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WMR-928NX pressure set up

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lds
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WMR-928NX pressure set up

Post by lds » Mon Mar 01, 2010 11:39 am

What is the correct way to set up pressure reading. It is very low compared to my other stations and aneroid barometer. I have set sea level setting, does the pressure data come only from indoor temp. sensor or is it in main console? It does not appear possible to calibrate local current pressure. I am talking about the station settings not Cumulus, I have not yet got data into Cumulus, awaiting USB serial adapter.
Cheers, Laurie.

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steve
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Re: WMR-928NX pressure set up

Post by steve » Mon Mar 01, 2010 11:54 am

The pressure sensor is in the little separate indoor unit. There's no way to actually calibrate the local/station pressure, you just have to adjust the sea-level setting until it reads correctly.
Steve

lds
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Re: WMR-928NX pressure set up

Post by lds » Mon Mar 01, 2010 12:02 pm

Many thanks Steve, Seems a bit Heath Robinson. So if I adjust sea level figure to some arbitrary value until local pressure is correct will Cumulus work OK with that ? Also what will the indoor LCD unit show, does the console "talk back" to it?
Cheers, Laurie.

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Re: WMR-928NX pressure set up

Post by steve » Mon Mar 01, 2010 12:40 pm

lds wrote:So if I adjust sea level figure to some arbitrary value until local pressure is correct
You adjust the sea-level pressure so that the indicated sea-level pressure is correct. You can't change the displayed local pressure.
Also what will the indoor LCD unit show, does the console "talk back" to it?
The little BTH unit always shows local pressure.
Steve

lds
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Re: WMR-928NX pressure set up

Post by lds » Mon Mar 01, 2010 2:15 pm

Thanks Steve. So it seems to me that this is a Mickey Mouse product. The local pressure reading on my unit is 11 mb too low. Surely this is not acceptable? Many thanks.
Cheers, Laurie.

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Re: WMR-928NX pressure set up

Post by steve » Mon Mar 01, 2010 2:49 pm

It's one of the reasons I replaced my WMR928 with a Davis. Quoted accuracy +/- 7 mb.
Steve

lds
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Re: WMR-928NX pressure set up

Post by lds » Mon Mar 01, 2010 10:37 pm

As 11mb seemed too far out I decided to investigate the possibility of adjusting the pressure reading to be more accurate. I opened the unit to see how pressure is measured. It uses a MPXM2102A sensor the output is fed into a dual op-amp. The PCB appears to have been designed to take a potentiometer so that the off set voltage of the op-amp can be adjusted, but it has been replaced by fixed resistors. I have changed the value of one of the resistors, I had to use three in series in order to get correct value to produce a more accurate reading. The original resistor was a 22K ohm I have replaced it with 2 10K ohm and 1 330 ohm a reduction in value of 1.67K ohms. I have only completed the modification a few hours ago so I don't know what the long term stability will be. BTW I have ordered a USB to RS232 adapter which uses FTDI chips which I am informed will work with Win 7 64 bit. Many thanks.
Cheers, Laurie.

lds
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Re: WMR-928NX pressure set up

Post by lds » Tue Mar 02, 2010 7:49 pm

Update:- Slight adustment carried out on resistor values, 330 ohm changed to 100 ohm now seems stable and accurate. USB to serial adaptor arrived. Worked first time with no intervention other than COM port setting.
Cheers, Laurie.

lds
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Re: WMR-928NX pressure set up

Post by lds » Thu Mar 04, 2010 11:18 am

I'm still a little confused as to correct set up for pressure. On my WS2300 the local pressure was set as relative pressure and displayed by Cumulus. My new WMR928 can only be adjusted for sea level. I have set this from figures on Met Office web site for Bridlington, which is nearest to me, ( about 70 miles east ) and as you will have seen from earlier posts in this thread I have modified my BTHR918N to display a more accurate local reading, again taken from Met. Office nearest being Bingley ( about 10 miles ). Cumulus seems to display the WMR928 sea level figure not local. Can someone explain please. Also what is the difference between "mb" and "hPa"? Many thanks.
Cheers, Laurie.

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Re: WMR-928NX pressure set up

Post by steve » Thu Mar 04, 2010 11:44 am

I'm pretty sure that all "hobby" weather stations are the same in this respect. They measure local ("absolute") pressure; this figure itself can't be adjusted, but they allow you to set an offset to adjust the value to sea-level ("relative") pressure (the Davis systems are slightly more clever, but the principle is the same). It's this sea-level pressure which is useful in meteorology, and it's this figure which Cumulus displays, regardless of the type of weather station.

Normally, the way you set the relative/sea-level pressure is by setting it to the same reading as a nearby known good source, preferably during a period of stable pressure (e.g. a large 'high'). The actual local/absolute value is then unimportant.

hPa and mb are the same thing (1 hPA = 1 mb) it's just that the Pascal is the SI unit of pressure.
Steve

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Re: WMR-928NX pressure set up

Post by lds » Thu Mar 04, 2010 11:50 am

Many thanks Steve.
Cheers, Laurie.

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