Author Topic: Setting versus calibrating a barometer  (Read 369 times)

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Offline gszlag

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Setting versus calibrating a barometer
« on: November 19, 2022, 04:30:13 PM »
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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« Last Edit: November 21, 2022, 06:58:22 PM by gszlag »
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Offline FW8379

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Re: Setting versus calibrating a barometer
« Reply #1 on: November 24, 2022, 11:28:23 AM »
Not replying to anything specific in this post - just want to say I appreciate your thoughtful posts regarding barometric pressures and the how-to's of calibrating/setting our stations depending on equipment.

Like you, I couldn't make heads or tails out of the instructions in my PWS manual as far as calibrating or setting the barometer. This led me to this forum where I was able to gain a better understanding of where to start and the basic principles of barometric pressure and how elevation affects it. I spent quite a bit of time researching what I could and testing things out based on my location (elev=2002 meters), nearest METAR, and equipment (out-of-the-box Ambient 2902c). After chasing around MSLP for a couple months, the conclusion I reached (again, for my location/equipment) was to calibrate to altimeter. The information in your posts has explained and confirmed many of the "things" I was observing and questioning as I tried to calibrate pressure. Thanks for the posts and keep them coming!

Offline gszlag

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Re: Setting versus calibrating a barometer
« Reply #2 on: November 24, 2022, 06:17:44 PM »
Not replying to anything specific in this post - just want to say I appreciate your thoughtful posts regarding barometric pressures and the how-to's of calibrating/setting our stations depending on equipment.

Like you, I couldn't make heads or tails out of the instructions in my PWS manual as far as calibrating or setting the barometer. This led me to this forum where I was able to gain a better understanding of where to start and the basic principles of barometric pressure and how elevation affects it. I spent quite a bit of time researching what I could and testing things out based on my location (elev=2002 meters), nearest METAR, and equipment (out-of-the-box Ambient 2902c). After chasing around MSLP for a couple months, the conclusion I reached (again, for my location/equipment) was to calibrate to altimeter. The information in your posts has explained and confirmed many of the "things" I was observing and questioning as I tried to calibrate pressure. Thanks for the posts and keep them coming!

2002 meters! You are way up there!
Many thanks for the kind words. Appreciate it!
Ambient Weather WS-2000
Ecowitt GW1000/GW1100
Ecowitt WS68: Anemometer, UV/solar
Ecowitt WH40: Rain gauge
Ecowitt WH57 Lightning sensor
Ecowitt WH32E: Outside T & H sensor
Stratus Rain Gauge (manual)
Raspberry Pi 3B+ (WeeWX/CumulusMX)
Raspberry Pi Zero 2W (WeeWX/MQTT/Belchertown)
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http://weewx.glenns.ca
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Offline TheBushPilot

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Re: Setting versus calibrating a barometer
« Reply #3 on: November 25, 2022, 10:47:00 AM »
Would going to the reference ASOS and adjusting the sensor in question to station pressure be a valid method of "calibration"? It obviously isn't a multipoint verification but putting them quasi-side by side and checking would work temporarily right?

Cheers
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Offline FW8379

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Re: Setting versus calibrating a barometer
« Reply #4 on: November 25, 2022, 11:19:20 AM »
I haven't tried this but have heard it suggested before.

I'm definitely no expert with this stuff, but it seems to me like a good way to calibrate a sensor to ABS pressure. As long as your home elevation offset (between ABS and REL) is calculated and entered correctly in your sensor, I would think the sensor should be calibrated and reading correctly/accurately for both ABS and REL when you got back home. (?)

Offline gszlag

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Re: Setting versus calibrating a barometer
« Reply #5 on: November 25, 2022, 12:31:02 PM »
This might help:

Alternative methods to calibrate a barometer
https://www.wxforum.net/index.php?topic=44512.msg452487#msg452487

Other than sending your barometer to a calibration lab (expensive), there are different methods to calibrate.

Before you wander around the airport with a black box and trailing a bunch of wires note the caveats in my post!!!

If you are close to a small airport (definition: no parking fees), contact the airport manager and maybe he will open up the equipment rack for you or at least give you a tour...
Ambient Weather WS-2000
Ecowitt GW1000/GW1100
Ecowitt WS68: Anemometer, UV/solar
Ecowitt WH40: Rain gauge
Ecowitt WH57 Lightning sensor
Ecowitt WH32E: Outside T & H sensor
Stratus Rain Gauge (manual)
Raspberry Pi 3B+ (WeeWX/CumulusMX)
Raspberry Pi Zero 2W (WeeWX/MQTT/Belchertown)
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http://weather.glenns.ca (pwsdashboard - live)
http://weewx.glenns.ca
http://glenns.ca/cumulusmx2/index.htm
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Offline CW2274

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Re: Setting versus calibrating a barometer
« Reply #6 on: November 25, 2022, 04:43:23 PM »
I see no reason for the need to be on the airport proper. Sitting outside a chain link fence at the airport would suffice just fine. I've suggested this method for years for those that can't just "dial it in". Just make sure you're there when the obs comes out (usually once per hour close to the top) for the best accuracy.

 

anything