I've always been somewhat resistant to selling things online,
mostly because I hate packing things up and lugging them off to
the post office. I'm always afraid the item in question will arrive
in pieces and that my terribly overpacked box will end up costing
a fortune to ship. One result of all this is that I don't own
a shipping scale. But that causes its own problems when I need
to compute the shipping cost ahead of time, such as selling on
eBay. The first time I just asked the guy at the corner store
if I could use his produce scale, but the second time I decided
to find a better solution. By better I mean an absurd and impractical
one, of course, not anything smart like just buying a scale.
So, how do you weigh something with household knick knacks? Despite
my household being the sort with a moped in the kitchen, a sea
of left over electronic bits from countless projects in my shop
and well over 150 computers everywhere in between, I didn't have
any sort of calibrated pressure sensor that could be pressed into
service as a scale. That left me thinking mechanical. How did
they do it in the days before electronic sensors? The answer is
a balance scale.
This simple device has been around for nearly as long as people
who haggle over prices. It consists of nothing more than a metal
rod balanced carefully on a smooth bearing, with equal weighted
pans hanging from each end. You just put the thing you want weighed
in one pan and place ever larger measuring weights in the other
pan until the two hang level. Add up the values on the reference
weights and you've got your measurement.
But what if you don't have a set of carefully calibrated reference
weights? Turns out there's a very easy option in every home. Tap
So long as you know its temperature (And strictly speaking, the
atmospheric pressure,) you can easily figure out the weight of
a given volume of pure water. 1 millilitre of water at 4 degrees
C is equal to 1 gram. At 20 degrees C it drops to something like
0.998 grams. That means that a good guess at the temperature should
be enough for a low precision application like this.
Great, now we have our reference weights, time to build a scale.
By combining a coat hanger, a shower curtain rod, a cheap carpentry
level and a plastic jug, we get something which is functionally
identical to the iconic balance scales of yore. What, were you
expecting wrought iron? The whole point of this exercise was to
be faster than going to the post office and a little more absurd.
I'd say it meets both those requirements.
So how did it work? Quite well in fact. I first balanced the
weight of the empty jug with a piece of scrap metal I had laying
around, then attached the package and began adding cold water
to the jug. When the level read, well, level, I used a kitchen
measuring cup to measure the amount of water in the jug. The result
was just over 4 and a half cups, which works out to 1200 millilitres
of water, or 1.2kg. That turned out to be just 200 grams more
than the post office's measurement of 1kg. The error was likely
due to the flexing and imprecise workmanship of the plastic coat
(Sorry for the ironically crooked picture)
Overall I would call it a success, so long as precision isn't
required. Acceptable for eBay auctions. Were I to do it again
I would probably use a wooden coat hanger or similar to see if
that reduces the margin of error.