increase accuracy of fine offset rain tipper

[SUBYDAZZ]

I have a Digitech XC0348 station.

Has anyone successfully made their fine offset type rain tipper gauge read more accurately by increasing the catchment area? I'm thinking about getting rain events down to 0.1mm instead of the existing 0.3mm.

Any dramas, downsides or concerns? Is it easy enough to do or do you have to be fairly handy?

[daj]

I recommend a search for "rain mod" in the fourms -- lots of discussion on this previously

[steve]

But note that increasing the catchment area doesn't make your rain gauge more accurate; most likely the opposite, in fact.

[Gina]

Quite true - just more sensitive. Registering light showers that may otherwise be missed. I wouldn't recommend increasing the catchment area where you get very heavy rainfall though steeper sides might help.

[beteljuice]

... doesn't make your rain gauge more accurate; most likely the opposite, in fact.

The beteljuice increased a LaCrosse pluviometer area by 10 fold with no problems (used a bit of guttering)

If you increase the area by 10, then the 'slack' for fine adjustment is also increased by 10.

[SUBYDAZZ]

But note that increasing the catchment area doesn't make your rain gauge more accurate; most likely the opposite, in fact.

OK, so it makes it more sensitive, but why wouldn't it also be more accurate? Assuming you add the factor in Cumulus to compensate for the added catchment area?

I just think it's a pity that it doesn't register any rain under 0.3mm or a rain rate of under 3.6mm/h or multiples of each.

[steve]

OK, so it makes it more sensitive, but why wouldn't it also be more accurate? Assuming you add the factor in Cumulus to compensate for the added catchment area?

There's a difference between accuracy and precision. You can't improve the basic accuracy of the rain gauge mechanism by increasing the size of the funnel. I do take beteljuice's point, though; if you then use the calibration factor in Cumulus you might be able get back to somewhere near the original accuracy, but in heavy rain it is definitely going to be worse. The advantage to having a larger funnel is, as Gina said, its sensitivity to small amounts of rain.

[nking]

Another consideration is by increasing the catchment area there is also a greater area for condensation/frost to occur and then when it thaws you will likely get more frequent readings of rain when there hasn't been any...... just a thought.

[Gina]

Another consideration is by increasing the catchment area there is also a greater area for condensation/frost to occur and then when it thaws you will likely get more frequent readings of rain when there hasn't been any...... just a thought.

A good thought but in practice, even with 9 times the sensitivity I've not had one bucketfull of moisture registered from frost. And since I've had a heater going inside the enclosure, no ice has formed on the funnel surface. Snow melt did register but the actual water content in snow is very small. That was before I had the heater so there were about 2-3 inches of snow in the funnel which produced about 0.1mm of "rainfall" when it melted - 3 bucket tips over several hours. A very heavy dew could be considered "rainfall" I would think, but again I have only seen one bucket tip with my 10" funnel from dew. That is 0.036mm (by calibration with real rain against a manual rain gauge - estimated accuracy 10%).

[SUBYDAZZ]

OK, so by adding a larger catchment area and using the calibration factor in Cumulus to compensate for this (eg: If I triple the catchment area of my gauge and then go under Multipliers for rainfall in Cumulus Calibration and put in 0.33) then it will be more sensitive to smaller amounts of rain and variations in the rain rate?

So far as I see it, this should give me slightly more accurate data with respect to having less likelihood of say 0.1 or 0.2mm of rain not being counted because previously it would only measure at least 0.3mm, and thereby giving me nil rain.

I take your point though on the accuracy of the device as far as measurement goes, if the original gauge is not perfectly accurate (and of course it wouldn't be as it's hardly a precision instrument) then any error will be propagated if you go changing the catchment area etc.

With respect to heavy rain, if you have a large catch area does this cause any problems like the reservoir overflowing because the bucket can't tip fast enough or any spillage through the bucket area as a result of the moving bucket itself?

[steve]

So far as I see it, this should give me slightly more accurate data with respect to having less likelihood of say 0.1 or 0.2mm of rain not being counted because previously it would only measure at least 0.3mm, and thereby giving me nil rain.

Yes, that's correct. I did in fact add a funnel to my Oregon rain gauge (my first station) because that had a 1mm bucket, and a lot of the time small amounts of rain just weren't being registered at all.

With respect to heavy rain, if you have a large catch area does this cause any problems like the reservoir overflowing because the bucket can't tip fast enough or any spillage through the bucket area as a result of the moving bucket itself?

I don't think it's likely to overflow, but one problem is that while it's tipping it's not measuring rain, so that rain is lost. This can be significant if it's tipping frequently.

[tjaliwalpa]

During this last week I have had extremely heavy rainfall rates recorded by my guage (Fine Offset with a modified larger catchment funnel). It has registered a rainfall rate of 131.9 mm/hr just a few days ago. The total rainfall I recorded matched that of an official weather station less than 1 km away to within 1-2%. I figure that is not a bad result. The calibration I currently use is .37, so bucket tips are recording as .111mm. I arrived at the .37 multiplier not by a calculation based on area of the modified and original collector alone, but by comparing the Fine Offset rain readings to a manual rain guage placed along side my stations modified rain collector. Over time I have settled on the .37 multiplier.

Bob

[SUBYDAZZ]

During this last week I have had extremely heavy rainfall rates recorded by my guage (Fine Offset with a modified larger catchment funnel). It has registered a rainfall rate of 131.9 mm/hr just a few days ago. The total rainfall I recorded matched that of an official weather station less than 1 km away to within 1-2%. I figure that is not a bad result. The calibration I currently use is .37, so bucket tips are recording as .111mm. I arrived at the .37 multiplier not by a calculation based on area of the modified and original collector alone, but by comparing the Fine Offset rain readings to a manual rain guage placed along side my stations modified rain collector. Over time I have settled on the .37 multiplier.

Bob

How much bigger than the original area is your catchment funnel?

[beteljuice]

... I arrived at the .37 multiplier ...

So approx. 3x

[tjaliwalpa]

How much bigger than the original area is your catchment funnel?

Approximately 5 times the area. I expected the multiplier to be .33, but in fact I needed it to be .37. This may have something to do with the water lost during the tipping process (as Steve suggested).

Bob