DI Water Systems In addition to the standards for water being very specific, DI Water is the single most important utility for good test results and insure the longevity of your instrument. Parts are replaceable, once the stainless is rotted that is much more serious and difficult to repair (but not impossible.) See "Repairing and Salvaging Your Rusty Ci35-65."
The very basics: municipal water supplies contain four basic contaminants in quantities your weathering test would care about, there are others, but I personally prefer to pretend they aren't in there (organic solvents, heavy metals, toxic materials.)
Particulate: dirt, rust, organic solids
Chlorine, Flourine, and other municipally added "contaminants"
Ionic: those substances which dissociate into charged particles when dissolved in water e.g., salt is Na+ and Cl-
Colloidal: those substances which carry no or a very small charge and are too small to be caught in a particulate filter as they are in the nm range in size.
Removing the contaminants:
Particulate is the least expensive to remove, a $30 housing and a $5 1-micron spun polypropylene cartridge will remove nearly all such contaminants.
Chlorine can be removed easily and just as inexpensively with a similar set up of an activated carbon filter.
Ionic material is a bit more expensive and for large quantities of water can really put a dent in your budget. It is, however, very cost effective for one or two Fade-Ometers or Weather-Ometers running dry cycles. Your residential "water softener" is an "ion exchange" system. Meaning less soluble Calcium and Iron molecules are replaced with far more soluble Sodium molecules. Service De-Ionization Tanks are basically "Ion Magnets" or removers. The charge particles float past the "charged" media in the cylinder and stick like iron filings to a magnet.
Colliodal material is the most difficult to remove as it is too small to get caught in the particulate filter, the activated carbon has no affinity for it, and Service De-Ionization cylinders really weren't designed for it. Silica is the most common and most annoying colloidal substance floating around in your water. It leaves a white powdery substance all over your lamp, test chamber, and samples. It does, however, carry with it a very small charge and the Service DI cylinders will capture it, to a degree.
When the cylinders are fresh, they do an admirable job soaking up the silica due to the small charge. However, as the media saturates, it releases the silica in favor of a stronger charged ionic molecule. This results in a large amount of silica being released all at once.
The good news is, as the cylinder begins to saturate, if you have a high quality conductivity style water quality meter such as the Hanna BL983324, the conductivity of the water is affected by the silica. A normal water quality of 18+ megOhms is possible with a recirculating Service DI system. As the silica is released the meter will drop off of 18, not due to ionic saturation but due to the influence of the silica. Setting the alarm on the Hanna will alert the operator to call for service. A "final cleanup" cylinder outside of the recirculation loop picks up the slack until the cylinders can be swapped out. Normally the "final cleanup" cylinder is put into the recirculation bank of four or more cylinders to make best use of the media. See a line drawing here of systems we have fabricated using Service DI as the only source of cleaning.
This all works wonderfully as long as procedures are followed. It is also expensive if using high volumes of water for multiple instruments. In this case, the initial expense of a "Reverse Osmosis" (RO) system will pay for itself many times over in maintenace and service fees of commercial DI services. For larger systems a "water softener" is also recommended (but not absolutely required) for protection of the "membrane" which is the heart of the system.
RO systems are essentially self-cleaning filters. Incoming water is sometimes softened, filtered of particulate and chlorine, and pumped up to a very high pressure to force it through the "membrane." The "clean" or "permeate" water is is brought down from the supply purity 60-400 ppm "total dissolved solids" to about 2-8 ppm tds and stored in an atmospheric or pressure tank. The "clean" water is then polished with service or purchased DI media to bring it to a nice sparkly 18 megOhms, silica free, before being fed to the instruments.
The only downside to RO systems is that for each gallon of pure water that is created, one to two gallons of "reject" water is put down the drain. The continuous flushing of the water is necessary to prevent damage and clogging of the membrane. For recommendations on manufacturers of RO systems, please use the e-Mail link below. We will also custom fabricate a system using your existing infrastructrure to save you money.
Note: If you use service DI, you should install a gallon meter on the feedwater. I have no proof, but I've long suspected they re-install cylinders from high purity locations such as hospitals into less stringent locations such as weathering chambers and humidity cabinets. You should get so many gallons out of a cylinder based on your incoming water quality. If your gallon through-put before the alarm goes off suddenly drops, I would be asking questions of your provider.
