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Vacuum sucks less - Alex Belits
Vacuum sucks less
I haven't updated this journal in a while, but I have solved the problem with vacuum the next day. Since a heat pipe has two openings when it is being filled, I simply connected both of them to the transparent pipe with valves on both ends. Upper end of this is connected to the source of heat transfer liquid, and lower one has a vacuum pump attached to it. After pumping the air out of this (valves are open) and filling with liquid I can simply measure the amount of liquid remaining in the system by tilting it, so all liquid moves to the transparent pipe. If I close the bottom valve I can measure precisely the amount of liquid by placing the transparent pipe vertically -- the difference between liquid volume and the volume of liquid that entered originally is the volume that evaporated to fill the rest of the system.

Then I can open the valves and tilt the device to put the liquid back into the heat pipe, seal the heat pipe, close all valves, disconnect everything and have the heat pipe with precisely measured amount of liquid and vapor at the room temperature. To fill the next pipe I can keep the valves closed until the heat pipe is filled -- I already know that the transparent pipe is filled by saturated vapor, so opening the valves when the vapor in the heat pipe is also saturated won't cause any additional evaporation or condensation. Whatever I will measure at this condition will be exactly the same amount of liquid that will end up in the heat pipe with given amount of liquid entering it even if valves to the measuring pipe were never opened -- all subsequent pipes will have the same amount of liquid and vapor as the second one.

Of course, another way to do it would be simply to calculate how much of liquid would have to evaporate to fill the whole system with liquid + saturated vapor, given that density of both is known and assuming that the temperature at the end of this process is the same as in the beginning. The problem is, I did not know the exact volume of all pipes and valves involved, and obvious way of measuring it (filling it with some liquid that remains liquid at a room temperature -- say, water or oil -- and then removing that from the system because it would otherwise contaminate the heat transfer liquid) would involve much more effort and risk than adding a pipe and two valves.
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