Topic: Advice for calibrating fleet of VantageVues in St. Thomas, USVI

[VIWeather]

Hello all from St. Thomas in the Virgin Islands!!

I just joined the forum this week and am happy to be accepted in to this community of weather enthusiasts! I am currently working as a climate research consultant for the University of the Virgin Islands, contracted (from Nov 2015 - June 2016) to develop real-time data collection and management analytics as well as a freely available data archive (which I will share upon its completion!) for long-term real-time microclimate monitoring. I am also teaming up with an astrophysicist to additionally investigate recent climate variability of the Leeward Islands, which we hope will steer the direction of our interpolation techniques for the active stations installed across the VI and ultimately create our own station management software that allows us to communicate directly with the stations.

I've recently successfully installed a fleet of 12 stations across the island and it is time to calibrate! I have been reading about numerous calibration techniques and procedures where one in particular has me stumped - the rain gauge. This project is being funded by the local university which was gracious enough to purchase a Kestrel 5000 Environmental Meter for the calibrations of the various station sensors; however, no more funds are available to purchase other calibration equipment, such as rain gauges.

Does anyone have a suggestion on completing simple yet effective rain gauge calibrations, without the use of a manual rain gauges...that would also require the least number of times I would have to physically access the stations? The fact that I am on an island creates a number of station accessibility obstacles that would make it difficult to access the stations, especially after a heavy rainfall event.

Any tips or suggestions would be greatly appreciated!! Many thanks! 

[CW2274]

Welcome. I'm curious as to why you feel the need to calibrate what I assume are "out of the box" ISS's. They're precalibrated from Davis when you get them. Also the meter you were given to do the calibration with is no more accurate than the Davis sensors them self.

[miraculon]

A "CoCoRaHS" type rain gauge is available at nominal cost from WeatherYourWay.com.
Despite the "no more funds" that you mention, you could probably invest in a few of these and place them to compare the rain fall. Expect them to differ somewhat. Search WxForum for "rain calibration" and there are long discussions on this subject.

Keep in mind that the VUE ISS requires a compromise between what is best for wind measurement and for rain. A high mount location which is best for wind, will result in an undercatch for rain.

Greg H.

[VIWeather]

Welcome. I'm curious as to why you feel the need to calibrate what I assume are "out of the box" ISS's. They're precalibrated from Davis when you get them. Also the meter you were given to do the calibration with is no more accurate than the Davis sensors them self.

Thank you for this - I completely agree and have tried to tell the university that calibrating "brand new" stations is not needed. There are a few stations; however, that they had some undergraduate students install in 2014 - it's those stations that I will calibrate.

As for the sensor, it's accuracy is slightly better than the Davis sensors according to specs.
Examples:
Temp - Davis accuracy = ±1°F, Kestrel accuracy = ±0.9°F
RH (for 0-90%) - Davis accuracy = ±3%, Kestrel accuracy = ±2%
Do I need an instrument(s) with higher accuracy?

[VIWeather]

A "CoCoRaHS" type rain gauge is available at nominal cost from WeatherYourWay.com.
Despite the "no more funds" that you mention, you could probably invest in a few of these and place them to compare the rain fall. Expect them to differ somewhat. Search WxForum for "rain calibration" and there are long discussions on this subject.

Keep in mind that the VUE ISS requires a compromise between what is best for wind measurement and for rain. A high mount location which is best for wind, will result in an undercatch for rain.

Greg H.

Thanks Greg! I checked out the site and I will definitely try to encourage the university to invest in a few. In the meanwhile, I might have to come up with an alternate calibration method. What are your thoughts on the method of slowly dripping a known accurate amount of water into the rain gauge and calculating the output?

Searching rain calibration is what brought me to this forum in the first place I really do appreciate your note about the mounting height. Rainfall variability across the island is of great interest to the university, so I might need to go back and reassess the locations of some stations.

[johnd]

The drip method of calibration is certainly an option and better than nothing, but if done carefully is laborious and still not necessarily very accurate. In addition, it obviously measures the intrinsic accuracy of the gauge rather than the effective accuracy in situ, it cannot correct for the very significant effects of wind speed on rain catch (hence the effect of mounting height).

A CoCoRaHS gauge costs only $30 or so http://www.weatheryourway.com/cocorahs/rgcoco.htm and you could get three for $100. If the university isn't prepared to accept this relatively modest extra cost in the interests of a step-change in rainfall accuracy then I wonder how seriously they're taking the study?

[miraculon]

A "CoCoRaHS" type rain gauge is available at nominal cost from WeatherYourWay.com.
Despite the "no more funds" that you mention, you could probably invest in a few of these and place them to compare the rain fall. Expect them to differ somewhat. Search WxForum for "rain calibration" and there are long discussions on this subject.

Keep in mind that the VUE ISS requires a compromise between what is best for wind measurement and for rain. A high mount location which is best for wind, will result in an undercatch for rain.

Greg H.

Thanks Greg! I checked out the site and I will definitely try to encourage the university to invest in a few. In the meanwhile, I might have to come up with an alternate calibration method. What are your thoughts on the method of slowly dripping a known accurate amount of water into the rain gauge and calculating the output?

Searching rain calibration is what brought me to this forum in the first place I really do appreciate your note about the mounting height. Rainfall variability across the island is of great interest to the university, so I might need to go back and reassess the locations of some stations.

