Barometric pressure correction the right way

[ml]

Hello, I need to correct the barometric pressure it is always 2 mbar to high regarding the other station arround (I compare barometric pressure with barometric pressure). But the absolut pressure seems to be right according to other devices in my envoiremnt. How do i correct it ???

[Andrew Milner]

When you compare to other stations in the area are they at EXACTLY the same altitude??  25 ft higher = 1 mb diffence.  Is your altitude set correctly for the altitude of the sensor?

If, after checking those items you can adjust the barometer (or pressure or altitude) in weewx by using the qcalibrate section of weewx.conf   

Remember that barometer is also temperature relevant.  Is your absolute pressure reading being taken at outdoor or indoor temperature - it makes a difference!!  If weewx is calculating barometric pressure it will use outdoor temperature from 12 hours ago I believe and the altitude defined in weewx.conf - errors in either could also cause correct absolute pressure but incorrect barometric pressure.

ie tread carefully before compensating and make sure it is indeed incorrect before compensating!!

[ml]

The pressure is measured inside how can i change the temperature which is used for the calculations?

[Andrew Milner]

You cant.

 

If you are just trying to make your readings the same as others in the immediate area just use the calibrate section and adjust either pressure or barometer depending on your other settings in weewx.conf

Eg if barometer is software generated then just adjust pressure whilst if it is hardware generated the adjust barometer if you are confident the absolute pressure is correct

 

The choice and options are all yours to pick between

 

 

 

Sent from Mail for Windows 10

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[ml]

Barometer value is calculated by Software. Can i Pick which Temperature ist used?

Thanks

[Andrew Milner]

You can change whatever formula you wish - it is open source software after all, but you will lose that change whenever weewx is updated (which is often a couple of times per year)..

Simplest just to say pressure = pressure - 2 (or whatever is the difference you require) in the calibrate section of weewx.conf  That is why the section exists after all!! You can only adjust data supplied by the station ie pressure and not derived entities eg barometer.

Have you checked and double checked your altitude in weewx.conf - that could also be 'tweeked' to force a change to barometer and 2 mb does not need much altitude change!!  GPS altitude can often be out, and i do not know if google earth uses ground level or height at top of roof for example (or even how accurate its altitude readout is)

[Thomas Keffer]

A few questions:

What hardware are you using? 

As Andrew notes, are you use that your altitude has been set correctly? Are you sure that the other station's altitude has been set correctly? What is that altitude?

Is the other station showing sea-level pressure (what weewx calls "barometer") or altimeter? Sea-level pressure (SLP) is pressure corrected for both the effects of altitude and air density. Perhaps the other station is showing "altimeter," which has been corrected only for altitude. See the Wiki for details.

-tk

On Fri, Jan 12, 2018 at 2:19 AM, 'ml' via weewx-user <weewx...@googlegroups.com> wrote:

Barometer value is calculated by Software. Can i Pick which Temperature ist used?

Thanks

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[ml]

Can you Tell meine the Formula? So i can calculat by myself?

[Andrew Milner]

http://keisan.casio.com/exec/system/1224575267#!

[Thomas Keffer]

Which formula?

All of the formulas used internally by weeWX are documented in the source code. In particular, look at wxformulas.py and uwxutils.py. 

-tk

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[ml]

Sorry i didn't get how the barometric pressure is calculated with the temperature, altitude and absolute pressure can you help?

[Andrew Milner]

Have you looked at the code as Tom suggested??

 

On the link I gave you there is a formula for the calculation which may help

 

 

 

Sent from Mail for Windows 10

 

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[ml]

Ok i got it....with the outside temperature value 3.2°C is 1025mbar with the inside temperature 23.2°C it is 1020mbar so what is now correct air pressure is measured inside

[Thomas Keffer]

The inside temperature does not enter the calculation. You are correcting for the weight of the air column outside, which is a function of outside density, which is a function of outside air temperature.

