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Van's steam gauges versus Dynon EFIS

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gerf(at)gerf.com
Guest

Posted: Mon Sep 25, 2006 1:43 pm Post subject: Van's steam gauges versus Dynon EFIS

At one point this weekend the barometer at the field where I'm 

building was exactly 29.92 - the Dyon EFIS read 62 feet, the 

official field elevation is 66 feet.  I also installed one of 

Van's "steam gauge" Altimeters - supposedly a fallback 

instrument if my wiring/plumbing goes belly up on the first 

flight.  The steam gauge reads minus 20 feet :-\



I know which gauge will be junked out of my panel at the first 

opportunity ...



__g__



========================================================== 

Gerry Filby                                  gerf(at)gerf.com

----------------------------------------------------------

J2j3h4(at)aol.com
Guest

Posted: Mon Sep 25, 2006 3:35 pm Post subject: Van's steam gauges versus Dynon EFIS

If you are just taking the reading from a physical barometer and setting  that in the window of your steam gauge, you will almost certainly not get the  correct altitude.  That reading must be corrected for temperature and  humidity.  The reading you get from AWOS or other FAA weather source  has this correction made.  For an explanation see http://www.exploratorium.edu/weather/barometer.html

  

 Jim Hasper

  

  

  

 In a message dated 9/25/2006 4:45:42 P.M. Central Daylight Time,  gerf(at)gerf.com writes:

  	  Quote: 	
		  -->    RV-List message posted by: Gerry Filby <gerf(at)gerf.com>

At one    point this weekend the barometer at the field where I'm 

building was    exactly 29.92 - the Dyon EFIS read 62 feet, the 

official field elevation    is 66 feet.  I also installed one of 

Van's "steam gauge" Altimeters -    supposedly a fallback 

instrument if my wiring/plumbing goes belly up on    the first 

flight.  The steam gauge reads minus 20 feet :-\



I    know which gauge will be junked out of my panel at the first 

opportunity    ...



__g__ 	


  [quote][b]

gerf(at)gerf.com
Guest

Posted: Mon Sep 25, 2006 4:01 pm Post subject: Van's steam gauges versus Dynon EFIS

Hmm, so you're suggesting that the Dynon is making those 

corrections ?  It does have an OAT temperature sensor, but 

obviously not a humidity sensor.



The two instruments have consistently differed by about 80 feet 

.



g



 	  Quote: 	
		    

 If you are just taking the reading from a physical barometer 

 and setting  

 that in the window of your steam gauge, you will almost 

 certainly not get the  

 correct altitude.  That reading must be corrected for 

 temperature and  humidity. 

  The reading you get from AWOS or other FAA weather source  has this 

 correction made.  For an explanation see 

 _http://www.exploratorium.edu/weather/barometer.html_ 

 (http://www.exploratorium.edu/weather/barometer.html) 

  

 Jim Hasper

  

  

  

 In a message dated 9/25/2006 4:45:42 P.M. Central Daylight Time,  

 gerf(at)gerf.com writes:

 

 

 

 

 At one  point this weekend the barometer at the field where I'm 

 building was  exactly 29.92 - the Dyon EFIS read 62 feet, the 

 official field elevation  is 66 feet.  I also installed one of 

 Van's "steam gauge" Altimeters -  supposedly a fallback 

 instrument if my wiring/plumbing goes belly up on  the first 

 flight.  The steam gauge reads minus 20 feet :-\

 

 I  know which gauge will be junked out of my panel at the first 

 opportunity  ...

 

 __g__

 

 


	


-- 

__g__



========================================================== 

Gerry Filby                                  gerf(at)gerf.com

----------------------------------------------------------

khorton01(at)rogers.com
Guest

Posted: Mon Sep 25, 2006 5:53 pm Post subject: Van's steam gauges versus Dynon EFIS

All he is saying is that there is a difference between barometric  

pressure and altimeter setting.  The original poster used the term  

barometer.   If he read a barometer, and it was at 29.92, then the  

altimeter setting would almost certainly have been something other  

than 29.92.   For example, if the pressure was as per the standard  

atmosphere, the altimeter setting would be 29.92.  The barometric  

pressure at sea level would be 29.92 in HG.   The barometric pressure  

at 66 ft would be 29.85 in HG.   So, if the barometric pressure at 66  

ft was actually 29.92, that implies the pressure was higher than  

standard, and the altimeter setting would be greater than 29.92.

