Dendritics Weighing Pan - Material Selection
It is hard to believe, but we went through an amazing number of materials for this simple part (the "Removable Pan") over the 11 years we were making Gemscales. The requirements initially were for strength and a custom shape that fitted onto the top part of the sensor (the "Fixed Pan") and allowed stones to pour well. Simple enough.
We started with ABS, the material used in the outer housing and in good quality telephones (designed to be thrown across the room). Turns out that this would snap in two when stepped on or repeatedly flexed.
We quickly switched to polycarbonate (Lexan®), the plastic used in bulletproof glass and in the draftshield. Its strength was fine, but we discovered that there was a static attraction that occured when we lowered the draftshield over stones in the pan, leading to as much as a half carat of wrong weight readings. Not good!
The next version was also polycarbonate, but filled with carbon fibers for conductivity. The fibers stuck out the edge of the pan a minute amount, and would ground the charges into your fingers when the pan was picked up. This worked like a charm, until we started hearing reports of wrong weights with diamonds. Just diamonds! Of course, when we had tested the pans we didn't use "real" diamonds. The problem was finally confirmed (in our presence) in Antwerp. So we got some diamond melee and found the problem was coming from a very strange property of diamonds. As semiconductors, it seems that they could generate charges when they rolled over each other as they were poured in and out of the pan. This charge was transferred to the pan. If the pan was being held by fingers at the time, there was no problem. But if the stones happened to be poured into the pan while it was on the scale, that was another matter altogether. Since the carbon fibers stuck out the edges of the pan only, there was no conductive path out of the pan and into the scale. We needed a still better conductive agent which worked in all directions (This is called "anisotropic" conductivity).
More research unearthed a new additive for the polycarbonate pans, a fine graphite dust or powder material. We switched to this material. A couple of years later, as we began our researches into Millicarat scales, we found that the carbon powder filler had its own problems. Being very small particles, they did not bind all that well to the polycarbonate, and could be wiped off if one tried. Old pans were not so good, or a pan would turn bad if cleaned thoroughly with a light abrasive such as a paper towel (You never thought of paper towels as abrasive, did you?).
So for the Millicarat scale, we used a more expensive process of chrome plating the pans. Instead of black, these pans were, well, chrome colored. They looked great, but were a bear in production. Well over half of them would bend or warp in the plating process. And the plating people never did get good control over the plating thickness. Seems no one else cared. But for us, the total weight of the pan had to be within a narrow range, to keep the scale in calibration. We had to inspect the pans for flatness as well as weigh each one. Yucch!
We continued to search for a better alternative, and finally found it in a newly discovered polymer additive which was molded into the pans and gave them a conductive nature. This was our last version, the best by far. These additives had existed for years, but they mainly work by attracting humidity in the air (a "hygroscopic" property). This works just fine in the lab (at 30% humidity). But when the humidity in the air isn't, the conductivity isn't either! In winter in a cold climate ... or in the desert. This is just when you need it. No one has static troubles in mid-summer in Alabama. The discovery (or invention) of the polymer additive that didn't need the humidity was quite recent. IN fact, when we found this material we changed the upper housing of the scales to use it too. IT was about three times the cost, but prevented any static problems from the housing as well.
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