There's a better and more absolute method to calibrate any tipping-
bucket gauge, as follows. This should be performed annually as the
adjustment does drift over time. If you haven't checked your TBR
calibration - undertake this simple check, it'll take just a few
minutes ...

This extracted from Andy Overton's very useful guide to settying up
and calibrating AWS, which can be found on the Royal Meteorological
Society's website:
---
"To calibrate using the method of measured amount introduced to the
raingauge, the area of the raingauge collector must first be
measured. For a square or rectangular collector multiply the lengths
in millimetres of the two sides together; for a circular collector
measure the diameter in millimetres, square it, divide it by 4 and
multiply the result by . Finally, divide 500 000 by this result to
give the amount of rainfall in millimetres which 500 ml of water
should register with your raingauge.

Next, it is necessary to measure 500 ml of water in a good quality
measuring jug, ensuring that the comparison of amount is taken from
the bottom of the meniscus of fluid. Measuring jugs are not perfectly
accurate but will suffice; if a more accurate measure is available,
use it. Pour the water extremely slowly into the raingauge. It
cannot be stressed too highly how slow this must be as errors in
registering become very significant with high precipitation rates.
The flow should be no more than a slight dribble, giving time for the
buckets to tip before introducing a fresh trickle. If a syringe is
available, use this to better control the flow. This will be time
consuming but the large amount of water goes a considerable way
towards equalling out the inevitable errors caused by uneven tipping.

Once all the water has been run through the raingauge the registered
precipitation should be noted and compared with the result to be
expected from the calculation above. It is good practice to repeat
the exercise to ensure accuracy and then average the results. If an
adjustment is necessary this must be done by manually calibrating the
raingauge assembly, an offset cannot be applied to the measurements as
the error is incremental and the introduction of the total offset for
500 ml of water in all circumstances would make a mockery of lesser
readings. The manufacturer's manual should be consulted for details
of how to make adjustments. For tipping bucket raingauges this is
typically by adjustment of the screws upon which the buckets rest in
the empty position. Lowering the screws will require more water in
the buckets to make them tip and so compensate for a raingauge which
is over-reading. Raising the screws compensates for a raingauge which
is under-reading. It is vitally important that the screws are
adjusted by equal amounts - it may be helpful to mark the screw heads
in some way to assist with equal positioning. If the raingauge is out
of calibration by as much a 10% or more do not be surprised if
considerable alteration in screw height is needed. Once adjustment
has been made it will be necessary to repeat the calibration process
to check what amount of water is now registered and make further
adjustments as necessary. A correctly adjusted raingauge should be
accurate to around 2%."


Stephen Burt
Stratfield Mortimer, Berkshire

I notice that in posting it lost the pi symbol ...

"For a square or rectangular collector multiply the lengths
in millimetres of the two sides together; for a circular collector
measure the diameter in millimetres, square it, divide it by 4 and
multiply the result by pi."

wrote in message
oups.com...
There's a better and more absolute method to calibrate any tipping-
bucket gauge, as follows. This should be performed annually as the
adjustment does drift over time. If you haven't checked your TBR
calibration - undertake this simple check, it'll take just a few
minutes ...

This extracted from Andy Overton's very useful guide to settying up
and calibrating AWS, which can be found on the Royal Meteorological
Society's website:
---
"To calibrate using the method of measured amount introduced to the
raingauge, the area of the raingauge collector must first be
measured. For a square or rectangular collector multiply the lengths
in millimetres of the two sides together; for a circular collector
measure the diameter in millimetres, square it, divide it by 4 and
multiply the result by . Finally, divide 500 000 by this result to
give the amount of rainfall in millimetres which 500 ml of water
should register with your raingauge.

multiply the result by pi = 3.14159

Will
--

Thanks very much Stephen (and Will for pi!). I will have a go at this, as I
reckon I have a 20% underread compared to a manual guage here.

Phil

Can I add just a small word of caution here. First, I wouldn't
recommend anyone altering the screw settings on their Davis rain gauge
unless they're really convinced that it is misreading significantly
and have completely ruled out other possible explanations. Often an
apparent miscalibration has another explanation, eg the gauge
increment is not correctly set on console or software or the reference
rainfall readings are not properly comparable, which can be for a
variety of reasons.And in practice it's pretty difficult to return the
screw heights exactly to their factory settings once altered if you
subsequently decide that the error wasn't as great as you initially
thought and you didn't keep an accurate record of any changes that
were made.

In general the VP2 gauges do record reasonably accurately though like
all less costly TBRs will inherently tend to under-record slightly and
will typically be mounted at eg 4ft height rather than at ground
level, so exaggerating this tendency a little. That said, I suspect
that there was a particular batch of VP1 gauges that were
mis-calibrated at the factory and this is primarily where the reports
of 20% under-recording come from.

