Air Pressure needs calibrating?

[Supermop]

Hi everyone.
My first post, so firstly hello and look forward to browsing around and learning lots from everyone here.
I bought on a bit of a whim a "Professional Weather Station" from Maplins and got the bug within hours, and within a week ordered the PC-connected version (which I understand to be the WS-1081 rebadged).

Set it up and installed Cumulus on my Netbook (A very professional piece of s/w that is a credit to the author, have to say )

I have also setup connections to Twitter, PWS Weather and Underground Weather and it is the 'PWS Weather' website info that is prompting my question ....

Looked at other PWS's fairly close to me (10-15 miles) and their pressures are a lot lower then mine. Do I need to calibrate the pressure reading in some way? Currently I am reading a pressure of 1014.6mb. Those other two guys have readings of 1002 and 1004 so I would have thought mine should be similar?

Probably worth confirming that my PWS IS set for REL pressure (the ABS reading right now would be 996.2)

If it helps, this is the PWS url for my Station - http://www.pwsweather.com/obs/BIRGHAM.html

Thanks in advance

David.

[robynfali]

Go to "Configuration" --- "Calibration"

And if yours is appearing a lot higher, you need to put a negative number in the pressure area, take an average of those around you maybe?

Another couple of things to bear in mind, is HOW far they are from you, and if you know if they are any higher/lower to sea level than you are

[robynfali]

take a look at this wunderground map around your area for an idea

[Orion]

HI David
First you need to set up the station pressure as on page 10(3) of the manual
Touch the pressure section 3 times and use the + and _ keys to set the value you require
If you find the pressure for 2 or 3 local stations and set your Wh1080 station the same you should not need any offsets

[steve]

It's best to avoid the pressure calibration setting in Cumulus, there are very few situations where you should need it. By far the best approach is to get your station to supply the correctly calibrated value. If your pressure is too high, then it sounds like the offset in the station has been set too high.

[robynfali]

oops, my bad, sorry Steve, think i had better do it in mine then

[steve]

oops, my bad, sorry Steve, think i had better do it in mine then

It's not exactly "wrong" to do it in Cumulus, it's just "better" to do it with your station. Then your console also displays the correct value, for example.

[Supermop]

thanks for the replies, chaps.

Of the two I looked at, one is around 13 miles directly east and very near North Sea Coast and probably a little lower elevation.
The other is about 10 miles NNW and I would guess a little higher elevation.

I will get some more detailed info re mine and their elevations and then apply a suitable adjustment as recommended

Cheers

[Ned]

It's not exactly "wrong" to do it in Cumulus, it's just "better" to do it with your station. Then your console also displays the correct value, for example.

I did that initially. Then when I had to reboot the station I couldn't be bothered trying to program that thing again
Using Cumulus to store the value is my easy way around that.....nothing at all to do next time (and I don't refer to the station for pressure)
Unless I'm missing something?

[hungerdunger]

Of the two I looked at, one is around 13 miles directly east and very near North Sea Coast and probably a little lower elevation.
The other is about 10 miles NNW and I would guess a little higher elevation.

If they are "professional" weather stations they should already be displaying relative pressure, so their elevation is irrelevant.
Try to wait for a day when the pressure is similar over your area so you can be fairly sure that the relative pressure at your station is the same as neighbouring ones. I find this chart quite useful for forecasting the right time, although there are others.

[steve]

Using Cumulus to store the value is my easy way around that.....nothing at all to do next time (and I don't refer to the station for pressure)
Unless I'm missing something?

As long as the station always comes up in a defined state, i.e. with the same pressure offset (zero?), then using the calibration setting is probably OK. It's more of an issue with stations which actually use the pressure themselves to determine other values, e.g. Davis stations.

Note that if you do want to do the sea-level compensation in Cumulus for Fine Offset stations, rather than in the station, a safer way of doing it is to use the EWpressureoffset setting in cumulus.ini. That way, the offset in Cumulus overrides the offset in the station, rather than being in addition to it.

[Station House]

I'm embarassed to admit 3 months in to the weather watching community that i totally missed the requirement to manually set the relative pressure at first set up

I know I should have read the manual more carefully, but having worked out whether to display absolute or relative pressure I stupidly expected the Station to already be calibrated to report the current pressure accurately - how wrong can one man be, but I guess the difference between an £80 unit and a £500 one! [subsequent edit - please ignore - Station does measure absolute pressure without requiring user calibration]

Whilst of course the trend movements whatever the start calibration pressure is have validity, there is nothing like having it show the 'correct' data, so huge thanks to Supermop for the pressure chart link - very useful.

Anyway, now recalibrated, my pressure chart now looks like a cliff face but I'll live with that as the trends will sort themselves out over the forthcoming days.

Just one question please Steve - in Cumulus, Station settings - does the altitude perform a function? I was presuming that this was used to enable to Cumulus to calculate the relative pressure based on the absolute pressure being measured by the Station.

Am now questioning that assumption, if the Station is in fact feeding Cumulus the Relative Pressure to start with?

A second daft one - I had expected such a dramatic pressure alteration (from 1028 to 1008 hba) to cause the weather trend symbol to start flashing withrain clouds - but it has stayed as it was (sunny symbol). A bit much to expect Steve to explain the inner workings of this product but given his knowledge I thought I'd raise - is it possible that a manual alteration is 'ignored' by the station when using pressure trends/speed of change to drive the forecast indicator?

[steve]

Just one question please Steve - in Cumulus, Station settings - does the altitude perform a function?

Currently the only thing Cumulus uses that for is for solar radiation calculations, with stations that have that ability.

I was presuming that this was used to enable to Cumulus to calculate the relative pressure based on the absolute pressure being measured by the Station.

As all stations supply sea-level pressure, one way or another (assuming they've been set up correctly), Cumulus doesn't bother trying to calculate it.

Am now questioning that assumption, if the Station is in fact feeding Cumulus the Relative Pressure to start with?

Fine Offset stations supply the absolute pressure and, separately, the offset to add to that to get the (more useful) relative pressure. Because of the occasional problem where the offset is corrupted when Cumulus reads it, the EWpressureoffset setting exists in cumulus.ini so that it can be overridden.

[Station House]

Thanks Steve

I've confused myself

Fine Offset stations supply the absolute pressure and, separately, the offset to add to that to get the (more useful) relative pressure. Because of the occasional problem where the offset is corrupted when Cumulus reads it, the EWpressureoffset setting exists in cumulus.ini so that it can be overridden.

How do they do that? I've re-read the manual and don't recall having input the height above sea level anywhere other than in Cumulus.

Of course, I'm likely mistaken!

Thanks

PS - edited my first post and added a thought re the weather forecast symbol

[steve]

Fine Offset stations supply the absolute pressure and, separately, the offset to add to that to get the (more useful) relative pressure. Because of the occasional problem where the offset is corrupted when Cumulus reads it, the EWpressureoffset setting exists in cumulus.ini so that it can be overridden.

How do they do that? I've re-read the manual and don't recall having input the height above sea level anywhere other than in Cumulus.

You set the relative pressure in the station so it corresponds to sea level. The station then supplies Cumulus with the absolute pressure and the difference between the relative pressure and the absolute pressure. Cumulus just adds them together again.

PS - edited my first post and added a thought re the weather forecast symbol

Presumably the station just uses changes in absolute pressure for its forecast. That was the kind of thing (the station's forecast) that concerned me about setting the pressure in Cumulus rather than the station, but clearly in this particular case it doesn't matter.