Oregon Scientific WMR-928 NX and barometer reading question

[kinder]

Hello.I am using an old Oregon Scientific WMR-928 with cumulus a couple of days.It works ok but i have a small problem.Reading of the barometric pressure doesnt show the decimal digits.Its always 1015,0 or 1022,0 or 1012,0.Is there any way to force cumulus to show the pressure with decimals after the commas?I know that my console doesnt show pressure like that but with other Weather Software like Virtual Weather Station or Weather Display,it was possible.

[steve]

The specification of the WMR928 says it has a resolution of 1 mb (and an accuracy of +/- 7mb!) and the spec I have for the serial protocol only allows for pressure in whole numbers, so I don't know where I'd get the decimals from to display them.

[kinder]

Ok.Sounds reasonable but i wonder why in barometric trends shows the decimals?For example Falling 0,3 hpa/hr or Rising 0,7 hpa/hr.

[steve]

Ok.Sounds reasonable but i wonder why in barometric trends shows the decimals?For example Falling 0,3 hpa/hr or Rising 0,7 hpa/hr.

It's calculated over a period of three hours. So 0.3 hPa/hr is a change of 1 hPa over the three hours, and 0.7 is a change of 2 hPa over three hours.

[apaneiro]

I've just come across the same issue while implementing the WM-918 protocol on an Arduino over USB (virtual serial) to send data from external Auriol sensors as long as from inbuilt barometric (BMP085) and humidity (HIH4030) devices.

The specs I've been following, on http://wx200.planetfall.com/wx200.txt, do state that the pressure is on @ 1 units, but the field itself, on bytes 1 to 5 of packet AF, is able to send @ 0.1 units, as "abcd.e".

Any chance we could see this estra digit (.1) beeing handle by Cumulus?

[steve]

The WM918 apparently only measures pressure to whole numbers (the local pressure)

AF. 0 HH all Group AF ----------------------------------------
AF. 1 DD all Barom Local: 'cd' of 795<abcd<1050 mb @ 1
AF. 2 DD all Barom Local: 'ab' of <abcd> mb
AF. 3 DD all Barom SeaLevel: 'de' of 795<abcd.e<1050 mb @ 1
AF. 4 DD all Barom SeaLevel: 'bc' of <abcd.e> mb
AF. 5 Bx 0,1 Barom Format: 0=inches, 1=mm, 2=mb, 3=hpa
AF. 5 Bx 2,3
AF. 5 xD all Barom SeaLevel: 'a' of <abcd.e> mb

So where does the decimal place come from in the sea level value? How does it calculate sea level pressure - do you supply it with a fixed offset, or does it do some kind of calculation? Either way, if it only measures to whole numbers, displaying a decimal place is spurious accuracy.

[apaneiro]

Yes, I understand that inconsistency, but on my device I'm just sending the barometric data on the Sea Level Barom, which seems what Cumulus is expecting for. I would be able to be more accurate than the real device and send an extra decimal digit, right?

[apaneiro]

I did some more checking using both Barometer fields, Local and Sea Level. These are my findings:

Both local and SLP being sent:
=========================
8F 00 00 00 00 00 00 00 57 00 00 00 00 00 00 00 00 00 00 00 62 00 00 00 00 00 00 00 00 00 00 00 00 00 48
9F 75 02 00 00 00 00 00 00 00 00 00 00 00 00 00 51 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 69
AF 11 10 08 01 21 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 FA
BF 00 00 00 00 08 00 00 00 00 00 00 00 C7
CF 11 50 22 14 50 22 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 D8

Local=1011
SLP=1010.8
CUMULUS Pressure=1010.0

Only Local being sent:
=========================
8F 00 00 00 00 00 00 00 57 00 00 00 00 00 00 00 00 00 00 00 64 00 00 00 00 00 00 00 00 00 00 00 00 00 4A
9F 76 02 00 00 00 00 00 00 00 00 00 00 00 00 00 48 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 61
AF 11 10 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 D0
BF 00 00 00 00 08 00 00 00 00 00 00 00 C7
CF 17 00 27 22 00 27 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 56

Local=1011
CUMULUS Pressure=0000

Only SLP being sent:
=========================
8F 00 00 00 00 00 00 00 58 00 00 00 00 00 00 00 00 00 00 00 64 00 00 00 00 00 00 00 00 00 00 00 00 00 4B
9F 76 02 00 00 00 00 00 00 00 00 00 00 00 00 00 50 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 69
AF 00 00 09 01 21 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 DA
BF 00 00 00 00 08 00 00 00 00 00 00 00 C7
CF 09 50 22 14 50 22 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 D0

SLP=1010.9
CUMULUS Pressure=1010.0

I also found the manual for WX-200 at http://support.radioshack.com/productin ... ns&Reuse=N.

They do have a way of setting the SLP and that's probably the reason why they send both values in the AF packet. Although inconsistent with the device accuracy, they must send the extra decimal digit to handle the SLP calculations in the station.

This raises me a question, Steve, why are you using the SLP field instead of the Local Pressure? Also, as this value is not being rounded, we get an extra display error...

[steve]

They do have a way of setting the SLP and that's probably the reason why they send both values in the AF packet. Although inconsistent with the device accuracy, they must send the extra decimal digit to handle the SLP calculations in the station.

So you can set the initial sea level pressure on the station, and then it does some sort of calculation from which they give the result with a spurious extra decimal point? Crazy. Anyway, I can't be bothered to argue about this, I've long ago given up trying to convince people that they can't get extra accuracy out of their stations just by displaying extra decimal places; I'll change the code.

This raises me a question, Steve, why are you using the SLP field instead of the Local Pressure?

Because meteorologists use SLP (for the most part); the actual value of the pressure at your station is of no use for meteorological purposes because it can't be compared to other stations unless you happen to be at the same altitude.

[apaneiro]

yeah, I ment why don't you use the Local Pressure and do your own SLP calculations in Cumulus? Anyhow, it would be great if you use the extra digit in SLP...

Thanks anyway for your support and for Cumulus itself.

[mcrossley]

Anyway, I can't be bothered to argue about this, I've long ago given up trying to convince people that they can't get extra accuracy out of their stations just by displaying extra decimal places; I'll change the code.

Steve, if I understand correctly, the extra accuracy is because he is not using the OS hardware, he is using custom hardware, but just emulating the OS protocol. You could argue that emulating a different protocol would allow him to do this without any changes to Cumulus

It was a case of thread high jack too which confuses things.

[steve]

Steve, if I understand correctly, the extra accuracy is because he is not using the OS hardware, he is using custom hardware, but just emulating the OS protocol.

Ah, I had misunderstood. I thought he was reading the data from a WM-918 using some custom hardware. So this still leaves the question of what the real hardware does. It's probably a reasonable assumption that the real hardware does put a valid number in the spurious extra decimal place, even if it's only ever zero, then I'm still OK changing the code to use it.

[apaneiro]

Yes, I'm using the Auriol external sensors with a custom Arduino based receiver with inbuilt pressure and humidity sensors. The people that started this project were using Cumulus with Easyweather.dat files and some other software to create the .dat files from the data received on the serial connection. I just thought it would be much easier to send the data directly through serial with some other protocol. I might have picked the wrong one though...

Anyhow, this setup is currently exporting fine to PWS, except for the pressure graphs, with those 1hPa steps.

Have a look on http://www.pwsweather.com/obs/APPSLV1.html