Incorrect TIT probe

[cbogie]

New owner trying to carefully and conservatively lean my new Bravo.  Could never get a TIT reading above 1620 at any power setting or altitude.  JPI tech service suggested putting the probe in freezing water and then boiling water. “If it accurate at 32 and 212 it will be linear all the way to 1900” , they stated.  Well, my probe read 34 and 206…….if you plot that out , you get about 100 degrees low at 1700, which approximates my error.  Called back JPI to report my Alcor probe model number which got the reply “ What!!?……we make and use our own probes. “. 
installed the correct JPI probe and now TIT numbers are as expected.  
 

I suspect a previous owner had a probe fail and had original ships probe still sitting on shelf …..since it fits, it must be correct …..right ?    Anyway , jpi and alcor probes are not interchangeable, at least for TIT. 

[LANCECASPER]

That's strange since when I had a JPI930 installed in an M20M I don't think there was a new TIT probe in the kit. 

[cbogie]

I did check and all the EGT and CHT probes are JPI branded as well. 
 

mine is the JPI 900

 

[N201MKTurbo]

I looked up both probes and they appear to be compatible. They are both K type thermocouples with the same lead stagger (short yellow long red) they are both grounded probes. 
 

I don’t think you have an incompatibility issue, just a bad probe. The failure mode is an internal short to ground somewhere other than the probe tip. This forms a secondary thermocouple not at the tip that can oppose the signal of the tip thermocouple.

[PaulM]

This is classic internal short behavior of thermocouples.   The Seebeck effect that generates the voltage difference is from where the 2 dissimilar metals begin to where they are joined.    The short simply joins them farther away from the probe tip.   Intermittent "lower temperature"  is a short... intermittent high/un-readable temp is an open circuit on one of the legs. 

All K type thermocouples are interchangeable.  It probably failed and the local shop had Alcor on hand. 

[LANCECASPER]

 1650 and over on the TIT will toast the probes on the M20M in a year or two. That was one of the original problems with the TLS before the Bravo. People were running it at 1750 since it was in the POH and the probes were no longer accurate so they were leaning even further and burning the exhaust valves after a couple hundred hours.

[cbogie]

Thank You for the excellent replies.