Martin, I disagree w
Martin, I disagree with some of your methods and conclusions. First, you cannot measure the diameter of thread (except for the very specific case of a round monofilament thread). That's because thread isn't round, and so has no "diameter." Moreover, as you partially recognised, in many instances the act of measuring affects the measurement. In statistical terms your measurements are reliable but not valid. In other words, you are consistently and systematically measuring the wrong, or a meaningless, value.
Denier, or decitex (a similar measurement in general commercial use), is the only way to reasonable assess thread "sizes" AND only when comparing threads of like material and construction. On the other hand, breaking strength can be reliably measured and so is a legitimate basis for thread comparisons. Of course one understands that even saying something is constructed of polyester filaments doesn't guarantee identical properties as the filaments are subject to scores of manufacturing tolerances. If you had total information about a thread's properties (such as specific gravity among others), one could calculate a cross-sectional area and hence an "effective diameter," for whatever that would be worth. For example, given the identical "effective diameter," a 1-ply thread would tie differently from a 2-ply one, a twisted one differently from a flat one, a waxed one differently from unwaxed; etc..
The "ought" system has been completely adulterated and so, at least on a global scale, is now completely meaningless. At one time responsible respoolers kept the numbers at least relative. But, because there was no standard, not even a definition, some respooling brands simply invented numbers to make their thread appear more attractive. In some cases they even gave the same thread different numbers to make it appear that they sold a range of sizes.
Is there any hope for standardization of fly-tying thread designations? No. It's a small market and the wild-west of "ought" designations suits some players. If you want to see a struggle to get some consistency in a "similar" angling product, one need only look at the case of monofilament line in the European market (relative to fly-tying thread a mega-market). Even in monos, there is variation in "roundness" (some are oval) and so pure diameter measurements aren't always possible.
What is true is that one should select thread based on the use (my booklet, How to Choose and Use Fly-tying Thread, can help here) and whether or not you trust the brand to provide accurate information. So while I disagree with what you say "will work" - besides, attempting to put that much information on a spool label is completely impractical - I applaud your attempt to try and clarify some of the issues.
Paul Marriner
Outdoor Writing & Photography. Owner: Gale's End Press. Member: Outdoor Writers of Canada. Author of: (new) Mahone Bay Mornings, Modern Atlantic Salmon Flies, 2nd Edition; A Compendium of Canadian Fly Patterns (co-author); Stillwater Fly Fishing: Tools & Tactics; How to Choose & Use Fly-tying Thread; Atlantic Salmon: A Fly Fishing Reference (OP); Modern Atlantic Salmon Flies (OP); Miramichi River Journal (OP); Ausable River Journal, and Atlantic Salmon (OP).
