Ambient Pressure Vs. Barometric Pressure?

[GlfWrVt]

The owner’s manual for my Garmin 60CSx only says that ambient pressure is the "un-calibrated current pressure". For the barometer it says "The calibrated current pressure". These two readings are greatly different most of the time. Why? I have an aviation background so I have a pretty good idea about how barometric pressure relates to pressure to determine altitude etc... Am I forgetting something? Little help!?

 

Thanks.

 

Mark.

 

[kb9nvh]

I'm curious as to how all this works too. Seems like if you set a known altitude then the gps would know the barametric pressure by virtue of that. I know that over time my gps get the barametric pressure wrong and I have to recalibrate.

 

Since the thing knows a gross altitude from the gps it seems it could get a pretty accurate pressure over time if it sits in one spot..??

 

The owner’s manual for my Garmin 60CSx only says that ambient pressure is the "un-calibrated current pressure". For the barometer it says "The calibrated current pressure". These two readings are greatly different most of the time. Why? I have an aviation background so I have a pretty good idea about how barometric pressure relates to pressure to determine altitude etc... Am I forgetting something? Little help!?

 

Thanks.

 

Mark.

 

[+Fred Flintstone]

I also have an Avation background but not as a pilot. The best I can figure out is that the Barometric pressure is the actual pressure or ambient. I know, I know, the gps does say ambient but it is never close for me. The Ambient pressure on the garmin cxs models I believe is the pressure that it would be if you were at sea level. I know standard psi is 29.92 hg. That is set at 18,000' and higher for flight. I am looking at my gps and Baro reads 29.70 and i am in the middle of a thunderstorm in NE Ohio. My alt is 1,167'. Ambient psi on my gps is 28.47 hg. I would think that if I could descend 1,167' I would be at sea level and both readings would be the same. This is only my the theory. I have not asked Garmin either. The owners manual doesn't really explain the differences. Give my theory some thought and see if you come up with the same answer or at least something similar.

Edited May 26, 2006 by Fred Flintstone

[Grasscatcher]

Ambient pressure is the actual pressure as felt by a guage.

 

Barometric pressure is that (ambient) pressure calibrated to sea level

 

ie Two locations, one at sea level and one on a mountain top, can have the same ambient pressure but the higher elevation one would show a lower barometric pressure at the same ambient pressure.

 

Thats the reason you must calibrate your (barometric pressure) altimeter to a "known" elevation.

 

edited for spelling....left out an "h"

Edited May 26, 2006 by Grasscatcher

[+Renegade Knight]

The owner’s manual for my Garmin 60CSx only says that ambient pressure is the "un-calibrated current pressure". For the barometer it says "The calibrated current pressure". These two readings are greatly different most of the time. Why? I have an aviation background so I have a pretty good idea about how barometric pressure relates to pressure to determine altitude etc... Am I forgetting something? Little help!?

 

Thanks.

 

Mark.

 

 

I don't think you are missing anything.

 

Garmin needed a way to differentiate uncallibrated elevations from calibrated elevations. Since it uses air pressure to calculate elevation if your GPS is equipped wiht the sensor, that's why the difference.

 

The reason the two readings are different is likely because air pressure varies greatly over time, but not so much in a short time. So your GPS altitue is wrong until you tweak it back by calibrating it. It sounds the GPS direclty correlates air pressure to elevation (someone could actually check this) so the callibration would adjust the air pressure reading to match the elevation it was calibrated to.

 

That's just a bunch of BS that came off the top of my head but it's a starting point until someone else can correct it.

[+seether106]

Maybe this is useful?

 

Ambient pressure is the pressure that the sensor is detecting. Barometric pressure is the calculated pressure for sea level based on your altitude. So at sea level the ambient pressure and Barometric pressure should read the same.

Air pressure can change due to altitude changes or changes in the weather.

 

As you change altitude both the Ambient and Barometric readings will to. If your altitude remains constant, but the GPS starts to show an elevation change it means the barometric pressure is changing, which can mean a change in the weather.

 

This is by no means exact, but gives you a good idea:

Over 30.20"

Rising or steady- Continued fair

Slowing falling- Fair

Rapidly falling- Cloudy, Warmer

 

29.80" to 30.20"

Rising or steady- Same as present

Slowing falling- Little change

Rapidly falling- Precipitation likely

 

Under 29.80"

Rising or steady- Clearing, cooler

Slowing falling- Precipitation

Rapid falling- Storm

[stevesisti]

Maybe this is useful?

 

Ambient pressure is the pressure that the sensor is detecting. Barometric pressure is the calculated pressure for sea level based on your altitude. So at sea level the ambient pressure and Barometric pressure should read the same.

Air pressure can change due to altitude changes or changes in the weather.

 

As you change altitude both the Ambient and Barometric readings will to. If your altitude remains constant, but the GPS starts to show an elevation change it means the barometric pressure is changing, which can mean a change in the weather.

 

Well, there's sort of a little conflict in what you are saying...although I don't understand this completely, here's my take....

 

As you change altitude the ambient pressure should change but the barometric pressure should stay the same, if it's definition is pressure at sea level. The only time it would change is if the weather brought in an air mass of higher or lower pressure. This would be true I think if the unit was in the mode where it didn't auto-calibrate eg. take altitude calculations convert to pressure and then subtract to re-adjust the barometric at sea level.

 

In auto cal mode, the unit would take a GPS altitude, convert it to an equivalent Known pressure for that altitude, see how it differs from the "ambient" reading you have, and then adjust to come up with the updated barometric at sea level.

 

A change of 1" of Mg in your reading represents 1000 ft approx (good ref. up to 10,000 ft or so) As an extreme example, if you left in the am for a mountain hike and hurricane gilbert blew over without auto or manual calibration the drop from 29.7 average nice weather day to Gilberts 26.7 would represent an error of 3000ft in altitude. Of course that drastic a drop doesn't happen that quickly, but the point is that it doesn't take much to have an error of sizeable proportion if the unit is not either manually (known elevation, known barometric station reading) or automatically (GPS derived altitude)calibrated. In the case of my 60CS, I believe the GPS derived altitude for one reason or another leaves a lot to be desired, as I believe it is the largest factor in the unit not being as accurate as it could be. Just using the barometer alone, it tracks very well with a barometer i have at home...the GPS derived altitude can vary by as much as 20% when several readings are taken within a minute or so while I am standing still. I would guess the very nature of being on one side of a steep mountain with a less than stellar satellite configuration could case the GPS derived altitude to be off target.

 

Steve