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» Online Classroom   » Tech Support   » Barometers   » ALL — Barometric pressure calibrations

   
Author Topic: ALL — Barometric pressure calibrations
david


 - posted June 17, 2003 01:47 PM      Profile for david           Edit/Delete Post 
There is a difference between "sea-level pressure" and what might be called "true pressure" or "absolute pressure." The first step in using a barometer when not at sea level is to decide what type of pressure you want to read. This article discusses this issue and tells step by step how to calibrate the barometer—there is also a short synopsis below.

Article on pressures (12 kb pdf)

If near water, your best source of pressure data is likely the national buoy data center. Find your nearest station, change "English" to "metric" to get mb, and then look to the table of data or click the graph icon on the left side of the line with pressure report. There are links near the bottom of the page for getting archived older data as well as a link that defines the terminology.

If inland, then data from an airport is likely best. See Metars section of Aviation Weather center. There is a link to the list of airports http://adds.aviationweather.gov/metars/stations.txt , which you need to find the right identifier so you can ask for the data. Also an option for what period of time you want. Quite nice.

The Metars table data (in normal format) can be copied, pasted into a text file, and then imported to Excel for analysis, plotting, etc.

As of Sept 3, 2005 we have found that this link is a good place to find all sources of pressure, inland, coastal and buoys: http://nowcoast.noaa.gov/ Drag and select the area of interest get a detailed map of your location, then select Meteorological data, then pressure, then real time, then press Go, and then when the yellow dots appear, click one of those for the data. Airports seem to only keep 4 days of data, so we need to keep that in mind when using those sources.

How to set your barometer
(1) Start a log book of pressure readings. Just record date, time, and pressure. Try to take the readings as close to an exact hour as possible, but it is not crucial if this is not convenient.

(2) After you get enough data that shows some variation in pressure, say at least a range of 10 mb or so, then locate the nearest reference station to you that is reporting accurate pressures—use one of the means described above.

(3) Most of these stations (and all of the buoy stations) include an archive of past pressures. Look up what the correct pressure was at the precise times you have recorded. You may have to interpolate for the minutes if you are off the exact hour very much. Record these next to the measurements you have.

(4) Subtract the measured values from the measured values, and you will get a list of how much your barometer was off. This will not vary with pressure for the instruments we sell, so any variation is just uncertainties in timing, or real differences from your location and the reference station. Average this list, to get your correction, and then set your barometer to that -- providing you do indeed want to read sea level pressure. See related articles we have here and online on this question.

(5) If the reference station is not close, then you may need to triangulate between 2 or 3 stations to get a good average for your location. Note that if you want to read sea level pressure, your own elevation does not matter here. It just means your correction will be large the higher you are. We have a table in the Weather Trainer that converts elevation to pressure correction, or you can find one online just to see that the large correction you have makes sense.

IMPORTANT: if you are using the "digital marine barometer" that we sell, then be sure to download our own instructions on how to set it. The manual is not wrong, but it is not as clear as it could be.

Notes on finding an airport for pressure references
Go to the metar page
http://adds.aviationweather.gov/metars/index.php
then, just above the map, click the METARs Java Tool. Then left click and drag a box around the area of interest to show the airports, then put the mouse on the airport to get its station number. then you can go back and get the data. Not all stations have archived data, however, and some only go back a few days. Keep this in mind when calibrating a barometer with only airport data.

From: Starpath School of Navigation, Seattle WA
david


 - posted August 21, 2008 05:24 PM      Profile for david           Edit/Delete Post 
Please see the note on pressure sources in the 4002 calibration article in this forum. There are some more notes on ways to get to metars.
From: Starpath School of Navigation, Seattle WA
david


 - posted June 09, 2009 12:27 AM      Profile for david           Edit/Delete Post 
Much of the above is now outdated by our new free barometer calibration resource. Go to www.starpath.com/barometers, and follow the instructions there for easy accurate barometer calibration, worldwide.
From: Starpath School of Navigation, Seattle WA


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