This post is to announce a new ebook on how, with simple materials and hand
tools, to make a HIGH PRECISION laboratory scale, a VERY unusual and
It's on eBay right now, and here's the link.
You can buy a commercial set of weights if you want to. Ohaus sells a
complete set of milligram weights for about $35. But this ebook also has
detailed instructions on how to make a HOMEMADE set of comparison weights.
You make them with things like plastic 35mm. film cans, lead buckshot, and
pieces of sheet aluminum cut from the bottom of a disposable cookie sheet
(sold by supermarkets and grocery stores).
The process is interesting and educational. The only caveat is that you'll
have to borrow a precision scale for a few hours to do it, and your homemade
weights will turn out approximately as accurate as the borrowed scale that
you use to make them. That is, if the scale you borrow is accurate to 1/10
of a gram, you homemade weights will be accurate to 1/10 of a gram. If you
can borrow an accurate centigram scale, then your homemade weights will be
accurate to approximately one centigram (1/100 of a gram). Thanks for your
On Sun, 29 Jan 2006 15:18:09 -0800, "David Sleeter"
Why not just buy a balance? If you look at a lab equipment dealer then
perfectly serviceable but old 4-figure balances are going in the skip
because everyone commercial wants a digital output. I've had a couple
of old wooden case ones that I converted to vitrines.
(Theodolites are dirt cheap too)
Of course it does, but I think you know that. I'm
not sure about centigram accuracy, but at the
milligram accuracy which is what a "precision
laboratory balance" provides, the weights used
even taken to account the weight of the air
displaced by the weight. And of course you need
an enclosure or air currents will blow the pans
Regarding what George said, David Sleeter replies:
Centigram balances are right on the edge of what might require a glass
enclosure. Therefore the companies that make the digital centigram balances
sold today usually offer the draft shield as an optional accessory.
In the process of perfecting the balance described in my eBook I learned
that it is INDEED very sensitive. To calibrate it and use it I had to turn
off the room air conditioner and close all the doors and windows because
tiny air currents would otherwise set the beam in motion. And I also found
it helpful to direct my breath away from the balance during operation.
Though I didn't mention it in the eBook, a simple 3-sided draft shield could
be made by simply cutting one side out of a large cardboard box. And of
course someone who's seriously into woodworking, could make a very nice one
from wood and glass. And a serious woodworker could also make a more
elaborate and attractive wooden stand. With the basic mechanical design
described in my eBook there's a lot of room for creative enhancements.
As a side comment, I'm 59 years old, and in the 1970s I built 5-string
banjos and repaired guitars for a living. At that time in my life I
developed a new kind of a banjo with a solid steel pot; it had a VERY nice
tone. Aiming at the high-end instrument market, I hand-crafted each one, and
designed my own distinctive peghead shape and inlay pattern for the peghead.
I built about 30 of those instruments before I moved on to other endeavors.
Today, with the rapid growth of the Internet, I'm amazed to see my inlay
designs being copied and sold in "kit"-form for people who want to inlay
their own instruments. I worked out the design in 1978 on the kitchen table
of my house in the mountains in Idyllwild, CA. I used to cut the patterns by
hand with a jeweler's saw from abalone and mother of pearl.
TODAY they are cut by a computer-controlled water-jet machine, and I feel a
distinct sense of regret as I note that craftsmen like I used to be have
been rendered obsolete by technology. If anyone's curious, here's the link
to the webpage of a company that sells these precut inlay patterns. You'll
find my D.G. Sleeter design about halfway down the page.
Speaking as a scientistist (my profession), "presision" balances
generally weigh to 3 decimal places (of grams, 10 mg, 1 centigram - not
a common measure of mass in science). "Analytical" balances are
generally 4-5 dp (0.1 to 1 mg resolution). Below this (6-7 dp) are
"ultra micro" balances. Anything below 3 dp generally requires a draft
shield to obtain a reliable result. Dave's balances, although good to
look at, and easy to explain how a "balance" works, are not common any
more, because the weights are susceptible to corrosion, and they aren't
because that's the official "metric term" for 10 mg. Regarding a draft
shield, as I explained in my previous post, a cardboard box with one side
cut out would be helpful but not absolutely necessary. Regarding corrosion
of comparison weights, ALL precision mechanical balances suffer from this
problem, and that's why the weights are traditionally handled with a pair of
forceps or a gloved hand.
Regarding their speed of use. Mechanical balances are INDEED slow. Unless
you manually stop the beam from rocking, the beam on my own balance will
take 30 seconds to a minute to come to rest. With practice and a delicate
touch, you can learn to stop the beam by gently raising a finger under each
bowl. And the more modern digital technology is FAR faster, in the range of
under 5 seconds.
I designed and perfected this balance while writing the now popular book
titled "Amateur Rocket Motor Construction". If you Google my last name
(Sleeter) and the title in quotes, you'll find more than a thousand
references to it on the Internet, numerous dealers selling it, and several
good reviews on Amazon.com. I'd originally intended to include the plans for
this balance as a separate chapter in that book. But by the time the book
went to press it was so large (8-1/2" x 11" & 514 pages) that there simply
wasn't room. The basic writeup and the photos for this balance languished on
my computer until about a month ago when I realized that it would make a
good ebook publication.
Regarding my own taste in equipment, in my workshop I have everything from a
Sartorius analytical balance (accurate to 1/10 milligram) to a Weightronix
platform scale that goes to 2,000 lbs. that I use for weighing heavy
equipment and loaded shipping crates. They are ALL digital, and I have to
admit that I've been totally spoiled by digital equipment.
But in terms of COST, it will cost you $10 to $20 to make the Homemade
Sleeter Centigram Balance. The Ohaus model AV212, which is the approx.
equivalent in accuracy and capacity will cost you somewhere between $425 and
Dave, you mean 30-60 seconds for each weight
addition. Which if you need 3 weights means 3
minutes. Not to detract from your balance which
seems pretty solid, if you added a swing limit and
and a swing release like analytical balances use,
weighing would be much faster. That feature could
be quite simple and would allow you to use the
single swing method of weighing which would speed
the process up to 5-10 seconds between weight
Actually my balance DOES have swing limiters. In the instructions on how to
make the balance I call them "beam stops", and you can see them in the photo
on the eBay page. They're the little black rectangles mounted on the sides
of the wooden upright. They extend out above the upper edge of the beam, and
each one is fitted with a plastic bumper cushion.
But using a beam stop or swing limiter is a double-edged sword. If you limit
the beam's motion to a VERY narrow range, you greatly limit the time it
takes to come to rest, but you also greatly reduce the weight differential
during which the beam can move. In practice you actually NEED to watch the
beam's motion and tilt to tell how close you're getting to the correct
combination of comparison weights.
In practice, though the unattended beam will take 30 to 60 seconds to come
to rest for each weight you apply, in reality, my learned-trick of gently
raising a finger under each bowl brings it to rest in less than 10 seconds.
rests. However determining weight by the position
the need stops at (beam stops rocking) is an
extremely tedious method. I suggest that you check
out various methods, including single swing or
similar. Sorry can't give you a URL on weigh
methods as I haven't been weighing stuff on a
daily basis for over 30 years.
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