Note: the weather folks also publish barometric pressures which are   

the station pressure, corrected to sea level.  But even these values  

are not the same as an altimeter setting.  The difference between the  

station pressure corrected to sea level and the altimeter setting  

will depend on the air density, which also depends on the temperature  

and humidity, as mentioned earlier.

I'm hoping the original poster actually used an altimeter setting,  

and that it came from a credible source (e.g. a control tower).   If  

it came from a less credible source, then the altimeter setting may  

have been in error.

Kevin Horton

On 25 Sep 2006, at 20:00, Gerry Filby wrote:

 	  Quote: 	

 Hmm, so you're suggesting that the Dynon is making those

 corrections ?  It does have an OAT temperature sensor, but

 obviously not a humidity sensor.

 The two instruments have consistently differed by about 80 feet

 ..

 g

>

> If you are just taking the reading from a physical barometer

> and setting

> that in the window of your steam gauge, you will almost

> certainly not get the

> correct altitude.  That reading must be corrected for

> temperature and  humidity.

>  The reading you get from AWOS or other FAA weather source  has this

> correction made.  For an explanation see

> _http://www.exploratorium.edu/weather/barometer.html_

> (http://www.exploratorium.edu/weather/barometer.html)

>

> Jim Hasper

>

> In a message dated 9/25/2006 4:45:42 P.M. Central Daylight Time,

> gerf(at)gerf.com writes:

>

> 

> At one  point this weekend the barometer at the field where I'm

> building was  exactly 29.92 - the Dyon EFIS read 62 feet, the

> official field elevation  is 66 feet.  I also installed one of

> Van's "steam gauge" Altimeters -  supposedly a fallback

> instrument if my wiring/plumbing goes belly up on  the first

> flight.  The steam gauge reads minus 20 feet :-\

>

> I  know which gauge will be junked out of my panel at the first

> opportunity  ...

>

> __g__

Kevin Horton

Ottawa, Canada

ogoodwin(at)comcast.net
Guest

Posted: Mon Sep 25, 2006 6:25 pm Post subject: Van's steam gauges versus Dynon EFIS

You have to remember that there is a tolerance for altimeters...even in jet 

transports with two or more altimeters, there will be a difference.  With 

the new glass cockpits and computerized instruments that may not be as true 

as with the old baro altimeters.

---

gerf(at)gerf.com
Guest

Posted: Mon Sep 25, 2006 7:04 pm Post subject: Van's steam gauges versus Dynon EFIS

The original poster was I 



The pressure - 29.92 - was read off a digital weather station 

located on the field at 66 feet ASL (my personal one, nothing 

to do with the airfield).  I was reasoning that since I was 

more-or-less at sea level (66 feet ASL) and the barometer was 

reading 29.92, this must be a "standard pressure" day ... 

therefore the altimeters should be reading the actual altitude 

of the field - 66 feet.  The Dynon EFIS read 62 feet and the 

steam gauge read minus 20 - thus the steam gauge is almost 90 

feet out.  I think ... ~()



g

 	  Quote: 	
		   

 

 

 All he is saying is that there is a difference between barometric  

 pressure and altimeter setting.  The original poster used the term  

 barometer.   If he read a barometer, and it was at 29.92, then the  

 altimeter setting would almost certainly have been something other  

 than 29.92.   For example, if the pressure was as per the standard  

 atmosphere, the altimeter setting would be 29.92.  The barometric  

 pressure at sea level would be 29.92 in HG.   The barometric pressure  

 at 66 ft would be 29.85 in HG.   So, if the barometric pressure at 66  

 ft was actually 29.92, that implies the pressure was higher than  

 standard, and the altimeter setting would be greater than 29.92.

 

 Note: the weather folks also publish barometric pressures which are   

 the station pressure, corrected to sea level.  But even these values  

 are not the same as an altimeter setting.  The difference between the  

 station pressure corrected to sea level and the altimeter setting  

 will depend on the air density, which also depends on the temperature  

 and humidity, as mentioned earlier.

 

 I'm hoping the original poster actually used an altimeter setting,  

 and that it came from a credible source (e.g. a control tower).   If  

 it came from a less credible source, then the altimeter setting may  

 have been in error.

 

 Kevin Horton

 

 On 25 Sep 2006, at 20:00, Gerry Filby wrote:

 

 > 

 >

 >

 > Hmm, so you're suggesting that the Dynon is making those

 > corrections ?  It does have an OAT temperature sensor, but

 > obviously not a humidity sensor.

