DEIONIZED WATER
Years ago, high purity water was used only in limited applications. Today,
deionized (Dl) water has become an essential ingredient in hundreds of
applications including: medical, laboratory, pharmaceutical, cosmetics,
electronics manufacturing, food processing, plating, countless industrial
processes, and even the final rinse at the local car wash.
THE DEIONIZATION PROCESS
The vast majority of dissolved impurities in modern water supplies are
ions such as calcium, sodium, chlorides, etc. The deionization process
removes ions from water via ion exchange. Positively charged ions
(cations) and negatively charged ions (anions) are exchanged for hydrogen
(H+) and hydroxyl (OH-) ions, respectively, due to the resin's greater
affinity for other ions. The ion exchanges process occurs on the binding
sites of the resin beads. Once depleted of exchange capacity, the resin
bed is regenerated with concentrated acid and caustic which strips away
accumulated ions
through physical displacement, leaving hydrogen or hydroxyl ions in their
place.
DEIONIZER TYPES
Deionizers exist in four basic forms: disposable cartridges, portable
exchange tanks, automatic units, and continuous units. A two-bed system
employs separate cation and anion resin beds.
Mixed-bed deionizers utilize both resins in the same vessel. The highest
quality water is produced by mixed-bed deionizers, while two-bed
deionizers have a larger capacity.
Continuous deionizers, mainly used in labs for polishing, do not require
regeneration.
TESTING Dl WATER QUALITY
Water quality from deionizers varies with the type of resins used, feed
water quality, flow, efficiency of regeneration, remaining capacity, etc.
Because of these variables, it is critical in many Dl water applications
to know the precise quality. Resistivity/conductivity is the most
convenient method for testing Dl water quality. Deionized pure water is a
poor electrical conductor, having a resistivity of 18.2 million ohm-cm
(18.2 megohm) and conductivity of 0.055 microsiemens. It is the amount of
ionized substances (or salts) dissolved in the water which determines
water's ability to conduct electricity. Therefore, resistivity and its
inverse, conductivity, are good general purpose quality parameters.
Because temperature dramatically affects the conductivity of water,
conductivity measurements are internationally referenced to 25șC to allow
for comparisons of different samples. With typical water supplies,
temperature changes the conductivity an average of 2%/ șC, which is
relatively easy to compensate. Deionized water, however, is much more
challenging to accurately measure since temperature effects can approach
10%/șC! Accurate automatic temperature compensation, therefore, is the
"heart" of any respectable instrument.
RECOMMENDED INSTRUMENTATION:
Hand-held instruments are typically used to measure Dl water quality at
points of use, pinpoint problems in a Dl system, confirm monitor readings,
and test the feed water to the system. The hand-held Myron L instruments
have been the first choice of Dl water professionals for many years. For
two-bed Dl systems, there are several usable models with displays in
either microsiemens or ppm (parts per million) of total dissolved solids.
The most versatile instrument for Dl water is the 4P or 6P ULTRAMETER,
which can measure both ultra pure mixed-bed quality water and unpurified
water. It should be noted that once Dl water leaves the piping, its
resistivity will drop because the water absorbs dissolved carbon dioxide
from the air.
In-line monitor/controllers are generally used in the more demanding Dl
water applications.
Increased accuracy is realized since the degrading effect of carbon
dioxide on high purity water is avoided by use of an in-line sensor
(cell). This same degradation of ultra pure water is the reason there are
no resistivity calibration standard solutions (as with conductivity
instruments).
Electronic sensor substitutes are used to calibrate resistivity
monitor/controllers.
Myron L manufactures a variety of in-line instruments, including
resistivity monitor/controllers which are designed specifically for Dl
water. Seven resistivity ranges are available from 0-200 KW to 0-20 MW to
suit any Dl water application. Temperature compensation is automatic and
achieved via a dual thermistor circuit. All models contain an internal
adjustable set point, piezo alarm connectors and a heavy-duty 10 amp relay
circuit which can be used to control an alarm,
valves, pump, etc. Available options include 4-20 milliamp output, 3 cell
input and 3 range capability.
Sensors are available constructed in either 316 stainless steel or
titanium. All cells are provided with a 3/4" MNPT polypropylene bushing
and 10' (3 meters) cable. Optional PVDF or stainless steel bushings can be
ordered, as well as longer cable lengths up to 100 feet (30 meters).
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Useful Conversions:
1/megohm = microsiemens (micromhos)
1/microsiemens (micromhos) = Megohm
Examples:
0.5 microsiemens = 2 megohm (2,000,000 ohms)
200 kilohm (200,000 ohms) = 0.2 megohm = 5 microsiemens
Click here for Ultra meter specifications and prices
December, 2005
Reprint of Myron L Co application bulletin.
Prices: If there is a difference in the prices
shown, customers will be contacted for approval before processing the order.
Prices do not include shipping charges.
Terms: Net 30 days
Visa, Master Card and American Express credit cards
accepted.
GWJ co
Hacienda Heights, California
Phone: 626-336-1134
800-336-1138 (USA)
Fax: 626-336-2409
Copyright © 2007 GWJ
Co. All rights reserved.
Other products and companies referred to herein
are trademarks
or registered trademarks of their respective companies.
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