Barometer sensor clarification


#1

I am quite confused regarding the Barometer sensor actual readings and options how to use them, so I would be happy to get some clarification.

There are two values coming from the sensor

  • pressure
  • altitude

Is the pressure, the absolute atmospheric pressure at the present location?
How is the altitude measured? Is it possible to calibrate it?
There is about 100m difference compared to the real altitude and this is similar for all Barometer sensors I have.

I guess, if there is no option to recalibrate, then I should recalculate it somehow, using the altitude shift.

Is the motivation to provide both pressure and altitude, to make it possible to get the barometric pressure at sea level?


#2

The altitude is computed from the atmospheric pressure in the sensor, so do not expect the altitude to remain constant while the pressure changes


#3

BTW, do not assume anything about precision of the sensors from looking at the altitude and referencing it to the typical altitude in your country – I’m living essentially at the SSL


#4

OK, thanks :slight_smile: Hmm I think I am still confused. Is the altitude then just some sort of estimate, perhaps useful in case of having absolutely no clue what the real altitude can be?

And the pressure is the absolute atmospheric pressure at the present location, so we only need information about the altitude to get the barometric pressure at sea level?

Is the deviation in altitude anyhow related to the deviation in pressure?
The thing is I have information from two other weather stations in my home providing pressure 15-18 hPa higher than sensors in BC modules.

And as the deviation seems to be consistent, perhaps it can be also explained and eliminated :slight_smile:


#5

The altitude computation uses the general knowledge that with increasing altitude the atmospheric pressure drops. There is even equation (or several equations) describing what the drop in pressure is.
Now, there exists something call SSL – standard sea level – (https://en.wikipedia.org/wiki/Standard_sea_level) which is essentially a set of physical constants taken as boundary conditions during calculations. These values, or constants to be exact, are just what they are – constants, providing some reasonable and widely accepted standard set of values. This is because the pressure field is variable and depends not only on the altitude, but also on many external factors. In those cases, physicists and people in the national institutes of standards and measures just define some abstract reference point, which is exactly what SSL is.

The sensor obviously does not know what is the current pressure and temperature at the 0 m level (which in real-life is fairly variable and in some sense, because of tidal flow and other things, does not even exist) so it takes the necessary values from the SSL set.
So when you look at the altitude output from the sensor, you have to interpret it in way that takes these things into account, i.e. something like this: "assuming the 0-level is now the same as SSL and assuming the pressure field is homogeneous (and perhaps assuming some other things), then the pressure the sensor indicates is equivalent to me having X meters altitude, relative to the ideal reference abstract location, i.e. the SSL.
You would need GPS to get something independent on the atmospheric pressure.

ad the calibration – that you can do easily manually, I think. There is an offset input on the sensor, but I don’t think it’s exported via the bigclown api. But you should figure out which sensor is more precise – what if the other ones are the bad accuracy sensors? What makes you believe it’s the bigclown sensor that is not measuring right? IIRC, the data sheet specs are quite good.


#6

Right now on Czech Television is Hyde Park, where they have Viliam Vartrt explaining different measurements of altitude related to sea “high” and he unified the measurements worldwide. Just in case … :slight_smile:


#7

Thanks for the detail explanation :slight_smile: The other sensors use some calibration, first (Netatmo) is taking the exact location, the second (WS-2800) in the beginning required reference value of the pressure from the nearest official station. They don’t provide the same results now but the difference is 3-4 hPa.

That’s why I expect the BC sensors without any calibration to be less precise. I got the answers that the altitude is not any kind of help with the calibration. It is just one of two measurements the sensor provides.

So the only thing to be still clarified is how to calibrate the sensors. Do we have anybody with such experience around? Otherwise, it is the time for research and experiments :slight_smile: