Controls on pressure and flow rate
- One or two vacuum pumps
There are two vacuum pumps connected in parallel which are independently controllable. One is a single stage and the other is a two stage. The two stage pump is a high vacuum low throughput type while the single stage pump is a low vacuum but high throughput type. Since the flow is not choked at or downstream of the test chamber, adding or removing a vacuum pump will decrease or increase the test chamber pressure, respectively.
- Two flow controllers
There are two mass flow controllers/meters (MFCs) that are used to finely adjust the pressure in the test chamber. Two controllers are used so that the flow rates through the saturator and dryer can be independently adjusted to control the frost point.
- Critical orifice
A critical orifice is used to expand the gas to the desired test chamber pressure. Since the pressure upstream of the orifice is normally about 150 torr and the downstream pressure is no more than about 10 torr, the orifice is choked. The orifice used is a VCR gasket with a ruby orifice insert, therefore different aperture orifices can be easily installed and removed. For maximum flow rates a normal VCR gasket without a ruby orifice insert is used.
Shown below are curves of chamber pressure vs. flow rate: