Thermometer

[N601RX]

My 67 F still has the original OAT through the windshield.  I would like to upgrade to a 201 windshield soon and was wandering what others are doing for a OAT?

[KSMooniac]

I'm using a Davtron M655 in place of my stock OAT gauge that resides on the left, top side of my panel.  It is a great instrument that gives F & C temps, pressure altitude and density altitude if connected to the altitude encoder, as well as voltage.  The probe is located in an inspection panel under the left wing, just outboard from the main gear. 

Whatever you do, don't put the new OAT probe in the cabin air intakes that are subject to heating from the engine and sun when on the ground.

[Parker_Woodruff]

My old one sticks through the pilot side window.

However I have one that's planted on the left side of the cabin under the window that goes to the G500.

[DaV8or]

I have both the one sticking through the windsheild and one sticking out of the fuselage under the pilot's window that's connected to the EDM 700. I'm thinking of removing the windsheild one and plugging the hole. It's hard to read and looks kind of goofy... oh, and all that drag!

[N201MKTurbo]

I always liked my Scott OAT. When flying in ice ): the ice would always start to melt as soon as the needle got 1/2 needle width off of zero. I have a 201 now with a davtron that works fine, but I don't trust it like the old Scott.

[Jerry 5TJ]

I put in a Davtron clock-OAT-Voltmeter recently and calibrated it.  It was within 1 degree F at 32F.   The Scott OAT poking thru the windscreen was reading 5 F too low at that temperature.  As the saying advises, "Trust, but verify."

Does anyone have advice on how to plug that hole once I remove the old Scott OAT?

 

 

[Piloto]

For accurate air temperature reading have the probe installed underneath the wing. Probes on the fuselage of a single engine will read slightly higher due to hot air coming from the engine.

José

 

[Jerry 5TJ]

Jose, are you suggesting the old Scott OAT on my 65 C was deliberately set to read low because Mooney expected hot air to spill from the cowl?  I suspect it just got a weak spring after 45 years.

 

 

[Immelman]

I just (yesterday) got a new OAT probe signed off (I did the work) and added to my JPI engine monitor. I recently had my windshield replaced and did not want the old "meat thermometer" OAT probe sticking out.

Before I put the new probe in I did a test flight with a little temperature measuring device that has two thermocouple inputs. I can confirm that the cabin air scoop is NOT a good place to mount an OAT probe - in flight the temperature there was 5-7dF higher than out under the wing where I mounted the other thermocouple (and then my new probe in the install I just did). Was the OEM scott gauge affected this way at the top of the windshield? Hard to say, but I was very surprised at how large the difference was in my little experiment. BTW, when I say cabin air scoop I mean on the pilot's side -- opposite the exhaust

[Piloto]

Like any other thermometer probe you want the probe installed in a shaded area to avoid sunlite heating. This is important for when you are trying to figure out density altitude before take-off. This rules out any location on top or side of the fuselage. On the bottom of the fuselage of a single engine the hot air coming from the cowl flaps or exhaust will increase the air temperature. This is why underneath the wing is the optimum location, specially for determining icing conditions. 

José

 

[Magnum]

I have the EDM probe in the air scoop (copilots side, no sunlight) and another OAT in the remote module of the Aspen. The factory OAT is mounted underneath the wing.

I have max 1 dC difference between the EDM and the Aspen. As the factory OAT is analgue and the instrument is very small it is not easy to read. But it always looks like the other two indications :-)

[Jerry 5TJ]

I calibrated the two airplane OAT probes by submerging them in a slury of ice and water, and compared those readings to a third, calibrated, lab thermocouple probe.  The new Davtron OAT read 33F, the Scott read about 28F, and the lab probe claimed 32.8 to 33.4F.    Engine was off and no sunlight was falling on the plane, which was in the hanger at the time.  The lab meter resolution is 0.1F, the Davtron reads to 1F, and the Scott has marks every 2 degrees F.

I am still interested to know if the probe is or is not in a "bad" location but the Scott is certainly reading way too low.

 

 

 

[carusoam]

Jerry,

How long does the probe extend from the windhield.  2 - 4 "? 

I don't believe that there is enough warm air to make a boundary layer that thick at the top of the windshield to alter your air temperature.  However when flying your mooney at Mach speeds, you may experience a warmer indication than usual....(insert smiley here).  I replaced my Scott thermometer with a new one after I purchased my M20C, but it was still impossible to read accurately or get valuable information out of it.  It worked better as a vibration meter.

I believe it will still be affected by sunshine, at least while standing still.....

Best regards,

-a-

[Immelman]

Quote: carusoam

I don't believe that there is enough warm air to make a boundary layer that thick at the top of the windshield to alter your air temperature.  However when flying your mooney at Mach speeds, you may experience a warmer indication than usual....(insert smiley here).

[Jerry 5TJ]

That's interesting, Jesse.  Thanks. 

So, I ran the numbers based on your formula:

 

C	C		knots
IAT	OAT	K	TAS
1.3	0	1	100
1.9	0	1	120
2.6	0	1	140
3.4	0	1	160
4.3	0	1	180
11.9	0	1	300
0.9	0	0.7	100
1.3	0	0.7	120
1.8	0	0.7	140
2.4	0	0.7	160
3.0	0	0.7	180
8.3	0	0.7	300
1.1	0	0.8	100
1.5	0	0.8	120
2.1	0	0.8	140
2.7	0	0.8	160
3.4	0	0.8	180
9.5	0	0.8	300
1.2	0	0.9	100
1.7	0	0.9	120
2.3	0	0.9	140
3.0	0	0.9	160
3.8	0	0.9	180
10.7	0	0.9	300

I used the K factor at 0.9, which Williams says is on the low end of the usual range.

So, the take-away is -- if you start picking up ice, just speed up to around 300 knots true and it will all melt right off.    Better yet, 450 knots will give nearly 25C rise.  I guess that's why 767s don't need boots.

[Piloto]

Quote: jerry-N5911Q

That's interesting, Jesse.  Thanks. 

So, I ran the numbers based on your formula:

 

C

C

 

knots

IAT

OAT

K

TAS

 

 

 

 

1.3

0

1

100

1.9

0

1

120

2.6

0

1

140

3.4

0

1

160

4.3

0

1

180

11.9

0

1

300

 

 

 

 

0.9

0

0.7

100

1.3

0

0.7

120

1.8

0

0.7

140

2.4

0

0.7

160

3.0

0

0.7

180

8.3

0

0.7

300

 

 

 

 

1.1

0

0.8

100

1.5

0

0.8

120

2.1

0

0.8

140

2.7

0

0.8

160

3.4

0

0.8

180

9.5

0

0.8

300

 

 

 

 

1.2

0

0.9

100

1.7

0

0.9

120

2.3

0

0.9

140

3.0

0

0.9

160

3.8

0

0.9

180

10.7

0

0.9

300

 

I used the K factor at 0.9, which Williams says is on the low end of the usual range.

So, the take-away is -- if you start picking up ice, just speed up to around 300 knots true and it will all melt right off.    Better yet, 450 knots will give nearly 25C rise.  I guess that's why 767s don't need boots.