Topic: VP2 Rain gauge adjustment

[David9723]

My VP2 is usually off by 1 or two tenths the manuel shows how to adjust it but i'm not sure how far to turn the screw. Do i simply turn it two lines on the dial?

[HamiltonNJWeather]

What are you gauging against? Why do you feel it's off?

[miraculon]

Search the forum for rain gauge calibration. There are several posts and instructions on how to check and adjust the gauge.

HorshamWeather is correct, unless you have a high quality manual gauge close to the VP2, you cannot conclude anything by viewing other nearby rain gauges. By close, I mean within a few feet.

The only time you should ever touch the adjustment screws is with a very precisely known quantity of water and a controlled flow rate that is representative of "real" rain fall rates. Even then, you will need to make multiple runs to ensure reputability.

Greg H.

[archae86]

Even careful calibration can only match one particular flow rate. The calibration won't be the same for a substantially different flow rate.

The effects of wind are even more complicated.  I run a Davis Vantage Pro 2, a Rainwise 8 inch electric reporting gauge, and a classic four-inch analog within a few feet of each other. For any given storm, the matching between them varies quite a bit, but apparently the calibration is not bad, because over the year of variation they come very close.

I fear it is very easy to degrade your accuracy by performing an inappropriate calibration procedure.

[David9723]

What are you gauging against? Why do you feel it's off?

multiple rain gauges placed by the VP2. They all read more than what the VP2 does.

And yes it depends on how hard the rain comes down.

[Jáchym]

My experience even with professional Vaisala rain gauges is that automatic ones always underestimate reality especially if it is very small long lasting drizzle or the opposite,  intense downpour

[ValentineWeather]

Yes, lots of information about calibrating the tipping bucket. The precise way is having both buckets tip with equal amounts of water by leveling the gauge. The bubble is ballpark only. Mounting on a pole that has any movement or slightest vibration you can forget calibrating, it won't matter. 
For these gauges to perform at best they need mounted at about 3 feet on a very sturdy platform and calibrated with some kind of precise measuring dropper so both buckets tip with equal amounts of water. You do this by adjusting the level. (Stand alone gauges this can be done)
Even after this is done you still have under reporting with heavy rain because of the nature of the tipping bucket while in the neutral position, the between tips you have lose.

I'm using the Rainwise 8" stand alone instead of the Davis 6.5", so far very happy with it.
4" Cocorahs gauge is a great backup gauge.

[Jáchym]

Especially during small long lasting drizzle you get substantial parallel evaporation so in this case no calibration will help too much unfortunately

[ValentineWeather]

Especially during small long lasting drizzle you get substantial parallel evaporation so in this case no calibration will help too much unfortunately

Yes that too.