Topic: Davis VP2 Rain Gauge Suddenly Reading Very High

[LABob]

I can hear a faint "click" sound when I manually swing the magnet by the reed switch, so I assume that's the contacts I'm hearing. I suppose I can listen for that click a bunch of times to verify that it's registering each time and only once each time. I wouldn't even know what to do if I discovered that the reed switch was bad. I'm not good with a soldering iron and I'm not really willing to have no station for two weeks while Davis repairs it. It looks like nobody sells the tipping assembly by itself. You can buy the tipping buckets by themselves, and you can buy the base without the assembly, but you can't buy the assembly itself. I guess my only option is to replace the entire rain collector base :/

[SLOweather]

I just went through the VP2 console and ISS PDFs, searching both on setup and maintenance, and I can't find any reference to setup mode being useful during maintenance, especially of the rain gauge.

The ISS is a transmitter only, and knows nothing of what any receiving console or Envoy is doing. I believe, as noted in other threads here, the rain reading is sent as a serial number of tips since last reset, and that it rolls over at 255(?) This assures that all rain will be recorded by a console, even if a packet or 2 is lost. The console stops receiving ISS transmissions in setup mode, which would stop erroneous wind, temperature, and humidity from being recorded, but when put back in Current Weather Mode, it will receive and record whatever rain fall value the ISS is sending.

So, as labob has noted, the correct thing to do when cleaning or messing with the rain gauge is to unplug it from the SIM board before even taking off the rain cone.

Just put your console in setup mode, as described in the user manual.

[SLOweather]

I'm pretty sure that I purchased a complete tipping bucket "sled" a few years ago, with the tipper, supports, reed switch and cable from Davis. Call them and ask.

I can hear a faint "click" sound when I manually swing the magnet by the reed switch, so I assume that's the contacts I'm hearing. I suppose I can listen for that click a bunch of times to verify that it's registering each time and only once each time. I wouldn't even know what to do if I discovered that the reed switch was bad. I'm not good with a soldering iron and I'm not really willing to have no station for two weeks while Davis repairs it. It looks like nobody sells the tipping assembly by itself. You can buy the tipping buckets by themselves, and you can buy the base without the assembly, but you can't buy the assembly itself. I guess my only option is to replace the entire rain collector base :/

[kobuki]

@LABob: I would imagine that any electronic repair shop would be able to replace the Reed tube for you if you brought in your rain gauge assembly. It is available separately (the Reed), I've seen it in at least 2 US shops in the last few months. Scaled Instruments seems to be popular in these forums, but at the moment they're blocked by my antivirus software (KAV) claiming it contains malware so I can't check.

[Weatherdata]

Yes, an electronic shop may be a good solution for replacing the reed switch, just have to make sure they don't over heat when soldering as is susceptible to heat.

Also Davis sell a mounting base with tipping bucket assembly, may be the easiest/best option Part #: 07852.804

[LABob]

Yeah, I'm trying to avoid replacing the base. I don't fancy taking the station down from the pole, dismantling it, putting it all back together, and remounting it on the pole just to get a reed switch replaced.

My bigger fear is that it isn't the reed switch. If it isn't that, I'll probably have to replace the whole assembly anyway because who knows what's going on with it at that point?

[dalecoy]

Am I correctly reading this thread? 

Previously, your Davis and your CoCoRaHS were in rather close agreement.

Your recent problem was that the Davis suddenly was measuring more rainfall than the CoCoRaHS.  You turned a screw.  Now the Davis is measuring less than the CoCoRaHS. 

But: "trying to avoid replacing the base. I don't fancy taking the station down from the pole, dismantling it, putting it all back together, and remounting it on the pole just to get a reed switch replaced."

Is that the situation?

[LABob]

It was reading more than the CoCoRaHS gauge for two rain storms ... way more. No wind to speak of. Then the next rain storm it was reading 8% less. The fourth storm it was still reading 8% less. Then I adjusted it by 1-1/4 turn on both screws to bump up the reading by ~7.5%. The fifth storm came and it still reads 8% less than the CoCoRaHS gauge.

EDIT: Correcting storm numbers.

[LABob]

Now we're not supposed to get any rain for at least 10 days. 

This is my plan of attack:

1. Rule out mechanical issues with the bucket assembly - I have a dropper bottle that dispenses 0.55mL per drop (I used a digital scale to measure the average of 100 drops three separate times). I suppose I can try that. Every 9 or ten drops should trigger a tip. It's not exact enough for calibration, but it should allow me to see if the tips are consistent.

2. Rule out mechanical/electrical issues with the reed switch - If the buckets pass test #1 I will move in to checking the operation of the reed switch. I'm open to options here. My first thought is to connect my cheap DMM on continuity test to the soldered ends and watch for a blip with every tip of the buckets. If that doesn't work I guess I'll have to take note of the rain readings for the day, month, and year and allow the console to count for me and then reset after the test is complete.

