Topic: Stratus or Tropo for calibrating Davis rain bucket ?

[npapadak]

if i pour 100 ml of  water

Tropo   measures 12.3 mm
Stratus measures 12.6 mm
Dif. of 0.3 or 2.44%

if i pour 200 ml of water

Tropo measures    24.6 mm
Stratus measures  25.2 mm
Dif. of 0.6 or 2.44%

According to the formula  V=H*πR2  100 ml of water in a 10.1cm cylinder has a height of  12.482mm

Both gauges seems to be identical with a diameter 10.1cm

Stratus reading seems to be much closer to the 12.482 

For Stratus to get the correct 12.482 reading , instead of the 12.6 mm, the diameter of the gauge should be 10.05mm
For Tropo to get the correct 12.482 reading instead of the 12.3 mm , the diameter of the gauge should be 10.17mm

It seems to me Stratus is more accurate from Tropo.

[johnd]

if i pour 100 ml of  water....

How are you measuring the 100ml water? Weight or volume? If weight then how accurate are the scales? If volume then what are you using - a graduated measuring cylinder? Is it definitely Class A glassware and an appropriate volume, eg 100ml for 100ml volume, maybe 250ml for 200ml? Are you trained or adept in judging the meniscus position to get an accurate reading? Not questioning your ability at all, just asking. And pointing out that even a reasonably accurate volume measure used carefully may have an error of ±1%. And it doesn't take much calibration or usage error to increase that to ±2-3%.

My point is that if the cylinder (or whatever) is under-delivering by even say 0.1mm height then that would take the 2 values to 12.4 and 12.7mm height and then you would favour the Tropo.

TBH I think the conclusion is that within experimental error then the 2 gauges perform equally and you could use either to measure water to drip through the gauge for calibration.

[npapadak]

  Ι measure by volume using a glassware 100ml with an error +/- 1ml at 20c
  The meniscus at the stratus its more visible than in Tropo. I agree with all possible minor errors plus the under-delivery by 0.1mm and i will add the slight possible difference between the guages diameter
Certainly , for Davis rain collector calibration , any of those two gauges will do the job .
Its raining today and i am doing a test in the field using the Stratus , the Tropo and the 16.5cm davis rain bucket by collecting the water . Volume test again and i will compare all three readings with the reading from the Davis digital station

[Mattk]

This sounds like a hair splitting exercise using simple comparisons to calibrate a Davis tipper which has a claimed accuracy of +/-4% to +/- 5% dependent on rate rates? I have never found a Davis bucket to measure "exactly" what the spec states and most are never exactly round either.

[ValentineWeather]

It seems to me Stratus is more accurate from Tropo.

You're correct. 

[npapadak]

nice rainning event yesterday...

My aim is to check and if needed , to calibrate the Davis vantage pro rain collector.

Tropo or Stratus will be permantly installed next to the Davis vantage pro and the readings will be used for following the Vantage pro rain reading accuracy over time

So Davis digital 16.5 cm vs Davis analogue 16.5cm

Tropo 10.1cm vs Stratus 10.1cm

Coleration Davis analogue readings to Tropo and Stratus reading.


Tropo (10.1cm)   reading by my eyes ~12.6mm .. The volume measured ~103ml and the formula gives 12.86mm

Stratus(10.1cm)  reading by my eyes ~13.4mm .. The volume measured  ~106ml and the formula gives 13.23mm

Davis bucket (16.5cm)       12.86mm    volume measured 275ml

Davis digital (16.5cm)      12.4 mm

If i calculate the 1ml error of the glassware

Tropo   103 ml or 12.86mm  with error (102-104) = (12.73-12.98)
Stratus 106 ml or 13.23mm  with error (105-107) = (13.11 -13.36)
 
if i calculate a 3ml error for  Davis bucket , since my measuring cylinder is 100ml and for 275mL i empty it 2 timees..three readings

Davis bucket 275ml or 12.86mm  err (272-278 )  (12.72-13)

Going back to my aims

Davis digital 12.4mm  vs Davis analogue 12.86mm  err(12.72mm-13mm)
                                   dif.  0.46mm  err (0.32mm-0.6mm)
                                   dif%  3.7%        (2.6%-4.8%)
                                  I may need to do a calibration of +2%


After this rain event considering its unique characteristics such as wind ,rain rate ,it seems my Davis digital underestimated the total accumulation.

