Topic: Rain Gauge

[George Richardson]

Hello,

As I rapidly approach my 1st anniversary with my Davis VP2 wireless weather station, I have encountered a situation I have never seen referenced on any of the boards. The rain gauge is doing strange things. First, I felt I had adjusted the VP2 gauge accurately relative to a 4" manual gauge. This spring I had at least two rain events where there was a pause in the action and I made a trip to the gauge site and found that with .6 to .9 inches of rain the two gauges agreed. At the end of the event, with total storm rainfall at approximately 1.3 inches, the VP2 read almost 5% high.

OK, so I thought maybe residual water in the big VP2 bucket collected at the end and backed the VP2 off a little. Guess what, another perfect storm and the VP2 reads 5% low below 1" of rain but is within .01 of 1.51" of total storm rainfall.

There does seem to be a consistancy of the VP2 bumping 5% at the 1" mark!?

Anyone ever heard of this or have any ideas?

VWS V12.08 p11 feeding WeatherUnderground KVAMONET1

VPLive 1.1.7.2 feeding CWOP CW6097

Thanks for your consideration.

George Richardson

[talbert1952]

You are sort of comparing apples to oranges for an individual event. Tipping bucket rain collectors are sensitive to rainfall rate. Running the exact same amount of water through the collector at a low rate vs a high rate will result a higher total at the lower rate. The manual gage doesn't exhibit this behavior.

I suggest taking readings of at least 5 rain events from both the Davis and 4" gage. Average the readings from each device and calculate the average error. Use the average error to adjust the Davis collector. Repeat the process after an adjustment.

Also check the buckets for crud build-up. It will affect the collector accuracy.

[kray1000]

The VP/VP2 is less accurate when it rains hard (will report less than the actual amount).  It's best to calibrate when the rain is light in intensity, but also when there's enough of it to give a good comparison.

Don't know what to tell you about the 1" thing.  You may have calibrated it a little on the high side, but with the heavier showers, it could still appear to be pretty accurate...

Just one man's opinion...  

[ncpilot]

Running the exact same amount of water through the collector at a low rate vs a high rate will result a higher total at the lower rate.

What's the technical reason for that?

[talbert1952]

It has to do with the tipping time of the buckets. Water from the funnel is still draining into the "full" bucket until the tipper can move far enough to start filling the other bucket. The extra water is discarded and isn't counted in the total. As the rain rate increases, the amount of extra water dumped into the "full" bucket while the tipper tips increases resulting in undercatch.

[SLOweather]

Plus rainfall is notoriously fickle. Set out a half-dozed identical cans (like small coffee cans) and compare the volumes collected. To get an accurate comparison, I'd use a lab grade graduated cylinder to measure the water in each can.

(Hmm.. Might have to try that next winter...)

[ncpilot]

That's what I was deducing, but wasn't sure...

Based on my testing for calibration, it doesn't seem to take much water in the cone before it backs up, and therefore I would assume given a constant drip rate through the hole (i.e., the drip rate will reach a maximum and not increase once the cone is full), the underflow error could start quite early in a precipitation event?

[Jim's Wx Sta]

I believe this is just the nature of the mechanics.  Last year at one of the Skywarn classes a Sr Forecaster from NWS in Sterling was deriding the value of the tipping buckets as under reporting.   During break I told him that I found the automatic was very accurate and only noticed a slight difference during periods of very heavy rain.

I have a  Davis Monitor II from 1999, and a 4" manual gage.  They're about 50' apart.  Unless it is raining very hard, the readings are identical.  Only when we have heavy rain does the automatic begin to lose a little ground...not enough to be significant.

My guess is that during heavy rain some drops get through before the rocker switch can react, and it slightly under reports.

Jim

[METARMan]

Typical tipping bucket rain gauges for the National Weather Service are certified to accuracy of +/- 5%.  Small errors should be nothing to be concerned about.

I've calibrated my VP 2 rain gauge to measure accurately in very heavy rain (several inches per hour rate).  That is because most of my rain comes from thunderstorms during the summer rainy season.  

Yes, my gauge reads "too high" during light precipitation events, but since 80-90% of my annual rain comes from thuderstorm and convective precip my current calibration works best for me.

[tinplate]

Also, as I found out recently after a discrepancy between the VP2 and a 4" manual gauge mounted at the same height on the same post, it's very important that the VP2's ISS unit be perfectly level. If not, it affects the accuracy of the tipping bucket mechanism. Since the tipping bucket mechanism is one of delicate balance, close to level is not sufficient. It should BE level.

[ncpilot]

Resurrecting this thread to ask... is there a preferred way to calibrate such that you don't record the data?

Just pull out the weatherlink module from the base unit? (after downloading all data of course!)

Other suggestions?

[SLOweather]

Unplug the rain gauge cable from the ISS (inside the white box).

[ocala]

You would have to keep the cable plugged into the ISS so you could see the result on the screen.
Your first suggestion of unplugging the data logger is probably the best way to go.
After your done you would have to clear the totals from the unit manually or I think it would send the data to the software when you reconnect the data logger.

[talbert1952]

Resurrecting this thread to ask... is there a preferred way to calibrate such that you don't record the data?

Just pull out the weatherlink module from the base unit? (after downloading all data of course!)

Other suggestions?

Unplug the rain collector and count the tips. The collector makes a distinctive noise when the bucket tips. Just don't lose count in the middle of a test.

[ncpilot]

While unplugging the cable in the ISS seems the simplest, to calibrate with 1" of "rain" means I'd have to count 100 "tips"..... I like the simplicity, but I'd hate to get distracted and miscount...  

Thanks for the suggestions...

I'm going with a higher volume, perhaps up to 2", because we had a deluge here over the weekend, had to have been at least 2"+ to give us the road flooding, but I only read about .45"...

