Rain Gague Accuracy

[arthurhh]

I have a FOHK 1081 which I have been evaluating for a little project.

I have all the sensors mounted on the cross arms provided and it works quite well (given the already known issues with the wind vane)

My problem is the unit appears to be over reporting rainfall by %50. I matched it against my main station which I was happy was accurate initially, then I got a Nylex 1000 main rain gauge to check and yep over reading it is.

Anyone else experienced this?

I will move the rain sensor down to the location of my Main station and manual rain gauge to see if it might be a vibration issue, aside from that any other ideas.

[harrym1byt]

I fancy the errors produced by the rain gauges location, will make a larger difference than any errors in the actual mechanism. Too be sure of proper location, the gauge really needs to be out in the open, clear of buildings, over hanging trees and with shelter from the effects of the wind. It also needs to be mounted so that wind cannot shake the gauges buckets producing false counts.

Its actual accuracy can be easily checked by working out the gauges funnel cross section and very slowly dripping 2" x (cross section) worth of water into it, then checking the result.

Cumulus does allow for some adjustment for errors in input.

[steve]

Typically, TBRs tend to under-read, particularly in heavy rain. Also, putting them up off the ground, which is where they tend to be with 'hobby' weather stations makes them under-read even more (due to wind etc). So, for yours to be over-reading suggests to me that changing its location isn't going to help.

Calibrating it as Harry suggests is a good idea. With both of the rain gauges I've owned (Oregeon Scientific and Davis) it is possible to adjust the tipping mechanism - I wonder if the Fine Offset gauges have something like that (even if it isn't documented)?

[arthurhh]

Yes the over read is puzzling. FOHK is a very small TBRG and I expected a certain lack of accuracy due to this as everyone has said on the under reading side.

Currently the TBRG is on the supplied cross arms (I want to make it simple as possible for my intended users to set up) and mounted on a pole attached to my main station gallows. There is no wind effect (other than the TBRG being higher than the other gauges) No vibration issues as it doesn't record any rain when it it is only windy.

I will move it down to the lower gallows on my main station tomorrow at Met Day roll Over (9am NZ) and see what happens when it is in the same location as my std gauge (La Crosse) and my manual gauge.

The prelim tests indicated exactly %50 percent over read 15mm for actual of 10mm sort of suggests some issue with the counting/measuring side of things.

[harrym1byt]

There is no mechanical adjustment that I could see. I think I recall reading something about a fault on a similar cheap system with an identical TBRG where the reed switch contact was 'delaminating' - a whisker of metal had come of the reed making it produce multiple counts for each tip of the buckets. If this is the case, they are easy and cheap to replace (few pence from Maplins).

[arthurhh]

Maplins ? oh yeah hang on i will just whip out the plastic and get a ticket to visit them, opps not so fast $2500 one way thats a bit steep

Okay tounge out of cheek. second run didnt produce the same results different error percentage so it may yet be both a position issue and a mechanical issue

[philcdav]

Morning guys.

Am also getting some odd " high "rain readings.

Am sure its NOT due to rain.

My best guess is that its down to wind blowing the 'buckets' about.

In an effort to stop spider access i have fitted some light fibre mesh across the rain outlets. This has stopped the spider ingress but doesnt seem to have stopped some dodgy rain readings.

If strong winds can whip the buckets about it may be that some form of external baffle may help.

Regards - Phil (G0DOR - Liverpool)

[Namrof]

I had a problem with my gauge over-reading due to wind.

Fixed it by covering 95% of the open base of it with duct tape. Just left some space for 'drainage out' rather than 'gale in'.

No problems since

Cheers - Phil
Gebbies Valley, NZ

[harrym1byt]

I have had no false rain recorded by the display, but lots of problems in the past with EW's false rainfall. My guage is mounted low down on the pole and in the shelter of an out building.

[arthurhh]

Mine is definitely not a wind issue, when I get a few minutes and its dry I will pull it apart and see whats what. I get no readings on windy days with no rain so it isn't getting blown around or vibrated.