-tk

Fat-fingered from my Android

On Jan 13, 2018 8:59 AM, "'ml' via weewx-user" <weewx...@googlegroups.com> wrote:

Ok i got it....with the outside temperature value 3.2°C is 1025mbar with the inside temperature 23.2°C it is 1020mbar so what is now correct air pressure is measured inside

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[Andrew Milner]

Tom - this has always baffled me - when the pressure reading is taken from an indoor console which is at a different temperature than outdoors.  Maybe Tom knows the answer - if one carries a console from inside at 20 degrees o outside at 3 degrees does the pressure recorded on the console change?  If it does then the 'pressure' reading from the console needs some adjustment for temperature change between indoors and outdoors before adjusting for altitude (altimeter) and outside temperature (barometer).

Or perhaps I am just misunderstanding (as usual).  The issue would not be an issue if the pressure reading was taken from an instrument already at outside temperature of course, but most pressure readings seem to come from indoor consoles rather than external instrument clusters.

On Saturday, 13 January 2018 20:50:27 UTC+2, Tom Keffer wrote:

The inside temperature does not enter the calculation. You are correcting for the weight of the air column outside, which is a function of outside density, which is a function of outside air temperature.

-tk

Fat-fingered from my Android

On Jan 13, 2018 8:59 AM, "'ml' via weewx-user" <weewx...@googlegroups.com> wrote:

Ok i got it....with the outside temperature value 3.2°C is 1025mbar with the inside temperature 23.2°C it is 1020mbar so what is now correct air pressure is measured inside

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[gjr80]

As far as I was aware the difference between pressure inside and pressure outside is negligible (unless of course you live in a sealed house). So deriving barometer and altimeter using inside pressure rather than outside pressure has negligible effect. Of course if you use inside temperature there may well be a significant difference to outside temperature so barometric pressure could vary significantly between inside and outside.

Gary

[Andrew Milner]

I ask the question because whenever I put the heating on I see a change to the barometer readings on my graphs which implies to me that the pressure is changed (it appears to fall as the temperature rises).  My console is 2/3 up the wall (too high to even read, but good for sensor reception!) and so warms quite rapidly as the hot air from the heating rises making the pressure change quite noticeable.  I have always put it down to being a quirk of FineOffset and have ignored barometer changes which appeared to be indoor temperature change related. However the variation does lead on to my conclusions re compensating indoor pressure readings to match outdoor temperatures.  Maybe I need to investigate this more and try and correlate 'pressure' to indoor temperature directly .......  I suspect it may be related to temperature differentials between temperature inside the pressure sensor and temperature outside the sensor ie if the temperature is uniform all is fine but when there is a difference the pressure reading is erroneous - which of course would mean that weewx is fine and the pressure sensor is naff!!!!

Why am I such a pedant I ask myself ......

[ml]

Would be nice to have it AS a Option in the next release. Thanks

[Greg Troxel]

Andrew Milner <andrew.s...@gmail.com> writes:

> I ask the question because whenever I put the heating on I see a change to
> the barometer readings on my graphs which implies to me that the pressure
> is changed (it appears to fall as the temperature rises). My console is

This seems like a sign of trouble. I see no effect on pressure readings
due to inside temperature changes or heating system on/off (with a
Vantage Pro 2), and I wouldn't expect any. (I just looked at last
night's data, and the heating system behavior is obvious from inside
temperature.)

> 2/3 up the wall (too high to even read, but good for sensor reception!) and
> so warms quite rapidly as the hot air from the heating rises making the
> pressure change quite noticeable. I have always put it down to being a
> quirk of FineOffset and have ignored barometer changes which appeared to be

That is a good theory :-)

> indoor temperature change related. However the variation does lead on to my
> conclusions re compensating indoor pressure readings to match outdoor
> temperatures. Maybe I need to investigate this more and try and correlate
> 'pressure' to indoor temperature directly ....... I suspect it may be
> related to temperature differentials between temperature inside the
> pressure sensor and temperature outside the sensor ie if the temperature is
> uniform all is fine but when there is a difference the pressure reading is
> erroneous

There are two things to understand about your situation.

