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OAT inaccurate

S

Stan Bahrns

Active Member
Am using GRT EIS & EFIS and have the OAT probe protruding out into the fresh air inlet. It seems to show hotter than the actual oat as I had the windshield ice up saturday. If I spray some insulation on the inside of the air inlet will that solve my problem? Has anyone tried this?
Thanks
Stan Bahrns
RV7-A
[email protected]
 
For what it's worth,,,

I've heard the same, so I have used a digital meat prob (love the BBQ!) and the OAT is 5 degrees cooler than what will display on my GRT EIS. I have my probe in the right vent. FYI.
 
I have the same problem

I also have an EIS 4000 with the probe mounted inside the air vent (left side) in an RV-8. My indicated OAT is always much warmer than true outside temp. I too suspect that the cabin heat vent warms the inner wall of the air vent/probe causing anomalous temps. Either that or there is a problem with the probe. Any ideas? Is there any way to calibrate the OAT temp readings with the instrument?

Dan Miller
RV-8 366 hours
 
This is a common problem with probes mounted in this area (mine included.) Between the heat from the cabin and the heat from the engine, it always reads warm.

You could re-locate the probe. I have my Dynon probe in the lower HS fairing and that seems to work well.
 
I have the GR EIS probe and the Blue Mountain EFIS probe mounted in the fresh air vent in an 8 and the EIS temp reads warmer than normal an the EFIS temp is correct. They both are correct on the ground. I've tried changing locations of each probe, both staying in the same vent, but the EIS probe remains warmer by about 15F.
 
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This is normal behavior for mounting an OAT probe there. Not only that, it interrupts the flow and purpose of the NACA fresh air duct.

Under the wing, out of direct sunlight and far away from exhaust is the best location.
 
A really good place on an -8 is in the fresh air intake under the right wing - outside of the engine heat slipstream, in the shade, and in my experience dead accurate. I have not heard of many folks having good luck in terms of accuracy with the side vents - too much engine heat coming back that way.

Paul
 
So faulty and useless were the readings when I installed the OAT probe inside a NACA vent, I relocated it to the center of an inspection plate underneath the horizontal stab. It reads far more accurately now. For optimum performance you would want to mount it as far outboard on a wing as practical.
 
Move the probe

Stan Bahrns said:
Am using GRT EIS & EFIS and have the OAT probe protruding out into the fresh air inlet. It seems to show hotter than the actual oat as I had the windshield ice up saturday. If I spray some insulation on the inside of the air inlet will that solve my problem? Has anyone tried this?
Thanks
Stan Bahrns
RV7-A
[email protected]
Click to expand...
I solved the same problem (same Van's model, same EFIS/EIS) by mounting it in the right wing's first inspection panel.
 
Rick6a said:
So faulty and useless were the readings when I installed the OAT probe inside a NACA vent, I relocated it to .....
Click to expand...

Now I'm not pickin' on you Rick, cause I know what you're talking about, but this is why this topic keeps coming up. People think the problem is "The NACA vent". Well, the problem is "The NACA vents on the forward side(s) of the fuselage"!

Talking about the NACA vent is misleading unless you say which model RV, and which vent....it's no wonder that people reading the archives get confused....

Paul
 
OAT is it worth the trouble?

I've got an OAT probe that came with my engine monitor but the same probe can also be used for monitoring Carb temp. So rather then buying another probe I've been thinking about not doing the OAT at all.

What's the real value of the OAT in everyday flying (except for knowing how hot it really is outside)?
 
Fix the problem without moving the probe

This problem is caused by the fact that most OAT probes (sensors) are metallic (often brass) and the main body is bigger than the small little probe it self. If you mount it in a NACA vent so that the body part of the probe is in the cockpit area, the conduction that takes place with the probe body causes temp. errors (the body has much more thermal mass than the little probe tip). The errors can be quite large in low temp conditions when cabin heat is being used.

One way many builders have fixed the problem is using some of the low expansion urethane foam in a can, to insulate the sensor.
Use a small Dixy type drinking cup to make a form for the foam, and then inject the cup with just enough foam to fill it.

The only down side is that it makes replacing a bad sensor a little tougher, but it is pretty rare for them to fail.

The best thing is to not put them there in the first place (spred the word),
 
Rick_A said:
What's the real value of the OAT in everyday flying (except for knowing how hot it really is outside)?
Click to expand...


If you have one of the EFIS devices (Dynon, etc) the OAT makes it possible for the unit to calculate not only temp (are we about to start icing?), density altitude, true airspeed, and winds aloft.

All in all, certainly worth having. :)
 
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