BMP180 Barometric pressure sensor - deviates about 8 full points from expected

Hello all,
I've bought a BMP180 barometric pressure sensor and used an existing
implementation of the conversion formule (as in the PDF mentioned below) to
see it work.
The problem is that the air pressure result is (currently) about 8 full
points off from what I could find on the 'Web that it should be. The
resulting temperature is in the middle of two other (though not as precise)
thermometers, and therefore looks to be ok.
I've made sure that the calculation is correct by taking the calibration, UT
and UP values straight from the PDF here:
formatting link

pull them thru the calculation and getting the exact intermediate results as
well as T and P values back as shown in the PDF.
My question:
Is the sensor supposed to be able to give that different results (internet
shows 1008, 1009 hPa around my position, the sensor shows 1000.47) ? I
thought that the calibration process and resulting data would not allow for
it ? What might be causing it ?
Regards,
Rudy Wieser
Reply to
R.Wieser
Loading thread data ...
In message , R.Wieser writes
Are you allowing for altitude ?. I was confused with my BME280, thinking it was under reading until I did a bit of reading up. Pressures usually seem to be quoted at sea level, and as you climb, the pressure drops. I'm at about 100M above sea level, and I was surprised by the drop over that height.
Adrian
--
To Reply : 
replace "bulleid" with "adrian" - all mail to bulleid is rejected 
 Click to see the full signature
Reply to
Adrian
The air pressure decreases around 1 hPa / 27 ft of altitude at surface altitudes. Are you at sea level?
--

-TV
Reply to
Tauno Voipio
Adrian,
Nope, forgot all about that. :-|
I'm 7 stories up, and allowing for the building being just a tiny bit above sea level (I'm from the Netherlands, which is almost /under/ sea-level :-) ) I should be at between 18 and 23 meters up.
Do you perhaps still know what the difference in pressure was for you ? Currently it looks that my 20 meters altitude translates to almost 8 points.
Regards, Rudy Wieser
Reply to
R.Wieser
In message , R.Wieser writes
My local airfield is giving 1004hPa (at 29m altitude), my BME280 is giving 994, so using the 1hPa/27ft quoted by Tauno Voipio, that suggests a 270ft (~93m) difference in altitude, whilst the map suggests 71m. Which doesn't sound right, but the trend is right.
Adrian
--
To Reply : 
replace "bulleid" with "adrian" - all mail to bulleid is rejected 
 Click to see the full signature
Reply to
Adrian
The vertical gradient isn't so precisely fixed, it depends on the temperature. Approximately varies between 11 and 13 Pa per metre at sea level, which means the error might be more than 10% right from the start. See
formatting link
Reply to
A. Dumas
Tauno,
I fully forgot about that. And no, I'm not. I'm 7 stories up, with the base of the building likely at sea level, or less than 5 meters up (there is a canal close by).
But something is still odd: that 27 ft translates to about 8 meters, and even when I'm generous thats adding about 3 hPa. That still leaves about 4 hPa unaccounted for ...
Regards, Rudy Wieser
Reply to
R.Wieser
Adrian,
I did some more googeling, and found an Arduino project* for the BMP180 which incuded a formule for altitude correction (and vise-verse). The result was that I'm now only 4...5 full points away from what it should be.
*
formatting link

Going the other way around it results me being at about 50 meters over sea level, instead of about 20 - and that doesn't seem right (I don't think my ceiling is 7 meters up. :-) ).
Regards, Rudy Wieser
Reply to
R.Wieser
On Tue, 30 Jun 2020 19:11:11 +0200, "R.Wieser" declaimed the following:
I believe weather reports (I presume that is what you are looking up) report sea level equivalent pressure. Have you compensated for your altitude?
Standard (not accounting for weather) sea level pressure is 29.92 inHg or 1013 mbar/hPa. 500 ft (153m) above sea level, the measured pressure would be 29.38 inHg or 995 mbar. I'm sure it is not a linear equation
formatting link
but for the first difference, that is coming out to 8.5 m/mbar (or 27.8 ft/mbar). You are stating an 8 mbar difference, which would correspond to an altitude of 68m (222 ft) above sea level.
{Strange -- Topo map shows my house at 650ft, calibrating my GPS altimeter for 650ft gives barometer 29.94 -- but setting my analog altimeters for 29.94 is showing altitude of 725ft. I suspect it is lack of temperature compensation -- the analog altimeters were consumer grade: Thommen Altitrek and Sun Altimeter 203... A few years ago I looked for the pro-grade Thommen, but all Thommen units had been discontinued. The pro-grade version included thermal compensation}
--
	Wulfraed                 Dennis Lee Bieber         AF6VN 
	wlfraed@ix.netcom.com    http://wlfraed.microdiversity.freeddns.org/
Reply to
Dennis Lee Bieber
[]
OP: Don't forget to check the datasheet for the accuracy:
formatting link

