OAT-indicator shows wrong temperature ....

[NicoN]

We have 2 OAT-indicators in our M20k:

1 is the factory OAT lin the left panel. The sensor seems to be in the wing.

2 is from the EDM830. The seinsor seems to be mounted in the left air inlet.

Nr. 2 shows today 6°C which is pretty close to the real temperature. The aircraft is in the hangar.

Nr. 1 shows a temperature clearly below 0°C ° !!!

Of course, I know that there mhight be differences in temperature due to high or low pressure effects, but inside the hangar there should not be a difference!

 

Can the OAT-Indicator be adjusted?

[carusoam]

They should be the same...

It is possible that one got calibrated...?

It is not normal to need calibration...

There are a whole bunch of reasons related to installation and age challenges, but normal drift doesn’t usually occur either...

Usually sunshine and warm cowling air, are reasons for odd OAT sensor behavior...

Good luck finding the culprit...

PP thoughts only...

Best regards,

-a-

[Marauder]

We have 2 OAT-indicators in our M20k:
1 is the factory OAT lin the left panel. The sensor seems to be in the wing.
2 is from the EDM830. The seinsor seems to be mounted in the left air inlet.
Nr. 2 shows today 6°C which is pretty close to the real temperature. The aircraft is in the hangar.
Nr. 1 shows a temperature clearly below 0°C ° !!!
Of course, I know that there mhight be differences in temperature due to high or low pressure effects, but inside the hangar there should not be a difference!
 
Can the OAT-Indicator be adjusted?

There may always be a degree or two difference due to limitations in accuracy. I believe most vintage Mooneys have Scott Air Model 2716A stuck in the plexiglass of the side window. These are not adjustable.

I don’t know much about your in panel OAT but if it is electronic, there may be a calibration mechanism. I don’t recall seeing any calibration area in the JPI software. There is an adjustment for MAP but I don’t recall seeing one for OAT.

Have you confirmed which one is accurate? Maybe the temp is really 3° and both are off 3°.


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[ArtVandelay]

The sensor in the wing is probably a thermomister, so extra resistance from corroded connectors can throw of the readings. Find all connections, take them apart, clean or replace.


Tom

[carusoam]

Interesting details...

First Compare your OAT... to all your CHT and EGT sensors... at rest, they should be reading the same number...  mine are usually within 2°F of each other...

Then read the JPI calibration procedure.... found here...

https://www.jpinstruments.com/FAQCategory/oat-probe/

Expect using a bucket of ice water to Best calibrate the OAT for defense against icing... a  few degrees really matters in some cases....

PP thoughts only, not a mechanic...

Best regards,

-a-

[NicoN]

Okay, the EDM830 seems fine. The OAT is/was indcated as 6°C, and all CHTs and EGTs were in the low fourties Fahrenheit, which should be close to the 6° OAT.

Indeed, the factory OAT seems to be far out of calibration.

[m20j]

Have seen reports that putting the OAT in the pilot side air inlet at knee level will cause it to read high during cruise, due to warm air spillover from the cowl.  Mine does in a J  

Further out and under the wing was said to be preferable. Wish I had had known that then.

 

Edited April 16, 2019 by m20j

[ArtVandelay]

Have seen reports that putting the OAT in the pilot side air inlet at knee level will cause it to read high during cruise, due to warm air spillover from the cowl.  Mine does in a J

Given the quantity of air flowing past the sides, compared to the warmth of the cowl, not sure about that.
The hot air cooling the engine appears to flows underneath.


Tom

[m20j]

48 minutes ago, ArtVandelay said:

Given the quantity of air flowing past the sides, compared to the warmth of the cowl, not sure about that.
The hot air cooling the engine appears to flows underneath.


Tom

The discussion was the warm air spill-over from the oversized cowl vents up front.  The side vent is in that path.  More air goes in than can flow through the cylinders, swirls around, and spills out the sides.

Lopresti made the cowl openings smaller in the initial J, but not small enough.

Edited April 16, 2019 by m20j

[spistora]

On 3/14/2019 at 9:15 AM, NicoN said:

Okay, the EDM830 seems fine. The OAT is/was indcated as 6°C, and all CHTs and EGTs were in the low fourties Fahrenheit, which should be close to the 6° OAT.

Indeed, the factory OAT seems to be far out of calibration.

Reviving an old thread, I have essential the same situation.  Installed an 830 and the JPI OAT was about 8c higher than factory OAT.  I rechecked today after ambient temps were normalized in the hangar overnight and the JPI OAT matched CHT/EGT temps and factory gauge was still about 8c lower.

@NicoNdid you find a solution?

I haven't checked factory probe connections for corrosion, that is my next step followed by an ice water check.

[carusoam]

Often, a new install with a large temp error...

It is possible that the wires have gotten switched, or improper wire was used...

Thermocouple wires are color coded... and a have a few rules to follow...

We have seen some extensions with copper wire, and the regular wires swapped on an extension...

PP thoughts about what we have seen here in the past...

Best regards,

-a-