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Author Topic: Digital Rain Gauge Confusion !  (Read 6692 times)

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Offline Antonio (stormboy)

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Digital Rain Gauge Confusion !
« on: 15 April 2010, 04:32:07 PM »
Well guys as posted a while back i got a neat weather station. It most of the times adds up to Ballina airports readings but i want to know one thing. Why is it that last night and other times Ballina AP would get like 15 to 20mm and my Digital < rain gauge would maybe receive 2 or 4mm?? To narrow it down even more. Last night 11mm fell at Ballina AP it was heavy here to but it stayed at 0.3mm?? Does it bounce of or something when it’s heavy?? I thought of adding a funnel above it so that maybe it drips in properly? Please all you experienced people can you help me. Thanks

Antonio
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Offline Michael Bath

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Re: Digital Rain Gauge Confusion !
« Reply #1 on: 16 April 2010, 01:51:43 AM »
Hi Antonio,   are the figures in millimetres?   Many US AWS have the metric value in centimetres, so 0.3 would be 3mm. Other than that, it looks like it it not measuring correctly.

Maybe test it by pouring some water in and see what values you get.

MB
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Offline Antonio (stormboy)

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Re: Digital Rain Gauge Confusion !
« Reply #2 on: 16 April 2010, 06:57:33 AM »
Hi. Yes I have made sure that my unit of measure was set to millimetres so that is right. I will test it over the weekend as it is 2 meters above my roof so I can’t do it easily. I do remember testing it once with 20mm in a medicine cup and it got 19.6so that was close to the true reading. As stated above I think that I don’t have a large enough surface area for my gauge.

But MB how is your rain gauge set? Also do calibration screws do anything? I have 2 on the bottom of my gauge. Also if I do get a funnel for above the gauge how big in diameter should it be ?

Antonio.
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Offline Michael Bath

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Re: Digital Rain Gauge Confusion !
« Reply #3 on: 16 April 2010, 09:39:10 AM »
You should not have to adjust anything or add any funnel. I have two AWS and they work without any changes apart from units of measurements controlled by the software.   You might want to check forums related to the AWS you have.

Michael

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Location: Mcleans Ridges, NSW Northern Rivers
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Offline Richary

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Re: Digital Rain Gauge Confusion !
« Reply #4 on: 16 April 2010, 04:25:51 PM »
A funnel wouldn't work as if it was bigger than the inlet it would increase the reading. Say the opening is 1cm. That gives a catchment area of about 0.785 square cm. If you had even a small funnel of 5cm diameter then you have a catchment area of about 19.6 square cm, and effectively your guage is collecting 25 times as much water (so would therefore register 1mm of rain as 25mm).

Depending on the rain you are getting it is quite possible Ballina AP could get 20mm and you get almost nothing, if it is from localised storms. If it is from a widespread rain band then I would expect readings to be fairly consistent.
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Offline Antonio (stormboy)

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Re: Digital Rain Gauge Confusion !
« Reply #5 on: 16 April 2010, 04:39:39 PM »
Richard most of these readings are during a downpour over my and Ballina AP are note. I’m 1 minute away from Ballina AP. Just now it rained 1.5mm on my gauge so now let’s check Ballina AP it rained 0.4mm there but I’m so close to the AP so why now is mine more? I’m very confused here.

Antonio
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Offline Richary

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Re: Digital Rain Gauge Confusion !
« Reply #6 on: 17 April 2010, 03:18:52 PM »
I guess the cheap option is to go to the local hardware and buy a cheap rainguage, or even just put a straight sided glass outside and see if it gets about the same amount of rain as your guage is reporting (only useful if more than a couple of mm though).

Not sure what your guage inlet size is, but I guess if the rain was happening while it was windy a small opening guage may under report the amount.

I'm not sure how the automated BoM guages work either, my guess is that some may not get checked regularly and could themselves get partially blocked by a leaf or something, or even gunk in the sensor. They do play up, for example the almost 500kph reading I spotted from Menai one night on the wind speed.
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Offline Antonio (stormboy)

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Re: Digital Rain Gauge Confusion !
« Reply #7 on: 17 April 2010, 04:23:36 PM »
Well I guess you are right since I have only had the station for 15 days I still don’t know the personality it has lol. For instance we just had a downpour I recorded 4.8MM and Ballina AP only 0.4 so I guess that it sometimes just happens and you have to deal with the fact that sometimes downtimes may occur.

One more thing. The sensor is in a white casing with a coil like look and it is high up with the other sensors. I get perfect night time readings but in the day I’m not so sure. It’s an average of 28 to 31+ is this because of the sun or is it really 2 or 3 degrees warmer near the coast?

Antonio - guess it works!
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Offline Richary

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Re: Digital Rain Gauge Confusion !
« Reply #8 on: 19 April 2010, 03:57:51 PM »
It's normally cooler nearer the coast. A "proper" weather station has the thermometers inside a specially built box designed to be consistent and allow airflow. A white tube will still heat up with the sun, ideally it should be in the shade designed in such a way as to allow the wind to blow past.

An external sensor as you describe could also be affected by rain or moisture sitting on the tube then evaporating, artifically cooling the temperature measured.
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Offline Antonio (stormboy)

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Re: Digital Rain Gauge Confusion !
« Reply #9 on: 19 April 2010, 05:03:52 PM »
Ok cool. Thanks for the info on that. Very excited now the station is fully set up 3M above roof on a what I think will act as a lightning rod!. Very soon I will be getting a waterproof web camera facing east and west and can be moved 180 degrees the link to it is here.. http://cgi.ebay.com.au/Wireless-OutDoor-IP-Colour-Camera-Webcam-CCTV-Night-Day_W0QQitemZ320477462471QQcmdZViewItemQQptZAU_Security_Equipment?hash=item4a9df1ffc7
I am hoping that MB will be able to show my webcam with his one as well.

Antonio.
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