When doing a Rice Test, you put your specimen in the Flask or Metal Canister, in water, and proceed to initiate a vacuum so the air going toward your pump is very moist. Many vacuum pumps will not perform correctly if they get water into the oil. So, the moist air is routed through the drierite in the air dryer. It absorbs the moisture and routes the dry air through the vacuum pump. It is a great alternative to filling flasks with drierite and putting in line with the pump.