Friday, July 17, 2009

Baby Scale Hack

I came home from work and promptly determined that Charlotte’s approximate weight was needed. I’ll spare you the long story behind why we needed her weight—just know that the desire was high.

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So how do you get your baby’s weight? Our bathroom scale isn’t accurate enough. We tried the ol’ weight you+baby, then weigh just you, then subtract and use the difference as the baby’s weight. We repeated that process several times and got her down to somewhere between six to eight pounds. That’s not accurate enough for anything.

We have a kitchen scale which is very accurate but it maxes out at four pounds. Charlotte’s about six to eight pounds so that obviously won’t work. So while Sarah was prepping dinner, I turned the tiny kitchen scale into a single-beam baby scale with a variable weight limit (I’ll explain).

The principle I’m using is leverage. I took an ironing board (a lever big enough to hold a squirmy baby) and balanced it like a see-saw on a rolling pin (the fulcrum). I placed the scale under one end and balanced it such that the scale read as close to 0 as possible like this:

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When balanced, the scale read about 10 grams with nothing but the board—I’ll consider that weightless so we can ignore the board’s weight for the rest of this. I tested my theory with something I could actually weigh normally on the kitchen scale: a can of soda. The soda weighed in at 13oz directly on the scale, so half-way between the pin and the scale, it should way 6.5oz. I verified this and marked the board’s midpoint:

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As you can see, there’s a linear relationship between the can’s location and it’s observed weight. The relationship is very simple:

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Get the ratio of red to blue by dividing the red distance by the blue distance. This is the multiplier to apply to whatever the scale reads to get the actual weight of the object on the scale. So if we’re half-way across, then we have a ratio of 2:1 or 2, so we double whatever the scale says to get the objects actual weight. If we’re just 1/4 of the way across, we quadruple whatever the scale says. That’s what I meant by a variable weight limit.  I know that Charlotte is less than eight pounds (half of eight is four, the scale’s weight limit) so I just went with the midpoint.

So did it work? Let’s see:

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Isn’t she lovely?

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Behold: The power of the swaddle!

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I can’t actually verify her real weight (thus this ridiculous contraption!) but my tests with the cans give me confidence that her weight is damn close to 2 x 3lb-2.65oz; or 6 pounds 5 ounces.