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Author Topic: A question about WD rain rate calculation  (Read 342 times)

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Offline atsio

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A question about WD rain rate calculation
« on: May 13, 2018, 12:11:59 PM »
Last Thursday, we've had a storm with extremely high rain rates and flood incidents at the city of Thessaloniki, northern Greece region of Central Macedonia.

WD has reported maximum rain rate 430.3 mm/hr, but WeatherLink which i run simultaneously using VPP has reported 606.4 mm/h!!!  8O

Look at the attached screenshots. there's a big difference between the two applications!   :scratch:

Which one is correct? Both programs, read same data coming from the ISS of my Vantage Pro2 Plus wireless weather station.

Thanks,

Thanos

Offline Dale Reid

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Re: A question about WD rain rate calculation
« Reply #1 on: May 13, 2018, 04:09:53 PM »
The spreadsheet is a helpful piece of information.

I cannot claim it is the reason, but if one program took the minute to minute value due to tips, and calculated on that number per minute, you found the highest rate at the time the most rain was accumulated in that minute, according to the data in your printout.

If the other system 'averaged' it over a couple minutes, for this very reason, it would have found the same number but averaged for a longer time, which of course reduces the peaks from what the real exact moment reading would be.

Welcome to the world of digital vs. analogue.....


Offline niko

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Re: A question about WD rain rate calculation
« Reply #2 on: May 13, 2018, 04:24:55 PM »
Exactly, it's not an obvious calculation and WD and Davis use different algorithms.

If you prefer the Davis method there is an option on the Davis tab of station setup to use the VP's rain rate instead.

Offline atsio

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Re: A question about WD rain rate calculation
« Reply #3 on: May 13, 2018, 05:40:00 PM »
If you prefer the Davis method there is an option on the Davis tab of station setup to use the VP's rain rate instead.

Thanks for the answers so far!

Niko, as you can see on screenshot, the option for rain rate in Davis setup has been already enabled!  :roll:

I believe Brian will explain how rain rate calculation in WD works!

Thanos
« Last Edit: May 13, 2018, 05:41:54 PM by atsio »

Offline niko

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Re: A question about WD rain rate calculation
« Reply #4 on: May 13, 2018, 05:44:09 PM »
...and hopefully someone will explain how rain rate calculation in WL works too  :wink:

Offline Weather Display

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Re: A question about WD rain rate calculation
« Reply #5 on: May 14, 2018, 01:37:09 AM »
maybe WD missed the LOOP data packet that had the very max rain rate value  :roll:

Offline atsio

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Re: A question about WD rain rate calculation
« Reply #6 on: May 14, 2018, 05:15:17 AM »
Brian, although I don't know how WD calculates rain rate, I am not sure that it lost that data packet,
because it gets data not from ISS directly, but from a common source(database?) like VVP together with WL.

As far as I know, VVP works like a "buffer" between ISS and datalogger to give the ability to other programs to run at the same time.

Does WD use an algorithm to calculate rain rate or just show the value read from the station?

Thanos

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Re: A question about WD rain rate calculation
« Reply #7 on: May 14, 2018, 07:16:34 PM »
it should be using the data provided in the LOOP data if you have set to use that

Offline steerage250

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Re: A question about WD rain rate calculation
« Reply #8 on: June 18, 2018, 01:41:39 AM »
I too am puzzled by WD's rain rate calculation.  I do NOT have the "use VP rain rate calc" option set (I only just realised it was there after looking at various rain rate threads).

I suspect WD might be being a bit ambitious in its rain rate calculations in heavy downpours, and suspect the time period over which the rate is calculated might need to be stretched.

A few weeks ago, the data recorded by MySQL showed (during a heavy rain shower) the daily rainfall increasing by 0.4mm from one minute to the next, but a 6mm/min rain rate.  Obviously, if WD was using one minute to the next, the rain rate would have been 0.4mm/min.  WD is receiving packets from my ethernet-connected Davis every few seconds, so a finer-grained rate calculation would appear to be being used.

A rain rate of 6mm/min, implies a 0.2mm tip every 2 seconds - which must just be about the mechanical limit for tipping speed.  If the packets are time-stamped to one second, then fractional-second differences in time of when the tip occurs relative to the second ticking-over can have huge differences to the calculation result.

I think a 2 second rate rate is a bit meaningless anyway.  If people are thinking "gee that's heavy rain, I wonder what the rate was?", then I would think 30 seconds (or a minute) would be a sensible period to do the rate calculation over anyway.

I think I'll try selecting the "use VP2 rain rate" and give that a try.