 >

 > The two instruments have consistently differed by about 80 feet

 > ..

 >

 > g

 >

 >>

 >> If you are just taking the reading from a physical barometer

 >> and setting

 >> that in the window of your steam gauge, you will almost

 >> certainly not get the

 >> correct altitude.  That reading must be corrected for

 >> temperature and  humidity.

 >>  The reading you get from AWOS or other FAA weather source  has this

 >> correction made.  For an explanation see

 >> _http://www.exploratorium.edu/weather/barometer.html_

 >> (http://www.exploratorium.edu/weather/barometer.html)

 >>

 >> Jim Hasper

 >>

 >>

 >>

 >> In a message dated 9/25/2006 4:45:42 P.M. Central Daylight Time,

 >> gerf(at)gerf.com writes:

 >>

 >> 

 >>

 >>

 >> At one  point this weekend the barometer at the field where I'm

 >> building was  exactly 29.92 - the Dyon EFIS read 62 feet, the

 >> official field elevation  is 66 feet.  I also installed one of

 >> Van's "steam gauge" Altimeters -  supposedly a fallback

 >> instrument if my wiring/plumbing goes belly up on  the first

 >> flight.  The steam gauge reads minus 20 feet :-\

 >>

 >> I  know which gauge will be junked out of my panel at the first

 >> opportunity  ...

 >>

 >> __g__

 >>

 >>

 

 Kevin Horton

 Ottawa, Canada

 

 

 

 

 

 

 


	


-- 

__g__



========================================================== 

Gerry Filby                                  gerf(at)gerf.com

----------------------------------------------------------

J2j3h4(at)aol.com
Guest

Posted: Mon Sep 25, 2006 9:32 pm Post subject: Van's steam gauges versus Dynon EFIS

I believe the Dynon is determining altitude by GPS.

  

 The following is a quote from the Dynon Forum: GPS altitudes will not  necessarily compare to barometric altitudes.  They will only be the same if  the atmosphere is at standard temperature all the way from the ground to the  test altitude, which is never the case. 

  

 While it is quite possible that your mechanical altimeter DOES have an  error, you won't know that until you set it using a corrected barometer reading  from an FAA weather report.  Even if you were AT sea level, you still  wouldn't have a standard reading unless the temperature and humidity were also  at standard conditions.

  

 Jim Hasper



 In a message dated 9/25/2006 7:03:03 P.M. Central Daylight Time,  gerf(at)gerf.com writes:

  	  Quote: 	
		  -->    RV-List message posted by: Gerry Filby <gerf(at)gerf.com>

Hmm,    so you're suggesting that the Dynon is making those 

corrections ?  It    does have an OAT temperature sensor, but 

obviously not a humidity    sensor.



The two instruments have consistently differed by about 80 feet    

..



g



 	  Quote: 	
		    

 If you are just taking the reading    from a physical barometer 

 and setting  

 that in the    window of your steam gauge, you will almost 

 certainly not get    the  

 correct altitude.  That reading must be corrected for    

 temperature and  humidity. 

  The reading you get    from AWOS or other FAA weather source  has this 

 correction    made.  For an explanation see 

    _http://www.exploratorium.edu/weather/barometer.html_ 

    (http://www.exploratorium.edu/weather/barometer.html) 

  

    Jim Hasper

  

  

  

 In a message    dated 9/25/2006 4:45:42 P.M. Central Daylight Time,  

    gerf(at)gerf.com writes:

 

 -->  RV-List message posted by:    Gerry Filby <gerf(at)gerf.com>

 

 

 At one  point    this weekend the barometer at the field where I'm 

 building was     exactly 29.92 - the Dyon EFIS read 62 feet, the 

 official field    elevation  is 66 feet.  I also installed one of 

 Van's    "steam gauge" Altimeters -  supposedly a fallback 

 instrument if    my wiring/plumbing goes belly up on  the first 

 flight.  The    steam gauge reads minus 20 feet :-\

 

 I  know which gauge    will be junked out of my panel at the first 

 opportunity     ...

