Deionization Filters and Resin
Deionization filters are used to prevent electrical arcing on the “laser” side of the heat exchange system. Many water cooled laser components, flashlamps for example, have high voltage exposed to the cooling water. For this reason, electrically conductive “city” water cannot be used on the laser side of the cooling loop.
City or tap water is full of positive ions such as calcium (Ca2+) and negative ions such as chloride (Cl-). These ions allow the water to conduct electricity. Large numbers of ions make water highly conductive; few ions make water resistive. Obviously, water must be made resistive in order to cool high voltage components without causing arcing. Therefore ions must be removed using deionization filters.
Deionization filters contain resins of small spherical beads. These beads have ion exchange sites throughout their polymer structure which capture ions from water. Typically, two types of resins are used: cation resin for removing positive ions and anion resin for removing negative ions. Some filters have separate reservoirs or “beds” for the two resins, while some filters have mixed beds.
The maximum resistivity level which water can reach is 18.3 Megohm/cm. Most lasers require cooling water resistivity of at least 1 or 2 Megohm/cm for trouble free operation. Below that level of resistivity, problems such as arclamp trigger failure will occur.
Deionization filters should be changed on a regular preventative maintenance schedule at least every 6 months. Many laser systems have in-line sensors to monitor water resistivity.
Particle filters are also required to prevent the build-up of particulates and debris on to pump cavity, laser lamp, flow tubes and the water cooling loop and system.
Particle filters have a nominal pore size, usually in the .2 – 10 micron range. Most laser systems use depth filters which are relatively inexpensive and filter throughout their bulky cross section. These entrap most but not all particles above the rated pore size. Some systems use the more expensive screen type, absolute filters. Screen filters entrap particles on the surface of the filter and remove all particles at or above the rated pore size.
Particle filters should be changed on a regular maintenance schedule at least once every 6 months, or sooner if an increase in pressure drop across the filter indicates clogging.
