The quartz sleeve is an important part of the UV water treatment system. It protects the UV lamp from coming in contact with the water while allowing the UV light to permeate the water. Minerals from the water will build up on the sleeve over time causing the sleeve to become cloudy not allowing the UV light to permeate the water as effectively. How quickly the minerals build up on the sleeve will vary depending on your water hardness, and if you have hardness of over 7 gpg you should install a water softener to treat the water before your UV system. Once the mineral layer on the quartz sleeve cannot be removed via cleaning, or the sleeve cracks or breaks it's time to get a replacement. US Water Filters USWF-350074 is a replacement for QS-330, for use in the VIQUA VT4, VTV/2, S2Q-PA, S2Q-PA/2 systems and the discontinued S2Q, S2Q/2, S2Q-GOLD, S2Q-GOLD/2, SSM-17, SSM-17/2 UV Systems. All quartz sleeves come with 2 replacement o-rings.