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From:
James Fischer <[log in to unmask]>
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Date:
Fri, 9 Nov 2007 00:43:34 -0500
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>  ... the only thing that looked interesting was a 
> 330lb platform scale meant to be used indoors ($65).

That sounds like a good deal, as many beekeepers have
used the old-school "shipping scales" with ad-hoc 
foul weather protection.

Others have bought the Maxant Hive Scale for $95.
http://www.maxantindustries.com/tools.html

> if such a scale were wrapped in plastic it would 
> survive under a bee hive?

I would think the key factor would be a good coat of paint
on all exposed surfaces, and special consideration for
the pivot point.  (I'm assuming that this is a non-spring
balance-type scale, as the spring type will rapidly rust
into an inaccurate hunk of junk.)

There was a set of plans floating around some time ago
for a hive base that set the weight of the hive on
two bathroom scales, and provided the scales some
protection from the weather.  One would read the
weight on one scale with a mirror placed under the
platform, and multiply by two.  Yes, the scales had 
springs, and over time, they both rusted and 
compressed into inaccuracy, but the idea here was 
that you simply bought new (cheap) bathroom
scales ($10 or so) every few years to replace them.

Not very elegant, but certainly a lower cost of
entry than other options.  And "engineering" is
the art of doing for $1.00 what any fool could
do for $2.00.  :)

> if a company can produce an electronic bathroom scale for 
> under $20 why couldn't someone put together a kit of say 
> four strain gauges that you could hook up a volt ohm meter 
> and  get a weight reading.  

I've looked at this specific problem over and over as different
technologies have appeared, and even got really close to what
looked like a viable product, only to be let down by over-hyped
materials, so let me run down the basic needs/criteria/problems,
and tell you more than you likely really wanted to know:

1) Wide range.  Hives can vary from roughly 30lbs to 
500lbs or more.  Strain gauges with wide ranges are
not cheap, and if you overload them too much, you
can damage them, so you need to over-spec them for
the real world.  Bathroom scales have the advantage
of being mass-produced, need only measure a limited
range (say up to 200 lbs), and are put under pressure
only for brief periods.  If they were used in hive
scale applications, they would not work very well,
and would not work for long.

2) Sensitive over the wide range.  Beekeepers expect
to see precision of 1lb or 1kg, rather than larger
increments.  Strain gauges so sensitive over such 
a wide range are not cheap. 

3) Accuracy and linearity over the wide range
are also important.  Accuracy is not precision, and
sensitivity is not linearity.  The trade-offs can
give you headaches.

4) Strain gauges need something to be under strain,
like a metal frame under a bottom board.  This is
an expensive part, as it must be precision-made,
and must be strong enough to support the weight
of the hive.  Tricks where only part of the hive
would be supported really only complicate matters,
as weight within a hive will not always be evenly
distributed.  This part is also bulky and expensive
to ship, due to its mass and size.

4a) The metal is never really allowed to "relax"
between weighings in the case of a hive scale,
so what you see, even with the most resilient
materials, is a gradual deformation of the
metal, a permanent change that makes readings
inaccurate (or "blind to") when then hive 
later loses weight.  A strain gauge is not
magic, it can only detect strain that is
present in the frame, and will never detect
the strain if the frame deforms or otherwise
becomes "less elastic", as there is no actual
strain to detect.  

4b) The same problem of constant load kills
all sorts of alternatives to strain gauges,
as none are really designed to sit there
and keep being compressed 24 x 7 x 365.

5) Tooling up versus quantity is a bear, in
that no product for beekeeping can ever pay
back most tooling investments, or even clear
a profit in the lifetime of the "inventor".
Beekeeping is simply a "niche" market, not
a mass-production one.

Given all of the above, I thought I had a great
solution, based upon the net pressure created 
by snaking a tube filled with hydraulic fluid
under the edges of a bottom board.  The display
was a simple pressure gauge.  It worked fine, but
the tubing was nowhere near as UV-resistant and
temperature-resistant as claimed by the vendor,
so after a few simulated winters and summers in
deep freezes and tanning salon beds, the tubing 
became hard and the unit no longer gave any 
reading at all.  Replacing the tubing was not
something one could do "at home" due to the need
to eliminate all air from the line before sealing
it, and remove all air from the internal works
of the gauge. 

I also was never able to swamp out the issue
of barometric pressure, so one had to look at
a barometer, and compensate a bit with a 
lookup table when taking readings.

I am waiting for "better tubing", but I am not
holding my breath, and neither should anyone else.

> Perhaps the owner would have to calibrate the kit, 
> and maybe even take a temperature reading to 
> adjust for temperature.

Oh, yeah, temperature can be an issue with both
electronics and with mechanical approaches.
One hopes to "design it out", but this is yet
another challenge, and it adds cost.

> I would love to see some sort of  cheap design worked up. 

Everyone would. Everyone also wants "cheap".

> It only needs to be precise to the pound and absolute 
> accuracy is not important.

See? You feel that you have very reasonable and "easy"
requirements, but I have been unable to create anything
that I was will to put my name on (and behind) that
was not far more expensive than the existing products.

I have a box full of sensors, strain gauges, and other
things that seemed temping enough to buy, but turned
out to be dead ends.  I've "wasted" a good number of
hours on the problem, too.  

Your turn. :)

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