Ambient and barometric pressure

[BacktrailJitters]

On my 76CSx I had an ambient pressure of 29.66"Hg and a barometer pressure of 29.89"Hg. I lied and told it I was at 0ft elavation. The GPSr then told me that both amb and baro were now 29.66"Hg. (They should be equal at mean sea level) I then told it I was at 400ft and it kept the ambient pressure the same but increased the barometric pressure to 30.05"Hg.

 

So, after doing the math, it looks as though the GPSr is adding about .01"Hg to the ambient pressure of my present elevation to give an estimated vaiue for the barometer reading at MSL.

 

Does this sound right? What's your thoughts on this?

[Hertzog]

On my 76CSx I had an ambient pressure of 29.66"Hg and a barometer pressure of 29.89"Hg. I lied and told it I was at 0ft elavation. The GPSr then told me that both amb and baro were now 29.66"Hg. (They should be equal at mean sea level) I then told it I was at 400ft and it kept the ambient pressure the same but increased the barometric pressure to 30.05"Hg.

 

So, after doing the math, it looks as though the GPSr is adding about .01"Hg to the ambient pressure of my present elevation to give an estimated vaiue for the barometer reading at MSL.

 

Does this sound right? What's your thoughts on this?

I don't quite follow your 0.01"Hg; when you told it you were at 400 ft it added 30.05-29.66 = 0.39"Hg, or 0.00097"Hg/ft, which is pretty close to standard temperature/altitude (slope for STA is 0.00106"Hg/ft from 0 to 1000 ft). Did you slip a decimal place in your original post, and leave out per foot? I've done some measurements at various altitudes (using mbars to get more precision) and get something like 0.00107"Hg/ft, but for all practical purposes assuming 0.001"Hg/ft is in reasonable agreement with your measurements, my measurements, the STA profile, and what Garmin is probably using in their calculations.

[BacktrailJitters]

Hertzog, you are correct. I was thinking of it as .01"Hg per 10ft of elevation, but neglected to say it. Thinking about it now, I suppose I should have used "Hg/ft.

Thanks for the response.

[Hertzog]

Hertzog, you are correct. I was thinking of it as .01"Hg per 10ft of elevation, but neglected to say it. Thinking about it now, I suppose I should have used "Hg/ft.

Thanks for the response.

Your results raise an interesting point. When I mentioned that I get 0.00107"Hg/ft, I was using a different approach. But when I duplicate your approach, I get a 0.44 or 0.43"Hg delta at 400 ft just sitting here. It's a small but significant difference from your measurement, and I get it for three GPSr's (60CSx, 60CS and Vista). I don't think the 76CSx would have a different algorithm, so that pretty much leaves two other variables: temperature and the actual ambient pressure (my readings happen to be at about 75 degF, and my current ambient is 30.08"Hg). It could be Garmin is adjusting for temperature, but I don't think so. I suspect they are maybe making some adjustments in slope based on the combination of ambient pressure and altitude, which makes sense now that I think about it. When I get a chance I will go somewhere to match your ambient pressure and see if I get your delta.

[BacktrailJitters]

Hertzog, you mentioned "(slope for STA is 0.00106"Hg/ft from 0 to 1000')", I'm new to this and so was wondering what it is above 1000ft, say to 2000ft.

BTW, When I took the pressure readings the temperature was about 66°F. I will do more readings with another GPSr also.

[Hertzog]

Hertzog, you mentioned "(slope for STA is 0.00106"Hg/ft from 0 to 1000')", I'm new to this and so was wondering what it is above 1000ft, say to 2000ft.

BTW, When I took the pressure readings the temperature was about 66°F. I will do more readings with another GPSr also.

Here's a link to a page giving the data for STA:

http://www.usatoday.com/weather/wstdatmo.htm#us

 

I went out today and ran this sort of test at ambient pressures down to 28.86"Hg, and it didn't make any significant difference in results (I ran some with elevation set to 400 feet, and then some with 1000 ft to get a bigger swing - I get a 1.11"Hg delta at 1000 feet if you want to compare results. Going between Fixed Elevation and Variable Elevation modes did make a small difference, but the 400 foot elevation deltas were between 0.42 and 0.48"Hg, nothing approaching your 0.39"Hg, so there is still some mystery there - but you might try looking at measurements in Fixed vs. Variable to see if you see anything.

 

When I got home I tried cold soaking the 60CSx in the refrigerator for about an hour (at about 32 degF) to see if temperature made any difference; absolutely no difference in readings.

[BacktrailJitters]

Tkanks for the link...very interesting. Using my 76CSx and 76S, I got the same results from both today with GPS mode off: 400ft delta=.43, 1000ft delta=1.08. Don't know why the low delta yesterday, couldn't make it repeat today. But then again...yesterday I didn't even know what "delta" was! I do see though that the 1000ft delta Igot is lower than your reading.

Thanks for your input.

[Hertzog]

1000ft delta=1.08 ... 1000ft delta Igot is lower than your reading.

Thanks for your input.

I wouldn't loose any sleep over it. I did in fact get one reading of 1.07 at 1000 feet, but most were around 1.11; I was playing around with recalibrating and switching between Variable and Fixed Elevtion at the time, so that may have had something to do with it.