Well here are the results;
The FO Rain Gauge is definitely built on the following numbers:
Collector area 5500mm2 or 55cm2;
Each tip of the bucket REPRESENTS 0.3 mm of rain falling into our 55cm2 collector.
To do a check calibration proceed as follows:
Accurately and carefully measure 55ml for a quick check or 110ml for a better check. This must be an exact measure using a laboratory measure.
55ml of water represents 10mm of rain. Therefore if you very carefully and slowly drip (don't pour )(and not directly into the small hole above the bucket, let it run down the side of the collector), that water into the gauge you should observe two things; one is you should hear the gauge tip 33 times. The second is you should see 10mm of rain register on your readout. If these things do not happen there is something wrong.
I have done this test 20 times now with both 55ml of water and 110ml of water. The gauge has consistently tipped 33 + or - 1 (or 66 + or - 2) times and read 10mm (or 20mm) on a readout. So this gauge at any rate is reasonably accurate.
If this does not happen check: a) wire connections; b) that the gauge is absolutely level, c) and that the tipping bucket is moving freely.
That is the accuracy check. Fairly good really.
Please remember the volume of water held by the tipping bucket is NOT the same figure as the amount of rain it represents. The figures are related arithmetically but are not the same numbers. (In the case of the FO Gauge each tip requires 1.66ml of water (55ml water / 33 tips = 1.66ml water) and this represents 0.3mm of rain which is the figure that really matters.)
Precision is another matter altogether. You will find this very difficult to achieve with this gauge. There is always going to be splash out given the design of the collector, and that will vary according to the intensity of the rainfall, wind etc etc. If you are tempted to put an extension on to reduce this problem be very careful how you do it. With such a tiny collector area (relative to professional gauges) a tiny increase (ie a few mm extra in any cross sectional dimension) will easily make the gauge over read by 10% or more. If you put a funnel or similar as your collector you will completely alter the recorded rain to the point that your figures mean nothing at all, unless your funnel has a cross sectional area of exactly 55cm2.
In summary the FO gauge will give a vague representation of rainfall over time. So long as this is accepted and the rainfall figures it produces not taken as a accurate, only representative; it is good value for money (@£4.99).
But if you want more accurate data consider an alternative such as Davis Rain Gauge or similar, remembering that each tip only on these better quality gauges represents 0.2mm (or 0.01" if it is an imperial gauge; NB: they are different and are setup differently mainly by the screw adjusters under each "Bucket" See attached) and one will need to alter the arduino sketch and / or the multipliers in Cumulus to get a correct rainfall figure. Davis or Texas Electronics ( and others are either calibrated as Metric or Imperial versions this is changed by altering the adjusting screws. Imperial gauges are calibrated to 0.01" or 0.254mm and metric to 0.2mm or 0.00787", as you can see there is a substantial 27% difference between the two and careful calibration is needed.
The FO Rain Gauge is definitely built on the following numbers:
Collector area 5500mm2 or 55cm2;
Each tip of the bucket REPRESENTS 0.3 mm of rain falling into our 55cm2 collector.
To do a check calibration proceed as follows:
Accurately and carefully measure 55ml for a quick check or 110ml for a better check. This must be an exact measure using a laboratory measure.
55ml of water represents 10mm of rain. Therefore if you very carefully and slowly drip (don't pour )(and not directly into the small hole above the bucket, let it run down the side of the collector), that water into the gauge you should observe two things; one is you should hear the gauge tip 33 times. The second is you should see 10mm of rain register on your readout. If these things do not happen there is something wrong.
I have done this test 20 times now with both 55ml of water and 110ml of water. The gauge has consistently tipped 33 + or - 1 (or 66 + or - 2) times and read 10mm (or 20mm) on a readout. So this gauge at any rate is reasonably accurate.
If this does not happen check: a) wire connections; b) that the gauge is absolutely level, c) and that the tipping bucket is moving freely.
That is the accuracy check. Fairly good really.
Please remember the volume of water held by the tipping bucket is NOT the same figure as the amount of rain it represents. The figures are related arithmetically but are not the same numbers. (In the case of the FO Gauge each tip requires 1.66ml of water (55ml water / 33 tips = 1.66ml water) and this represents 0.3mm of rain which is the figure that really matters.)
Precision is another matter altogether. You will find this very difficult to achieve with this gauge. There is always going to be splash out given the design of the collector, and that will vary according to the intensity of the rainfall, wind etc etc. If you are tempted to put an extension on to reduce this problem be very careful how you do it. With such a tiny collector area (relative to professional gauges) a tiny increase (ie a few mm extra in any cross sectional dimension) will easily make the gauge over read by 10% or more. If you put a funnel or similar as your collector you will completely alter the recorded rain to the point that your figures mean nothing at all, unless your funnel has a cross sectional area of exactly 55cm2.
In summary the FO gauge will give a vague representation of rainfall over time. So long as this is accepted and the rainfall figures it produces not taken as a accurate, only representative; it is good value for money (@£4.99).
But if you want more accurate data consider an alternative such as Davis Rain Gauge or similar, remembering that each tip only on these better quality gauges represents 0.2mm (or 0.01" if it is an imperial gauge; NB: they are different and are setup differently mainly by the screw adjusters under each "Bucket" See attached) and one will need to alter the arduino sketch and / or the multipliers in Cumulus to get a correct rainfall figure. Davis or Texas Electronics ( and others are either calibrated as Metric or Imperial versions this is changed by altering the adjusting screws. Imperial gauges are calibrated to 0.01" or 0.254mm and metric to 0.2mm or 0.00787", as you can see there is a substantial 27% difference between the two and careful calibration is needed.