Topic: New Method of calibrating your barometer Part II (for the intermediate user)

[gszlag]

In Part I we talked about calibrating a personal weather station’s barometer from the perspective of a beginner. The procedure that was discussed in Part I was simple by design – no discussion of correcting pressure due to temperature, no discussions about air density or pressure reduction equations, etc.

I also recommended matching the airport’s Altimeter setting (or matching QNH if your region uses METAR “Q” codes). rather than the airport’s SLP/MSLP reading. The Altimeter(setting) ignores observed temperatures, air density (warm air vs cold air), humidity and other factors.

The usual cautions apply:

It is very important to match the airport Altimeter reading only when you and your airport are in the same weather system (the same high or low pressure zone) and have the same atmospheric pressures.

For the intermediate user that wants to expand their meteorological knowledge a bit more and extract more precision and accuracy out of their barometric sensor(s) – read on.

We will use a different approach and focus our attention to our station pressure (ABS); then correct for sensor error and then match our airport’s METAR Altimeter or QNH. We have already calculated our fixed offset/elevation offset for our location so there is no need to recalculate it. As you might recall from Part I, the elevation offset was 29.7 mb for a 250 m elevation.

Calculated¹ pressure readings like Altimeter(setting), QNH and METAR SLP are based on a calibrated barometric sensor that produces an accurate and precise station pressure measurement. Keeping that goal in mind, let’s start by calibrating your barometric sensor.  For the intermediate user we take a different approach by correcting/adjusting your current ABS (station pressure) reading.

¹ Calculating Altimeter/QNH and MSLP/SLP will be discussed in a future post for the advanced user.

Ambient & Ecowitt display consoles:

Check your console’s pressure readings. If ABS = REL, that’s probably the original factory default settings, If you believe you’ve messed them up badly — you can switch your pressure units back to inHg temporarily and put in the pressure reading from the WH32B (units will be inHg) back into both the ABS and REL fields. You are now back to the factory default pressure readings. Then switch back to hPa units to start the calibration process once again.

Now adjust the REL in your display console’s calibration page so that REL is 29.7 higher than your current live ABS reading. Refer to your weather station manual for directions how to do this. Your REL is now lock-in-step with your ABS and REL will always be 29.7 higher than the ABS reading.

Compare with your airport’s Altimeter (setting). If the airport reading is higher than your REL reading you will have to increase your ABS to match your REL reading with the airports Altimeter(setting).. When you increase/change your ABS, you will notice that your REL reading will still be exactly 29.7 higher than your ABS. Do not change the REL reading by pushing any buttons – you will ruin your 29.7 offset. In other words, if you want to change your REL reading you change your ABS reading. The two readings always move lock-in-step.

Ecowitt GW Series WiFi Gateway:

If you don’t have a display console and are using the Ecowitt GW series WiFi Gateway as your console, things are even simpler - you can’t mess up your default pressure settings. You calibrate your barometer using the WSView smartphone app or depending on the GW model, you use a web browser to configure the settings. In the GW system, you do not change the ABS or REL values directly – you apply an offset/correction to change their values. For example, if ABS is too low and you wanted to add 2.0 hPa to compensate, you would add 2.0 as an ABS offset in the calibration settings page. Similarly, if ABS is too high by 2.0 hPa you subtract an offset/correction of 2.0 hPa (you enter a -2.0 as the ABS offset).

Remember the elevation offset for a 250 m elevation that we calculated previously? It was 29.7. Enter that number into the REL offset field in the calibration screen in the WSView app or in the web browser. Now check your current REL pressure. It should always read 29.7 hPa higher than your current ABS pressure.

Now compare your REL reading to the airport’s Altimeter(setting). Chances are is that the two readings will not be exactly the same (unless your barometer happens to be perfectly calibrated). If the airport’s reading is too high compared to yours, you will have to add a ABS offset to match. For example, if the airport’s Altimeter setting is 2.0 hPa higher than your REL reading, you need to add 2.0 hPa as an ABS offset. It is important not to change the REL offset as that number is tied to the specific elevation of 250 m. Always calibrate your barometric sensor by adjusting the ABS offset and set your barometer to your specific elevation offset using the REL offset.

Let’s continue with our discussion of the Earth’s atmosphere.

Meteorological considerations – atmospheric pressure:

Pressure, temperature and weather for our discussion, refers to the bottom layer of the atmosphere < 11,000 meters (troposphere).

