Topic: Question about calibration

[blaforest]

Hey all. I was just curious as to how accurate the VP2 rain gauge is out of the box and whether or not it could use some additional calibration. Some information I've found suggests that Davis rain gauges, at least in the past, were usually off +-10 to 20% when compared to other gauges. Does this still hold true or are the newer units pretty accurate out of the box?

My new unit should arrive tomorrow and while my site isn't necessarily the best setup I at lease want the rain gauge to accurately report rainfall for its location. I also purchased a Stratus 4" gauge that I plan to mount right next to and at the same height as the ISS to compare readings and to have an additional source for rainfall measurements. Can't wait until tomorrow!! 

[SlowModem]

Hey all. I was just curious as to how accurate the VP2 rain gauge is out of the box and whether or not it could use some additional calibration. Some information I've found suggests that Davis rain gauges, at least in the past, were usually off +-10 to 20% when compared to other gauges. Does this still hold true or are the newer units pretty accurate out of the box?

My new unit should arrive tomorrow and while my site isn't necessarily the best setup I at lease want the rain gauge to accurately report rainfall for its location. I also purchased a Stratus 4" gauge that I plan to mount right next to and at the same height as the ISS to compare readings and to have an additional source for rainfall measurements. Can't wait until tomorrow!! 

It's my experience that the rain gauge could use a little tweaking (I just used the bias in the software).  However, you will also notice that the accuracy will vary depending on the rainfall rate.  It may tend to under-report during high rainfall rates.

[George Richardson]

Hey,
Your VP2 could be very accurate, or, not very accurate. I won't tell you how to calibrate your VP2, I'll tell you how I calibrate mine. I have my 4" gauge mounted close to my VP2 ( not same pole which can cause some difference). I have recorded every rain event since Sept. '06 and every time I get an accumulated total of .5 to 1' precip., I record the 4" gauge and compare it to the VP2 total. This way I average different types of rain events. I make small adjustments to the VP2, never more than 1 full turn of the adjusting screw. You will get lost in the woods if you make 2 or 3 turns. Remember to adjust each screw the same amount.

Congrats on a great purchase.

George

[d_l]

My VP2 rain gauge has been perfectly accurate when compared to a nearby CoCoRaHS gauge.  However, Greg is right that accuracy is a function of rainfall rates and will under report during high rainfall rates.  We don't usually have high rainfall rates here and all my comparison tests were at rates of much less than one inch hour.

Another factor that will affect the accuracy is wind speed during the rain fall.  IMHO, the Davis collecting cone with it larger diameter collection area will be more accurate than a 4 inch CoCoRaHS gauge if the wind speeds are moderately high during the rain.  In this instance the CoCoRaHS gauge will under report the rainfall.

[ocala]

I use roughly the same set up as George Richardson. The only difference is that my gauge is mounted on the backside of the 4x4 and the Davis gauge is mounted on the front.
I borrowed a Novalynx calibrator from a forum member and calibrated mine.
Since doing the calibration the two gauges are about .01 to .02 off with the Davis gauge always lower.
As mentioned above during heavy rains the Davis gauge will under report.

[johnd]

...The only difference is that my gauge is mounted on the backside of the 4x4 and the Davis gauge is mounted on the front...

Have say that I initially read 'the 4x4' as referring to your SUV and that you were perhaps trying trials to test the effect of different wind speeds on rain catch. But I guess I was wrong and the truth is more mundane than that.

[ocala]

Just to drive home the point about heavy rain. Last night I had a quick intense rain storm that lasted about 5-7 minutes. The Davis gauge recorded .09. The plastic gauge .23.
Two days prior I had a long sustained rain for about an hour. The Davis gauge recorded .53. The plastic gauge was .54.

[bwh13]

Don't know if this thread is dead yet, but I'll put my 2cents in:
I've worked with a handful of different tipping buckets, and I've noticed something that I'm calling 'instrument interception' with just about all of them.  This is the left over rain that sticks to the cone, or the debris screen, or is left in an untipped bucket.  This rain eventually evaporates and doesn't get recorded.

Has anyone ever seen how much screen there is on a TE-525?  There is probably just as much surface area on the screen as there is on the cone.  I've checked on these gages after a storm and seen enough drops hanging on the debris screen to count for a couple of tips.  My guess is that in low intensity rainfall events, these tippers are under-representing the total.

I've haven't done the research on this, but I would imagine that a similarly sized orifice on a bulk gage, would probably record more rain than a tipper would.

[George Richardson]

I'll leave it to our scientific experimentors, but with 108.8 drops per tip of a VP2 bucket, I seriously doubt more than 1/2 of a bucket tip would ever stick to the cone/screen.

JMO

George

[ocala]

I would have to agree with George  here. Just can't see a couple tips being stuck to the walls or the screen.
It's probably not a bad idea to clean the bucket itself though. Also putting something like RainX on the inside would insure any residual drops would run down.

[bwh13]

Okay, so maybe I was trending toward hyperbole, however, I still stand by my assertion that low intensity, short duration rainfalls are under-represented in part due to instrument interception / evaporation.
You can just make out the screen in this pic.  There is a whole lot of screen there.