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difference between my altimiter and gps

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byoungplumbing(at)gmail.c
Guest

Posted: Thu Jul 05, 2012 9:03 am Post subject: difference between my altimiter and gps

The one instrument that does vary is my MicroTim, a little altimeter.   I got the model with no connection to the pitot, just a little hole in the  housing, and after takeoff it's always a couple of hundred feet different from  my Garmin GPS.  With the semi- enclosed cockpit there apparently is reduced  pressure at the instrument panel.  I have to be careful there because a lot  of my flying is under the 22 or 19 ceiling of ABE's airspace so I place my trust  in my GPS there.



 dave and kolbers...

  

 it seems I always have a 150 to 200 ft difference  between my altimeter and gps... the gps reads high...   so the last time I  flew by an atc,  I explained the difference and asked what altitude my mode  c blind encoder was reporting via the transponder.   they replied the  exact reading on my altimeter,     best I can figure, is my  blind encoder and altimeter are connected to the same static source, and it is  running a bit high in pressure.  the static source is a small tube with a  hole drilled through both sides...   maybe a small washer soldered on  the tube just before the holes,   has anyone had experience with this  problem.

  

 boyd young

   [quote][b]

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russk50(at)gmail.com
Guest

Posted: Thu Jul 05, 2012 12:53 pm Post subject: difference between my altimiter and gps

FWIW in my limited experience, I've found GPS elevations unreliable, except for the military units. And found to my surprise a GPS won't read thru heavy snowfall!Russ K 



On Jul 5, 2012, at 1:02 PM, b young wrote:

 	  Quote: 	
		  The one instrument that does vary is my MicroTim, a little altimeter.  I got the model with no connection to the pitot, just a little hole in the housing, and after takeoff it's always a couple of hundred feet different from my Garmin GPS.  With the semi- enclosed cockpit there apparently is reduced pressure at the instrument panel.  I have to be careful there because a lot of my flying is under the 22 or 19 ceiling of ABE's airspace so I place my trust in my GPS there.



dave and kolbers...

 

it seems I always have a 150 to 200 ft difference between my altimeter and gps... the gps reads high...   so the last time I flew by an atc,  I explained the difference and asked what altitude my mode c blind encoder was reporting via the transponder.   they replied the exact reading on my altimeter,     best I can figure, is my blind encoder and altimeter are connected to the same static source, and it is running a bit high in pressure.  the static source is a small tube with a hole drilled through both sides...   maybe a small washer soldered on the tube just before the holes,   has anyone had experience with this problem.

 

boyd young

 	  Quote: 	
		  



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undoctor

Joined: 21 Oct 2008
Posts: 212
Location: Bethelhem, PA

Posted: Sat Jul 07, 2012 7:27 am Post subject: difference between my altimiter and gps

Boyd,

     

     I guess it's human nature but I blindly trusted the hugely expensive     satellites and technology of our military over my little plastic     MicroTim.  May be a naive choice!  I'll check it further to find     out.  Thanks.

     

     BTW, that little tail wheel you sent me after my flying system     flipped on its side in the hurricane just follows me all over the     sky.  Great bunch of guys and gals on this list, and you're one!!!

     

     Dave Kulp

     Bethlehem, PA

     FireFly 11DMK

     

     

     On 7/5/2012 1:02 PM, b young wrote:

     

     [quote]                            The one instrument that does vary is my MicroTim, a little         altimeter.  I got the model with no connection to the pitot,         just a little hole in the housing, and after takeoff it's always         a couple of hundred feet different from my Garmin GPS.  With the         semi- enclosed cockpit there apparently is reduced pressure at         the instrument panel.  I have to be careful there because a lot         of my flying is under the 22 or 19 ceiling of ABE's airspace so         I place my trust in my GPS there.

       

       dave and kolbers...

        

       it seems I always have a 150 to           200 ft difference between my altimeter and gps... the gps           reads high...   so the last time I flew by an atc,  I           explained the difference and asked what altitude my mode c           blind encoder was reporting via the transponder.   they           replied the exact reading on my altimeter,     best I can           figure, is my blind encoder and altimeter are connected to the           same static source, and it is running a bit high in pressure.            the static source is a small tube with a hole drilled through           both sides...   maybe a small washer soldered on the tube just           before the holes,   has anyone had experience with this           problem.

        

       boyd young

        	  Quote: 	
		  

	
     [b]

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ceengland(at)bellsouth.ne
Guest

Posted: Sat Jul 07, 2012 10:00 am Post subject: difference between my altimiter and gps

Static port placement can have large effects on both airspeed and     altitude measurement. Using a port on only one side of the plane can     have a big effect, too.  Try setting your altimeter to field     elevation while sitting on the ground, then fly a low pass over the     runway at full speed & check the altimeter during the pass. If     it's different by more than your height above the runway, you've got     a static port 'issue'. In faster planes like RV's, it's common to     see an altitude reading that's 50 feet or more below ground level     during a high speed pass. A byproduct is a significantly lower     indicated airspeed than reality. The fix on RV's  is  a 'bump' in     the skin at the static port.

     

     Another possibility is, I read years ago that non-WAAS GPS's     sometimes use an idealized spherical earth model, & local GPS     altitudes may be off quite a bit from reality (the earth isn't     exactly round).

     

     Charlie

     

     On 07/07/2012 10:25 AM, David Kulp wrote:     [quote]              Boyd,

       

       I guess it's human nature but I blindly trusted the hugely       expensive satellites and technology of our military over my little       plastic MicroTim.  May be a naive choice!  I'll check it further       to find out.  Thanks.

       

       BTW, that little tail wheel you sent me after my flying system       flipped on its side in the hurricane just follows me all over the       sky.  Great bunch of guys and gals on this list, and you're one!!!

       

       Dave Kulp

       Bethlehem, PA

       FireFly 11DMK

       

       

       On 7/5/2012 1:02 PM, b young wrote:

       

        	  Quote: 	
		                                      The one instrument that does vary is my MicroTim, a little           altimeter.  I got the model with no connection to the pitot,           just a little hole in the housing, and after takeoff it's           always a couple of hundred feet different from my Garmin GPS.            With the semi- enclosed cockpit there apparently is reduced           pressure at the instrument panel.  I have to be careful there           because a lot of my flying is under the 22 or 19 ceiling of           ABE's airspace so I place my trust in my GPS there.

         

         dave and kolbers...

          

         it seems I always have a 150 to             200 ft difference between my altimeter and gps... the gps             reads high...   so the last time I flew by an atc,  I             explained the difference and asked what altitude my mode c             blind encoder was reporting via the transponder.   they             replied the exact reading on my altimeter,     best I can             figure, is my blind encoder and altimeter are connected to             the same static source, and it is running a bit high in             pressure.  the static source is a small tube with a hole             drilled through both sides...   maybe a small washer             soldered on the tube just before the holes,   has anyone had             experience with this problem.

          

         boyd young

          	  Quote: 	
		  

	
       	
       

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