Pmerems

Pmerems

Well Known Member
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Gents,

I have a 2.25" UMA 20K Altimeter I just received for my RV-7A (finishing kit). This my backup altimeter just in case my Dynon fails.

I just checked the barometric pressure in my shop and confirmed it with the local news broadcast (29.93"). I set the pressure window to 29.93" and the altimeter reads 2200 ft. Unfortunately my shop is at 2700 ft. That is 500 ft error.

Now I understand that these single needle altimeters aren't as accurate as the three needle models but 500 ft sure seems to be a significant error.

I choose the 20K model because I was told by UMA that the 10K could be damaged if one exceeded about 11,000 ft.

I don't know about you other fliers but when your being chased by a MIG-21 in your RV, you might need to go vertical and exceed 11,000 ft and damage the 10K model (It's getting late and I am beginning to have delusions :confused: ).

Does anyone have any experiences to share about the accuracy of the UMA 2.25" altimeters? I will be calling UMA on Monday to find out if this error is normal or is the altimeter out of spec.

Any info would be appreciated.
 
Should be a little screw by the set knob. You unscrew it then pull the set knob out. Now you can change what the altimeter reads relative to the set window.
 
The altimeter setting IS NOT the local barometric pressure. The altimeter must be referenced to the barometric pressure at sea level.
 
Mel said:
The altimeter setting IS NOT the local barometric pressure. The altimeter must be referenced to the barometric pressure at sea level.
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How do you set yours before your takeoffs from Dallas? What reference do you use?
 
UMA Altimeter

I have the single needle 20K UMA 3-1/8" version I[m going to use in my -9A.

When I set the baro correctly (based on FSS nearby), it pretty much agrees with my GPS & topo map for my location.

Seems fine to me.

Daver
 
Use ATIS

Paul... I'm not sure if I would trust the TV news too much... :)

Set it to the TUS ATIS at 520-741-1177 and see what you get.

gil in Tucson
 
Setting the altimeter

I'd agree with the asav8tor above. I've set my altimeter using the method he describes. Set your altimeter to the correct field elevation, remove the set screw next to the adjustment knob. There is a tab that this set screw holds in place. It should slide slightly up. This unlocks the altimeter from the baro adjustment. Now set in the correct barometric pressure. (Your altitude should not change while this adjustment is being made.) Slide the tab back to align with the hole for the set screw. Install set screw. The correct barometric pressure will now correspond to the field elevation. I seem to remember being able to set it to within about 30 feet or so.
 
Smaller altimeter...

C. Brenden said:
I'd agree with the asav8tor above. I've set my altimeter using the method he describes. Set your altimeter to the correct field elevation, remove the set screw next to the adjustment knob. There is a tab that this set screw holds in place. It should slide slightly up. This unlocks the altimeter from the baro adjustment. Now set in the correct barometric pressure. (Your altitude should not change while this adjustment is being made.) Slide the tab back to align with the hole for the set screw. Install set screw. The correct barometric pressure will now correspond to the field elevation. I seem to remember being able to set it to within about 30 feet or so.
Click to expand...
That's the procedure for a standard 3 1/8 altimeter that has the little cut out in the bottom left corner. The screw holds a little plate on that is part of the altimeter that fits in the panel cut out.

This 2 1/4 inch altimeter does not have the cut out in the panel, and from the pictures, it does not seem to have a plate there.

Perhaps the knob will just pull out to adjust.... try that Paul.... :)

gil A
 
captainron said:
How do you set yours before your takeoffs from Dallas? What reference do you use?
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My barometer was calibrated with the local airport altimeter settings. I live exactly 1/2 way between two controlled airports (TKI & GVT) and averaged the 2 over a period of time.
BTW the adjustment you guys are talking about is NOT approved by anyone but a certified instrument shop, and not all of them!
 
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Gil was right

az_gila said:
Paul... I'm not sure if I would trust the TV news too much... :)

Set it to the TUS ATIS at 520-741-1177 and see what you get.

gil in Tucson
Click to expand...

I called TUS ATIS and found that indeed the TV broadcast and my barometer (cheap barometer/temp/humidity indicator) were not reading accurately. When set to the ATIS pressure the altimeter reads right:). The resolution on the 20K altimeter is about 200' per increment so it is reasonably close.

Keep in mind this a my backup altimeter. Sure would be nice to find a low cost three needle 2.25" out there.

Thanks for the help.
 
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