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Fine Offset Rain Sensors
#1
How have most got on with the rain sensors we use from Fine Offset? I purchased one new and have just in the last week put it out properly. When I tested it by pouring water out of my manual rain gauge, I was getting no-where near accurate readings.

What I discovered is that the switch is not being activated on each pass of the magnet.

To test this I removed the cover, and rocked the bucket by hand. I found that if I pulled the buckets on their pivot toward me as I rocked them (which works out to be away from the switch assembly, the gauge reliably registered 1mm for every three tips, give or take 0.1 - 0.2 each set. Which I believe equates to what I read is around 0.300mm per bucket.
Then if I rocked the buckets on their natural pivot level, I was lucky to even see 0.300mm advancement.

I know the gauge is set level as I used a spirit level when setting its bracket. Has anyone else encountered this, and how did you resolve it? I will hopefully have time at the end of the week to look at it more closely, and see if any adjustments can be made without compromising the rocking motion. Otherwise I may have to cut my losses and try another unit.
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#2
Mine seems fine.
Maybe the magnet and/or the reed switch need to be closer.
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#3
That's the theory I'm thinking and hoping. It is funny how pulling the buckets away seem to make it better, unless there is some flexing to the assembly inside the switch house. Hopefully tomorrow I will have some time to open that up and see if I can adjust their seating. Of course there may even be a poor connection in there.
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#4
The verdict is not great. Firstly I confirmed that the switch was only being activated by one bucket, and it was always the same one. I found that by holding the PCB containing the switch away from the magnet corrected the issue. I put a blob of hot glue on the PCB to make a spacer. Now the switch 'ticks' on both bucket tips.

However now I discovered the other issue. One bucket requires considerably more water to tip, effectively requiring just over 2mm at least to make 3 tips to register the 0.9 to 1mm.

I thought perhaps some hot glue to the offending bucket would balance this out. It has taken a lot of messing about to bring it close on the bench. Back in the field it just was not going to cut it. I checked with my spirit level again, and the sensor is definitely level sitting on the bracket.

I think I am just unlucky with this particular unit, and will have to cut my losses.
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#5
(27-05-2015, 11:52)qldbureau Wrote: How have most got on with the rain sensors we use from Fine Offset? I purchased one new and have just in the last week put it out properly. When I tested it by pouring water out of my manual rain gauge, I was getting no-where near accurate readings.

I can't say if your unit is faulty or not, but...

Unless the water collector area of your manual rain gauge be exactly equal to the collector area (not likely) of the Fine Offset rain gauge, dropping the water from the first on the second don't give you the same readings.
Don't worry, this is a common misunderstood, and there are lots of info about this on the Cumulus forum.
MeteoCercal - Air Quality Data
Click here to watch at my ThingSpeak channel



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#6
Thanks Werk_AG. I am going to give it the benefit of the doubt and wait for rain and see how it compares. Might be a little while before we have any rain though.
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#7
Correct way to calibrate and check Rain Gauge:

The tipping-bucket mechanism is a simple and highly reliable device.

The rain gauge must be calibrated with a controlled rate of flow of water through the tipping-bucket mechanism. The maximum rainfall rate that most tipping bucket rain gauges can accurately measure is 25mm of rain per hour (approx. 36 seconds between bucket tips). Therefore, the rain gauge is calibrated using a water flow rate equivalent to, or less than, 25mm of rain per hour (more than 36 seconds between bucket tips). If the flow rate is increased beyond 25mm / hr, a properly calibrated instrument will read low. Decreasing the rate of flow will not materially affect the calibration. The reason for this is obvious. Watch the tipping bucket assembly in operation. With water falling into one side of the tipping bucket, there comes a point when the mass of the water starts to tip the bucket. Some time is required for the bucket to tip (a few milliseconds). During the first 50% of this tipping time water continues to flow into the filled bucket; the last 50% of this tipping time water flows into the empty bucket. The amount of water flowing during the first 50% of time is error, the faster the flow rate the greater the error. At flow rates of 25 mm/hr or less, the water actually drips into the buckets rather than flowing. Under this condition, the bucket tips between drips, and no error water is added to a full moving bucket.

To Calibrate:
NB: Use a vernier caliper accurate to 0.02mm.

1) First work out the cross sectional area of your rain gauge. Measure the internal diameter of your rain gauge receiver if it is round use: A=πr^2. Assume a for arguments sake our rain gauge has a diameter of 6” or 152.4mm.

i) 152.4 / 2 = 76.2

ii) 76.2 x 76.2 x π = 18241.469 mm2

iii) 182.41 x 2 = 365 ml of water will represent 20mm of rain falling on our gauge.

