The gas expands using a needle valve as mentioned earlier. One controls the pressure by finely tuning the orifice opening with a large lever/knob. I made a large lever because the standard knob handle is too small for fine control. Furthermore, as the temperature drops one finds difficulty turning the knob because it gets frozen in position. A large lever makes this fact moot.
I extended the stem so it would reach the outside. I insulated the connection to the stem with Teflon to reduce any heat transfer through the stem from the outside. Below is a picture of the Generant needle valve. This is the part that drops the temperature
This is a 303 stainless screwed bonnet needle valve. Just Google the part for distributors near you. The part number for a 1/4" Female-Female NPT is GE 3000-4-SS.