Considerations and questions for a DIY heat pipe; how to choose a working fluid? I plan to build a DIY heat pipe! For more info on what a heat pipe is, see Wikipedia.
I would like to use it to cool a desktop graphics card I have. I am having trouble deciding how to choose the working fluid for the pipe. I plan to build it based on this design. Using gravity to return the fluid to the hot end. 
Working fluid considerations
My graphics card consumes 250 watts at 100% load and 15-20 watts at idle, the maximum temperature is 90º Celsius and the temperature at idle is 30º Celsius.


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*How does this translate to a choice of working fluid? I would assume the fluid has to have a phase transition temperature just lower than that of the lowest operating temperature of the card, which can be changed by based on the PV=nRT law? So just whatever substance will boil between 30–90ºC at the given pressure in the pipe?

*How can I decide how much fluid to use? As long as the pipe is filled with only the vapor of the working fluid at the proper pressure, then would the only amount of fluid you need be just enough to cover the surface area of the boiling point on the hot end? Would it be a good idea to add a little more?
 A: The mathematical model for a heat pipe in equilibrium is quite complicated. Since if the temperature difference between the hot and cold end increases, the heat transfer should too. This will increase the internal pressure, since the pipe is a closed system and the evaporated liquid tries to take up more space. However the volume inside the pipe might expand when the temperature increases, the boiling point of a liquid will increase with pressure and heat of evaporation will drop with pressure. The gas state can probably be approximated with the ideal gas law as long as the temperature/pressure will not get to high. But we haven't even started about the flow of the gas/liquid.
But your main question is about how to choose your work fluid. This will depend on which operating pressure you want to use, since water can also be used at lower pressure so it will boil at a lower temperature. But different work fluids might affect the effective boiling point when the heat transfer is increased. I assume that you would like that this increases as slow a possible, such that it will remain below the 90C for as high heat transfer as possible?
A: You can find information on working fluids here 
From the table, the working ranges are:
Ammonia     213-373
Freon 21    233-360
Freon 11    233-393
Pentane     253-393
Freon 113   263-373
Acetone     273-393
Methanol    283-403
Flutec PP2  283-433
Ethanol     273-403
Heptane     273-423
