Calibrating a Barometer ?

[bmiller]

I'm looking for a barometer to install in the boat. Most say they are calibrated for up to 3,500 feet. My boat spends summers at 9,000 feet, winter trips to sea level. Will going up and down affect it's accuracy, will I need to re-calibrate annually? If so how is that done, send it in?

[Alan Wheeler]

Will going up and down affect it's accuracy

Absolutely. Even 3000ft will make a difference. But then, you don't always need to know the actual pressure. It is the trend that tells you the most information. However, if you want "real" pressure readings, you will need to calibrate each time you move altertude. It's easy enough to do, you just need an authority to tell you the real time pressure and set the barometer to suit. I small margin of error will not be a problem.

[Hud3]

Yep, Wheels is right. When I moved to Nevis my little electronic barometer went from 15' above sea level to 450'. I thought the movers had broken it until I realized that it was the altitude effect. 435' made a noticeable difference.

[Paul Elliott]

On my barometer there is a little screw in the back you turn to calibrate the reading. You can get the current air pressure from the Weather Service broadcasts. Or, if there is an airport nearby, you can often find their weather report, including local pressure, on the internet. If the reporting location is nearby but at a different elevation than you, as long as conditions are settled you can do an altitude correction on the reported pressure. Here in a link to air pressure vs altitude I just googled:Pressure Conversion Table - Transwiki

I like the disclaimer on that page: The following information may have errors; It is not permissible to be used by anyone who has ever met a lawyer.

[Topazken]

Wheeler is correct. It is not the current reading that really matters. It is the changes that indicate impending weather variation. Your worrying should be directly proportional to the speed and severity of the change!

[svstrider]

This is good info on barometers 101.
I forget where I came by it, maybe this site as I have been lurking here nonstop for the past 2 weeks, filling in time at work (1/4 of the way to cape town) waiting for some medical problem to happen and hoping it won't.

If it is already found somewhere else on these discussion boards, my apologies.

Anyway it is from the NZ meterological site, but don't hold that against the info. In these days of complicated electronic thingys on boats, it is nice to use a piece of kit that is as simple as a wheelbarrow.
Come to think of it, it took humans 100 K years to perfect that piece of machinery. see attached pdf file

[s/v Jedi]

Call the meteo at the nearest airport and just ask them for current barometer readings at sea level (they have that).

These guys finally get someone on the phone who is interested in what they do and they will help you and like doing it.

ciao!
Nick.