another cylinder and see if the issue follows the probe. May be an issue with 830 itself. 
Skip

[bradp]

Yeah k type thermocouples have a defined resistance can temperature curve.  Measure the resistance at know temps and you’ll get a general idea if it’s working or not. 
 

The thermocouples are pretty robust. I’m on my same set going on 12 years. Something is off - could the wires be getting singed, vibration excessive, something corroding the tip, etc. 

[N201MKTurbo]

1 hour ago, bradp said:

Yeah k type thermocouples have a defined resistance can temperature curve.  Measure the resistance at know temps and you’ll get a general idea if it’s working or not. 
 

The thermocouples are pretty robust. I’m on my same set going on 12 years. Something is off - could the wires be getting singed, vibration excessive, something corroding the tip, etc. 

Thermocouples  don’t change resistance with temperature, they generate a voltage proportional to the temperature difference between the cold junction and hot junction. All the systems we use have temperature sensors in there amplifiers that form a pseudo cold junction. 
 

https://en.wikipedia.org/wiki/Thermocouple

You seem to be describing a thermistor. Our stock CHT and oil temp probes are thermistors. All the aftermarket sensor systems use thermocouples.

[bradp]

I got it reversed thanks Rich. 
 

I guess an easy way to remember is thermocouple is a couple of metals, therm/istor/ sounds like resistor. 

[N201MKTurbo]

48 minutes ago, bradp said:

I got it reversed thanks Rich. 
 

I guess an easy way to remember is thermocouple is a couple of metals, therm/istor/ sounds like resistor. 

[takair]

22 hours ago, Joe Larussa said:

Just had my #2 cylinder exhaust probe fail for the third time in about 400 hours. It’s the only one that ever goes bad. Thoughts?

Do you have pictures of the old probes?  Probes can erode, but that typically takes hundreds of hours.  They can be damaged if they are mounted near a slip joint, such that expansion actually makes the joint act like a bolt cutter.  Then there is the harness, as other suggested.  This is often the week link.  I suspect the probes are not failing, but a connector somewhere in the harness.

[N201MKTurbo]

There are only two things a system can tell about thermocouples, they are open or they are shorted to ground.

both are wiring issues. It is possible there is vibration shaking your wires apart.

[carusoam]

Very helpful if you can post JPI data to savvy...

the graphs may indicate where errors are occurring during the flight...

If the JPI is finding something, the data will probably be showing it...

A typical test for an EGT sensor... is to turn on the JPI in the hangar to see if  all the EGTs and CHTs are showing about the same room temperature as the OAT...

PP thoughts only, not a mechanic...

Best regards,

-a-

[bradp]

Simple thing Joe - make sure that the star washer is between the probes.  They read funky if not installed this way. 

[Joe Larussa]

On 2/27/2021 at 4:45 AM, bradp said:

Simple thing Joe - make sure that the star washer is between the probes.  They read funky if not installed this way. 

Hmm.... I’m pretty certain I didn’t put the washer there! Thanks

[ArtVandelay]

Hmm.... I’m pretty certain I didn’t put the washer there! Thanks

Me thinks you had a bad AP doing the installation. While you’re at it, check the probe wires are not cable tied together with the ignition wires, another amateur mistake. And your fuel flow sensor is horizontal with no 90° bends on in/out ports.

[MikeOH]

The voltages involved with thermocouples are VERY low; a twenty degree F temperature change is around a half of a millivolt!

Connections and wire placement are critical.

[carusoam]

Surprisingly,

It doesn’t mention the materials being used...

Is that star washer specific to the TC metals?

Is it anything special to be aware of?

So a different voltage doesn't get generated by the junction...

PP wonderment only, not a mechanic...

Best regards,

-a-

[MikeOH]

40 minutes ago, carusoam said:

Surprisingly,

It doesn’t mention the materials being used...

Is that star washer specific to the TC metals?

Is it anything special to be aware of?

So a different voltage doesn't get generated by the junction...

PP wonderment only, not a mechanic...

Best regards,

-a-

Not saying there isn't a special washer, but the voltages generated will be cancelled out as long as both sides of the star washer are at the same temperature; which is pretty certain!

Additional thermocouple trivia/minutiae:  Common misconception is that the thermocouple voltage is generated at the junction between the two different metals; that is NOT true.  If you think about it, it can't be because you would have some, albeit small, voltage across close to zero ohms which would generate an enormous current!

The voltage is generated along the length of the two wires; since the wires are different metals, the voltages generated are slightly different.  That difference is the thermocouple voltage.  This is the reason for both aging and why there is 'thermocouple wire' and 'thermocouple extension wire' available.  Thermocouple wire is highly purified and homogenous.  Extension wire, not so much.  The reason for aging is that the wire's remaining impurities migrate over time, in part due to the temperature difference along their length, and thus change the voltage generated.  Thermocouple extension wire operates at a much lower temp (usually room temp) over its entire length.

[RobertGary1]

20 years on my jpi probes and never changed one