Setting versus calibrating a barometer
Are they one and the same?
In this forum, I have have been guilty in describing the entire process as calibrating a barometer. I am also guilty of distinguishing between the terms: calibration and setting. So what’s the difference?
In Dr. David Burch’s book “The Barometer Handbook” he states that “setting” and “calibrating” are two different things. He describes calibration as comparing your barometer with a reference barometer and creating a calibration table or graph illustrating the deltas. “Setting” as he calls it, is choosing a pressure to optimize the calibration. I should mention that Dr. Burch runs a calibration lab.
Keep in mind though - Dr. Burch is referring almost exclusively to aneroid barometers circa late 1990’s in his book rather than our more modern digital barometric sensors of today.
Today’s digital sensor errors are primarily linear vs the calibration curves of even the best aneroid barometers. Setting or calibrating may have different meanings depending how your weather station manufacturer has set up the firmware.
Unlike a Davis console where you set the elevation in the console, in an Ambient or Ecowitt console, you do not set your elevation because there is nowhere to put it in! For our Fine Offset manufactured consoles, out-of-the-box ABS = REL tells us that elevation is zero and we need to set/calibrate to our specific elevation. Elevation is not stated but implied by calculating a fixed offset from an elevation of 0 meters (mean sea level) to our location’s elevation. If you are following your manufacturer's manual, calibration is done by comparing with a reference barometer (usually an airport).
Setting the barometer is the process to calculate the fixed offset to determine Relative pressure (REL). Basically, you are manually calculating a pressure offset to correct for your elevation at your location. If we are using a standard atmosphere model to set/calibrate (we should) we are in effect, choosing a pressure (sea level pressure of 1013.25) to optimize the calibration.
Dr. Burch though is clear on one point. In his view, calibrating a barometer is the process of comparing with a reference barometer (preferably a lab grade barometer – NIST traceable) at various pressures where your barometer and the reference barometer is in a pressure chamber. Pressure is increased (or decreased) and the differences between the reference barometer and the test barometer readings are logged. A calibration curve can be plotted and/or a calibration table can be generated.
In my view, if you are calibrating a barometric sensor, you are checking or should be checking for sensor error. If the reference barometer is indicating that the pressure is 1000 mb and you are showing 1002, you are out by 2 mb. Checking other pressures should yield the same 2 mb discrepancy. Our digital sensor error is linear and is always reading 2 mb too high at all the tested data points. We compensate by subtracting -2 mb from our raw pressure reading. This can be done in software, firmware or maybe even at the chip level.
How about an illustration? You are at home somewhere in the Caribbean at exactly 0 meters elevation and glance at the console which shows 1013.2 ABS. We touch a button and a REL of 1013.2 is displayed. No surprise there as we know that ABS = REL at 0 meters and because we had our barometer just back from the lab, we know that ABS is accurate. At sea level the only calibration required is to compensate for sensor error by making a small adjustment to ABS or the ABS offset.
But let’s say that for work reasons, we now have to move and we bring our weather station with us. The elevation is 500 meters at our new home but our console is still showing that ABS = REL. Obviously, something is amiss. Our barometer is perfectly calibrated (you’ve already adjusted ABS by -2 mb based on the calibration table from the lab) but it is not showing the correct REL. That’s because the console still thinks it is located at an elevation of 0 meters. REL for these machines is supposed to display a calculated sea level pressure (SLP). To spare our readers some math, the pressure correction or offset for a 500 meter elevation works out to be about 58.6 mb. Having done this calculation manually, REL is set (by you) to always be 58.6 mb higher than the current live ABS.
So is setting the elevation of your barometer considered to be “calibration” or a “setting”. Perhaps not for the esteemed Dr. David Burch, but for this forum, I think we can simply things by referring to calibration as the entire process as long as we know it is at least a two part process of checking and adjusting for sensor error and making adjustments for elevation at our particular location.
This process needs to be done just after unpacking your new weather station or moving your weather station and checked annually thereafter.
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