grounded will show zero resistance, ungrounded will show between 5 and 10 ohms
That would mean the probe contains a resistor.
Not impossible but I have designed various thermocouple instruments and have never seen that.
Is the Q on probe interchangeability?
I am now looking into the topic of TIT/EGT probes.
I have a JPI Slimline TIT instrument.
In their FAQ, in the section about probes, JPI themselves state that:
– Alcor probes are OK for JPI EDM devices, as long as the same type is used (which will most likely be K – with red and yellow wires); I understand that to also include probes from EI and other manufacturers
– units manufactured after 1985 use grounded probes (which are less expensive and allegedly last longer)
– to see if a specific probe is grounded or ungrounded one should measure resistance between the wires and between the yellow wire and the probe body; grounded will show zero resistance, ungrounded will show between 5 and 10 ohms
The JPI FAQ on probes is here: https://www.jpinstruments.com/FAQCategory/probes/
A local PDF of the page made on November 12, 2022 at 2125 UTC is attached for reference: 221112_JPI_FAQ_Probes_pdf
I came across this potentially useful table which shows thermocouple types and the connector colours
Unfortunately there is just the right amount of ambiguity 
However, in GA, it tends to be just J or K type.
Thermocouple is K as original ones.
I moved the probe on a different channel, and saw the same.
I think I don’t get your suggestion of wrapping ?
Anyway, the conclusion is that they are not proper for my set up.
they under read by 40-50°F on the instrumentation compared to Alcor measurement
That would concern me. It might be a ground loop i.e. the probe output voltage is being reduced by a voltage drop in the ground connection caused by some other current flowing. Can this be reproduced with the two probe types alongside each other, wrapped up in a rag? It could also be a different type of thermocouple is involved (J vesus K etc – there are about 7 types).
Update: P110F works with G1000 engine monitor.
But:
- they are less responsive than Alcor 86317/86343
- they under read by 40-50°F on the instrumentation compared to Alcor measurement
It’s only a dumb thermocouple Any ungrounded probe of the same type (J or K) will work, subject to suitable mounting etc.
Someone is charging $400 “because they can”. Could also be that they make very few of some obscure product which is contracted out to somebody who doesn’t want to do it anyway…
GA suffers a lot from this very small batch production issue. Years ago I saw some details of a business for sale in the USA, consisting of an 80 year old guy sitting in a shed, making annunciator panels for a particular very old US GA brand whose trademark is an upright vertical stabiliser He was still turning over six figures, which is not bad with a ~90% margin. Throw in anal interpretations of type certificate “requirements”, an A&P who won’t (possibly can’t) read, and you have a great business model
Peter wrote:
Something similar came up before here.
Yes, at the time, I wondered if I could switch to grounded probes, which I didn’t do.
But, two new probes later, and with prices still going up, I thought I might give another try.
All the help I can get is welcome, to identify compatibility between Garmin GEA71 (engine monitor unit) and P-110 probe.
Have tried to contact Garmin to ask them about compatibility with GEA71
Answer is “Cessna holds the Type Certificate, only them know…”
Will report what I find on the topic.
I asked EI why their ungrounded probe was $100, while Alcor’s one is $400. Here is their answer:
We design and build our own probe in house, this is how we keep prices so low. I think we make the best probe in the Industry.
When EI first got going, they were selling quite a thick EGT probe. They claimed it would last a lot longer than the JPI one, which was undoubtedly true because the JPI ones seem to last only a few hundred hours (much depends on usage I am sure). But the thicker probe was slower responding.
