Re: [weewx-user] Re: Barometric pressure correction the right way

[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

On Wednesday, 1 May 2019 12:00:25 UTC-7, V. Kelly Bellis wrote:
>
> @Louis De Lange, thanks for mentioning this. By any chance, do you recall 
> where you read that FineOffset was using indoortemp for pressure 
> calculations?
>
> On Sunday, January 14, 2018 at 2:35:33 PM UTC-5, Louis De Lange wrote:
>>
>> 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.  
>>
>

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Re: [weewx-user] Re: Barometric pressure correction the right way

[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?

On Sunday, January 14, 2018 at 2:35:33 PM UTC-5, Louis De Lange wrote:
>
> 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.  
>

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Re: [weewx-user] Re: Barometric pressure correction the right way

[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!!

On Sunday, 14 January 2018 21:35:33 UTC+2, Louis De Lange wrote:
>
> 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
>
> On Sunday, 14 January 2018 07:17:31 UTC-8, Greg Troxel wrote:
>>
>>
>> Andrew Milner  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. 
>>
>

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Re: [weewx-user] Re: Barometric pressure correction the right way

[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

On Sunday, 14 January 2018 07:17:31 UTC-8, Greg Troxel wrote:
>
>
> Andrew Milner  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. 
>

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Re: [weewx-user] Re: Barometric pressure correction the right way

[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

On Sun, Jan 14, 2018 at 7:17 AM, Greg Troxel  wrote:

>
> Andrew Milner  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.
>
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Re: [weewx-user] Re: Barometric pressure correction the right way

[Greg Troxel]

Andrew Milner  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.

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Re: [weewx-user] Re: Barometric pressure correction the right way

['ml' via weewx-user]

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

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Re: [weewx-user] Re: Barometric pressure correction the right way

[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 ..



On Saturday, 13 January 2018 23:38:33 UTC+2, gjr80 wrote:

> 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
>

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Re: [weewx-user] Re: Barometric pressure correction the right way

[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

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Re: [weewx-user] Re: Barometric pressure correction the right way

[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"  > wrote:
>
> Ok i got itwith 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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>
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RE: [weewx-user] Re: Barometric pressure correction the right way

[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" 
wrote:

Ok i got itwith 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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RE: [weewx-user] Re: Barometric pressure correction the right way

['ml' via weewx-user]

Ok i got itwith 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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RE: [weewx-user] Re: Barometric pressure correction the right way

[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

From: 'ml' via weewx-user
Sent: 13 January 2018 17:54
To: weewx-user
Subject: Re: [weewx-user] Re: Barometric pressure correction the right way

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

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Re: [weewx-user] Re: Barometric pressure correction the right way

['ml' via weewx-user]

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

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Re: [weewx-user] Re: Barometric pressure correction the right way

[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

On Fri, Jan 12, 2018 at 10:13 AM, Andrew Milner  wrote:

> http://keisan.casio.com/exec/system/1224575267#!
>
>
> On Friday, 12 January 2018 19:35:01 UTC+2, ml wrote:
>
>> Can you Tell meine the Formula? So i can calculat by myself?
>
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Re: [weewx-user] Re: Barometric pressure correction the right way

[Andrew Milner]

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


On Friday, 12 January 2018 19:35:01 UTC+2, ml wrote:

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

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Re: [weewx-user] Re: Barometric pressure correction the right way

['ml' via weewx-user]

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

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Re: [weewx-user] Re: Barometric pressure correction the right way

[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-user@googlegroups.com> wrote:

> Barometer value is calculated by Software. Can i Pick which Temperature
> ist used?
>
> Thanks
>
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Re: [weewx-user] Re: Barometric pressure correction the right way

[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)



On Friday, 12 January 2018 12:19:10 UTC+2, ml wrote:

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

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RE: [weewx-user] Re: Barometric pressure correction the right way

['ml' via weewx-user]

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

Thanks

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RE: [weewx-user] Re: Barometric pressure correction the right way

[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

From: 'ml' via weewx-user
Sent: 12 January 2018 11:49
To: weewx-user
Subject: [weewx-user] Re: Barometric pressure correction the right way

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

Am Freitag, 12. Januar 2018 09:05:07 UTC+1 schrieb 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 ???
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[weewx-user] Re: Barometric pressure correction the right way

['ml' via weewx-user]

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

Am Freitag, 12. Januar 2018 09:05:07 UTC+1 schrieb 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 ???
>

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[weewx-user] Re: Barometric pressure correction the right way

[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!!



On Friday, 12 January 2018 10:05:07 UTC+2, ml wrote:

> 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 ???
>

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