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  #1  
Old 01-17-2021, 03:59 PM
MGR1977 MGR1977 is offline
 
Join Date: Jul 2020
Location: Seattle
Posts: 30
Default 2.25" Altimeter Calibration?

I just bought myself a pre-owned non-sensitive, 2.25" UMA altimeter. All looks to be in order, however the subscale is out and it under reads by about 900ft. So I need to calibrate it. Anybody knows how I do that...? It doesn't have the adjustment screw that the bigger ones generally have. Here's a picture:
https://photos.app.goo.gl/8Q81MQSP8sAtXdjK9
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  #2  
Old 01-17-2021, 04:17 PM
BobTurner BobTurner is offline
 
Join Date: Dec 2011
Location: Livermore, CA
Posts: 7,184
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I donít know about this one. But Iíve seen others where there is a screw (maybe itís internal) that disconnects the rotating dial with the local altimeter setting from the knob mechanism. So you set the Kollsman window to the local pressure, loosen the screw, turn the knob until you have the displayed altitude equal to the field elevation, then re-tighten the screw. You just have to find it!
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  #3  
Old 01-17-2021, 04:42 PM
MGR1977 MGR1977 is offline
 
Join Date: Jul 2020
Location: Seattle
Posts: 30
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Quote:
Originally Posted by BobTurner View Post
I donít know about this one. But Iíve seen others where there is a screw (maybe itís internal) that disconnects the rotating dial with the local altimeter setting from the knob mechanism. So you set the Kollsman window to the local pressure, loosen the screw, turn the knob until you have the displayed altitude equal to the field elevation, then re-tighten the screw. You just have to find it!
Exactly. To find that screw... The bigger 3-1/8" are easier and they have that screw. But I don't see it on this one.
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  #4  
Old 01-17-2021, 08:19 PM
JDeanda JDeanda is offline
 
Join Date: Apr 2013
Location: Ventura, CA
Posts: 209
Default Just a Thot

Your favorite avionics tech might know how to tweak that altimeter. Sometimes they do that during static checks.
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  #5  
Old 01-18-2021, 02:59 AM
KayS KayS is offline
 
Join Date: Jul 2009
Location: lake constance
Posts: 324
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I had the same problem with my 3 1/8. After lifting the dial knob we had to insert a safety wire or small screwdriver to unlock the linkage and to do the adjustment. Can you lift the knob?
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  #6  
Old 01-18-2021, 08:38 AM
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Snowflake Snowflake is offline
 
Join Date: Oct 2009
Location: Sidney, BC, Canada
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Quote:
Originally Posted by KayS View Post
Can you lift the knob?
This was my guess as well. Does the knob pull out perhaps? Not much, maybe only 1/8" or so, but it may be spring loaded or have a detent that lets it come out to a position where turning it moves the sub-dial independently of the front face?
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  #7  
Old 01-19-2021, 03:23 PM
MGR1977 MGR1977 is offline
 
Join Date: Jul 2020
Location: Seattle
Posts: 30
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Okay guys, I spoke to UMA tech support directly. The only way these altimeters can be calibrated is for them to physically open it up at the workshop and do the calibration. According to the techie I spoke to, this is one of their most complicated instruments. Will only cost about $50 plus the shipping, so I'm inclined to do just that. Thanks again for everyone's advice.
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