OAT probe issue

[Gary F]

I had a Garmin G500 installed in my DA40. The original Davtron temp gauge with a separate OAT probe was not removed. The temp readings by the Garmin G500 agree with ASOS and the Davtron temp gauge until I crank the engine then within a few minutes and during the flight the G500 temp reads 2 to 4 degrees C higher then the Davtron gauge. I initially thought that the Garmin G500 OAT probe was being heated by exhaust gas but the shop informed me that it Davtron OAT probe is on the same side as the exhaust and the Garmin probe is farther out of the exhaust steam on the other side of the airplane. The shop that installed the G500 told me that the G500 OAT is corrected for airspeed and temperature. This does not make sense to me. Any ideas?

 

[denverpilot]

Garmin probe malfunctioning as voltage rises, perhaps?

See if it follows battery voltage after start-up and the Alternator is putting out power.

 

[flyingcheesehead]

The shop that installed the G500 told me that the G500 OAT is corrected for airspeed and temperature. This does not make sense to me. Any ideas?

The temperature is corrected for temperature? That seems fishy too, unless they mean temperature of the avionics bay throwing off the transducer reading somehow. But I would think that in that case, or the case of correcting for airspeed, that the Garmin would read LOWER, not higher. So, I don't buy it.

 

[Gary F]

The temperature is corrected for temperature? That seems fishy too, unless they mean temperature of the avionics bay throwing off the transducer reading somehow. But I would think that in that case, or the case of correcting for airspeed, that the Garmin would read LOWER, not higher. So, I don't buy it.

I'm sorry, I meant to write airspeed and altitude. I don't see how altitude would really effect the probe. I suspect that the probe is a thermistor and that there may be some heating or cooling of the probe which can affect the accuracy but I don't buy the 2 to 4 degree C difference between two separate devices.

 

[denverpilot]

I'm sorry, I meant to write airspeed and altitude. I don't see how altitude would really effect the probe. I suspect that the probe is a thermistor and that there may be some heating or cooling of the probe which can affect the accuracy but I don't buy the 2 to 4 degree C difference between two separate devices.

Yeah, still makes no sense. Make 'em fix it.

 

[RotorAndWing]

http://en.wikipedia.org/wiki/Total_air_temperature

 

[denverpilot]

So is the Garmin an OAT or a TAT? Are you saying it's a TAT? What does the manual say?

I'm just going off the OP's assertion that it's an OAT gauge.

If it's supposed to be doing something else other than OAT, and he didn't read his own avionics manual, my comment "get it fixed" is rescinded for the avionics and should be applied to the OP.

 

[flyingcheesehead]

So is the Garmin an OAT or a TAT? Are you saying it's a TAT? What does the manual say?

Screw the manual... The display itself says "OAT." (To the left of the HSI, below the wind vector)

But the manual says the following:

The Outside Air Temperature (OAT) is displayed to the left of the HSI. The OAT can be displayed in oF or oC, which is configured in the Aux System Setup Page. The temperature is derived from the Temperature Probe on the aircraft. The displayed temperature is the Static Air Temperature reported by the Air Data Computer. This temperature value is corrected for ram air heating effects.

So, that would seem to indicate that it really is OAT and not TAT. Also, the TAT article linked to says:

In practice the ram rise is negligible for aircraft flying at (true) airspeeds under Mach 0.2

That's about 132 KTAS, so the DA40 can go just a hair faster, but we're still so close to the "negligible" range that I doubt it'd be changing as much as it is.

Gary, any word from the shop or Garmin yet?

 

[TMetzinger]

Screw the manual... The display itself says "OAT." (To the left of the HSI, below the wind vector)

But the manual says the following:



So, that would seem to indicate that it really is OAT and not TAT. Also, the TAT article linked to says:



That's about 132 KTAS, so the DA40 can go just a hair faster, but we're still so close to the "negligible" range that I doubt it'd be changing as much as it is.

Gary, any word from the shop or Garmin yet?

On the whole OAT/TAT thing, On my recent flight from KBDR/KJYO in a DA 40 I'm 99% sure that I saw OAT on the PFD, but TAT was also available on one of the AUX pages. There was about a 1 degree difference cruising along at 140 TAS.

 

[Gary F]

Gary, any word from the shop or Garmin yet?

I am still trying to process everything. I had not heard of TAT although the display shows OAT. I am most concerned about being to determine when I am risk for icing while in IMC near freezing. I noted on a recent trip in warmer conditions that the OAT difference was only about 2 degrees C. I think that this suggests that the probe is being warmed by exhaust gas. Although the exhaust is more in line with the Davtron OAT probe, I think that the prop wash slipstream may direct the exhaust more to the Garmin OAT probe. Here is an excerpt from the shop's response.

I cannot explain this anomaly on the ground. Once you are airborne, the OAT is corrected for speed and altitude. My only possible thought is that it is the air flow of the exhaust still. We could try to use a “temperature gun” on the ground to see how accurate the temp really. However, not having tried this, I do not know how accurate this will be?

This is a very reputable shop but we are both uncertain on how to sort this out. I considered swapping probe locations but would not be easy according the shop where the annual is being done..

Swapping the probes themselves would be relatively painless. However rerouting the wiring for the factory probe would be fairly involved. It goes right into a conduit near the probe, so it would have to be completely redone on the other side of the aircraft. The Garmin probe wiring that Mayday installed looks like it would work in either probe location.

 

[flyingcheesehead]

On the whole OAT/TAT thing, On my recent flight from KBDR/KJYO in a DA 40 I'm 99% sure that I saw OAT on the PFD, but TAT was also available on one of the AUX pages. There was about a 1 degree difference cruising along at 140 TAS.

That's more like what I'd expect.

And sho'nuff, according to the manual, on the Trip Planning page (Aux-1) in Automatic mode, there's TAT. Cool.

 

[flyingcheesehead]

We could try to use a “temperature gun” on the ground to see how accurate the temp really.

Might as well try.

 

[TMetzinger]

One other possibility... are you SURE that the probe installed is the proper one for the Garmin? and are you SURE that the wiring/connections are right?

As I recall, these probes are thermocouples that offer variable resistance to current flow based on temperature. If the probe isn't the correct make/model, it may be offering a different resistance and slope than the G1000 is expecting. Same thing can happen with a wiring issue.

The way to check these (at least in the auto world) is to put them in a liquid of a known temperature - boiling is 100C and ice water is around 1C and measure the resistance at the probe with an ohmmeter.

 

[Gary F]

I think we found the problem. My DA40 had the 1000 hour inspection recently with includes a comprehensive test of the grounding system. As is typical for the DA40 there were several problem connections and the OAT temps agree after they were fixed. I suspect that there was a slight voltage difference between the ground connections for the different OAT sensors.