Standard Atmosphere (model of the atmosphere):

15°C @ 0 meters (sea level)
1013.25 hPa/mb @ 0 meters (sea level)
Temperature declines linearly at the rate of 0.0065°C per meter with altitude.
Pressure declines in a non-linear fashion (it’s on a curve) with altitude (determined by the use of an equation, e.g. hypsometric equation)

Important: Note that for standard ISA conditions (aka “standard conditions”), 15°C is the temperature at sea level – not at your location..Use the digital dutch Standard Atmosphere calculator to calculate the elevation offset for your location.

Refer to Part I – how to calculate elevation offset.

Coming up! The excitement starts. Controversies rage. We take a deep plunge into the shark-infested waters of Altimeter(setting), QNH and MSLP/SLP (Sea Level Pressure).

.… continued in Part III - a guide for the advanced user.

[golftango]

i'm new to ambient weather and i read the part 1 of this subject and it lost me because i can't see there's any way to set my ws-2902d to hpa mode. i personally am fine with setting a relative reading to match a local airport as described in the manual, but once i signed up with cwop and they are qc'ing my data i'm getting what they call a level 3 spatial error in the barometer reading. when i signed up i made a typo on my elevation and input 532m instead of 582m which might be causing it, but despite me following all the instructions, they haven't corrected this yet. hopefully it will get corrected and that's the problem but in the meantime i'm reviewing the calibration to see if i can get it to match nearby stations because that's what they said this error meant--my station didn't match other stations in the area.

what i'm wondering is that given the airport is about 150-200 feet lower in elevation, is there a compensation or offset i can use to compensate for this difference. you say that if it's in a different weather system but don't address any elevation difference.

if you would like to suggest a way i could calibrate it more accurately than using the airport or a nearby afb i'd appreciate it. the manual also says you can use a calibrated barometer and i have a kestrel weather instrument which i think has certificates (nist?) for some functions, but unsure if the barometer is certified. in any case it reads so much higher than any of my other barometers i'm not inclined to trust it.

[later note] i really trust the kestrels--i've had a half-dozen over the years. so i dived into that manual and they said to set the altitude on the barometer panel, note the reading, and then plug that into the altimeter panel. then you can sync them although i'm not sure i want to do that unless i'm somewhere my elevation is changing.

i have one civil airport about 5 miles from me and it's reading 29.77. i have a huge afb 5 miles another direction and it's reading 29.71. i'm reading 29.78 on my aw console so i feel a little more confident i'm close to being as accurate as possible.

[sorry, even later note] found this in the kestrel manual. evidently if i set altitude to zero i can see station pressure and then try to find out how to calculate the relative pressure from that. but evidently this is what the kestrel does internally so me setting my aw console to the kestrel relative pressure should be all i need to do and no calculations necessary since i want relative and not station.

"If you want the altitude to display correctly and have station pressure, you will need the current barometric pressure and enter that as the reference barometric pressure. The reference altitude should remain zero. Sync Alt should be OFF."]

thanks in advance for any help. /guy (73 de kg5gt | wqpz784)

[Gyvate]

"hpa" mode means you change the unit display of pressure in your console to hPa.
See the WS-2902B manual page 22 (Chapter 6.3 set mode). It shows which buttons to press on your console to toggle between pressure (and also other) units.
You can choose between hPa, mmHg or inHg - hpa mode would mean choose hPa as pressure unit. 

by the way - elevation means the elevation of your place above sea-level. If your ground level elevation is 582 m, then you have to add the height between ground level and your console position to get the final altitude needed for the calculation of the offset. The online calculators also offer you to enter the actual average (!) temperature of the past 12 hours (!) at your location to compensate for temperature related pressure changes if you want to walk down the after the decimal point accuracy road (reference: hPa).

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on the picture the airplane "equals" your console

[gszlag]

Gyvate is quite correct. You need to calculate the total elevation of your barometric sensor above sea level.This number is the one the that should have been sent to CWOP. The elevation of the airport is not relevent with any of our calculations.

Since CWOP expects that you will be uploading Altimeter setting(REL value)  we can ignore 12 hour averge temps, air density and whole bunch of complications associated with changes in outside temperatures.

The idea is to match your weather station's Altimeter setting with your airport's Altimeter setting and subtract a relative offset (based on your sensor elevation)to calculate your ABS value. Use the online ISA calculator I mentioned in my original post to calculate your offset (the difference between REL and ABS).

I am not a member of CWOP so I don't know if CWOP calculates anything from your wrong elevation. There should be a mechanism somewhere at CWOP to corect errors in elevation or if a member changes addresses, etc. so I would recomend you contact CWOP and give them your correct sensor elevation.