One is if the inside pressure and the outside pressure are actually
different. This seems very unlikely. Surely someone on this list has
set up pressure sensors in and out and compared them - hopefully they'll
admit to that and share results :-)

The other is which device is doing the reduction to sea level pressure
and if it is doing it correctly. This is about the weight of the air
and is related to the outside temperature only. It seems this is done
by weewx.

https://github.com/weewx/weewx/wiki/Barometer,-pressure,-and-altimeter

So I don't understand why you are having trouble. I would look at the
raw station pressure data. If that is changing correlated with the
heating system, then either it really is changing or your station is
behaving funny.

It might help to post graphs of computed baromtric pressure and inside
and outside temperate, along with barometric presssure reported by a
nearby station.

[Thomas Keffer]

​The gauge in your station always reads absolute pressure.​ But, to compare it everyone else's barometer, which may or may not be at the same altitude, you "reduce" is to sea-level altitude. That corrects for altitude, and improves your ability to compare your pressure to a nearby neighbor. The result is commonly called "altimeter pressure." Pilots refer to it as QNH.

But, it turns out air pressure is also a function of the density of the air column above. A lot of hot air between sea-level and your station means lighter air, and the correction should be a little less. A lot of cold air, and it should be a little more. Because nearby stations tend to be at the same temperature, this is a small error when comparing to your neighbors, but it can be a massive correction when calculating basin-wide horizontal pressure gradients across temperature fronts. If you do this correction, you have what is commonly called "sea-level pressure" (SLP), or what pilots call QFF. 

Note that you are correcting for the density of the outside air. The inside temperature has no effect. 

If your barometer changes with varying inside temperature it's because it was not temperature compensated properly. It's a flaw in the instrument. You could correct for that, but that's a whole different issue having everything to do with instrumentation and nothing to do with atmospheric sciences.

Hope that helps.

-tk

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[Louis De Lange]

As a previous owner of a Fine Offset station I can confirm that I noticed this same behavior.  Living in Canada where our homes are heated, I could clearly see the altimeter reading (my station did not support barometer correction to sea level) seesaw with the furnace operating and shutting down.  

Reading up on the issue it appeared that the Fine Offset hardware was doing a temperature correction internal to the indoor unit, but that quick changes to the indoor temperature threw the reading off.  There is no way that you can correct for that with calibration - it is a flaw in the hardware.  

This and other quirks in the Fine Offset hardware eventually drove me to buy a Vantage Pro and be done with it.  If you are a stickler for accuracy you will save yourself o lot of frustration by getting a Vantage Pro

[Andrew Milner]

Louis - thanks for restoring my sanity!!  Yes, it is a FineOffset which I have got.  In the summer I see the issue when I turn the AC on and in winter when I light the kerosene stoves!!  So, it's a faulty station design - which was, I will admit, a conclusion I was coming to following Tom's patient answer.  I suspect the issue is that the sensor is not insulated well enough - but if the same sensor is used for temperature - which would not require the same insulation - the issue is unresolvable!!  Maybe I will try and get a separate pressure gauge and take readings from that instead!!

[V. Kelly Bellis]

@Louis De Lange, thanks for mentioning this. By any chance, do you recall where you read that FineOffset was using indoortemp for pressure calculations?

[Louis De Lange]

I do not recall where I came across the fact that it is temperature compensated. At some point I thought perhaps the indoor sensor was trying to provide a barometer reading and I placed it outside in a sheltered area, and imported it into weewx as barometer and still it was all over the place.  Recently I build a constant temperature box and placed the sensor inside, and for the first time it followed my local airport altimeter exactly.  

  Conclusion - garbage.

I ended up building a BYOD station which I am very happy with