OAT probe question

[Don]

Here's a simple question, or I hope it is. I'm just starting on wiring my 9A and I want to pretty well understand it before I actually start running wires. So I'm laying things out on the bench and I see both my Horizon Hx and the Garmin GTX 330 have places to connect the leads from an OAT probe. I was surprised to see the 330 wanted information on the outside air temp. At this point I have one OAT probe that was going to send data to the GRT EFIS.

Is it possible to connect both units to the same OAT probe or do I need to install a second OAT probe?

A quick glance at the 330 manual seems to indicate that the unit only reports the OAT, which seems kind of quirky to me. If I can't use one OAT probe for both the EFIS and the transponder, can I ignore those two connections on the 330 without encountering other problems?

[bcondrey]

With GRT you can connect the OAT probe to either the EIS or AHRS unit. If you've got a dual AHRS, each can have its own OAT if desired. When you connect up the transponder you'll actually be sending air data from the EFIS to the transponder which in addition to the altitude info, will contain air data.

Bob

[vfrazier]

This isn't directly related to your question, but might help someone else avoid an small error.

The OAT probe should be mounted in the wing, away from the fuselage, if possible. Many (maybe all?) RVs and Rockets are much warmer around the fuselage, up to 10 degrees or more in the wingroots of my plane. The temperature error will mess up your TAS and other calculations.

For this same reason, any future airplane I build will have the cabin air vents out in the wing also. The extra 10 degrees isn't really needed when the summer heat hits 90 degrees or higher!

[Don]

Thanks Bob, that answers my question.

Vince, I'm mounting my OAT sensor in the wing for the reasons you stated.

And, I do have a dual AHRS but I think I'll stick with one OAT probe, at least initially. It is redundancy that I can live without and more importantly to me, I can imagine two side by side sensors giving different readings and I don't want to sort out which one *might* be telling the truth. Maybe later...right now I want to get this bird in the air.

[Rupester]

Once you read all of the booklet that comes with a 327 or a 330, you'll be amazed at how many parameters it can calculate and/or display. You just discovered one of them. I would stick to one OA sensor wired to the GRT, since it needs that info for a number of things. Save the 330 display for parameters you can't read elsewhere, or don't want to add to the already busy EFIS display.

[bcondrey]

Don,

If going with a single OAT probe wire it in through the EIS, the EFIS will pick it up from there regardless of which AHRS is being used. If you wire it through one of the AHRS and that AHRS fails, I don't know if EFIS would have any OAT data. You could ask GRT but it's easier to just wire through the EIS.

Bob