A top quality, professional brewer's set of Plato saccharometers (hydrometers) can be had for under $50, and a homebrew hydrometer for $6, and these will do the exact same thing. If you want a to use a small sample, a gadget, and your phone, a digital refractometer and a free online calculator will do the same thing with an even smaller sample -- just a couple of drops -- for a third of the cost. But it's your money.