Bill, Typically, as D.I. beds begin to fail (exhaust) either the cation or anion (as a function of your H2O constituents) exhausts first which causes a pH imbalance. This occurs increasingly below approx 500K, so 1M builds in a margin of safety that the water quality will be within reasonable boundaries. If you want to confirm this, simply measure the pH (inexpensive pocket meters are available from Fischer Scientific) as yor beds fall below 1M and plot the curve pH vs. resistivity. Hope this helps! Regards, Ed Popielarski QTA Machine 10 Mc Laren, Suite D Irvine, Ca. USA 92618 Ph: (949)581-6601 Fx: (949)581-2448 E-Mail: [log in to unmask] ################################################################ TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c ################################################################ To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TechNet <your full name> To unsubscribe: SIGNOFF TechNet ################################################################ Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information. For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312 ################################################################