[JT118]

Has anyone measured the cross sectional area of the FO rain gauge receiver funnel? It would be interesting to get a series of readings doing the calibration above from different gauges.

[AllyCat]

Hi,

IIRC the collection area is between 60 and 65 square cms, but it's not of much significance unless you know the volume of the "buckets". Also, the main "complaint" about the FO rain gauge is the very low gradient of the "funnel" and the low height of the side walls. So the recorded value can be low due to "splash out".

Personally, I have added a 14 cm diameter funnel, which increases the sensitivity by a factor of about 2.5. But unfortunately it doesn't actually give any earlier indication of "first rain" unless using a treatment such as "Rain X" to prevent surface tension holding drops in the funnel.

Cheers, Alan.

[werk_ag]

Also, the main "complaint" about the FO rain gauge is the very low gradient of the "funnel" and the low height of the side walls. So the recorded value can be low due to "splash out".

Trying to minimise this problem, I raised the height of the side walls, gluing some plastic material.

"Rain X" - Nice idea! I always use this material in the front glass of my car.

[JT118]

I am afraid that the cross sectional area of the collector is of primary importance to enable an accurate calibration. It is from this that all the other arithmetic is derived. Refer to my previous post but you will need the area accurate to within a few mm2 (not cm2) and do the process as described to work out accuracy, once you have done that you can work out the precision with which that accuracy is repeated. The FO gauges equate theoretically to 0.3mm rain per tip,(and have a 55cm2 collector area) therefore after 67 tips ( and 110ml water ) you should see 20mm appear on you readout.

If you have a EXACTLY 140mm internal diameter funnel feeding your gauge, 70x70xpi=15400mm2 (154cm2). Therefore 308ml water = 20mm rain. Proceed as per previous post. the water when released into the gauge in the method prescribed should result in 20mm registering on your readout. It will not and you are not getting 0.3mm per tip, you will get 187 tips and 56 mm rain on your readout, which will be incorrect, as actually your 154cm2 collector only got 20mm rain (308ml water).

The precision (which will be affected by variables such as splash out) of the gauge is determined by how often it repeats the same result when the same exercise is carried out a number of times.
Of course this exercise will allow you to work out the rain each tip represents. We have a known number in the case of our 308ml we KNOW this equates to exactly 20mm of rain through our 140mm diameter collector. If we release this water into the gauge (nb: only in the manner prescribed) and end up with say 100 tips we know now that each tip is equivalent to 0.2mm rain. Or 70 tips means 0.285mm per tip and so on. You cannot just double the collection area, this does not increase sensitivity but does mean more water for a given amount of rain going through the gauge. Consequently more tips for a given amount of rain. If you alter the collector cross sectional area in any way at all, you will have to alter the arithmetic in your nano sketch.

It is likely with your 140mm diameter collector that a FO gauge will read 2.8 times more tips (the collector is 55cm2), or 187 tips per 20mm rain. and therefore the readout will register 2.8 times more rain than it ought to. The nano sketch will need changing to assume that each tip registers only 0.1074mm of rain per tip not 0.3mm with a 55cm2 collector area.

If you use the FO gauge as is, without the funnel, the cross sectional area is 55cm2 then 110ml of water is the same as 20mm of rain. If the gauge is working as designed then dripping this into the gauge in the prescribed manner should result in 67 tips and 20mm rain registering on the readout. ie 0.3mm rain per tip.

Remember in heavy downpours tipping bucket gauge error increases ( see previous post) so a very large collector could increase error in heavy rain, which is why you must not simply pour the water into your gauge, doing so will make the results of this calibration completely incorrect.

The actual volume the bucket holds is really irrelevant, however if you want work it out, 55cm2 figure is correct area of the collector, 110ml. / 67 tips = 1.64ml in a bucket BUT this REPRESENTS 0.3mm of rain for recording purposes. Do not mix up bucket volume with the amount of rain each tip represents. They are related to each other by the arithmetic described but will not be the same number.

Please see last post in this thread, the FO gauge is 55cm2 not 65cm2 and all volumes need to be altered accordingly.

[JT118]

I have, today, ordered a Maplin Replacement Rain Gauge for N25FR/N96FY/N96GY stations for the grand sum of £4.99.

http://www.maplin.co.uk/p/maplin-replace...96gy-n77nf

It should be here by the weekend with luck; if it is, I shall measure it up and do the calibration for both accuracy and repeated a few times for precision and report back to this thread with the results.

[werk_ag]

Raising the height of the side walls, don't affect the cross sectional area of the collector. Just avoid the losses by splashes.
It seems that FO recognized the problem, at least their new weather stations come with rain gauges with large side walls.

When possible, I recommend doing this procedure.

[JT118]

Moo

Raising the height of the side walls, don't affect the cross sectional area of the collector. Just avoid the losses by splashes.
It seems that FO recognized the problem, at least their new weather stations come with rain gauges with large side walls.

When possible, I recommend doing this procedure.

I realise that, but if, as others have done, you put a 140mm funnel as a collector the area alters considerably and so does all the arithmetic, remember too that if you merely glue an extension to the outside of the collector you have possibly added 2mm to every dimension. This would mean that your collector area would go from 55cm2 to over 71cm2 nearly a 10% increase in area and necessitate a rework of the arithmetic in the arduino sketch. Accurate calibration in last post of this thread.

[qldbureau]

I saw much improved results over the last week with the replacement sensor over the original I had. Instead of having wild variations (which always trailed) against even my manual gauge in the same location, and another further up the street, readings came to withing 1 - 2mm of each other. Until I neglected to empty my manual gauge at 9:00AM rollover the last day it rained, I was seeing generally a 0.5mm difference between the two.

Not a bad outcome in the end I think.

[JT118]

Is your replacement another FO or clone thereof or another type and make altogether? Will attempt to do some test on the FO one I bought a few days ago, I expect it will be a bit variable.

Going to do the calibration as per previous posts on the basis of a 110ml of water being the equivalent of 20mm of rain based on a 55cm2 collector area. I assume I should get 67 clicks after this amount of water has gone through assuming a 0.3mm per click specification.

See this thread, there seems to be two threads on the same topic. http://www.meteocercal.info/forum/Thread...FineOffset

[werk_ag]

See this thread, there seems to be two threads on the same topic. http://www.meteocercal.info/forum/Thread...FineOffset

Sorry, there isn't two threads on the same subject! This one is only about Fine Offset Rain Sensors, the other is about alternatives to the FO Rain Gauges. Here is supposed we talk about FO rain gauges, at the other, it's supposed we talk about alternatives to FO rain gauges (other brands, models, etc).