Ellipse rain Gauges are slightly more complex; Area = π x a x b;

Where semi-major axis of length A and semi-minor axis of length B. (These semi-major axes are half the lengths of, respectively, the largest and smallest diameters of the ellipse.) Type area of ellipse into google for the calculation and do the same arithmetic as iii) above ie: divide by 100 and multiply by 2 to get the amount of water equivalent to 20mm of rain.

2) Obtain a plastic or metal container of at least one litre capacity. Make a very small hole (a pinhole) in the bottom of the container.

3) Place the container in the top funnel of the Rain Gauge. The pinhole should be positioned so that the water does not drip directly down the funnel orifice.

4) Pour exactly 365ml ( or the amount you have calculated for your rain gauge) of water into the container. Assume each tip of the bucket represents 0.2 mm of rainfall. Then your gauge should have tipped 100 times. (this will vary according to make but should be in the Specification Sheet, some are 0.3mm per tip in which case the bucket will tip 67 times ) But you should see 20mm of rain register on your readout. If you are unsure about the bucket size manually tip your bucket until 20mm shows up on the display; if it is after 67 tips it is a 0.3mm bucket, if it is 100 tips it is a 0.2mm bucket etc.

5) If it takes less than one hour for this water to run out, then the hole (from step 1) is too large. Repeat the test with a smaller hole.

(Adjusting screws on Davis and the better quality gauges are located on the base of the Rain Gauge housing underneath the buckets. Turning the screws clockwise increases the number of tips per measured amount of water. Turning the screws counterclockwise decreases the number of tips per measured amount of water. A 1/4 turn on both screws either clockwise or counter-clockwise increases or decreases the number of tips by approximately one tip. Adjust both screws equally; if you turn one a half turn, then turn the other a half turn.)

6) Repeat Steps 3–6 as necessary until the Rain Gauge has been successfully calibrated.

Some interesting numbers; 1mm of rain falling over 1 hectare is 10,000 litres of water or 10 tonnes of water.
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#8
Thanks JT for this really useful and re-assuring information. It makes sense, and I did wonder how much water was 'in error' at the cusp of bucket tips; especially if it was at a higher rate. I went about it incorrectly. One thing though, the switch was definitely not activating on one of the bucket tips, which I discovered when manually tipping them. That glue spacer holding the circuit board slightly away from the bucket solved that.
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#9
After some reasonable rainfall on different days two weeks ago, I yielded poor results on my sensor. It was mostly 50% behind my manual rain gauge. The manual gauge was spot on each time with another station located further up my street.

I have just taken delivery of a replacement sensor, this time bought from another supplier. The thing that stands out with the new one when I opened the funnel, is that the magnet protrudes out of its holder in the buckets, and is positioned very close to the reed switch chamber. On the previous one, the magnet is molded into the buckets, and not positioned so close to the reed chamber. The magnet is not even visible.

I have changed them over, and will now await some precipitation for the first test. We are forecast for possible showers later in the week.
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#10
(22-06-2015, 11:21)qldbureau Wrote: After some reasonable rainfall on different days two weeks ago, I yielded poor results on my sensor. It was mostly 50% behind my manual rain gauge. The manual gauge was spot on each time with another station located further up my street.

I have just taken delivery of a replacement sensor, this time bought from another supplier. The thing that stands out with the new one when I opened the funnel, is that the magnet protrudes out of its holder in the buckets, and is positioned very close to the reed switch chamber. On the previous one, the magnet is molded into the buckets, and not positioned so close to the reed chamber. The magnet is not even visible.

I have changed them over, and will now await some precipitation for the first test. We are forecast for possible showers later in the week.


I have one of these which I intend to hook up to the weatherduino.

http://texaselectronics.com/products/rai...ucket.html

(I hasten to add I did not pay that kind of money; got it off ebay for a fraction of the price, some old research project selling up)
I have to say that I think the accuracy of the FO gauges is fairly "ordinary" compared to a davis gauge and certainly compared to the Texas instrument. The "sharpness" of the tipping bucket top will determine the accuracy of the gauge at the critical cusp. Additionally, I am not sure if the FO gauge can be adjusted and therefore calibrated even at the fairly basic "field" level described above.

Eventually I intend to put the weatherduino on my website and run both the davis system and weatherduino simultaneously on separate page and do a comparison. www.ayrshireweather.org

I have got some very nice anemometers, again research grade, old(1990's) but beautifully made which will also go on the weatherduino once I have sorted out how they can be read. See thread ( http://www.meteocercal.info/forum/Thread...nemometers )
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