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Coat hanger scale


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 water.

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.

Ta dah!
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 hanger.

(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.