Regarding Disc Weights
The Hydroscopic Effect
By Jason Southwick
Almost a decade ago, Marshall Street became the first disc golf retailer to weigh every disc. It was right after we purchased our first digital scale, along with a 200-gram weight to check its accuracy. We noticed the weights that manufacturers pen on the bottom of discs were often off by one or more grams.
One thing I imagined for years, was that the Innova people who weighed the DX discs must be an entirely different group than the people who weighed the Champion and Star. The DX crew consisted of Eagle Scouts (presumably CE Eagle Scouts), and important voices in the church choir. The kind of people who could find food on Survivor, and would give you the bigger portion, because that's what perfect people who weigh every disc perfectly would do on a desert island.
By contrast, the Star and Champion weighing contingent is holed up in one of their parents' basement. A broken scale sits on a table next to a bong, out of which curls a wispy band of smoke. One guy holds a disc in his hand, as if to judge its heft, and inquires to no one in particular, "167?" He gives it another little shake and declares, "Definitely 167...Who's got the Sharpie?"
Another thing violating our sense of propriety were Discraft's weight range stickers (e.g., 167-168), the ones that adhere to the inner rim as if their life depended on it. We understand that a gram or two doesn't make a noticeable difference in the physical universe of disc golf. Nevertheless, knowing the exact weight of the disc in your hand seems like a fundamental right, as well as psychologically soothing.
Then there's Gateway, founded and managed with a light hand by my friend David McCormack. Their weights have always been the least accurate. One reason for this, I learned, was that on Gateway's production line, discs are first weighed and THEN trimmed. David insists that they can tell whether the extra plastic that gets squeezed out along the parting line - where the top and bottom halves of the molds meet - is a half gram, a whole gram, or a gram and a half. Oh really?
In this early phase, most likely after weighing a bunch of Wizards, I offered to send Gateway an accurate digital scale. David said they had tried a bunch of $100-$200 digital scales, and all of them broke. (Gateway uses triple beams.) We proceeded to buy, use and break several $100-$200 digital scales in very short order. Then we purchased the OHaus SC Series 200g digital scale, that'll weigh discs up to 200 grams to 1/10 of a gram. We've used them every day since 2009, and we've replaced maybe two or three.
It was about this time that David McCormack, writing on the PDGA's Discussion Forum, talked about the hydroscopic effect - the ability of certain plastics to absorb and retain water, and hence weight. David claimed this could add a gram or two to a disc, but he was contradicted by a seemingly knowledgeable poster who claimed David's math was off by a decimal point or, in other words, by a factor of 10.
We'd eventually realize that David was right and that the seemingly knowledgeable guy was wrong. We had begun to notice that many of the discs that had been sitting on our racks for a long time had gained up to a gram. We began calibrating our scales more often.
Then in 2013, Prodigy Disc, deeply involved with reinventing the wheel, noticed something strange. Multiple PDGA World Champion and Prodigy executive David Greenwell told me a story about weighing stacks and stacks of brand new Prodigy discs. Then moving the discs, and the same scale, to a new location and weighing them again a few weeks later, only to find out that the discs were now, on average, two grams heavier. It was baffling.
Subsequent conversations with disc golf manufacturers corroborated our suspicions. Certain plastics are indeed hydroscopic. So a premium plastic disc produced, say, in Sweden, and stamped accurately with a weight of 168, might tip the Marshall Street scales at 170 by the time it reaches Leicester.
Finally, one Facebooker asks, " ...I've had discs marked 175 that weigh 178-179 on my calibrated scale. I tend to think manufacturers don't like to grind up overweight premium plastic discs that are almost in spec."
Agreed, and I'd also be hesitant to grind up a perfectly good (albeit technically illegal) disc, especially since once you start playing with it, no one's ever going to know, because no one ever checks. Or, for that matter, really cares...as much as we do at Marshall Street.
