Things haven't been going well for your marauding band of savages lately. The last two raids have come up with very little food and the war boys are running low on silver spray paint. As the cook, you are catching a lot of blame for the situation and you are starting to worry about getting sent to Valhalla earlier than expected. You have one last chance to get on the good side of the war chief, Thok'gar, and fortunately you stored away the supplies to bake him a nice cake for his birthday; Thok'gar loves cake. Unfortunately, the recipe you have has all the ingredients listed by weight, and electronic kitchen scales are a thing of the distant past...
Sound familiar? Well worry not, a reasonably accurate kitchen scale can be made from some stuff you already have lying around. I used a strip of the inner tube from the
handle bar grip project, a round of
wood cut on a miter saw, some string, and two wooden dowel pins carved down from serviceberry sticks to make this prototype.
The scale is accurate to about 2 grams with a max weight of 100g. Nothing amazing, but not terrible for a first pass at it. I installed a stick on the wall with the graduations on it so that it can be replaced and recalibrated if the torsion part of the scale changes in any way. For calibration, I used a handful of pennies and quarters, which way 2.50g and 5.67g respectively. I had
enough to get up to the full 100g, and I made a lot of marks at various test weights because the scale is pretty non-linear. It is quite repeatable though, so with lots of calibration points it works alright.
The scale operates by twisting the elastic element at the top and unwinding a thin string from the upper wooden pulley. A large-ish diameter pulley is good because it increases the sensitivity of the scale. In place of the rubber strip, some thin rope or braided string would work too (sort of like the old rope-powered catapults). The elastic element is pulled through slots on the dowel pins and pinched tight, I also added a couple dabs of hot glue for security. I made a small hook from baling wire to hang the scale cup from, and then pinched in a piece of zip-tie and hot glued that as well to act as an indicator for readout. The cup is the bottom of a tin can with a short loop of baling wire holding it on.
I was going to make a large free-standing version of this that has more range and more accuracy, but I'm not sure I'll have time to get to it, so prototype it is for now. Because of the non-linearity of the scale, it is good to approach the weights from the bottom. In other words, it is good to pull the scale down past its reading, and let it gently spring back up, otherwise there are certain points on the scale that are a bit unstable.
This is part of the
MoPID series of permaculture innovations that I am working on during my time at
Wheaton Labs. Check out the
thread if you'd like to follow along.