Show Relative Pressure instead of Absolute Pressure

[Miguel Ángel]

Hi, I have recently installed WeeWX v3.9.2 in my Raspberry Pi 2, with the PCE-FWS 20. All is working, but this station has Absolute and Relative pressure, and in the section Barometer, WeeWX show me the Absolute pressure and I want to see the Relative pressure.

I have tried changing names of sensors in index.html, but doesnt work, and I dont know how to change the source of the barometer. I use NeoWX, as skin (Standar show absolute pressure too).

Miguel. 

[Thomas Keffer]

Take a look at the Wiki article Barometer, pressure, and altimeter, and see if that answers your question. If not, perhaps you can tell us what you mean by "relative pressure."

-t

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[Miguel Ángel]

Thanks for your reply Thomas, I had read this article and that's why I changed the names of sensors to try something. With relative pressure I want to say the absolute pressure plus an offset. (for example, the absolute pressure show 950hPa, and the relative pressure show 1011hPa, both in the station, but weewx only show 950hPa.) Actually I am not an expert in this topics, but I've always seen the pressure on these figures (1000-1020hPa), and now I get lost (with 940-960hPa).

Miguel.

El domingo, 22 de septiembre de 2019, 14:53:42 (UTC+2), Thomas Keffer escribió:

Take a look at the Wiki article Barometer, pressure, and altimeter, and see if that answers your question. If not, perhaps you can tell us what you mean by "relative pressure."

-t

On Sun, Sep 22, 2019 at 5:43 AM Miguel Ángel <milo...@gmail.com> wrote:

Hi, I have recently installed WeeWX v3.9.2 in my Raspberry Pi 2, with the PCE-FWS 20. All is working, but this station has Absolute and Relative pressure, and in the section Barometer, WeeWX show me the Absolute pressure and I want to see the Relative pressure.

I have tried changing names of sensors in index.html, but doesnt work, and I dont know how to change the source of the barometer. I use NeoWX, as skin (Standar show absolute pressure too).

Miguel. 

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[Andrew Milner]

I believe that with your station absolute pressure is the pressure as seen at your station.  relative pressure on your station you will have manually set in your station by adjusting for altitude (and will usually be a higher value).  The station only outputs the absolute value to weewx - and this is called pressure.  weewx will use the altitude specified in weewx.conf to calculate the altimeter value - which should correspond to 'relative' pressure on your station.  barometric pressure also involves a temperature compensation.  if you want to see the same value on your webpages as the relative value displayed on the station then use altimeter in the webpages.

the correct value to display though is barometer.  for this to be correct the station altitude must be specified in weewx.conf

[Thomas Keffer]

I think I understand what you are asking.

By default, the Fine Offset driver reports only absolute pressure. WeeWX uses this, as well as altitude and temperature, to calculate "altimeter" and "barometer". Relative pressure is not emitted by the driver at all.

If for some reason you really want "relative pressure", here's what you would need to do.

1. Figure out the relative pressure offset used by your console. From your description, it sounds like it is 1011 - 950 mbar, or +61 mbar.

2. Figure out what unit system you are using. This is under section [StdConvert] in weewx.conf. Unless you changed the default, this will be "target_unit = US".

3. Convert the offset you got in step #1 to this unit system. For example, +61 mbar is +1.801 inHg in the US system.

4. Use this number for a new [StdCalibrate] expression in weewx.conf. For example, enter this in your weewx.conf:

[StdCalibrate]

  [[Corrections]]

    rel_pressure = pressure + 1.801

Restart weewx.

Now you will have relative pressure as observation type "rel_pressure". You can then use this in your template as $current.rel_pressure. If you want historical relative pressure as well, you will have to add rel_pressure to your database schema.

But, frankly, I don't know why you would want to do this. The Fine Offset "relative pressure" is a poor substitute for altimeter and sea-level pressure. 

-tk

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[Miguel Ángel]

Thanks Andrew and Thomas for your replies and your help, now I think I am near of my solution. Okey, let me tell you the news.

First I have checked the current info about the station with wee_device --current, and the relative pressure doesnt appear, only the absolute, in this point I figured I would have to calculate it.

Second I did that you say, Thomas, and added rel_pressure = pressure + 1.801 and edit the template, but then, WeeWX show 20.02345324 (I dont remember the decimals exactly), a lot of decimals and not the unit (hPa).

Then, I added rel_pressure = pressure + 984.59 (the difference) and now, it show the pressure I want but with a lot of decimals and without the unit, and I dont know yet if this data will be good in the future or next days.

I adjunt a screenshot about the status of barometer now, and the output of wee_device --current.

And yeah, I know that Absolute Pressure is more "reliable", but I want to see the relative pressure because in my country, all is expresed in relative pressure, Meteo Channels, basic meteo stations, etc and I am accustomed to see it.

Here is an example of WeeWX working near of my country (http://www.colladojermoso.com/colladojermoso.com/meteo/meteo/index.html), it show the relative too.

Sorry if my english is not so good, it isn't my native language.

Miguel.

[Thomas Keffer]

Your weather station is based on the Fine Offset series of weather stations. It has a very simple way of correcting for altitude: it simply adds a constant to the absolute pressure, to produce what it calls "relative pressure." This is not what commercial or government weather stations do. They use much more complicated formulas. 

WeeWX also uses these more complicated formulas. The result is "altimeter", corrected for altitude only, and "barometer" corrected for altitude, temperature, and humidity. These are what you should use to compare with Meteo channels, etc.

The "relative pressure" produced by the Fine Offset series is generally worthless. I don't know why anyone would want it.

-tk

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