Dennis: There are two altitudes which can be quoted:
MSL: above mean sea level
HAE: height above reference ellipsoid
In Edinburgh there's a 50m difference according to CGPS on two systems here, and that seems too high to me.
formatting link
--
Cheers, 
David 
 Click to see the full signature
Reply to
David Taylor
Altitude?
meteorogically all barmoeters have to be adjusted for that. Just as alitimeters have to be adjusted for meteorological pressure
--
"Women actually are capable of being far more than the feminists will  
let them."
Reply to
The Natural Philosopher
Modern houses are roughly 2.5 meters per storey.
What is the tolerance on the sensor anyway?
--
"The great thing about Glasgow is that if there's a nuclear attack it'll  
look exactly the same afterwards." 
 Click to see the full signature
Reply to
The Natural Philosopher
As most all of you mentioned, I foolishly forgot all about not being at sea level and that pressure goes down the higher up you are.
So I adjusted for that height too - only to conclude I still missed 4 hPa.
Yeah, right ... After some more pondering I realized that I was simply chugging on in the direction I entered this question with, and I should probably look at it from the other direction.
Assuming that the readout of the sensor is correct (which is an easy assumption, seeing that its carefully calibrated) I must conclude that my (roughly estimated) altitude is the one thats incorrect. That I compared my sensors readout with some data from the internet which was #1. rounded to whole numbers #2. had to be interpolated for my current location will surely not have helped either I also just took the canals waterlevel as being at sea level. Which ofcourse isn't quite true.
An around 32 meter difference (4 hPa difference between my altitude-adjusted pressure and the one I guestimated from the internet) does seem like a lot, but missing a few meters here and there quickly adds up.
Grumble ... As the powers-that-be have not placed a weatherstation next to my building there is ofcourse no way to do some direct un-compensated data comparision ... :-)
Thanks for the help guys.
Regards, Rudy Wieser
Reply to
R.Wieser
I refer you to A Dumas' point elsethread - to quote:
Add that 10% to your other sources of error and I think you'll find you're doing quite well to be as close as you are.
--
Steve O'Hara-Smith                          |   Directable Mirror Arrays 
C:\>WIN                                     | A better way to focus the sun 
 Click to see the full signature
Reply to
Ahem A Rivet's Shot
For a rough and ready altitude, GPS in your smart phone can tell you to within about 10 meters
formatting link

I got reading from 95m -115m...
--
  ?A leader is best When people barely know he exists. Of a good leader,  
who talks little,When his work is done, his aim fulfilled,They will say,  
 Click to see the full signature
Reply to
The Natural Philosopher
Ahem,
...
Thanks. 32 meters (4 hPs) +/- 10% still sounds like a lot though (wouldn't want to jump it, not even into water).
Regards, Rudy Wieser
Reply to
R.Wieser
On Wed, 1 Jul 2020 10:07:21 +0200, "R.Wieser" declaimed the following:
Well, there IS the act of taking the unit down to ground level, to minimize estimated building height.
You didn't give a more precise location earlier, so looking up elevations isn't going to be that useful (from my view).
formatting link
has the Amsterdam region varying from -20 (airport to the west) to +60 feet (a +70 ft spire near "Amsterdam-Slatervaart" [if I read the map correctly]).
--
	Wulfraed                 Dennis Lee Bieber         AF6VN 
	wlfraed@ix.netcom.com    http://wlfraed.microdiversity.freeddns.org/
Reply to
Dennis Lee Bieber
Dennis,
I was thinking of that too, but as that would not give me any kind of a (new) reference I did not want to roll out my extension cord for that :-)
Alas, I got JS disabled on my machine, which makes that site rather unusable to me.
Question: Assuming that I put another OS installation on a new MicroSD card specifically for browsing tha intarwebz, is there any way to erase /all/ of the data Chromium stores ? Having to reinstall everything every time isn't quite my thing ...
Regards, Rudy Wieser
Reply to
R.Wieser
On Wed, 1 Jul 2020 18:49:21 +0200, "R.Wieser" declaimed the following:
On what machine? Location of files may vary.
I run Firefox on Windows... However, since I have (for another reason) an R-Pi running in a VNC viewer, I will comment that Chromium has, on the "..." drop-down menu:
New incognito window
which opens to (much better formatted in the browser )...
""" You?ve gone incognito Now you can browse privately, and other people who use this device won?t see your activity. However, downloads and bookmarks will be saved. Learn more
Chrome won?t save the following information: Your browsing history Cookies and site data Information entered in forms Your activity might still be visible to: Websites you visit Your employer or school Your internet service provider """
Or... delete everything in .config/chromium (if on an R-Pi or other Linux system)
pi@rpi3bplus-1:~$ ls .config/chromium BrowserMetrics-spare.pma InterventionPolicyDatabase SafetyTips CertificateRevocation 'Local State' ShaderCache CertificateTransparency MEIPreload SingletonCookie 'Crash Reports' NativeMessagingHosts SingletonLock Default OriginTrials SingletonSocket Dictionaries 'Safe Browsing' SSLErrorAssistant FileTypePolicies 'Safe Browsing Cookies' 'Subresource Filter' 'First Run' 'Safe Browsing Cookies-journal' pi@rpi3bplus-1:~$
Though I haven't tested to see if some of those directories/files are needed to even run Chromium. "Default" is likely the current user specific data.
--
	Wulfraed                 Dennis Lee Bieber         AF6VN 
	wlfraed@ix.netcom.com    http://wlfraed.microdiversity.freeddns.org/
Reply to
Dennis Lee Bieber
Dennis,
My apologies. An RPi (ofcourse), most likely using noobs 3.2.x (easy to do a full reinstall with).
You can even do that from the comandline or a shortcut
Its just that, looking at who the major developers of Chromium are, I'm not too sure that either that "Incognito" mode or the "temporary profile" one work as advertised. ("evercookies" and "User IDs for advertising purposes" come to mind). Hence my question in regard to erasing /every/ file it writes to.
Regards, Rudy Wieser
Reply to
R.Wieser

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.