[Mark / Ohio]

While unplugging the cable in the ISS seems the simplest, to calibrate with 1" of "rain" means I'd have to count 100 "tips"..... I like the simplicity, but I'd hate to get distracted and miscount...  ...

Just start singing 100 bottles of beer on the wall and pour your beer very slowly into the top catching it in a mug as is alternatively runs out each drain in the bottom.  You won't loose count that way.    

[ncpilot]

I figured I'd lose count because I'd be talking to Jim Beam while the water dripped...  

[ncpilot]

I got my 4" rain gauge, and checked calibration today.

Something is seriously wrong with my VP2, or I've overlooked something major.

Initially I was getting about 1/3 the reading of actual rain, e.g., .3" in the cylinder would register about .11 on the VP2.

Thinking that might be a bit too crude, to just dump all the water in at once... I got a dropper, and one by one, dropped water into the cylinder until I got to .02", then added more to .03", then to .04".

I get about 38-39 drops per .01".

Next I used the dropper to drop water directly into the rain bucket, and it took anywhere from 87 to 91 drops (depending on cup) to tip the bucket.

Then I adjusted the screws upward (counter clockwise) until they were about to fall out, and still can't get it close.

Yes, I checked the level in all directions, and the fact that the difference between buckets was only about 4 drops leads me to believe it's level, otherwise there would be a greater disparity between volume required to tip the buckets.

I can't figure out what the problem might be... it just doesn't make sense...

[Mark / Ohio]

Sounds like maybe the reed switch is missing passes of the magnet when it tips.  Also make sure the plug from the switch to the ISS is good and clean.  Some corrosion in there could reduce the voltage change enough the IC chip in the ISS might not be able to detect the closure of the reed occasionally.

Just some thoughts...

[ncpilot]

That's not it... I was dropping the water directly into the buckets, not the funnel, so I could see when the bucket's tipped... I wasn't using the console for feedback...

I guess that wasn't clear in my other post, except for the original dump of .3", everything else I did was with the funnel removed, and dropping water directly into the buckets...

Also I checked level on top of the funnel, and I also checked level with the funnel removed... (on the main body of the ISS)...

[jaded]

Are you sure the bucket is absolutely free to tip?  Is everything clean, no sticky spider webs, pine tree pitch, or insecticides on any of the moving parts?    If you tip the bucket by finger, can you feel any "stickiness" such as a sudden release of the bucket mechanism, or a "slow spot" in the tip?  Is the bucket itself clean, with no bird droppings glued to it?  

It sounds like the pivot is sticking somewhere, possibly inside the tube where the hinge pin goes through the bottom of the tipping bucket.  I've not ever taken mine apart (no need yet) but it looks like you can press the pin out from the side and remove it.  Check the pin for corrosion, and clean it thoroughly with soap and water on a cloth (don't use sandpaper, you don't want to damage its finish.)  Run a soap-and-water covered pipe cleaner through the hinge pin hole and clean it throughly, too.  Make sure the bottoms of the buckets are clean, especially where they contact the screw heads.  Clean the screw heads, too.

If you can positively identify the sticky substance (such as pitch) you may need a special chemical to clean it up.  But be very careful that any cleaner you use won't attack the plastics of the bucket or the ISS.

Dry everything before reassembling it (blow some dry compressed air through the pivot hole.)  A single drop of light household oil in the pivot hole should both protect and lubricate the hinge pin.  You could try WD40 as that should displace any water, too.  Don't use any type of grease or motor oil, as they're too thick for such a delicate mechanism.

[ncpilot]

I was only about a foot a way from the mechanism while I dripped water into the buckets, and didn't see anything like you mention... and I was tipping it by hand, and didn't notice any impediments.

Before doing anything else--like taking it apart, I'll call Davis on Monday to see if they have any ideas...

Could be, living basically on the coast, that there's an issue of corrosion from salt?

I'll take a closer look tomorrow with all that in mind...

[ncpilot]

Yeah, 4 AM... it hit me, as I was taking a pee, (the pee didn't hit me...), that my fatal assumption was that an inch = inch no matter what the measuring device. (water I mean, a fisherman of course has problems with measurement)

Forgot to include volume as related to the surface area of the collecting tool... 1" of rain in a 4" collector, if dumped into the VP2, will not be 1" to the VP2...

I'll bet the ratio of the surface area between the two collectors will be close to 3:1...... but I'll leave that for later this morning after I go back to bed, then get up later for coffee...

[Ravenstar]

as I was taking a pee

TMI  <------

[Mark / Ohio]

.....Forgot to include volume as related to the surface area of the collecting tool... 1" of rain in a 4" collector, if dumped into the VP2, will not be 1" to the VP2...

I'll bet the ratio of the surface area between the two collectors will be close to 3:1...... but I'll leave that for later this morning after I go back to bed, then get up later for coffee...

http://www.weatherstations.co.uk/vp_faqt2.stm

"Calibration by simulating rainfall

An alternative calibration approach is to drip a known volume of water through the raingauge and to oberve the resulting reading. The required volume may be easily calculated as follows: The effective internal diameter of the Davis VP raingauge aperture is ~165mm, giving an area of 21382mm^2. One inch (25.4mm) of rainfall would therefore correspond to a volume of 544ml (18.39 ounces US). This volume needs to be measured out as accurately as possible, preferably using scientific volume measures, and dripped through the rain gauge over a period of 30-60 minutes, so as to stay within the maximum equivalent rainfall rate for maximum accuracy. (NB Even this calculation is not strictly accurate as the tipping buckets actually work on the weight of water and not its volume. The accuracy of the raingauge calibration will therefore vary the temperature of the water used for calibration or more strictly with its density)."