There is no doubt to me that Tropo measures less than the Stratus but the question now is if Tropo readings are more accurate comparing to the Stratus one in coleration to the Davis 16.5cm gauge

In this test it seems Tropo reading is more close to the Davis analogue one

I have to wait another 2 or 3 rain events in order to decide if calibration is needed to the digital Davis by taking the averages and which , Tropo or Stratus , will be used as a guide .Actually , both can be used , but which one is more close to the Davis reading (easy to calculate )

Your thoughts ,hints,comments will be appreciated 

[ValentineWeather]

There's been other threads on this. Being a Cooperative Observer for the NWS have tested the Stratus and Tropo against the Official 8" diameter NWS gauge.  The Stratus is closest, with the Tropo being furthest off, reading 4% low consistently. You'll find people here defending the Tropo by weighing the water etc. What I care about is the end result, which gauge performs closest to the Official 8" gauge.

I've done some testing this last winter with the 4" vs 8" and the 4" under-catches snowfall. I was surprised by how much. Wind has a lot to do with it, especially with snowfall, but from what I can tell the 4" diameter under catches snowfall in the 30-40% range vs the official 8" diameter gauge. I'm no longer doing Cocorahs since taking on the Coop Obs and now official snow measurements every 6 hours during snow events.

[npapadak]

hhm  ?

my Stratus has a single cap and records only mm ,not , inches

[davidmc36]

hhm  ?

my Stratus has a single cap and records only mm ,not , inches

I didn't realize Stratus came in MM.

[npapadak]

The mm Stratus was bought many years ago from weatheryourway

[CW2274]

There's been other threads on this. Being a Cooperative Observer for the NWS have tested the Stratus and Tropo against the Official 8" diameter NWS gauge.  The Stratus is closest, with the Tropo being furthest off, reading 4% low consistently. You'll find people here defending the Tropo by weighing the water etc. What I care about is the end result, which gauge performs closest to the Official 8" gauge.

All I know is since recalibrating both my 8" RW gauges to 824ml in January, is how remarkably close they and my two Stratus's now are. Granted, the two Startus's are slightly different, one 200ml per inch, the other 204ml, but the four of them have been virtually within 0.01" of each other, and that's up to an inch of rain for a couple of events. I don't own a Tropo, but don't see any reason my results would be different. Soooooo.....

[Mattk]

Davis Digital v Davis Analogue. Could somebody clarify those two?

[npapadak]

Davis Digital v Davis Analogue. Could somebody clarify those two?

[npapadak]

After weeks with no rain, finally i got a nice rainy day with moderate winds

So i got the opportunity to do another  test .

My goal is to verify that my Davis digital vantage pro records precipitation correctly and callibrate if needed.

The most appropriate way at my opinion is to do the comparison with a rain collector with the same diameter .

So i collected rain using a Davis rain collector bucket .Tropo and Stratus was also installed next to Davis.

The Davis digital recorded 14.8mm of rain . I opened the bucket and i collect a 0.05mm from the spoon. 14.85 in total

The water collected from the Davis "analog" bucket  was 315 ml and using the cylinder volume calculator with Davis bucket
diameter 16.5cm i am getting a 14.73mm of rain.

So my conclusion is the Davis digital records rain accurately and no callibration is needed.

My task is to use the Tropo or Stratus as a guide , instead of collecting every time the rain from the analogue Davis bucket.

Tropo (dual inches + mm ) measured 14.6mm and Stratus mm only ,measured 15.4 mm. Tropo measured 5.5% less rain than Stratus or/and Stratus measured 5.2% more rain than Tropo.

Is it possible?

So i decided to empty the internal tubes of each gauge and measure the volume.

Tropo gave me 120ml and Stratus 122ml !!

My conclusion is both gauges are collecting the same amount of water BUT Tropo is more accurate in the way the readings in the internal tube are printed

I am waiting a rain episode with expected rainfall more than 50mm to do my final test ,but up to now.. Tropo is more close to the my Davis recording and it can be used as a guide

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