I have a vacuum chamber that I would like to backfill with 15 mTorr of argon. The problem with accomplishing this is that most pressure regulators can't do pressures that low. Are there regulators that accurately regulate mTorr level pressures, or is there some other approach to backfilling a vacuum system at low pressure?
The simplest way to do this would be to design your own.
A commercially available spring, with an OD of 9mm, a length of 19mm, and a wire dia. of 1mm has a rate of 3.15 N/mm, with 11.5 mm compression length. Putting the lowest possible regulator in front of your homemade regulator, set a large diaphragm across a 1m diameter expansion chamber.
Then, place this commercially available spring on both sides of the diaphragm. At 15 mTorr, the springs should deflect a controllable 0.5 mm, which could be used to regulate the poppet valve. The key is a very large expansion chamber and a very small spring with a highly precise spring rate. To prevent the spring from moving, the expansion chamber will need to be evacuated, and the small springs need to be placed carefully to ensure the edges do not bind, but they do not collapse or buckle. This shouldn't be a problem, as the springs are well designed, but it's something to keep in mind when designing the expansion chamber.