Topic: Vantage Pro 2 low rain readings

[desrae20]

Hi,
 I have been monitoring the rain collection on my new Vantage Pro 2 for nearly 2 months, and believe compared to a co located manual gauge and other Australian BOM reading in the area it is reading about 20% lower.
Is there any means of manual adjusting the tipper mechanism to counter this low reading.
Thanks,
Des

[George Richardson]

Des,
All Davis sensor values can be calibrated, either in the console or, as is the case with the rain buckets, at the sensor. The adjusting screw for the rain bucket is 6% per revolution. However, speaking from bad experience, I would urge you to not adjust more than 1 revolution, allow several rain sessions to calculate the new offset and adjust again. If you say that 20% should be 3 1/3 revolutions, I'm afraid you will be way off (at least I was).
Good Luck,
George

[desrae20]

Thanks for your reply, but where is this adjustment screw, I have found no reference in manual or am I only have a "man" look ?
Des

[moehoward4]

Take the rain bucket off the base....at each end of the tipping bucket, mounted in the base are TWO adjustment screws(one each end), use a 3/16" wrench to adjust....there is a small diagram molded into the base giving you some help(+/-, %). Like George said, DO NOT get carried away with massive adjustments......

[ericfynne]

http://www.davisnet.com/product_documents/weather/manuals/07395-275_IM_07852.pdf

The screws are clearly shown on the diagram, and adjustment is described on pages 6 and 7.

Eric

[moehoward4]

One more thing....EQUAL amount of turns each screw !!!!

[miraculon]

My previous advice (consistent with what has been given here basically) is here: http://www.wxforum.net/index.php?topic=27966.msg270283#msg270283 The only thing that I would add is to calibrate with a known volume of water under controlled conditions. If you look at "rain gauge calibrators" they have a water reservoir that is fed through precision orifices to deliver the water under a constant, controlled rate.

This might be helpful as well: http://www.weather-watch.com/smf/index.php?topic=46062.msg374384#msg374384

I have not attempted to calibrate my current VP2 rain gauge, although I tried on an earlier Davis rain collector, with mixed results. I mounted a wind measurement set close to my lSS and found that there is undercount due to a rain shadow from the SE. Also, I think that the tapered cone aerodynamics may be contributing to a lift when wind is present, also lowering rain count.

I calibrated my NovaLynx 8" gauge using home-brew equipment consisting of a adapted PVC reducer and gas-jet orifice. I used the same 1g resolution scale that I use for CoCoRaHS snow SWE measurements to determine the water volume. This was an all-day project and I made several passes even after settling out on the final value to determine repeatability.

I would make comparisons between gauges only when there is fairly gentle to moderate rain and no wind.

Greg

[desrae20]

Thanks Greg, George & Eric,
I have removed the collector cone and I can see the adjustments you are referring to.
In the manual 'Integrated Sensor Suite" that come with the unit there is no mention of the adjustment screws, but in the Rain Collector II that you linked me to it is quite clear.
I will heed your advice and adjust a little at a time.
Thanks again.
Regards,
Des

[desrae20]

Hi again,
I did a rough test, using a syringe, and now find that each buckets takes about 5ml of water to tip, and then using formula h(cm)=v/ 3.14x("r"squared) gives a rough figure of .23mm per tip. (.023(cm)=5/(3.14x8.25x8.25)) Refer "The Weather Observer's Handbook" pg 306.  r=8.25cm is figure for Davis Vantage Pro funnel hole radius.
If this is right I still need some adjustments, but will wait for the real stuff to fall first.
All interesting
Thanks again,
Regards,
Des

[CW2274]

Hi again,
I did a rough test, using a syringe, and now find that each buckets takes about 5ml of water to tip, and then using formula h(cm)=v/ 3.14x("r"squared) gives a rough figure of .23mm per tip. (.023(cm)=5/(3.14x8.25x8.25)) Refer "The Weather Observer's Handbook" pg 306.  r=8.25cm is figure for Davis Vantage Pro funnel hole radius.
If this is right I still need some adjustments, but will wait for the real stuff to fall first.
All interesting
Thanks again,
Regards,
Des

I have no idea what you just said, but the correct amount per tip for a VP2 is 544ml.