[golftango]

we must have different manuals: "Version 1.28 ©Copyright 2022, Ambient LLC. All Rights Reserved." there are 4 instances of 'hpa' and none of them address how to see the bp in hpa units. but under the calibration section it says to hold. pressure and wind to see the absolute (station?) reading and i suspect hpa might show up if you do that procedure. but they warn you in 3 separate places that relative calibration is the recommended method.

my elevation is a little tricky as i live in a small hamlet in a very rare hilly area of west texas. of course the hills are not that high but it means you could easily be off by a dozen or more feet. but the figure i feel most comfortable with is 1910 feet for several reasons. it's the figure that pops up most when you search my location, it's the number for the post office which is about 2 blocks away and it's flat between me and it, it's what my kestrel claims is my 'altitude' although as with any altitude instrument it's a dance between bp and altitude, and it's also the number my apple watch ultra gives me and that watch purportedly designed for climbers has way more accurate gps than an iphone or many gps units.

there wasn't a link to the isa online calculator in the original article but i just now searched and discovered that there are dozens of 'isa' calculators scattered around. i used the one at aerospace web and after adding 5 feet for my sensor's height above ground i asked it to show the pressure result in inhg and here's the result: 27.908 inhg.

next i checked the airport and not being a pilot i don't know what elevation figure they use. they give a field elevation of 1790 and a tower elevation of 1940 so i don't know where their sensor is. if it's in the tower i wouldn't have to make much if any correction. i plugged in the field elevation which says it's surveyed and is the figure i think a pilot would see and got a pressure reading of 28.036.

i'm ignoring temperature as suggested by gyvate.

so is this enough? the difference is 28.036 - 27.908 = 0.128 inhg. but that's absolute pressure, right? my console shows 27.99 absolute and 29.94 relative right now. the airport is 29.90 relative right now. but none of these calculators so far offer to convert absolute to relative and i'm lost in all these figures and still don't know how to use them specifically to adjust my console. and hopefully we can leave hpa out of this entirely!

oh, and it appears that cwop only does updates once a week on tuesday, so hopefully my elevation mistake will be corrected next week. i discovered that the level 3 spatial error isn't uncommon because i got a couple more for my dew point reading. evidently madis/cwop compares your readings to neighboring station and tosses out the least 'reliable' or the one that differs most. i only got the dewpoint one for two update cycles and it went away, so i'm hoping that once my entered elevation gets corrected the pressure one will go away as well.

"Barometer
Average barometer error:   0.0 milliBars
Error standard deviation:   0.0 milliBars
These values are within range, so your sensor is probably calibrated correctly."

also, nearly everything you could ever want to help in troubleshooting can be found at this link and it says my barometer is accurate. so i might be getting that error because of errors at one or more nearby stations.

https://weather.gladstonefamily.net/site/AW140?days=3

/guy

[Gyvate]

why do you want to convert or calculate absolute pressure.
Absolute pressure is what your console measures.
Now you only need to find the corresponding relative pressure.
And, depending on what your console requires, enter either the relative pressure or the offset (difference) to the absolute pressure.
For this you need two things:
1. the altitude of your (sensor's) place (=console) above sea level (ground level + height of the barometer above ground)
2. a calculator which converts this altitude and the absolute pressure of your location into the relative pressure.
you can use: https://keisan.casio.com/exec/system/1224575267
enter the abolute pressure your console shows, enter your altitude and stay with 15°C - the calculator will provide the relative pressure at your altitude.
If you need the offset, subtract your absolute pressure (local pressure measured by the console) from the relative pressure calculated
Go to https://ambientweather.com/manuals.html to find your manual.
One of the WS-2902 manuals should match your console ...
then you can set your console to hPa which will give you more accuracy as the steps are smaller than with mmHg or inHg.

[golftango]

i've worked in tech occupations my entire life so i'm comfortable with conversions. but there are way too many different units of measure used in these procedures and by the time you've found ways to convert them to ones you can understand you've lost the plot. the calculator you linked to gives the result in hpa and leaves you on your own to go find another calculator to convert that to inhg. and i do have the right manual for my console which is the 2902D version which is the latest one and it does not show me how to see readings in hpa unless perhaps i do a factory reset and go through the initial setup again. there's literally no way to keep up with what you're doing by the time you've waded through a half-dozen essential data points all in different units of measure and tried to reconcile them.

i suspect i already have my barometer calibrated close enough by using the two airports and my kestrel to yield readings to compare to.

here's all the readings just now in relative and in inhg:
29.94 my console
27.99 my console absolute
29.94 kestrel (nist certified)
27.96 kestrel absolute (0 altitude)

29.94 (cheap local sensor monitor)
29.94 (another cheap local sensor monitor)

29.85 dyess afb (3 miles north)
29.90 kabi local airport (3 miles southwest)

i'm assuming the two airport readings are outliers because they are about 200 feet below me in elevation but i appear to be unable to confirm that's true due to not being able to deal solely with relative readings in inhg units.

