Rain Bucket Modification - Fine Offset WH1081

[AllyCat]

This means that if all our collectors are the same size then we are exaggerating our rainfall by 9.9%.

Hi,

Welcome to the forum. Fine Offset claim +/- 10% rainfall accuracy, so you appear to have a FO device that's within its specification!

But seriously, you are absolutely correct to try to calibrate your unit. However, do bear in mind that it is basically a very "cheap and cheerful" sensor, so absolute accuracy (and particularly, repeatability between different units) cannot be guaranteed (or even expected). I think the majority of people who have tested their FO rain sensors actually find it under-reads somewhat (perhaps due to splash-out). You may find this thread of interest, it's not specifically concerned with the calibration of the FO tipping bucket sensor, but it does include a number of comparative measurements made with it and a "manual" rain gauge.

You might have already found other threads on this forum (or elsewhere) about rainfall accuracy and in addition to Steve's comments above, here IMHO are a few other "important" factors: Any (tipping bucket) sensor must be mounted very firmly (not on a swaying pole!), absolutely horizontal and preferably close to ground level (but not near to anything that might screen the rain collection). Then, the FO version generally does need some modifications to reduce splash-out and to prevent the entry of spiders, etc..

Cheers, Alan.

[denlong]

Thanks Alan & Steve for your quick replies.
I have been using Cumulus for about 3 years and it wasn't until I downloaded the latest version and was able to use the new log reader that I was able to look back at old data (i.e. Flood episode of 24/11/12) that I was able to look seriously for the parameters which I could use to try to predict flood events in my area.
The UK Met Office (Hadley Cente) is only about 6 miles from here and many employess live nearby. You can imagine how exposed I am feeling trying to use what may seem like a toy to them for the serious task of measuring weather conditions in the micro climate area that is my home village. Hence my desire to make this equipment as accurate as possible, and that is the reason I wanted to modify the rain collector to be as accurate as possible.
I hoped to do away with the need for correction factors (which I know is possible with Cumulus), because the receiving console of the WH 1081 is not able to apply correction factors, as far as I know. I want to use that as the primary reporting and hence alarm sensing device for my project. the EasyWeather programme is quite useful for setting up alarms in the console which will continuously monitor the weather conditions here without the need to keep a computer running all the time.
I take the point about more accurate calibration - I only have a 10 ml syringe which does deliver a drop at a time but it makes my arm ache when I do it slowly so I decided three lots of 7.5 ml was enough and in any case the bucket tipped 5 times in every case - so the repeatability was very good. I understand now that is a dynamic calibration albeit for a reasonably average rain rate (equivalent to 4.5 mm rain in 15 minutes).
Has anyone else reported a calibration like mine? I have searched and diligently read a lot of the forum and I cannot find anything along these lines.
Cheers - Den