DIY Scale (and some other fine instruments)

Ok, you probably can’t make the baby grand in this picture, and even the metronome is likely to be a serious DIY challenge — but you can definitely make a pretty accurate DIY scale, and you can do it cheaply and easily.

I needed an accurate scale for a science project and knocked this baby together (based on this design) using found parts.  I was able to easily measure to centi-gram precision and with a little care, a scale like this could be tuned to measure to milligram precision.

Precision (the ability to discriminate between differences in mass) is largely a matter of careful construction — accuracy (the ability to weigh to an agreed upon standard) is another matter altogether,  and it basically hinges on having an accurate reference.  Fortunately, a great institution, born of Philly — the U.S. Mint — was wise enough to make Nickels and Pennies in rather convenient dimensions.  It turns out that nickels are 5.000 grams and pennies are 2.500 grams — so you not only have sub-milligram accurate references of convenient size — you also have an easy way to cross-check your scale by using nickels to weigh pennies and vice-versa.

Details of DIY Scale

The zoomed in photo shows most of the essential elements of construction.  Basically, I used a threaded 10-24 rod for the balance (10-32 would have been a better choice).  I used a wall-board razor as my knife-edge pivot point.  Two angle-brackets served as a hard, flat surface for the knife edge.  A nickel with a hole in it and some thread served as a reference weight (I wound up with a whole array of perforated nickels and pennies). A wall-board T-square served to measure the distance from the pivot to the reference weight.  I used an index card and a small mirror to make a sliding mirror in order to read the position of the weight w/o parallax error.  The whole shootin’-match was held on a stand that was salvaged from a cheap drill-press.  Measurements were performed by reading the distance between the movable weight and the pivot point, and entering that value in a Google Docs spreadsheet.

I definitely could have purchased a milligram scale for far less than this cost me in terms of spare time, but I learned a lot about scales in the process.  Almost all of it was stuff that I knew “in principle” — but actually building the scale infused my arm-chair knowledge with real-world experience, yielding an alloy whose properties seem to have exceeded its constituent parts.

The scale was nowhere near large enough to measure my satisfaction, but I estimate that this exercise yielded just about one metric ton of fun.

3 Responses to “Instrument DIY”

  1. Dave says:

    That’s an awesome scale, nice job!

    I would be cautious about using coins as reference weights. I’ve weighed 3 different pennies and found 3 different weights, differing by a couple tenths of a gram. I assume all the coins from a given year / style are pretty similar to each other.

  2. Mike Hogan says:

    Yeah, I did a little research after the fact and found that the standard deviation of pennies and nickels of the same minting are in the several hundredths of a gram range.

    For this experiment (conversion rate of varying sucrose concentrations to microbial cellulose), we could easily tolerate a few percent error in accuracy — but we needed pretty high precision to distinguish between individual samples because the total yields are relatively small.

    I’m kind of obsessed now with designing an open-source analytical balance that is compact, elegant, precise and easy to build — probably a hybrid with a micro-controller and a galvanometer (the galvanometer can be used for active damping and also pick off the milligrams portion of the measurement).

  3. Dave says:

    That would be pretty awesome, particularly if it could be modified such that it would weigh at truly milligram or better precision and similar accuracy. Cool!

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