 

 __g__

 

 


	


--    

__g__



=================================    

Gerry Filby                                        gerf(at)gerf.com

---------------------------------------------------------es  Day       -->                - NEW MATRONICS WEB FORUMS -

bsp;    -->  nbsp;                - NEW MATRONICS LIST WIKI -

_-=  ;  p;               - List Contribution Web Site  ;                             =========================



	


 

  

  [quote][b]

khorton01(at)rogers.com
Guest

Posted: Tue Sep 26, 2006 1:54 am Post subject: Van's steam gauges versus Dynon EFIS

Ah.  Sorry about the confusion.



Let's assume that your digital weather station is accurate.  Does it  

correct for field elevation, or does it provide a raw barometric  

pressure?  If it is providing a raw barometric pressure, then a  

reading of 29.92 represents the pressure you would expect to see at  

sea level on a standard day.  If you put in an altimeter setting of  

29.92, you have just told your altimeter that the pressure at the  

field elevation is the same as a standard day at that altitude.  So,  

the altimeter is seeing a pressure of 29.92, and it should display  

sea level (0 ft), as that is the altitude at which the pressure would  

be 29.92, on a standard day.  In this case your steam driven  

altimeter is closer than the Dynon.



If your weather station corrects for field elevation, then the Dynon  

is probably closer than the steam driven one.



But, both altimeters have means to adjust them to correct for small  

errors.  The Dynon has a setup menu item, and the steam driven  

altimeter has a way to change the relationship between altitude and  

altimeter setting.  These small errors will be corrected when you  

have your static system check done.



Kevin



On 25 Sep 2006, at 23:02, Gerry Filby wrote:



 	  Quote: 	
		   

 The original poster was I 



 The pressure - 29.92 - was read off a digital weather station

 located on the field at 66 feet ASL (my personal one, nothing

 to do with the airfield).  I was reasoning that since I was

 more-or-less at sea level (66 feet ASL) and the barometer was

 reading 29.92, this must be a "standard pressure" day ...

 therefore the altimeters should be reading the actual altitude

 of the field - 66 feet.  The Dynon EFIS read 62 feet and the

 steam gauge read minus 20 - thus the steam gauge is almost 90

 feet out.  I think ... ~()



 g

>

> 

>

> All he is saying is that there is a difference between barometric

> pressure and altimeter setting.  The original poster used the term

> barometer.   If he read a barometer, and it was at 29.92, then the

> altimeter setting would almost certainly have been something other

> than 29.92.   For example, if the pressure was as per the standard

> atmosphere, the altimeter setting would be 29.92.  The barometric

> pressure at sea level would be 29.92 in HG.   The barometric pressure

> at 66 ft would be 29.85 in HG.   So, if the barometric pressure at 66

> ft was actually 29.92, that implies the pressure was higher than

> standard, and the altimeter setting would be greater than 29.92.

>

> Note: the weather folks also publish barometric pressures which are

> the station pressure, corrected to sea level.  But even these values

> are not the same as an altimeter setting.  The difference between the

> station pressure corrected to sea level and the altimeter setting

> will depend on the air density, which also depends on the temperature

> and humidity, as mentioned earlier.

>

> I'm hoping the original poster actually used an altimeter setting,

> and that it came from a credible source (e.g. a control tower).   If

> it came from a less credible source, then the altimeter setting may

> have been in error.

>

> Kevin Horton

>

> On 25 Sep 2006, at 20:00, Gerry Filby wrote:

>

>> 

>>

>>

>> Hmm, so you're suggesting that the Dynon is making those

>> corrections ?  It does have an OAT temperature sensor, but

>> obviously not a humidity sensor.

>>

>> The two instruments have consistently differed by about 80 feet

>> ..

>>

>> g

>>

>>>

>>> If you are just taking the reading from a physical barometer

>>> and setting

>>> that in the window of your steam gauge, you will almost

>>> certainly not get the

>>> correct altitude.  That reading must be corrected for

>>> temperature and  humidity.

>>>  The reading you get from AWOS or other FAA weather source  has  

>>> this

>>> correction made.  For an explanation see

>>> _http://www.exploratorium.edu/weather/barometer.html_

>>> (http://www.exploratorium.edu/weather/barometer.html)

>>>

>>> Jim Hasper

>>>

>>>

>>>

>>> In a message dated 9/25/2006 4:45:42 P.M. Central Daylight Time,

>>> gerf(at)gerf.com writes:

>>>

>>> 

>>>

>>>

>>> At one  point this weekend the barometer at the field where I'm

>>> building was  exactly 29.92 - the Dyon EFIS read 62 feet, the

>>> official field elevation  is 66 feet.  I also installed one of

>>> Van's "steam gauge" Altimeters -  supposedly a fallback

>>> instrument if my wiring/plumbing goes belly up on  the first

>>> flight.  The steam gauge reads minus 20 feet :-\

>>>

>>> I  know which gauge will be junked out of my panel at the first

>>> opportunity  ...

