Viva Recharge "brew stop": how does it work?
September 10, 2021 5:05 PM Subscribe
Could somebody explain to me how the "brew stop" system in this water bottle works?
The bottle in question is the Viva Recharge. There's a central plunger that features rubber gaskets around the top and bottom, as well as a metal filter on the bottom. (Similar to third-party metal filters for the Aeropress.) When brewing tea, you're supposed to push/screw the plunger all the way in to stop the brewing process. Presumably, this means that the water in the lower chamber won't touch the water inside the plunger.
I couldn't figure out what trick prevented the tea water from touching the drinking water, so I bought one to find out. Turns out: there's no trick! It's exactly what it looks like in the illustrations. The metal filter never touches the bottom, and nothing seals it shut. And yet... it does seem to somehow work. If I fill the plunger with water when pushed all the way in, it doesn't seem to leak into the bottom chamber at all. On the other hand, if I pull the plunger out and fill it with water from the faucet, it leaks immediately.
I assume there's a physics reason for this interesting interaction. What is it? Something to do with pressurized air in the lower chamber, maybe? If so, why isn't pressure lost through the metal filter?
The bottle in question is the Viva Recharge. There's a central plunger that features rubber gaskets around the top and bottom, as well as a metal filter on the bottom. (Similar to third-party metal filters for the Aeropress.) When brewing tea, you're supposed to push/screw the plunger all the way in to stop the brewing process. Presumably, this means that the water in the lower chamber won't touch the water inside the plunger.
I couldn't figure out what trick prevented the tea water from touching the drinking water, so I bought one to find out. Turns out: there's no trick! It's exactly what it looks like in the illustrations. The metal filter never touches the bottom, and nothing seals it shut. And yet... it does seem to somehow work. If I fill the plunger with water when pushed all the way in, it doesn't seem to leak into the bottom chamber at all. On the other hand, if I pull the plunger out and fill it with water from the faucet, it leaks immediately.
I assume there's a physics reason for this interesting interaction. What is it? Something to do with pressurized air in the lower chamber, maybe? If so, why isn't pressure lost through the metal filter?
Presumably, this means that the water in the lower chamber won't touch the water inside the plunger.
I don't think that's a safe presumption. I agree with wyzewoman - the compressed tea leaves below the filter clog up any movement of water within them, and the tiny surface area of tea exposed to the upper chamber via the small and relatively sparse filter holes slows the brewing process to a very low rate. It's a matter of making the water inside the plunger not touch the tea leaves much.
This isn't really the same thing that happens if you fill the plunger with water after pushing it home, rather than pushing it through water that's already in the cup.
If the screen at the bottom of the plunger has air underneath it rather than water, then only very small amounts of water are going to make it through to the underneath before water completely wets and covers the screen.
The holes in the screen are very small, and the additional pressure at the bottom of a 20cm water column is only 2% of an atmosphere. Such a small pressure differential acting over such a small area translates to a very small force, one that I would expect to be easily resisted by the surface tension of the water that spans the hole.
Even if that expectation is wrong, it would only take 2% of the space below the screen to fill with water before the air there got compressed enough to equalize the water pressure completely, at which point surface tension across the screen holes would certainly be enough to stop further air or water movement through the screen.
Tilting the cup and very carefully filling the plunger with a slow stream, so that the filling stream hits the screen with a fair degree of speed but is controlled enough to prevent the fill water from spreading out over the whole screen, should also let you get quite a lot of water through to the bottom before surface tension clogs the works.
I would also expect that filling the plunger and then capping and shaking the cup would cause turbulent pressure peaks at the screen that would be enough to overcome surface tension in the holes and let the air underneath bubble through to the top.
Conversely, I would expect shaking to add almost no additional brew strength to a cup of tea made by steeping loose leaves in the cup and then compressing the tea leaves into a puck below the filter screen, because in that case it's solid tea leaves, not mere surface tension, clogging the filter screen holes and diffusion of solutes through the compressed though thoroughly wet puck is going to be very slow.
posted by flabdablet at 2:33 AM on September 11, 2021
I don't think that's a safe presumption. I agree with wyzewoman - the compressed tea leaves below the filter clog up any movement of water within them, and the tiny surface area of tea exposed to the upper chamber via the small and relatively sparse filter holes slows the brewing process to a very low rate. It's a matter of making the water inside the plunger not touch the tea leaves much.
This isn't really the same thing that happens if you fill the plunger with water after pushing it home, rather than pushing it through water that's already in the cup.
If the screen at the bottom of the plunger has air underneath it rather than water, then only very small amounts of water are going to make it through to the underneath before water completely wets and covers the screen.
The holes in the screen are very small, and the additional pressure at the bottom of a 20cm water column is only 2% of an atmosphere. Such a small pressure differential acting over such a small area translates to a very small force, one that I would expect to be easily resisted by the surface tension of the water that spans the hole.
Even if that expectation is wrong, it would only take 2% of the space below the screen to fill with water before the air there got compressed enough to equalize the water pressure completely, at which point surface tension across the screen holes would certainly be enough to stop further air or water movement through the screen.
Tilting the cup and very carefully filling the plunger with a slow stream, so that the filling stream hits the screen with a fair degree of speed but is controlled enough to prevent the fill water from spreading out over the whole screen, should also let you get quite a lot of water through to the bottom before surface tension clogs the works.
I would also expect that filling the plunger and then capping and shaking the cup would cause turbulent pressure peaks at the screen that would be enough to overcome surface tension in the holes and let the air underneath bubble through to the top.
Conversely, I would expect shaking to add almost no additional brew strength to a cup of tea made by steeping loose leaves in the cup and then compressing the tea leaves into a puck below the filter screen, because in that case it's solid tea leaves, not mere surface tension, clogging the filter screen holes and diffusion of solutes through the compressed though thoroughly wet puck is going to be very slow.
posted by flabdablet at 2:33 AM on September 11, 2021
It's basically a french press / cafetière: the screen prevents most of the water from mixing with the leaves. If you drank all the available tea quickly then held the cup at a 45° angle for a bit, you'd likely get some fairly sour tea oozing out the filter. In normal use, that doesn't happen in a noticeable way.
posted by scruss at 7:33 AM on September 11, 2021
posted by scruss at 7:33 AM on September 11, 2021
Response by poster: FWIW, I experimented with it a bunch yesterday. Neither leaving the bottle upside down nor shaking it caused any leakage from the lower chamber, and there weren’t even any tea leaves involved.
posted by archagon at 1:37 PM on September 11, 2021
posted by archagon at 1:37 PM on September 11, 2021
Response by poster: Wow, great find! I'm going to mark this as best answer because that does indeed sound very similar to what Viva is doing, regardless of whether there's a direct connection between the two companies.
posted by archagon at 8:34 PM on October 14, 2021
posted by archagon at 8:34 PM on October 14, 2021
This thread is closed to new comments.
posted by wyzewoman at 6:55 PM on September 10, 2021 [2 favorites]