3. If the buckets mechanically tip reliably with the same amount of water for each tip, and the reed switch operates correctly with each bucket tip, I won't know where to go next. Let's hope I don't make it to step #3   

[ValentineWeather]

For 75$ you might be able to avoid a lot of hassle with a standalone unit. Provided you have a place for it.  https://www.scaledinstruments.com/product/davis-7852-rain-collector-for-flat-surface-mount/

[LABob]

I don't understand how it would get rid of the hassle unless there's something inherently flawed with the design of the ISS-integrated version of the rain bucket.

[ValentineWeather]

I don't understand how it would get rid of the hassle unless there's something inherently flawed with the design of the ISS-integrated version of the rain bucket.

NEVERMIND, continue..............  

[dalecoy]

Your tests should be a good start.

For Test #2 - most DMMs (and probably yours) on continuity test will "beep" on contact closure, so that should be helpful.  And as an alternative to direct connection to the reed switch, consider using a telephone "jack" to connect to the plug that normally connects the rain bucket assembly to the ISS.  You can buy a suitable "wall jack" at most hardware stores.

[LABob]

OK. I ran the drip test for 9 trials on each side of the bucket. Counting to ~100 18 times gets old (it's 0.0525 mL per drip, not 0.55 mL). Here are the results:

Trial	North Bucket	South Bucket
1	94	109
2	91	110
3	81	112
4	88	110
5	86	112
6	88	107
7	85	113
8	83	108
9	84	93
Mean	86.667	108.222
Median	86	110
SD	4.062	6.037
Avg. Dev.	3.185	3.704

I was counting very slowly and deliberately out loud so I wouldn't mess up. It's possible to have miscounted by one here or there, but not by five or ten. So that got me to thinking that perhaps the drops aren't a uniform size. A simple test will clear this up. If the variation seen in the rain bucket trial is due to variations in drop size, we should see similar variations when we measure the mass of 100 drops over ten trials.

Trial	Mass
1	5.18
2	5.23
3	5.22
4	5.28
5	5.24
6	5.21
7	5.18
8	5.36
9	5.35
10	5.40
Mean	5.265
Median	5.235
SD	0.079
Avg. Dev.	0.066

The average deviation in drop size is 1.25% while the average deviation in the number of drops required to tip a bucket is 3.68% for the north bucket and 3.42% for the south bucket. If you subtract the average variability in drop size it becomes 2.43% for the north bucket and 2.17% for the south bucket. It looks like that is well within Davis' 4% ± 1 tip specification for the rain gauge. I am forced to conclude that the mechanics of the bucket assembly are working properly.

Am I missing something before I go on to the next step?

EDIT - I think it's interesting to note that the north bucket requires much less to tip than the south bucket, but we've known that the buckets are intentionally unbalanced to some degree. What is also interesting to me is that the average amount of water to produce one tip in the south bucket and one tip in the north bucket is 10.26 mL (5.13 mL per tip). That should mean the Davis is reading about 6% higher than the CoCoRaHS gauge.

[LABob]

Wanna watch someone go down the rabbit hole? I just got the bright idea to use my digital scale instead of counting drops. I'll weigh the dropper bottle before, drip water until the bucket tips and then weigh the bottle again. Duh. What's wrong with me? Be back in a bit with more data. At least be entertained by my OCD 

[LABob]

OK. I guess I can't count worth a darn, or the way I'm making the drops at the ISS isn't as reliable as how I'm making them at the scale. When I test by weight, I get an average of 5.402 mL for the south bucket to tip and 4.532 mL for the north bucket to tip. That means I should be reading about 9% high. That's almost perfectly how much I turned the adjustment screws.

Either way, it looks like the error is not in the tipping mechanism.

[CW2274]

Will it still count tips in setup mode? I tried using setup mode to do maintenance on the ISS before and while I was outside working on the station the console reverted to regular mode (is there a time limit on setup mode?) and I got all kinds of wacky readings. Now I just unplug all the sensors from the ISS when I do work.

No and Davis' new software limits the shutdown to 30 min. which frankly I find irritating.

[LABob]

Well, we've finally had the first measurable rain since the last time I posted. As you recall, my original problem was the Davis reading much more rain than the CoCoRAHS gauge. Then it read much lower on two separate cases so I bumped it up by ~9% with the adjustment screws. So that's where the VP2 stands as our latest rain began. This rain has been "perfect" as far as comparing gauges: Never over 0.12" per hour and literally zero wind.

Last night we received 0.36" according to the VP2 and the CoCoRAHS registered 0.31". I calculated an 18% error from that. Then this morning I took another reading where the VP2 registered 0.99" and the CoCoRAHS 0.84", which is 18% again. That's good. Two different observations that agree down to a tenth of a percent under ideal conditions. When this rain is over I'm going to do a full 18% adjustment and let you all know how it goes.