/guy

[Gyvate]

I don't see the conversion issue:
set your console to hPa, do the calibration. Once done, if you prefer inHg or mmHg, set the console to these units and the console does the conversion for you.

The reason for using hPa for the calibration (especially in a country which is still one of the few where many don't yet use SI system units) is that the scaling in hPa is more granular - and the console will often only use only digit after the decimal point. Meaning, your results created on hPa basis will be more accurate.
Once the calibration is finished and your preference is other units than mbar/hPa, let the console do the work for you by choosing the unit system of your preference.

Anyway ...
Given that there is anyway an additional impact of temperature, the numbers you have found look reasonably good enough.

i do have the right manual for my console which is the 2902D version which is the latest one and it does not show me how to see readings in hpa unless perhaps i do a factory reset and go through the initial setup again.

I suggest reading and applying page 27 of the WS-2902D manual 
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[golftango]

thanks. and i finally found how to set the console to hpa units in an article on the ambient weather website. and you are not going to believe this, but the reason i couldn't find it is that they spelled it 'hap' in the manual and it's only in one place jammed in with commas and my search wasn't finding it!

ok, so i will be able to see the absolute and relative readings on my console in hpa and i understand what you're saying about accuracy, but none of my sources are going to yield a reading in hpa units and i'd have to convert their relative or absolute readings to hpa manually and thus i'd lose the accuracy. what am i still missing? do i need to return to the calculator and get an elevation-based figure for my location in hpa? then plug in my source airport elevation and get that reading hpa and then subtract them to get an offset?

here's the section in the manual about relative calibration:

"Barometer Calibration
We found an official station "KABI" 12.6 mi away and a difference in elevation of 1371 feet. You can use its relative pressure: 29.83 inHg to configure yours.   Try another"

how they get a 'difference ... of 1371 feet' i have no idea. that airport is only about 200 feet below my 1910 foot elevation.

i did find a way to read that airport data in hpa units (both relative and absolute) so all i need is a conversion for elevation. the pilot's calculator shows it at 1791 feet and i'm at 1910, so the difference is only 1910 - 1791 =119 feet. 

[Gyvate]

forget about the airport
set your pressure display in your console to hPa, then
just take, as said in my earlier post, the absolute pressure your console provides and the altitude of your console
(ground level elevation plus distance to your console - depending on where you have put your console something between one and 16 meters (or more, depending on the house you're living in).
With these two pieces of information go to the keisan link I provided earlier, enter the altitude and the absolute pressure (says Atmospheric pressure there) and press "Execute".
You will then get your relative pressure (sea-level pressure P₀ in the tool).
In your console just change the relative pressure (see the picture from the manual) and set the pressure to what you got as sea-level pressure P₀.
Then you are set.
Prefer to see in numbers in in or mmHg - change the display units (also on the posted page).

[golftango]

ok, so here's what we have.

1013.1 console relative reading at my elevation of 1910 feet
946.6 console absolute reading

1012.0 airport relative reading
1009.4 airport sea level pressure

elevation difference is that i'm 119 feet higher.

ok, i input my abs value of 946.6 hpa and 582 meters into the calculator and got a sea-level value of 1,013.89 hpa. i switched my console back to relative and have 1013.0. which means i was very close already as that's only an 0.89 hpa difference although since i don't work in those units, i don't know how significant that is but i went ahead and calibrated it at 1013.9 because my console only has one decimal point.

i then set it back to relative and let's do the area comparison again:

29.94 console reading
29.94 kestrel
29.92 cheap sensor monitor #1
29.91 cheap sensor monitor #2
29.89 airport i used for reference (would the difference be the elevation difference?)

if i did all this right i think i was so close before that it didn't matter much. but i can't tell you how much i appreciate your patience walking me through this--i was lost despite being a weather spotter for decades and still one to this day. the main point is that now i feel comfortable with my calibration being as accurate as i can make it. as a bonus, i can 'calibrate' those sensor monitors to my console now instead of the airport.

/guy

[Platokidd]

This stuff can be complicated at times. I found a simple way to calibrate pressure and most all the time it matches kestrel.

https://www.starpath.com/calc/Weather%20Data%20Calculators/baroheight.html  Just fill in the blanks.

I calibrate my barometers a few times a year as seasons change. Today used 60f for temp.

This gives me my off-set. Once that data is set in the console I compare to the nearest airport.

If you need to adjust to the airports rel pressure adjust the absolute until the relative pressure matches.