How do I know when my DI cartridges need to be replaced? You watch the resistivity meter. As the ion exchange resins become saturated with captured ions, their ability to remove new ions declines. The meter will show a falling resistivity reading. For most general lab purposes, a rule of thumb is to replace cartridges when resistivity drops below 1 to 5 megohm-cm, depending on your application. For critical trace analysis, you may change cartridges as soon as resistivity falls below 18.0 megohm-cm. Some modern DI systems automatically monitor total liters processed or cumulative ionic load and alert you when capacity is near exhaustion. Another warning sign is a slowly increasing conductivity reading (the inverse of resistivity) on the display. Do not rely on a calendar schedule alone; a lab that processes 100 liters per day will exhaust cartridges far faster than a lab that processes 10 liters per week. Also, if you notice a sudden change in taste, odor, or the appearance of algae or slime in tubing or storage reservoirs, that indicates bacterial growth, not necessarily exhausted resins—but contaminated cartridges should still be replaced promptly. https://www.molewater.com/products/laboratory-deionized-water-system