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How are you calibrating the barometer?


Team Chinook
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So what is the common form of barametor calibration?

 

How often does it need to be done?

 

My iFinder Hunt manual suggests calibrating it at an airport because they are supposed to be on the money as far as elevation. Our airport is supposed to be at 800 feet and my Hunt said 796 feet so I didn't change it. That was the only time I checked it.

Edited by DWBur
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If you wish to use an airport, you must know where on the airport you are, most runways are not 'level'.

Go to http://www.acukwik.com/PowerSearchAirports.aspx or http://flightaware.com/resources/airport/browse/MA and find your airport.

You must have permission to be inside any security area, so go get it.

Or you could go into the tower and check the weather. If that airport has an ASOS you can usually get the barometric pressure with an aircraft band receiver if you are close enough. I live close enough to KFIT to pick up their transmissions.

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Ok. Sounds like it is more common to use the elevation/altitude calibration rather than the baro pressure..unless you're near an airport.

 

Anyone have an idea on how capable the home weather system baramoters are? I have a weather station...

 

it reads 29.68 / 1018.8

whereas weatherunderground reports downtown Walnut Creek reading 29.95 in / 1014.1 hPa and falling.

whereas weatherunderground nearest airport (10mi away) reads 29.96 / 1015 - but data is an hour old...

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Ok. Sounds like it is more common to use the elevation/altitude calibration rather than the baro pressure..unless you're near an airport.

 

Anyone have an idea on how capable the home weather system baramoters are? I have a weather station...

 

it reads 29.68 / 1018.8

whereas weatherunderground reports downtown Walnut Creek reading 29.95 in / 1014.1 hPa and falling.

whereas weatherunderground nearest airport (10mi away) reads 29.96 / 1015 - but data is an hour old...

 

It all depends on elevation, but in your situation the 29.68 seems a little off if there is little change in elevation.

Edited by ryguyMN
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So what is the common form of barametor calibration?

 

How often does it need to be done?

If you have a Garmin with barometric altimeter (I can't speak for other brands), just make sure you have "auto-calibration" turned on, and then most of the hard work is done for you.

 

Autocalibration uses the GPS elevation to periodically adjust the built-in barometer to allow for real-time barometric pressure changes. On older software versions, the auto-calibration process was a bit flaky, and sometimes tended to over-correct even when GPS accuracy wasn't great and the GPS elevation might not have been accurate. Later software versions seem to do a much better job of not over-compensating, unless the GPS accuracy is good, and I find I can generally maintain about plus or minus 5 metre vertical accuracy all day.

 

I manually calibrate my GPSr (Summit HC) at the start of each day's recording, and have auto-calibration on. Most days start from home, and I know my true elevation there (partly from good topo maps, partly from a few months' experimentation with my GPSr.) When I am away from home on a trip, I will make a note of my elevation at the end of each day's travels, and then manually recalibrate the altimeter at the start of the next day. Whenever I start recording at a location with no known elevation, I will allow the GPSr to get a good 3D fix, then use the GPS elevation to initially calibrate the altimeter. Even if the GPS elevation you use for your initial calibration is not good, the unit will auto-calibrate itself to better accuracy as the day goes on.

 

If you forget to calibrate at the start of a day, don't sweat too much - the auto-calibration routine should have the barometric altimeter back on track to a few metres accuracy within half an hour or so, but the elevations shown in the first half hour or so of track-log may not be accurate. Once the altimeter has auto-calibrated itself correctly, it should hold very good elevation accuracy all day.

 

Hope this helps!

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The other day, a local survey company was taking some measurements right outside my apartment building, and they were using a particular geographical point on the sidewalk. I asked them if the elevation of that point was known, and they said yes and gave me that elevation, and that's the figure I use to calibrate my barometer.

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Pressure trivia:

 

29.92" Hg is considered to be "Standard" pressure. Pressure readings higher than that are considered to be "High" pressure and readings below that are "Low" pressure.

 

HIGH pressure areas have clear skies, cooler temps and generally are associated with good weather. Wind moves in a clockwise motion around the H on a weather map.

 

LOW pressure areas are usually associated with cloudy skies, warmer temps and airborne moisture such as fog, clouds or precipitation. Wind moves in a counterclockwise motion are an L on a weather map.

 

Typically, the barometer reading "falls" as poor weather approaches and the pressure "rises" as good weather is approaching.

 

That "barometer reading" or whatever you choose to call it is really called the "Altimeter Setting" and it can be obtained from any airport that has an automated weather system. To find an airport online that is close to you, go to http://www.airnav.com/airports/ and you can just type in the airport name OR the city/town the airport is located in. For example, the airport in Fitchburg, MA...I can type "Fitchburg" or "KFIT" (which is it's aviation airport identifier) and both will get me to the airport information page. There's a couple of items that are cool here:

 

1. The first is a phone number you can call which will allow you to listen to the weather broadcast that pilots use before they land or take off at an airport. You'll find it on the left side near the top of the page and it reads "WX ASOS: 135.175 (978-343-9121)

The WX ASOS stands for "Weather Automated Surface Observation System". The "135.175" is the frequency a pilot would tune to hear the weather. The next number is obviously the phone number you can call that will patch you in to that broadcast message. It's usually a robotic voice that repeats the current weather conditions and it updates about every minute so it's as close to real time weather as you can get.

 

2. The second cool piece of information you can get from the airnav site is if you scroll down, on the right side you'll see a photo of the airport, and below it you'll see the word "METARS". That's a textual version of whatever the ASOS is currently saying. This one can be as much as a hour old, but never more than that. It looks like a bunch of code and nonsense, but it really a string of very useful weather information. The part that concerns you here is the little piece that says "A2992" or "Axxxx" with the 4 x's being numbers. That too is the altimeter setting. The "A" stands for "Altimeter" and the 2992 stands for "29.92 inches of mercury"

 

One more thing to know is how pressure settings pertain to accuracy: one inch of change is equal to 1000 feet of change. So let's say you're at an airport that's at sea level. If you have your GPS set to 30.23" and the actual pressure is 29.23", then it's going to think you are 1000 feet above the airport.

 

Good luck!

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HIGH pressure areas have clear skies, cooler temps and generally are associated with good weather. Wind moves in a clockwise motion around the H on a weather map.

 

Yes.

And if you ever want to drive up Mt Wachusett to see Boston ~40 miles away, or Pack Monadnock to see Boston ~60 miles always do it on a day when you have high pressure.

 

P.s. From that first hill twice a year for a few days straight you can see the setting sun reflect off the Prudential building, a great big 50 story mirror ~40 miles away. What a great sight. When I first saw it I thought that Boston was nuked, that actually was about 2 months after 911. Exact seeing dates might be findable within some of my cache find logs that I might mention in my profile. I can't remember if I ever showed dates for Mt Wachusett, center of Princeton, Worcester Airport, and any other spots that can see Bostonwithin the funnel zone that it's possible for a reflection to go. And that last bit does depend of the orientation of the buildings that are the mirrors, so I must do some calculations for the Hancock which should reflect down to Rhode Island, if RI wasn't such a flat state.

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