US rain collector on Vantage Vue

[katdoc]

Ah great.thanks a lot!!!
It all started when i thought that rain that fell one day was much less than expected.I used my old national geographic station and the amount was 10% at least more than davis rainmeter.
So every tip now with the metric adapter is 0,2 mm so i will have to check it again somehow.Is there a known volume of water and speed that gives some certain mms?

[prodata]

Ah great.thanks a lot!!!
Is there a known volume of water and speed that gives some certain mms?

Take a look at our VP knowledgebase:

http://vp-kb.wikispaces.com/Sensor+Errors3

NB These notes are a year or two old now and an update is planned. But the key facts should still all be relevant.

[katdoc]

544 ml= 25,4 mm

Youre the man .ill have work to do tomorrow.

[katdoc]

Ah great.thanks a lot!!!
Is there a known volume of water and speed that gives some certain mms?

Take a look at our VP knowledgebase:

http://vp-kb.wikispaces.com/Sensor+Errors3

NB These notes are a year or two old now and an update is planned. But the key facts should still all be relevant.

SO.i made the test using 500ml of nacl 0.9% and it gave me 22.4mm instead of 23,3 that it should.its 4% less than expected.Do you think i should make something else or am i ok??

[prodata]

SO.i made the test using 500ml of nacl 0.9% and it gave me 22.4mm instead of 23,3 that it should.its 4% less than expected.Do you think i should make something else or am i ok??

Not quite sure why you chose saline rather than pure water, but at 0.15M the specific gravity should be <1% different from distilled water.

But taking this single result at face value, I would conclude:

1. Within experimental error, your rain gauge is correctly calibrated;

2. If you want a more exact answer then you have to do some repeat tests - at least 3 and preferably more. It is important that the water drips in only slowly, eg it should take 30-60 minutes for 500ml to run through. If it's much faster than this then the gauge will tend to under-record.

Doing some repeat runs will give you an idea of the repeatability of your test regime. I suspect you'll find that it may vary by at least 2-3% between runs even if you carefully standardise your technique.

It's difficult to conclude that an error of 3-4% is sufficient to justify recalibrating the gauge. You have to start thinking about the possible errors in your test. For example, how did you measure the 500ml - is it possible that there was an error of eg 1-2% in this measurement? Everything that you do experimentally potentially contributes to the overall error. Personally I would say that 4% error is within specification.

[katdoc]

I heard that saline water is similar to rain's and it was easy for me because i got it from the pharmacy package of 500ml.Do you think it will be better with normal water or distilled?
I have also iv system that can fix the amount of the water that drops.I took your advice and made a rainfall rate about 18-24mm/hr which is about the time (1 hr)that needs the test to finish.
I think ill count carefully the 500ml with a tube and try it some more times.
Thank you for all the help and knowledge and ill come back with the final (hopefully) conclusion!!!

[prodata]

I'd guess that the quoted 500ml may not necessarily be very accurate - I wouldn't have thought it a critical element of the product to have the volume measured precisely. But I could be wrong.

It's often difficult for users outside of a lab to measure 500ml very accurately. Even if you have a suitable measuring cylinder or large burette plus good technique for using them it's difficult to achieve an accuracy of better than 1-2%. As per the comment in the notes I linked to, the most accurate option available to many users will be to weigh the water with a good electronic balance (used eg for cooking) and assume that 1g=1ml (which is an excellent approximation in fact). But even here there could be an error of eg 2-3%, if not a bit more.

This is the point that I keep trying to emphasise: When you perform an experimental calibration like this, you must take account of all the possible contributory sources of error in the test process - it's not difficult for these to add up to 4-5% in total. So if your calibration test gives an answer within 4-5% of the expected value you don't know whether the error is in the rain gauge or the calibration process. There's little point in adjusting the rain gauge to equal the nominal calibration amount if you can't have complete confidence that your test amount is exactly correct - the gauge might already be more accurate than the test amount.

So the drip test is good for confirming that the calibration is approximately correct, but difficult to use for any more precise attempt at calibration. Probably a better calibration method in practice is to get a good manual reference rain gauge (eg a CoCoRaHS gauge) and mount it with the rim at exactly the same height as the VP2 gauge and very close by - touching if you like. Then take daily rainfall readings over eg one month (and at a time of year when snow is unlikely) over different types of rain - light drizzle to prolonged heavy rainfall - and compare the results