>>>

>>> __g__

>>>

>>>

>

> Kevin Horton

> Ottawa, Canada

>



 -- 

 __g__



 ==========================================================

 Gerry Filby                                  gerf(at)gerf.com

 ----------------------------------------------------------




	


Kevin Horton

Ottawa, Canada

pgroell

Joined: 24 Jun 2006
Posts: 4
Location: France

Posted: Tue Sep 26, 2006 3:18 am Post subject: Van's steam gauges versus Dynon EFIS

Just to add a bit more. It would have been interesting to make a reading 

by having the altimeter set to the QNH.

The QNH being the pressure you have to set on the altimeter to read the 

field's altitude on the altimeter. (the QNH being rarely  if never equal 

to the real sea level pressure due to the difference in density between 

the ideal standard atmosphere between sea level and the airport and the 

real atmosphere we have out there).

By being not to precise one could say that your QNH whas around 29.92 +  

0.06 (for 66ft) : 29.98.

By tuning the altimeter to 29.98 your reading would have gone up around 

66ft (all this is not exactly exact due to density adjustments no being 

taken into account)and I concurr with Kevin the mechanical altimeter 

would have been nearer to the real altitude than the Dynon.

To finish two remarks :

- on my big aircraft (Airbus A330 and A340) the tolerance is +- 25ft 

between normal altimeters on the ground and +-200ft between those 

altimeters and standby altimeter.

- it is quite difficult to understand how altimeters truly work, but try 

to be aware that this is a very "tricky" instrument, using a standard 

table for reference when the atmosphere is never standard. And we use 

settings which are sometimes not fully understood (QNH shows the 

airports altitude when set on the altimeter and aircraft on the ground, 

in France we sometimes use QFE which shows 0 when set on the altimeter 

and aircraft on ground). Those settings are correlated to measured 

barometric pressure but are obtained after some calculation or reading 

in a table.



For example you can try to solve the following little puzzle : two 

airfields are only a few miles apart. Field A altitude is 275ft, field B 

altitude is 1900ft. Temperature throughout the whole atmosphere is ISA + 

20° Celsius. QNH at A is 30,47 inHg.

What would the QNH be at field B?

Same as in A, more or less?



Pascal



Kevin Horton a écrit :

 	  Quote: 	
		   



 Ah.  Sorry about the confusion.



 Let's assume that your digital weather station is accurate.  Does it 

 correct for field elevation, or does it provide a raw barometric 

 pressure?  If it is providing a raw barometric pressure, then a 

 reading of 29.92 represents the pressure you would expect to see at 

 sea level on a standard day.  If you put in an altimeter setting of 

 29.92, you have just told your altimeter that the pressure at the 

 field elevation is the same as a standard day at that altitude.  So, 

 the altimeter is seeing a pressure of 29.92, and it should display sea 

 level (0 ft), as that is the altitude at which the pressure would be 

 29.92, on a standard day.  In this case your steam driven altimeter is 

 closer than the Dynon.



 If your weather station corrects for field elevation, then the Dynon 

 is probably closer than the steam driven one.



 But, both altimeters have means to adjust them to correct for small 

 errors.  The Dynon has a setup menu item, and the steam driven 

 altimeter has a way to change the relationship between altitude and 

 altimeter setting.  These small errors will be corrected when you have 

 your static system check done.



 Kevin



 On 25 Sep 2006, at 23:02, Gerry Filby wrote:



> 

> The original poster was I 

>

> The pressure - 29.92 - was read off a digital weather station

> located on the field at 66 feet ASL (my personal one, nothing

> to do with the airfield).  I was reasoning that since I was

> more-or-less at sea level (66 feet ASL) and the barometer was

> reading 29.92, this must be a "standard pressure" day ...

