If I remember right, Mike was inspired by another well-known sharpness tester (I believe it was Ankerson or Cliff Stamp over on BladeForums) who used fishing line, and there were a number of such contraptions popping up around in the early 2010s. As far as I remember, the reason Mike produced the BESS media is because fishing line was just too inconsistent for our purposes, even with the same manufacturer. A lot of guys at that time would end up looking at batch numbers to try to maintain consistency.
The initial prototype they used was the KN100, which was a much different contraption. I actually did some of the prototype testing for that device. The KN100 would suspend the filament above a cutting edge that was clamped down on the device, and then there was a platen above that you would place a cup on. You would rest the thread on the knife edge with no weight on the platen, and gradually add weight via supplied lead shot. One of the problems with this, is that the rate at which you added weight would greatly effect the results, because the loading was dynamic. In other words, if you took a 100 gram calibration weight, and stuck it on the platen and it cut the media, you could also gradually fill the cup with 100 grams of shot and it would not cut, and the difference was basically the
sudden application of the 100 grams versus the gradual build-up. Because of that, the scores you get depend greatly on the rate at which you add weight. The other major issue that they solved with the PT50 was the loading of media in the yoke, because people found that depending on how taught you made the media could also greatly change the score; so the PT50 was designed to use disposable clips which had a consistent tension of the filament. I see recently they also released another yoke to use the media on a spool, and I'm not sure if they mitigated the issues of inconsistent tension, or if maybe those yokes are meant for "in-house" comparisons where the tester finds some way to maintain a consistent tension.
When they released the PT50, they changed the operation significantly, mostly to account for the difference in platen weight and filament tension. Instead of lowering the platen and a weight onto the stationary knife edge, you place the tip of the knife on a pedestal, and then lower the edge down through the test media. The problem with this method is that you need a scale that takes frequent measurements, and records the highest load read. An ordinary kitchen scale probably won't do the trick, because as soon as the media is cut, the weight will be released. You might then be tempted to keep a close eye on it and try to remember what the weight was when the clip severed, but depending on your own reaction time and the speed at which the scale will take measurements, you're likely to get inconsistent readings. This is one of the reason that the scale itself differs between their home and professional models, with some taking readings more more rapidly than others.
Not sure if you've found the thread by now, but here is the initial test thread on Bladeforums
https://www.bladeforums.com/threads/edg ... r.1285538/
I have frequently thought that the issue of dynamic vs static loading could be pretty easily solved in the original design by using some kind of hopper to drop weight on to the platen more consistently than doing it by hand. So if you're going to go with an ordinary kitchen scale, you'll probably need to use the KN100's approach and apply the weight with a platen of some kind so that when the filament severs you don't lose your reading. Perhaps you could also figure out a more consistent way to load the weight too.
I still have my KN100 and use it from time-to-time, but ever since they changed the operation they also had to make a new BESS scale. With the KN100, the weight of the platen is included in the BESS score in the end, and since the platen weighs about 54 grams, you must add 54 to the BESS number you get on that device for it to be comparable to BESS scores taken on the PT50 series.
This has the unfortunate effect of making BESS comparisons among different users very unclear. For example, on the BESS Exchange forum, some people have tried to make charts describing what BESS levels are hair whittling, hair shaving, etc. and so on. (
https://www.bessex.com/forum/showthread.php?tid=185) However, among the many issues of consistency here, is that there's no telling which BESS scale they're using unless they specify it. Here it does say they're using the PT50 (so it would be the BESS-C scale), but even then, there are some pretty big discrepancies between this scale and other peoples' observations. For example, my beard hair gets whittled at <=150 BESS-C (the scale the PT50 uses) or <=96 BESS-A (the original scale used by the KN100), whereas the user who put together these comparisons lists that as only achievable down at the 25-30 area. No telling what causes the discrepancy; maybe I have wire edges, maybe my hair is so coarse it just gets whittled super easy, maybe I'm making it easier by whittling towards the follicle rather than away from it, etc. *shrug* Plus you again have to consider the issue of dynamic load vs static load, and that you can get lower scores by severing the media at different rates, temperature of the media, humidity, et al.
So, unfortunately, I still don't find the BESS scale to actually be super useful for us to be able to compare sharpness results with different people; though a lot of this is due to no fault of the system, but rather just the results and tests people are comparing it to. On the other hand, as you pointed out, it's still a useful tool to make observations against your own edges and uses where you can control the factors which can lead to inconsistencies. However, as I said before, this whole system was inspired by similar fishing-line testers that were popular online years earlier, and the consistency of that media versus the BESS media apparently leaves a lot to be desired. I'm guessing you could probably spend just as much money trying to find the perfect fishing line as you'd spend just buying a spool of the BESS certified media.
You may have seen me mentioning some BESS scores testing the CPM 15V Manix 2 here:
viewtopic.php?f=2&t=93878&start=100#p1681889 I adjusted all my scores to the BESS-C scale and you can also see where the numbers can get really inconsistent at times. There will be weird instances where the media won't get severed no matter how much weight gets added, or when it suddenly takes significantly more weight than previously, and then takes the normal weight again right afterward. So all of the scores I discuss are actually an average of 3 different measurements taken, usually with any outliers thrown out if they were significantly different than the rest of the set.
Anyway, I hope some of this info is helpful and I'll be curious to see your progress on your DIY version.