Negative Rain

[Karen K]

First I realized, that rainRate > 0 but rain == 0. And it was raining. So I looked into it.

weewx.conf says:

[Station]

    ...

    station_type = WeatherLinkLiveUDP

[WeatherLinkLiveUDP]

    wll_ip = XXX.XXX.XXX.XXX

    poll_interval = 10

    driver = user.weatherlinkliveudp

    hardware = Davis Vantage Pro2

Before midnight, no error is logged into syslog. All is normal. Starting with 00:00:05 today WeeWX complains to syslog about a negative rain value:

May 26 00:00:05 LokalWiki weewx[365882] WARNING weewx.qc: 2021-05-26 00:00:06 CEST (1621980006) LOOP value 'rain' -0.14960629921259844 outside limits (0.0, 10.0)

This message is repeated for every loop packet. The value increases during the day a little bit.

May 26 21:05:50 LokalWiki weewx[365882] WARNING weewx.qc: 2021-05-26 21:05:50 CEST (1622055950) LOOP value 'rain' -0.015748031496062992 outside limits (0.0, 10.0)

When I realized all that, I stopped WeeWX and started it again. No more messages from that moment on. And rain is registered again as normal.

Reading weewx.sdb results in:

sqlite> select dateTime,rain,rainRate from archive where dateTime>=1621980000 order by 1;

1621980000|0.0|0.0

1621980300|0.149606299212598|0.0

1621980600|0.0|0.0

...

1621980300 == 26.05.2021 00:05:00 CEST, which is the time, when the messages started. For all other timestamps up to the restart rain is 0.0

Can someone draw sense out of that?

[Susan Mackay]

Weewx expects the rain amount added since the last 'loop' record but *my guess* is that the weather station is providing the volume of rain since some reset time (midnight?). Therefore the driver is remembering the last volume the station provided and subtracts that from the latest volume to pass on the difference to weewx. (This is how my driver - not the one you are using - handles this.)

Further I'm guessing that the driver will try to match the weather stations reset time.

As the problem starts just after midnight, it could well be that there is a difference in the times that the weather station sets the rain volume for the day back to zero and when the driver tries to do the same. If the driver has a certain value from just before midnight; the weather station resets to zero but the driver does not; the next rain volume seen by the driver will be loer than the last one it has recorded and so will pass on a negative 'increment'.

I would check that both the weather station and the system running weewx have the same time and therefore record the 'midnight reset' together.

(This is all wild supposition on my part - please treat it as such!)

Susan

[Karen K]

Thank you, Susan, for your thoughts. 

I use WeatherLinkLive hardware from Davis. I looked around how to adjust the clock time of that device. Nothing found.

Then I checked the time that is displayed on the console. It was some minutes behind. So I adjusted the time of the console. 

We have 2 consoles. I checked the second console, too. It was behind, too. And adjusting the first console did not affect the second console. I adjusted it, too.

After that - surprise - the WeatherLinkLive device is sending the correct time, too. I do not understand, how that works.

I will see, what happens this night.

[Karen K]

2 hours of work to fix the database from the logging messages.

[Karen K]

After I adjusted the time of the console no further error messages occured. All is working as normal again.

I looked into the driver. From what I saw there, the time sent by the device is used to decide when to reset the rain counter. So it should work even if the clock is misaligned. But it did not. 

The Davis weather station sends data wireless. The consoles are not connected to the network. Only the WeatherLinkLive device is. So I wonder how the WeatherLinkLive device could learn about the time adjustment of the console.