h a l f b a k e r yRecalculations place it at 0.4999.
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You know those metal tea balls, with a bunch of holes in them for brewing loose leaf tea without the leaves getting everywhere? This would be one of those, but constructed of a material that slowly expands when heated, so after 5 minutes the holes are completely sealed off. To 'reset', simply remove
it from the hot liquid.
[link]
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Good one. Some hydrophilic plastics or silicones could probably be engineered to do this. Coffee too? Lord knows we really need another way to make coffee. |
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What if it worked the opposite way? You could carry one in your pocket and when you dropped it in hot water the holes would open. When you removed it it would revert to the closed mode. |
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[a1] Sorry I did not communicate my deep sarcasm clearly enough, referring to the 10,001 stupid yet interesting ways. I'll be more careful from now on. |
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Minor, I like that a lot for travel use, but it doesn't prevent over-steeping, which was the primary driver for the idea. |
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You could make the ball out of a first metal. Each hole would be an incomplete cut, leaving a tab which could be bent back to open the hole. Each tab is coated with a second metal, thus each tab becomes a bimetallic strip. |
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The bimetallic tabs are in the closed position below a threshold temperature (e.g. 85ºC) and in the open position above this temperature. |
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So the brewing/steeping starts at 100ºC and stops when it gets to 85ºC (after about 5 minutes). |
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I knew there was someone here who could science the hell out of this! |
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I was thinking cork, but the water inside would keep it swollen shut way too long. |
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From what I've read online, aluminum seems to have the highest coefficient of thermal expansion. I wonder if you just could drill holes in a thin aluminum shell small enough for them to just... close up when it's heated, and reopen when cooled? |
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I was thinking of two concentric shells, which are both pierced, and which can slide past each other. When cold they would be in the start position with the holes closed. When put into hot water the thermal expansion drive would start to slide them open. However there would be a slow-running damper which the sliding shell would reach halfway through the cycle, when the holes were open, and which would then slow down the movement, taking a couple of minutes to reach the end of travel (with the holes once again closed on the far side of the travel). A thumb screw with gradated markings could be used to advance or retard the damping mechanism to allow for customised steep times. |
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sorry just skimmed. wouldn't holes get bigger? |
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Not if the metal is expanding. I think... |
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[21Q], I think [po] is technically correct in a geometrical and physical sense. You can simulate the expansion by taking a photo of the holey surface and then doubling the size of the photo - everything doubles in size, the circumference of the holes, the diameter of the holes, the distance between adjacent edges of the holes, the distance between the centres of the holes... |
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I'm reminded of the SR-71 aircraft. The metal frame had an expansion rate of several inches as it warmed up, and they never did manage to design a fuel system that could cope with the expansion AND contain fuel, so prior to takeoff it's said to have looked like it was raining under the aircraft because it leaked so much. |
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As suggested, a sphere of the same metal expands uniformly outward more than the hole expands inward. This is why you can remove a bearing from a shaft by heating it. |
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If area immediately around the hole is of a different metal, both weaker and with a higher thermal expansion coefficient than the body of the shell, it would tend to expand inward. (Aluminum in a stainless steel matrix should work for this). |
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In practice, however, it's really hard to choke of a round hole by pushing inward uniformly. Probably your best bet would be to make each "hole" actually a narrow slot, with a separate piece of aluminum on each side, again still embedded in that stainless matrix. In that setup, the steel keeps all of the expansion towards the slot, and there's no structural shape in the aluminum fighting you. |
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For the sealed, unsealed, sealed version, the first version I can think of is a triple shell of steel-aluminum-steel. The three shells are held rigid to each other, and the holes in the aluminum are slightly offset from the ones in both steel layers. The aluminum is tight to the inner layer when cold, and tight to the outer layer when hot. The steeping happens in between the two states, when it's loose. The problem is the total relative expansion is relatively low, so your total open area is small, even in the midstate, and reltively little water moves through. |
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My only concern with the shutter idea is metal fatigue from repeated bending. You wouldn't want one of those things snapping off in your tea. |
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Why do you assume a sphere? Because it is called a "Tea Ball?" |
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Make it an elongated triangular tube and the bimetallic expansion and contraction will be much more pronounced. One vertex has a split backed by a screen that opens in boiling water and closes as it cools. |
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That's most efficient in terms of containing the most tea with a minimum of material. Once the curvature starts introducing complications in shuttering holes, it might be more cost effective to simplify the engineering, making a flat-sided container - perhaps even approximating a tea bag. |
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(perhaps a CRS frame, 2 Al sheets, outer sheet attached to one side, inner sheet to the other side of the frame. The tiny holes in the sheets align at your preferred temperature.) |
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So Lurch, instead of a ball it would look more like a perforated snuff box? |
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You all know PG Tips pyramid bags. Stretch one vertically, allowing enough internal volume for circulation and the bimetallic seam opens, holding the tea leaves behind a screen. Add a short handle or lanyard and wear it like a watch fob, instantly ready for a cuppa. |
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I like the idea of drive through coffee shops using these. I hate getting tea from them because it's such a hassle and mess to safely open the lid on the travel cup to remove the bags 5 mins down the road. |
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