Build a Precision Laboratory Scale


Hi Folks:
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 educational project.
It's on eBay right now, and here's the link.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&itemu85874244
David Sleeter
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David Sleeter wrote:

Does it need any calibrated weights to be purchased?
--
Dave K

http://www.southminster-branch-line.org.uk /
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wrote in message

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 inquiry,
David Sleeter
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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)
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Dave (from the UK) wrote:

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 all over.
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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. http://www.luthiersupply.com/pegheadinlaypage.html .
David Sleeter
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David Sleeter wrote:

[big snip, segue]

FWIW, I for one would like it very much to read a web page with a detailed description of how you did/do that. And I'm sure others would, as well.
er
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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 fast enough.
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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 $500.
David Sleeter
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David Sleeter wrote:

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 additions.
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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.
David Sleeter
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David Sleeter wrote:

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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