I'm trying to dry air by leading it through calcium chloride.
I'm measuring the resulting humidity with a simple capacitive humidity sensor. I do not see measurements below 10% relative humidity, but it is quite possible that it is a result of a calibration error (The sensor is not actually calibrated.)
But how dry can I get air by exposing it sufficiently long to calcium chloride dihydrate?
"Completely dry"? No, because molecules are always moving and there will be some exchange between desiccant and air. If you mean "below 10% relative humidity (RH)", there are a number of ways to do so:
Consider that another way to discuss humidity is the dew point. Use a cold trap: after drying the air with a desiccant, pass the air slowly through a U-tube (or better, a through a spiral condenser) cooled with ethanol/dry ice or similar bath. As the air emerges, the dew point, ~200 K for dry ice, remains that of the bath, but as it warms to room temperature, the RH would fall.
Dry the air with desiccant at room temperature, then heat the air (not desiccant) and the RH drops.
Synthesize air from ~80% liquid nitrogen and ~20% liquid oxygen. If you're a stickler for accuracy in the simulation, add a dash of liquid argon, and a smidgen of liquefied CO2 (not dry ice, which, being open to air, quickly condenses water). These liquefied gases can probably be obtained from a welding supply shop.
BTW, don't forget to bake your apparatus before use! Metal, and particularly glass, adsorb water quite well, and slowly release it at low RH or pressure.