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)
--