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Davis Vantage Pro 2 Plus - Non-recording of Little Rain

Discussion specific to Davis weather stations
Flosex
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Davis Vantage Pro 2 Plus - Non-recording of Little Rain

Postby Flosex » Sat Jun 24, 2017 9:17 am

I have been using a Davis Vantage Pro 2 Plus weather station for about a year that replaced a Fine-offset WH3080 station that I had used for about three years before that. I have been very pleased with the Vantage apart from a niggle when recording (or rather, not recording) very small amounts of rain.

I don't ever remember an occasion when using the WH3080 station when it rained very lightly or for a very short time, that nothing was recorded by the rain sensor, but on a few occasions with the Vantage station when it has rained very lightly or for a very short time, nothing has been recorded. I find this strange because the smallest amount of rain recorded with the WH3080 is 0.3mm, whereas on the Vantage it is only 0.2mm, and so I thought it would be better and more accurate for recording rain than the WH3080. With all other types of rain, I am sure the Vantage has been recording accurate rain levels because the readings have been consistent with neighbouring weather stations.

Is this non-recording of small amounts of rain normal with Vantage weather stations? If the answer is no, what can I do to overcome this problem, please? I have ensured there is no blockage in the rain funnel or anything else that might prevent small amounts of rain from being recorded. I have also experimented with dropping water droplets into the funnel to try and ascertain how much water initiates a first recording, and have to use more than I would have thought was necessary. Once the first (0.2mm) recording has registered, it doesn't take as many droplets to cause it to register the second (0.4mm) and subsequent recordings, which again makes me think something may be wrong with it.

If the answer is yes and the Vantage doesn't record very small amounts of rain, what should I do with my weather records if it actually rains a little, but the Vantage doesn't register anything? Cumulus will record the day as a 'dry day', whereas I know it has rained and should be a 'rain day'. This will result in false recordings for dry spells and wet spells. Should I keep to what Cumulus says or should I override it using my own observation that it has rained? If I do the latter, then if I happened to be out when it rained a little, then I would not know that and would not therefore have cause to override it! This seems to question the whole aspect of recording what is a 'dry day' and what is a 'rain day'.

I know the Met Office use 1mm to define a 'rain day' and presumably ignore amounts of rain below this. Is there a generally accepted standard for amateur weather stations to record what is a 'dry day' and what is a 'rain day'? Any amount of rain during the day causes Cumulus to record it as a 'rain day'.

Thanks for any help and / or advice.

Chris Page
(West Sussex, UK)
West Sussex, UK - <www.g4bue.co.uk/en/Weather/>

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steve
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Re: Davis Vantage Pro 2 Plus - Non-recording of Little Rain

Postby steve » Sat Jun 24, 2017 10:35 am

I'm not aware of this being a problem with Pro2 stations. I think you need to check that the gauge is level, and that the mechanism is correctly positioned - perhaps it is sticking, and the first tip frees it up somewhat.

There are screws for adjusting the gauge, perhaps yours are unevenly set (so a tip one way requires more than a tip the other way). It would also be worth trying a full calibration check - very slowly pour a measured amount of water in and check what the gauge records - there's a standard amount that is usually used for this, I can't remember the figure but a search should find it.

Note that the threshold for a rain day in Cumulus is configurable, via a setting in Cumulus.ini (RainDayThreshold).
Steve
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Flosex
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Re: Davis Vantage Pro 2 Plus - Non-recording of Little Rain

Postby Flosex » Sat Jun 24, 2017 1:46 pm

Thanks for your helpful reply, Steve. I will check what you suggest.

In the meantime do you (or anyone else) have any comments on whether I should I keep to what Cumulus says about no rain being recorded or override it using my own observation that it rained, albeit a very little?

Chris
West Sussex, UK - <www.g4bue.co.uk/en/Weather/>

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steve
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Re: Davis Vantage Pro 2 Plus - Non-recording of Little Rain

Postby steve » Sat Jun 24, 2017 2:13 pm

The standard for manual recording if you know it rained but not enough to be measured is to record "trace", but of course you can't do that with Cumulus. So personally, I wouldn't change the readings you're getting unless you're absolutely sure of your correction, as otherwise you're just replacing one dubious reading with another. Just my opinion.
Steve
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Re: Davis Vantage Pro 2 Plus - Non-recording of Little Rain

Postby PaulMy » Sat Jun 24, 2017 2:25 pm

(or anyone else) have any comments on whether I should I keep to what Cumulus says about no rain being recorded or override it using my own observation

I have a 4" manual gauge and send 24 hour rainfall to CoCoRaHS and quite often that amount is different than my VP2, and sometimes no rain reported by the VP2 due to debris plugging the rain gauge, which is then reported as a greater amount when I clear the gauge. After initially trying to make both show the same I have just accepted that both CoCoRaHS and VP2/KomokaWeather data is what it is. If I am absolutely sure what the amount should be I will make the change, otherwise just leave as is.

Remember, that for most of us it is still a hobby. We would like/want our data to be as accurate as possible but Environment Canada is not checking my weather station data to record historical data. But... but... there has been occasions when the weather service providers have used my data, such as on the local snowfall :o

Enjoy,
Paul

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Re: Davis Vantage Pro 2 Plus - Non-recording of Little Rain

Postby RayProudfoot » Sat Jun 24, 2017 5:34 pm

Chris,

Another VP2 owner here. I've set 0.02" as the trigger for a 'rain day'. I find in very humid conditions dew can reach 0.01" which would mess up rainfall records.

It does take a reasonable amount of rain to trigger a single bucket trigger. The bucket may be dry if it hasn't rained for a while or if there is some in it from earlier rain may trip the bucket almost immediately.

Unlike manual records where rain has fallen but not measurable 'trace' would be recorded. Cumulus can't do that so I tend to treat 0.01" as a trace. All precipitation is added to the various totals of course.
Cheers,
Ray, Cheshire.

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Flosex
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Re: Davis Vantage Pro 2 Plus - Non-recording of Little Rain

Postby Flosex » Sun Jun 25, 2017 7:54 am

Thanks again to Steve for his comments, and to PaulMy and Ray Proudfoot for their additional comments. I now have a better understanding of recording very small amounts of rain.

I am yet again very impressed with this Forum, both with the knowledge of other members and their willingness to share it with less knowledgeable members like me.

Regards

Chris
West Sussex, UK - <www.g4bue.co.uk/en/Weather/>

RayProudfoot
Posts: 2402
Joined: Wed May 06, 2009 6:29 pm
Weather Station: Davis VP2 with Daytime FARS
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Re: Davis Vantage Pro 2 Plus - Non-recording of Little Rain

Postby RayProudfoot » Sun Jun 25, 2017 8:33 am

Flosex wrote:Thanks again to Steve for his comments, and to PaulMy and Ray Proudfoot for their additional comments. I now have a better understanding of recording very small amounts of rain.

I am yet again very impressed with this Forum, both with the knowledge of other members and their willingness to share it with less knowledgeable members like me.

Regards

Chris


We all started as beginners Chris. :D I'm still a novice with many aspects of capturing and storing wx. Fortunately there are lots of people here willing to help. Excellent place to learn! :)
Cheers,
Ray, Cheshire.

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