A New Project?

I've been wanting to add some kind of physics experiment to my rotation of hobby projects, and I think I've picked one out. But I don't want to go into that just yet, because I'll be concentrating on the equipment prerequisites first. The most interesting thing I'll need is a way to measure tiny amounts of force - on the order of mN (milliNewtons) or even μN (microNewtons). Weighing scales are the most common force-measuring tools out there, so it makes sense to convert this force to a weight or mass. The amount of mass that produces a μN of force under standard Earth gravity is 0.000102 grams, or 0.102 milligrams.

Representation of a black hole, courtesy NASA.

Digital postal scales tend to have a resolution of 0.1 oz (2.8 g), which simply won't do. But there are also cheap digital scales intended for weighing jewelry, powders, etc. which claim resolution of up to 0.001 g (1 mg). Scales like this are all over sites like Alibaba and Amazon for less than $25 ... but they're short of the sensitivity I might need to see the effect. Going up an order of magnitude in price will get me an order of magnitude more precision, from this laboratory scale for example. For the really serious scales with a resolution of 1 μg, I would have to lay out over $10K. Technically that's within my grasp, but I'm not that invested in this experiment ... and I don't think I could claim to operate this blog on a "shoestring budget" anymore if I made such a purchase!

But wait! There's one more option: "build your own." Dig around a bit, and you can find some how-tos for building a scale that "is easily able to get around 10 microgram precision out of a couple of bucks in parts." The crux of the idea is to repurpose an old analog meter movement, adding circuitry that measures the electric current needed to return the needle to a neutral position after a weight is placed on it. The author of that Hackaday article "can’t really come up with a good reason to weigh an eyelash," but I can ... so now I'm really tempted by this build. It seems challenging but doable for someone with electronics knowledge. I don't really believe the budget would be 2 bucks ... any Digikey order costs more than that ... so let's figure $25.

To sum up, my options are as follows:

Consumer jewelry scale (Resolution = 0.001 g): ~$25
Laboratory scale (Resolution = 0.0001 g): ~$250
Home-built needle scale (Resolution = 0.00001 g): ~$25 + blood/sweat/tears
Professional microbalance (Resolution = 0.000001 g): ~$12000

Ideally, I'm thinking I should make the needle scale and buy the laboratory scale, so I can cross-check them against each other. As a home-made piece of equipment prone to all sorts of unexpected errors, the needle scale will benefit from some degree of corroboration, even if it can technically achieve a higher resolution than the lab scale. And either of these would put me in the right range to measure effects of a few μN, without breaking the bank. I really don't need the microbalance, thank goodness.

I'm not sure where I'll fit this into my schedule, but I've already got some analog meter movements, courtesy of my dad's extensive junk collection. So stay tuned to see if I can weigh an eyelash.

Until the next cycle,
Jenny