PRESSURE READING

[Terry66]

G'Day, new here and to weather stations. I hope I am in the right forum. I have a WS3083 up and running. My pressure reading on the station is set hpa absolute. The altitude of the station is, as best I can work it out, 195 ft asl. I note when comparing my pressure reading with stations in the general area, I am under reading by about 7 - 8 hpa. I have adjusted the offset in Cumulus to add that amount. Right or wrong?

Regards, Terry

[steve]

You should be looking at the relative pressure on the station, and comparing that to the corrected sea-level pressure in your area. If it needs adjusting, you should do that on the station (instructions are normally in the manual; there's a PDF copy in the wiki if you don't have one, but the procedure might be slightly different on WS3083).

If you do need to adjust the relative pressure on the console, you then need to restart Cumulus to pick up the change. You don't normally need to use the pressure calibration setting in Cumulus.

[Terry66]

Steve, Thanks for your help. I think I now have the problem sorted. Regards, Terry

[Gazzadp]

I have to say that having updated to the new version of Cumulus, this is a similar problem to what I have got.

As when using the last version, I am pretty sure that my display (on the computer) matched with what was shown on the LCD display. As I had it set to display the "absolute" air pressure.

I have got 2 other psuedo weather stations (the type which just display air pressure, temp and humidity) one of them has a outdoor temp sensor. I also have an old fashioned barometer, all of those show exactly the same reading as the LCD Display set to absolute. But the Cumulus software console is only showing the relative pressure.

Now perhaps I am just a bit dumb or going senile (well I am in my 40's) but I cannot for the life of me find an easy way to change the Cumulus display to "absolute" or more importantly find a way to get to display the same as the LCD display.

I have set up the Cumulus software with the Longitude & Latitude of where I live which is Speen, Berkshire UK!

So any help would be welcomed by this seemingly OLD & DUMB user!



Just for reference as of the time of composing this message, Saturday 20 April, time 15:21 the air pressure was shown as:

1020.6 hpa (abs) - on the LCD Display.
1043.8 hpa/mb - on the Cumulus Display.

[steve]

Cumulus displays sea-level pressure, it always has done. This is the what the Fine Offset calls 'relative'. It's the station pressure (what the FO calls 'absolute') corrected to what it would be if you were at sea level. This is done so that the pressure at one station makes sense when compared to the pressure at another station, regardless of any difference in altitude between the two stations.

You need to set the relative pressure on your station so that it is correct. The sea-level pressure where you are now is about 1031 hPa. Having corrected the relative pressure on the console, you need to restart Cumulus.

[mcrossley]

Hmm, Steve posted whilst I was typing - he's got quicker fingers than me!...

I think you need to read this - http://wiki.sandaysoft.com/a/FAQ#Cumulu ... et_console

Absolute pressure is pretty useless, it is the actual pressure measured at the station, so if you were at the top of a mountain it would be very low compared to a station at the bottom. What we normally use is the relative pressure, that is the pressure corrected as if the station was at sea level. Using relative pressure allows meaningful comparisons between stations, and comparison with the Met Office which also quotes surface pressures as 'relative'.

[Gazzadp]

Thanks for the replies... I think I have got it sorted out now!