Finally, I agree - the drip method is a good way of checking the
calibration - volume figures for the Davis gauges can be found on our
website towards the bottom of the page at:

http://www.weatherstations.co.uk/vp_faqt2.stm

(NB Apologies - this page is now a few years old and really needs
updating soon but the figures are still all correct AFAIK)

Don't underestimate the care and patience that's needed to run the
drip method reliably - although it's reasonably straightforward to do,
things like measuring (or weighing) the total volume of water
accurately, not exceeding the suggested drip rate etc all need
attention. Unless you're meticulous with the method, it's still not
easy to get a precision on repeat calibration runs of better than
4-5%.

John Dann
www.weatherstations.co.uk

In message , Phil Layton
writes
Thanks very much Stephen (and Will for pi!). I will have a go at this, as I
reckon I have a 20% underread compared to a manual guage here.

Phil
Repeated thanks to all, particularly Stephen Burt and John Dann.

In tests, it took about 40 mins for 500ml to run through my improvised
feed and the result?

Well, calculations said that 500ml should translate into 23.64mm and the
average of two runs was 23.1mm - close enough for my peace of mind.

That still leaves me wondering at the 20% anomaly - the other gauge (1m
away and 0.6m lower) checked out as accurate too. Can that separation
cause such a large disparity?

Anyway, it all goes to prove that John Dann's advice about not twiddling
anything until error is proven!
--
Chris

Just one correction to my own message...

Well, calculations said that 500ml should translate into 23.64mm

That should read 23.38mm

--
Chris

"Zak" wrote in message
...
In message , Phil Layton
writes
Thanks very much Stephen (and Will for pi!). I will have a go at this, as I
reckon I have a 20% underread compared to a manual guage here.

Phil
Repeated thanks to all, particularly Stephen Burt and John Dann.

In tests, it took about 40 mins for 500ml to run through my improvised
feed and the result?

Well, calculations said that 500ml should translate into 23.64mm and the
average of two runs was 23.1mm - close enough for my peace of mind.

That still leaves me wondering at the 20% anomaly - the other gauge (1m
away and 0.6m lower) checked out as accurate too. Can that separation
cause such a large disparity?


Hi Zak. In a word - yes!!!

If you look back through British Rainfall at the beginning of the 20th Century
when folk were trying to decide on best gauge design and site, it was quickly
concluded that elevated gauges would under-read due to air currents deflecting
rain and greater exposure to wind generally.

I have exactly the same problem as you my Davis reads 20% lower than my rain
gauge 2m away and just over 1m lower.

Cheers,

Will
--

The strange choice of an elevated design of these Davis AWS gauges
will lead to under-reading owing to wind eddying effects, as Will
correctly points out. The difference between this type of gauge and a
standard adjacent checkgauge (rim at 30 cm) will vary significantly
with both site and weather conditions, so a windy wet day in an
exposed site will lead to greater losses than a day's rain with light
winds - the differences could then be expected to vary from almost nil
to perhaps 30% or more, and adjusting the tip capacity may not be the
right answer.

Sheltered sites may see much smaller differences between these gauges
and a standard checkgauge - but of course both may be under-recording
if the site is seriously under-exposed, particularly to the main rain-
bearing winds.

I'd suggest before tinkering with the calibration settings themselves:

1. Maintain a standard five-inch checkgauge in a reasonable exposure
to provide the benchmark against which to judge the accuracy of the
tipping-bucket readings. They're not expensive, perhaps £150 for a new
one, less second-hand (check on e-bay): it'll last 50 years with a bit
of care. Don't forget the measuring cylinder too.

2. Do the drip test as outline in my earlier post. If this comes out
OK, and 5% is probably the limit of accuracy without laboratory
equipment, look elsewhere for the reasons for any under-reading.

3. If your exposure is limited, check the readings from your
checkgauge with that of a local gauge in a standard location. Readings
can vary significantly over short distances in hilly locations, and in
showery situations, so compare over weeks and months rather than days.
There's enough COL sites in the major population areas to get an idea
of the true fall in your locality, or ask for help on this site. If
you feel your readings show a previously unsuspected dry spot in your
neighbourhood, check against large-scale average annual rainfall maps.
If your percentage of average over 3-6 months is significantly
different from nearby gauges, your gauge is under-reading, probably
owing to sheltered exposure.

4. If the checkgauge readings look OK, the calibration test is OK and
you're still under-reading on your TBR, it's your exposure - most
likely the height that's causing the problem. Use the elevated gauge
readings only for time/intensity readings and use the checkgauge as
the absolute values, or if you want automatic gauge readings to agree
more closely, disconnect the elevated gauge and plug in a standalone
0.2 mm unit instead - mounted on the ground.

HTH.

Stephen Burt
Stratfield Mortimer

And just to emphasise the points Stephen made about the effect of wind eddies.

My site is well exposed but is often windy and the rain is often dense and fine
(many droplets swirling around). Here is a comparison for annual rainfall of
Davis AWS (1.4m above ground) versus manual 5" check gauge on the ground in
short grass.

2005 AWS 1263 mm Gauge 1406 mm (9% deficit with AWS)
2006 AWS 1333 mm Gauge 1544 mm (14% deficit with AWS)

I'll have to live with that, using the manual gauge for the definitive record
and the AWS for recording interesting rainfall event timings etc.

Will (Haytor, Devon, 1017 feet asl)
--