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Author
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Topic: Barometer calibration curves
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David Burch
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posted September 19, 2006 10:27 PM
A question came up in our online weather course that is important to all mariners who care about marine weather, so we are sharing the answer in this section.
We discuss the calibration curve part of your question separately as it is a separate topic.
The type of "calibration curve" you refer to is discussed in the Weather Trainer in detail but it applies to aneroid barometers.
Though not stated in the quiz question—it should have been—the barometer we are calibrating in that question is a digital marine barometer and these barometers do not have a calibration curve... they have no curve at all. They are "linear" meaning once you find the proper offset as we are doing in this example, they will be right at all other pressures.
This is not the case for aneroid barometers (as discussed in the weather trainer). Those barometers, even ones costing 300 or 400 dollars, would have have one correction at one pressure, ie + 1 mb at 990 mb, and different ones at other pressures, ie - 2.5 mb at 1020 mb and so on. Thus to use them to optimum value you need a table of corrections vs indicated pressures, and if you plot this data as a curve it is called a calibration curve.
This data almost always had to be plotted because in the early days we did not have the Internet and it was difficult to get good calibrations, and consequently we had only a few data points to go by and thus we had to interpolate between them, which was best done with a plotted curve.
Also unlike the digital marine barometer that we can "calibrate" with just a few pressure readings at any pressure at all, for an aneroid device if you want to know the correction, you have to be there! So how often do you get to 990 mb or to 1035 mb etc. ie not too often, so you make a curve and project it into those regions... or there are professional services that can use a test chamber to calibrate any barometer.
Electronic barometers are easy to use and usually accurate. The only disadvantage is they are still electronic devices, subject to the rigors of a life at sea. The alternative is to select a high quality aneroid barometer—one that has a certified accuracy and a track record of tested use at sea. There are not many options in this category that are affordable, but after much searching worldwide we do now carry one. See Precision Aneroid Barometer
Having a quality aneroid to back up your electronic barometer is like having a sextant to back up your GPS.
From: Starpath, Seattle, WA
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