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)
--

"Zak" wrote:

After carefully comparing rainfall using both the Davis VP and a standard
manual gauge it seems, in calm weather, my VP is under-reading by 20%. The
tipping mechanism is clean and moving freely.

I know the two screws under the tipping buckets need to be turned
anti-clockwise in order to lessen the amount of rain required to tip, but
can anyone suggest the amount they need to be turned in order to achieve
the 20%?

Sorry to come to this thread so late ... and following all
the other excellent advice all I am going to do is repeat
the Eden mantra which applies to anyone and everyone
with an AWS ... however fantastic your AWS is,
please add a traditional manual rain-gauge to your site;
it is the only way you will achieve a rainfall record
under standard conditions, comparable with all official and
historical data (with the proviso that your site characteristics
should also be as close to the official standard as you can
manage).

Philip

Philip Eden wrote:

"Zak" wrote:

After carefully comparing rainfall using both the Davis VP and a
standard manual gauge it seems, in calm weather, my VP is
under-reading by 20%. The tipping mechanism is clean and moving
freely.

I know the two screws under the tipping buckets need to be turned
anti-clockwise in order to lessen the amount of rain required to
tip, but can anyone suggest the amount they need to be turned in
order to achieve the 20%?

Sorry to come to this thread so late ... and following all
the other excellent advice all I am going to do is repeat
the Eden mantra which applies to anyone and everyone
with an AWS ... however fantastic your AWS is,
please add a traditional manual rain-gauge to your site;
it is the only way you will achieve a rainfall record
under standard conditions, comparable with all official and
historical data (with the proviso that your site characteristics
should also be as close to the official standard as you can
manage).

Philip

The amounts measured by the raingauge on my Davis VP average about
10-15 percent lower than what the manual gauge measures but there is
great variability. Sometimes the measurements are almost identical and
sometimes the AWS is as much as 30 percent lower. The variability seems
to be related both to rainfall intensity and to wind direction. In
prolonged slight to moderate rain the AWS compares very well to the
manual gauge but in heavy rain it under-reads quite markedly.

The AWS rainfall measurements are interesting but I agree that a manual
gauge is essential for reasonably accurate measurements.

Norman.

--
Norman Lynagh
Chalfont St Giles, Buckinghamshire
85m a.s.l.
(remove "thisbit" twice to e-mail)