> therefore the altimeters should be reading the actual altitude

> of the field - 66 feet.  The Dynon EFIS read 62 feet and the

> steam gauge read minus 20 - thus the steam gauge is almost 90

> feet out.  I think ... ~()

>

> g

>>

>> 

>>

>> All he is saying is that there is a difference between barometric

>> pressure and altimeter setting.  The original poster used the term

>> barometer.   If he read a barometer, and it was at 29.92, then the

>> altimeter setting would almost certainly have been something other

>> than 29.92.   For example, if the pressure was as per the standard

>> atmosphere, the altimeter setting would be 29.92.  The barometric

>> pressure at sea level would be 29.92 in HG.   The barometric pressure

>> at 66 ft would be 29.85 in HG.   So, if the barometric pressure at 66

>> ft was actually 29.92, that implies the pressure was higher than

>> standard, and the altimeter setting would be greater than 29.92.

>>

>> Note: the weather folks also publish barometric pressures which are

>> the station pressure, corrected to sea level.  But even these values

>> are not the same as an altimeter setting.  The difference between the

>> station pressure corrected to sea level and the altimeter setting

>> will depend on the air density, which also depends on the temperature

>> and humidity, as mentioned earlier.

>>

>> I'm hoping the original poster actually used an altimeter setting,

>> and that it came from a credible source (e.g. a control tower).   If

>> it came from a less credible source, then the altimeter setting may

>> have been in error.

>>

>> Kevin Horton

>>

>> On 25 Sep 2006, at 20:00, Gerry Filby wrote:

>>

>>> 

>>>

>>>

>>> Hmm, so you're suggesting that the Dynon is making those

>>> corrections ?  It does have an OAT temperature sensor, but

>>> obviously not a humidity sensor.

>>>

>>> The two instruments have consistently differed by about 80 feet

>>> ..

>>>

>>> g

>>>

>>>>

>>>> If you are just taking the reading from a physical barometer

>>>> and setting

>>>> that in the window of your steam gauge, you will almost

>>>> certainly not get the

>>>> correct altitude.  That reading must be corrected for

>>>> temperature and  humidity.

>>>>  The reading you get from AWOS or other FAA weather source  has this

>>>> correction made.  For an explanation see

>>>> _http://www.exploratorium.edu/weather/barometer.html_

>>>> (http://www.exploratorium.edu/weather/barometer.html)

>>>>

>>>> Jim Hasper

>>>>

>>>>

>>>>

>>>> In a message dated 9/25/2006 4:45:42 P.M. Central Daylight Time,

>>>> gerf(at)gerf.com writes:

>>>>

>>>> 

>>>>

>>>>

>>>> At one  point this weekend the barometer at the field where I'm

>>>> building was  exactly 29.92 - the Dyon EFIS read 62 feet, the

>>>> official field elevation  is 66 feet.  I also installed one of

>>>> Van's "steam gauge" Altimeters -  supposedly a fallback

>>>> instrument if my wiring/plumbing goes belly up on  the first

>>>> flight.  The steam gauge reads minus 20 feet :-\

>>>>

>>>> I  know which gauge will be junked out of my panel at the first

>>>> opportunity  ...

>>>>

>>>> __g__

>>>>

>>>>

>>

>> Kevin Horton

>> Ottawa, Canada

>>

>>

>>

>>

>>

>>

>>

>

> --__g__

>

> ==========================================================

> Gerry Filby                                  gerf(at)gerf.com

> ----------------------------------------------------------

>



 Kevin Horton

 Ottawa, Canada

ddcuster(at)wmv-co.us
Guest

Posted: Tue Sep 26, 2006 5:42 am Post subject: Van's steam gauges versus Dynon EFIS

The barometric altimeter "computes" the altitude using the correlation  between air pressure and altitude; there are lots of opportunity for error. The  GPS computes altitude based on the satellite information. The GPS should be real  close to the actual elevation. 

  

 Barometric altimeter error rapidly builds as it moves away (distance  and time) from the corrected reporting station. That is why ATC or FS  always gives you the closest reporting station altimeter reading when you  contact them. 

  

 Since the difference between your barometric altimeter and the GPS if  fairly constant, I would suspect that there is some kind of mechanical error in  the barometric altimeter. Aircraft altimeters coming out of the shop should be  within some specified error. There has to be some tolerance. It is unusual to  find any science based instrument with no error. The error should be small,  however. Anyone out there know the tolerance for a VFR/IFR altimeter. I know it  varies with the altitude.

  

 Doc Custer

Retired software engineer and sometime  Audiologist

Building RV9-A. Fuselage

 [quote]   ---

sbuc(at)hiwaay.net
Guest

Posted: Tue Sep 26, 2006 6:00 am Post subject: Van's steam gauges versus Dynon EFIS

J2j3h4(at)aol.com wrote:

 	  Quote: 	
		    

 I believe the Dynon is determining altitude by GPS.


