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The BorgCo Invisible eggtimer has the classic "hourglass" shape expected of such devices.
It is filled with tiny, perfect polished spheres of transparent silica instead of just plain fine sand.
Instead of air, the device is filled with a colourless transparent low viscosity liquid with exactly
the same refractive index as the silica beads.
Thus, when the device is inverted, the silica grains start to flow through the pinch in the usual way, displacing the fluid which flows in the opposite direction. The pinch is sized to compensate for this counterflow, thus the time for all the beads to pass from the upper to the lower bulb is exactly three of your Earth minutes.
It's just that you can never tell when that is ....
||Clever [+]. However, once you stop hearing the tiny pings of the grains landing on the pile below, you know the 3 minutes is up.
||[the porpoise], we mere humans, not having super hearing
like porpoises, might need to add a microphone and
amplifier and speaker, in order to hear those pings.
||An interesting novelty egg timer exists which has your standard egg timer fitted snugly inside a tall tube filled with water. The egg timer is buoyant and floats to the top. When you invert the tube, the timer is now at the bottom and is top-heavy, so tries to flip over. But the tube isn't big enough for it to flip, and so it grips the sides of the tube. Only when enough sand has reached the lower bulb of the timer does the centre of gravity move, the timer no longer wants to flip, and suddenly floats to the top.
||So, if your invisible egg timer was configured like this, you would still get a visual indicator of when the time is up.