[desrae20]

I don't think so as 500ml is about 2 cups of water. ?
Des

[CW2274]

I don't think so as 500ml is about 2 cups of water. ?
Des

Oops! Forgot my decimal point, 5.44ml.

[desrae20]

No worries, I thought that may be the case.
So my experiment that resulted in approx 5ml of water per bucket is not far out, and confirmed via that formula.
Thanks again for your help and interest.
Des

[Mattk]

So is this setup as an imperial or metric rain collector? Reference to BOM and being AU one would suspect it is a metric collector? Hence the 5.44ml would not be correct? 

[CW2274]

So is this setup as an imperial or metric rain collector? Reference to BOM and being AU one would suspect it is a metric collector? Hence the 5.44ml would not be correct?

5.44 ml equates to 0.01 inch of rain.

[Mattk]

So is this setup as an imperial or metric rain collector? Reference to BOM and being AU one would suspect it is a metric collector? Hence the 5.44ml would not be correct?

5.44 ml equates to 0.01 inch of rain.

Yes but for a Metric 0.2mm tipper setup the amount will be 4.26mm per tip. Australia is a metric country, rainfall is measured in mm's, hence the AU Davis version will be most likely be metric 

[CW2274]

So is this setup as an imperial or metric rain collector? Reference to BOM and being AU one would suspect it is a metric collector? Hence the 5.44ml would not be correct?

5.44 ml equates to 0.01 inch of rain.

Yes but for a Metric 0.2mm tipper setup the amount will be 4.26mm per tip. Australia is a metric country, rainfall is measured in mm's, hence the AU Davis version will be most likely be metric

I figured smart folks like yourselves would get the conversion.

[desrae20]

Yes my rain gauge is set up for metric and the formula I referred to earlier confirms that the Davis gauge, to be exact, should tip with 4.26ml of water or .2mm of rain if the readings are to be accurate.
Thanks all for your interest I have appreciate the discussion.
Regards

[Mattk]

20% low doesn't surprise me, that's about par for the course.

[desrae20]

A follow up, I consider my rain gauge is now adjusted as close as I will get it and it eventually took 3.25 turn of the adjustment screws to complete this.
I emailed Davis re no mention in manuals supplied with the unit relating to the adjustment screws, and they replied that they would pass it on but they did not expect it to need adjusting as it was set in the factory.
Let it rain, and it has, as here in South Australia we have already had more rain than we normally have in a full year.
Regards and thanks for your help,
Des

[Mattk]

Yes I have seen screw adjustment of 5 turns and by then the screw is just about out of the thread. Davis appear to have a fixed screw height setting depending on the model but did also hear there is some specific angle the tipping bucket is set to. In reality the only way check/validate is to do a calibration then adjust. One thing for sure is there is not a linear change between the number of turns and % change, must be changed gradually, re-calibrating each time.

PS Maybe Aussie rain is lighter than American rain     

[WheatonRon]

A follow up, I consider my rain gauge is now adjusted as close as I will get it and it eventually took 3.25 turn of the adjustment screws to complete this.
I emailed Davis re no mention in manuals supplied with the unit relating to the adjustment screws, and they replied that they would pass it on but they did not expect it to need adjusting as it was set in the factory.
Let it rain, and it has, as here in South Australia we have already had more rain than we normally have in a full year.
Regards and thanks for your help,
Des

This post is now a couple of months old. Curious. Do you think you fixed the calibration of your rain gauge?

[desrae20]

Yes I believe it is now adjusted, the main issue was knowing what to adjust as the manual supplied did not detail this, and thanks for the help from others,
In Sept South Australia has had it highest daily rainfall on record, over 100mm.
Regards,
Des

[WheatonRon]

Yes I believe it is now adjusted, the main issue was knowing what to adjust as the manual supplied did not detail this, and thanks for the help from others,
In Sept South Australia has had it highest daily rainfall on record, over 100mm.
Regards,
Des

That's a lot of rain! The reason I was askng about adjusting the VP2s is I have two of them and am disappointed in their rainfall accuracy so I need to calibrate. Glad you were successful, hope I will be too!

[desrae20]

Its a slow process, particularly if its not raining.
I have a manual  gauge mounted along side of the Davis and have been comparing and adjusting for sometime, trying to do a little at a time, fortunately winter has been a lot wetter than normal and it has given me the opportunity to compare and adjust.
Good Luck,
Des