This could work

[Feign]

Feign, it definitely would work, and negate the need for auto focus laser. I cant quite get my head around the linearity of Masterada's design.... I have a feeling (need to do the maths), that the resin level would fall...... but both ideas need exploring.

It's understandable that it looks like that, but you'll find that the water level in the inner container relative to the top of the outer container actually rises a negligible amount, while your design indeed keeps the level completely constant. (though the need for a gasket introduces some potential for mechanical problems.)

[masterada]

It's understandable that it looks like that, but you'll find that the water level in the inner container relative to the top of the outer container actually rises a negligible amount, while your design indeed keeps the level completely constant. (though the need for a gasket introduces some potential for mechanical problems.)

That's not necessarily true. The weight of the inner container (including the weight of the water in it) is always equal to the weight of the water in the outer container surrounding the inner container. (http://en.wikipedia.org/wiki/Archimedes%27_principle)

Ignore the italic part, the next part is clearer
Imagine the inner container is very thin. In this case, dropping a very small amount of water will not effect the weight of the inner container (so the container itself doesnt move at all), but will raise the level of the water, thus the resin will be higher the originally.
Now imagine the inner container is just marginally smaller than the outer container (both containers have very high walls). Now siphon 10mm height of water from the bottom of the outer container. The resin falls by 10mm (there is 10mm less water below the inner container). Hold the inner container, so that it cant sink. Pour the siphoned water into the inner container. The resin level rises almost 10mm (you just poured that much water into the inner container). Let go off the inner container. As it is now heavier by 10mm worth of water, it will displace that amount of water from the outer container (Archimedes' principle). That means 10mm water from below the inner container will move to the gap between the inner and the outer container, effectively shrinking the inner-container and the resin for another 10mm.

So:
very thin inner container -> resin level rises
very thick inner container -> resin level falls

My head starts hurting.

Lets imagine there are 3 containers. "A" is the inner container. "B" is the water in the outer container, thats near the inner container. "C" is the water in the outer container thats below the level of the bottom of the inner container. Due to Archimedes' principle, "A" and "B" must always weight the same. If you move x amount of water from the "C" container into the "A" container, then you must also move the same amount of water from "C" to "B". That means, that regardless of the size of the inner container, for every mm of water moved from the bottom of the outer container into the inner container, the inner container will shrink 2 mm.
So if the inner base territory of the inner container is exactly half of the inner base territory of the outer container, then the resin never moves. The thickness of the wall of the inner container doesnt matter, neither does its weight.

I thought it would be more complicated then that

[mike_biddell]

That's not necessarily true. The weight of the inner container (including the weight of the water in it) is always equal to the weight of the water in the outer container surrounding the inner container. (http://en.wikipedia.org/wiki/Archimedes%27_principle)

Ignore the italic part, the next part is clearer
Imagine the inner container is very thin. In this case, dropping a very small amount of water will not effect the weight of the inner container (so the container itself doesnt move at all), but will raise the level of the water, thus the resin will be higher the originally.
Now imagine the inner container is just marginally smaller than the outer container (both containers have very high walls). Now siphon 10mm height of water from the bottom of the outer container. The resin falls by 10mm (there is 10mm less water below the inner container). Hold the inner container, so that it cant sink. Pour the siphoned water into the inner container. The resin level rises almost 10mm (you just poured that much water into the inner container). Let go off the inner container. As it is now heavier by 10mm worth of water, it will displace that amount of water from the outer container (Archimedes' principle). That means 10mm water from below the inner container will move to the gap between the inner and the outer container, effectively shrinking the inner-container and the resin for another 10mm.

So:
very thin inner container -> resin level rises
very thick inner container -> resin level falls

My head starts hurting.

Lets imagine there are 3 containers. "A" is the inner container. "B" is the water in the outer container, thats near the inner container. "C" is the water in the outer container thats below the level of the bottom of the inner container. Due to Archimedes' principle, "A" and "B" must always weight the same. If you move x amount of water from the "C" container into the "A" container, then you must also move the same amount of water from "C" to "B". That means, that regardless of the size of the inner container, for every mm of water moved from the bottom of the outer container into the inner container, the inner container will shrink 2 mm.
So if the inner base territory of the inner container is exactly half of the inner base territory of the outer container, then the resin never moves. The thickness of the wall of the inner container doesnt matter, neither does its weight.

I thought it would be more complicated then that

Masterada... you are correct, it is Archimedes principle..... a floating body displaces its own weight of 'water'. Whether it works or not does depend upon the relative areas/sizes of the inner and outer.