Author Topic: CoCoRaHS Gauge Reading  (Read 1101 times)

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Offline LABob

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CoCoRaHS Gauge Reading
« on: January 10, 2018, 10:46:38 AM »
On several scales that are well-calibrated, I discovered that the CoCoRaHS gauge collects 2g of water for every 0.01" of rain. The math checks out. A precisely 4" round container would capture 2.059g per 0.01". It's interesting to me because math vs. reality suggests that the CoCoRaHS gauge has a 2-3% variance. This is because when I fill the tube so that the meniscus is precisely on the 1.00" mark, it contains 201g of water. If the gauge were precisely 4" in diameter and perfectly round, 1.00" should be 206g.

I find this to be a much easier way of reading amounts greater than 1.00" than decanting. Every time I perform both measurements, the decanting loses 0.01" for every pour. Anyone else try weighing their captured rainfall?

Of course you can't really weigh it in the middle of a storm, but I think you could just pour the water into another container and take it inside. The nice thing about that approach is it's every easy to measure any loss that occurs transferring water from the gauge to another container.
« Last Edit: January 10, 2018, 10:54:57 AM by LABob »

Offline LABob

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Re: CoCoRaHS Gauge Reading
« Reply #1 on: January 10, 2018, 11:17:18 AM »
Now I'm wondering which is wrong, the shape and size of my gauge or the markings on the inside tube. This morning I measured 620g of water in the gauge, which would be 3.10" using the 2g per 0.01" I observe with testing, but would be 3.01" using the amount calculated for a 4" circle capture area.

Offline miraculon

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Re: CoCoRaHS Gauge Reading
« Reply #2 on: January 10, 2018, 01:56:48 PM »
Since you are in LA, you probably don't need to worry about snow much, but I have been using the Escali scale from WeatherYourWay for a couple of years now for SWE. It makes a much quicker job of it than melting it and pouring it into the inner cylinder. I use the 201g/inch standard myself.

I do revert to the normal method when warmer weather returns. If I weigh, I make a note in my comments about "SWE by weighing method", per the advice of my regional coordinator. I asked CoCoRaHS for a "tick box" for this, but there was some database issue and they have not implemented the suggestion (at least yet).

Greg H.



Blitzortung Stations #706 and #1682
CoCoRaHS: MI-PI-1
CWOP: CW4114 and KE8DAF-13
WU: KMIROGER7
Amateur Radio Callsign: KE8DAF

Offline LABob

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Re: CoCoRaHS Gauge Reading
« Reply #3 on: January 10, 2018, 06:13:38 PM »
Interesting. When I get home I'm going to take my vernier calipers to the CoCoRaHS gauge to see if it's really precisely 4" in diameter and actually round. I'm not sure what I'll do if I discover it's not round as calculating the area of an arbitrary shape is not easy. Is it all overkill? Of course, but it's interesting ... to me anyway.

Offline LABob

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Re: CoCoRaHS Gauge Reading
« Reply #4 on: January 10, 2018, 11:58:37 PM »
Welp, the funnel ranges from 4.07" to 4.075" diameter depending on where you measure. If I simplify that to an ellipse with a semi-major axis of 4.075" and a semi-minor axis of 4.07", I get a collecting area of 13.026 square inches. Every 0.01" of rain captures 2.135g of water. My 620g captured this morning with the funnel on, then, is really 2.90", while the gauge read closer to 3.07. An error close to 6%.


Offline LABob

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Re: CoCoRaHS Gauge Reading
« Reply #5 on: January 11, 2018, 12:32:02 AM »
The main body is 4.105" inside diameter most any way I measure it. That's even worse. It's 5.32% greater collection area than the specs suggest.

Not sure what to do with this information.

Offline LABob

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Re: CoCoRaHS Gauge Reading
« Reply #6 on: January 11, 2018, 12:49:08 AM »
After careful measurement with the inner graduated cylinder, I can confirm that the markings are 2.02g per 0.01".

So, if I want to report my CoCoRaHS the way the gauge would read, I divide the total mass of water collected by 202.22. If I want to report more accurately, if the funnel was on I divide the total mass of water collected by 213.458, and if the funnel was off I divide by 216.879.

Today's rain is 3.07" by direct reading, and 2.90" by correcting for more accurate funnel area. If I had collected that rain in the main tube without the funnel on it would have been 2.86". That amount of variation seems significant to me.

Offline miraculon

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Re: CoCoRaHS Gauge Reading
« Reply #7 on: January 11, 2018, 08:42:47 AM »
I have three outer cylinders because of the snow measurements. I typically swap the cylinder out with snow collected and substitute a clean one from the garage. The other is used to get the core sample from the snow board. (aren't you glad that you don't have to deal with all this?)

Hence, I have taken the empty weights for each gauge and labelled and numbered them. I "tare" the scale before I place the gauge and make note of which cylinder number it was. I have spreadsheet and just put the collected mass + gauge and the spreadsheet subtracts the empty reading and calculates the inches from the mass.

I also keep a 500mg weight as a reference and recheck the scale before and after the gauge measurements. FYI, the Escali scale is prone to drifting around at low temperatures, Escali says the scale is designed for use above 40F.

Greg H.


Blitzortung Stations #706 and #1682
CoCoRaHS: MI-PI-1
CWOP: CW4114 and KE8DAF-13
WU: KMIROGER7
Amateur Radio Callsign: KE8DAF

Offline SLOweather

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Re: CoCoRaHS Gauge Reading
« Reply #8 on: January 11, 2018, 08:55:28 AM »
LABob, did you measure the diameters at different temperatures? Polycarbonate has a fairly high coefficient of thermal expansion.

Offline LABob

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Re: CoCoRaHS Gauge Reading
« Reply #9 on: January 11, 2018, 10:05:49 AM »
I haven't tried different temps yet, but I don't think mine is polycarbonate. I'm fairly certain it's acrylic, which is even worse. I measured indoors at 67F. I wonder if this is part of why we feel like we have so much trouble with the Davis rain gauge - we're trying to measure something precisely with yardsticks that are worse than 5%.
« Last Edit: January 11, 2018, 10:11:02 AM by LABob »