A quirk of collecting vintage (or new, for that matter) mechanical watches is that we enthusiasts and collectors are constantly worried about their accuracy — when we buy them, when we wear them, and when we're about to sell them. This process is important for sussing out whether a watch is functioning accurately or whether it needs service. If you can check a particularly watch before purchasing it, you can potentially save yourself the hassle of an immediate, expensive trip to the watchmaker.
Checking accuracy via a watch movement's amplitude is possible at home via a desktop system like a Timegrapher, but these aren't exactly portable, nor do they look particularly slick on a desk.
A new company called OneOf is hoping to change this. Their Accuracy2 is a portable watch measurement tool that consists of a sensor pad, a USB cable, and an adaptor for your phone or iPad — when used in conjunction with a proprietary app, you can use it to measure your watch's accuracy. The sensor weighs just 20g, and you can throw it in a bag and conceivably take it on the go. At $320, it's less expensive than something like a Timegrapher, and much easier to move.
OneOf also has two other systems in its lineup: the Accuracy Boutique Edition, which includes a demagnetizer ($1,270) and the Accuracy Pro ($3,330), a serious tool geared toward watchmakers that includes multi-position sensing. For more info, check out OneOf's website below.