	




The Dynon does not use GPS for any of its calculations. It has pressure 

transducers for both altimeter and airspeed solutions.



Sam Buchanan

jack.lockamy(at)navy.mil
Guest

Posted: Tue Sep 26, 2006 6:29 am Post subject: Van's steam gauges versus Dynon EFIS

Gerry,   

Before the first flight in my RV-7A, I had the airspeed indicator, altimeter and transponder checked for IFR tolerances.  The Dynon EFIS-D10 was right-on altitude and within 2-3 KTS accuracy on airspeed from S/L up to 40KFT.  The mechanic doing the certification test was extremely impressed with the accuracy of the Dynon EFIS-D10.  You can also adjust the Dynon altimeter up/down in 10 foot increments to correct any errors that may be present.  Mine is set at "0" and is nuts on....  

The same can not be said for the Van's/UMA back-up steam gauges I installed.  The airspeed indicator has a 10 KT fast error that can not be adjusted.  The altimeter was over 300 FT off but CAN be adjusted.  There is a small screw just left of the adjustment knob on the altimeter.  If I remember correctly, I adjusted the altimeter to the known field elevation, loosen the screw, then turn the adjustment knob to the correct barometric reading (ie. 29.92, etc.) in the window.  Once the field elevation and the current barometric reading are both correct.... tighten the adjustment screw and you are good to go...  

Jack Lockamy  

RV-7A N174JL (240 hrs)  

Camarillo, CA  

www.jacklockamy.com     [quote][b]

dbris200(at)sbcglobal.net
Guest

Posted: Tue Sep 26, 2006 6:35 am Post subject: Van's steam gauges versus Dynon EFIS

The altitude that the air traffic system uses is based on the air pressure that is measured by the barometric altimeter, not on the true altitude that you get from GPS. When ATC tells you to fly at 8000 feet, they're talking about pressure altitude, not actual altitude. So, unless the GPS altitude is somehow corrected for pressure, you'll be at the wrong altitude when using the GPS altimeter.

 

 Dave

 EAA Technical Counselor

 

 

 Doc Custer wrote: [quote]            The barometric altimeter "computes" the altitude using the correlation between air pressure and altitude; there are lots of opportunity for error. The GPS computes altitude based on the satellite information. The GPS should be real close to the actual elevation. 

    

    	  Quote: 	
		  

	
 [b]

lessdragprod(at)aol.com
Guest

Posted: Tue Sep 26, 2006 7:19 am Post subject: Van's steam gauges versus Dynon EFIS

There are three different altitudes commonly used in aviation.

    

   1.) Below 18,000' the station pressure is used to adjust the altimeter for local variations in pressure.

   2.) Above 18.000' a pressure setting of 29.92 is used.  This is also called the pressure altitude.

   3.) Density altitude.  The pressure altitude can be adjusted for the OAT to obtain the density altitude.  The Dynon with an OAT can provide the density altitude readout.

    

   Density altitude is used for performance flight testing.  (Station altitude and pressure altitude, by themselves, are useless for performance testing.)

    

   I understand that GPS horizontal position is supposed to be accurate within 60 feet.  And the vertical position is less accurate.

    

   Our pilot training provided a common altimeter base so we would be likely to have the same altitude reference while flying.

    

   Doing flight performance testing at a fixed density altitude can be interesting, since the station altitude can be 2,000' different.

    

   Regards,

   Jim Ayers

  

 --

rv9jim(at)juno.com
Guest

Posted: Tue Sep 26, 2006 7:30 am Post subject: Van's steam gauges versus Dynon EFIS

Gerry,

        I use to be an instrument instructor in the Army for helicopter

pilots.  Our rule was if the "K" factor was 7 or less, to note it on our

flight plan and apply that correction factor to any barrometric reading

while in flight.  29.75 +,-  .07  is an example of acceptable.   More

than the 7 points was reason to have the altimeter replaced.   Of course

this is only my opinion and is for my use only as a disclaimer.



Jim Nelson

ddcuster(at)wmv-co.us
Guest

Posted: Tue Sep 26, 2006 7:49 am Post subject: Van's steam gauges versus Dynon EFIS

Good to know.

  

 Doc Custer

Retired software engineer and sometime  Audiologist

Building RV9-A. Fuselage

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