Next to water filtration and “softening,” de-ionization is probably the next most common water treatment. Ions in water interfere with many cleaning processes and cause water spots on drying after rinsing. The common measure of the amount of ions in water is “resistivity.” The more “resistive” water is, the less ions it contains. Water from a municipal water supply very seldom has a resistivity greater than a few thousand ohms and may have a resistivity as low as 200 ohms or less. You might imagine, then, that sea water (which contains salt) has a resistivity of only a fraction of an ohm (about .2 Ohms). Water with virtually no ions (high purity DI water) may have a resistivity up to 18 megOhms (18,000,000 ohms) which is the resistivity of absolutely pure water. As you can see, this is a huge range for water which all looks pretty much the same to the naked eye.
The electrical resistivity of water is measured using an instrument called a “resistivity meter.” Very simple and inexpensive resistivity meters use a familiar orange neon bulb with electrodes immersed in the water being metered. An electrical circuit causes the neon light to stop glowing when a preset resistivity is not achieved. This type of resistivity meter is commonly used when the resistivity target is up to approximately 1 megOhm (1,000,000 Ohms). Measuring resistivity above 1 megOhm requires increasingly sophisticated instruments with commensurately higher prices. More sensitive instruments also require more maintenance in the way of calibration and tend to have increasingly more delicate sensing probes. Choosing a resistivity meter that is sensitive enough to do the job yet not overly complex is an important consideration.
Similarly, although achieving the required resistivity for a particular application is important, more is not necessarily better! Plain tap water or tap water that has been “softened” is totally adequate in most cleaning and many rinsing processes. Things start to get tricky when water leaves “water spots” on parts. Water spots are simply dissolved minerals left behind after beads of water remaining on the parts after rinsing evaporates. It is generally agreed that “spot-free” drying requires water with a resistivity between .5 and 1 megOhm but this target varies depending on the surface texture of the parts and several other considerations. An earlier blog discussed “spot-free” in somewhat more detail.
Many critical cleaning applications, particularly in the manufacture of medical equipment, optics, and electronic chips require water at or near the ultimate resistivity of 18 megOhms.
The next blog will describe ways of making de-ionized water. But before we do that, it is important to recognize that de-ionized water, especially that approaching 18 megOhms, has some very unusual physical properties. Water, being the “universal solvent,” wants desperately to attract and dissolve ions. Creating de-ionized water is difficult enough but maintaining that water in an ion-free state is often next to impossible. Even stainless steel will corrode when in contact with high-purity de-ionized water. The quality of de-ionized water in a stainless steel tank will degrade in a matter of minutes unless it is either continuously recirculated through a de-ionizing process or replenished with freshly de-ionized water on a continuous basis. In fact, high purity deionized water left in an inert container (glass, teflon, plastic) with its top open to the atmosphere will, literally, absorb ions from the air! Maintaining high-purity de-ionized water almost demands that it be manufactured on-site. I am not aware of any delivery trucks pulling into manufacturing facilities delivering certified 18 megOhm purity de-ionized water.
– FJF –