See this: http://www.wxforum.net/index.php?topic=27402.msg264111#msg264111

and the followup here: http://www.wxforum.net/index.php?topic=27839.msg268957#msg268957

A bit of a warning, this was an all-day project with multiple iterations to get it right on this single rain gauge. I adjusted the stops less than a turn at a time near the end of the process. When I was "done" I ran back-to-back repetitions to make sure that I was getting consistent results.

I do not own, nor have I adjusted the VUE tipping spoon gauge. I did try to "calibrate" my original Davis rain collector, but I really had it out of wack for a few weeks. I eventually used a small calibrated medicine dispenser to calibrate it.

The more recent method with the controlled orifice and measuring the water in the beaker by weight worked well. I use the same "Escali"  scale for CoCoRaHS snow measurements.

My advice: make small adjustments (if any) after making several repeatability iterations. Find out the exact amount in mL that the VUE funnel/bucket is set up for per unit measurement. Someone had this figure for the VP2, but you might need to hunt it down or call Davis for the VUE numbers.

This is from the Davis spec:

Rain Collector Type  .......................................Tipping spoon, 0.01" per tip (0.2 mm
with metric rain cartridge, Part No. 7345.319), 18.0 in2 (116 cm2) collection area

so you could do the math. There are formulas out there for this, I was lazy and used the cookbook number for the VP2 and NovaLynx gauges.

Greg H.

[SnowHiker]

I think it would have been interesting and have given you an idea as to how well they were calibrated to set up all twelve stations at the same site when you first received them and see how they compared each other before sending them to the final sites.  That way you could have calibrated them altogether.

[SnowHiker]

I think it would have been interesting and have given you an idea as to how well they were calibrated to set up all twelve stations at the same site when you first received them and see how they compared each other before sending them to the final sites.  That way you could have calibrated them altogether.

I guess one problem would be if they're wireless twelve stations together would interfere, you could only do up to eight(?) at a time. 

[CW2274]

Welcome. I'm curious as to why you feel the need to calibrate what I assume are "out of the box" ISS's. They're precalibrated from Davis when you get them. Also the meter you were given to do the calibration with is no more accurate than the Davis sensors them self.

Thank you for this - I completely agree and have tried to tell the university that calibrating "brand new" stations is not needed. There are a few stations; however, that they had some undergraduate students install in 2014 - it's those stations that I will calibrate.

As for the sensor, it's accuracy is slightly better than the Davis sensors according to specs.
Examples:
Temp - Davis accuracy = ±1°F, Kestrel accuracy = ±0.9°F
RH (for 0-90%) - Davis accuracy = ±3%, Kestrel accuracy = ±2%
Do I need an instrument(s) with higher accuracy?

The 2014's should be OK too as sensor drift should be minimal.
As far as a more accurate calibrator, it's all about what you want to spend and achieve. I guess what you have is better than nothing, but remember to get the best comparison, the calibrating sensor should be inside the ISS's sensor chamber. Good luck with that.

[VIWeather]

I think it would have been interesting and have given you an idea as to how well they were calibrated to set up all twelve stations at the same site when you first received them and see how they compared each other before sending them to the final sites.  That way you could have calibrated them altogether.

I guess one problem would be if they're wireless twelve stations together would interfere, you could only do up to eight(?) at a time. 

Calibrating them together would have been ideal. I wish I had joined the forum earlier prior to installing the stations! However here is where they are all located now:



My main concern with calibrating these stations is accessibility combined with the extreme variability in rainfall on the island. The island is only 13mi X 4mi, but the microclimates present make it very challenging for to know the duration of rain event at any of the station locations. This will make retrieving gauges very difficult (if gauges were used for calibration), not including the obstacle of actually getting to the station post-rain due to the condition of the roads.

Though there are some outstanding accuracy issues with the drip method, I am starting to lean more towards it as the most practical calibration method. I hate to even allude to practicality over accuracy, but working on an island has brought about many issues that seem not to be taken into consideration in past forum discussions.

[VIWeather]

Welcome. I'm curious as to why you feel the need to calibrate what I assume are "out of the box" ISS's. They're precalibrated from Davis when you get them. Also the meter you were given to do the calibration with is no more accurate than the Davis sensors them self.

Thank you for this - I completely agree and have tried to tell the university that calibrating "brand new" stations is not needed. There are a few stations; however, that they had some undergraduate students install in 2014 - it's those stations that I will calibrate.

As for the sensor, it's accuracy is slightly better than the Davis sensors according to specs.
Examples:
Temp - Davis accuracy = ±1°F, Kestrel accuracy = ±0.9°F
RH (for 0-90%) - Davis accuracy = ±3%, Kestrel accuracy = ±2%
Do I need an instrument(s) with higher accuracy?

The 2014's should be OK too as sensor drift should be minimal.
As far as a more accurate calibrator, it's all about what you want to spend and achieve. I guess what you have is better than nothing, but remember to get the best comparison, the calibrating sensor should be inside the ISS's sensor chamber. Good luck with that.

How often do you suggest stations should be calibrated then? At this point, it seems like I'll be preparing a calibration manual for the university for future calibrations....as well as a shopping list

[CW2274]

Don't really know. I replaced my SHT11 after about 5 years because of perceived drift. Frankly, I wouldn't even mess with them unless you see something obviously wrong.