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same reference is used for both the pH and the ORP sensors. Both
pH and ORP will indicate 0 for a neutral solution. Calibration at zero
compensates for error in the reference junction.
A zero calibration solution for ORP is not practical, so the P
OOL
P
RO
uses
the offset value determined during calibration to 7 in pH calibration (pH
7 = 0 mV). Sensitivity of the ORP surface is fixed, so there is no gain
adjustment either.
5.
Sources of Error
The basics are presented in pH and ORP/Free Chlorine, pg. 59, because
sources of error are much the same as for pH. The junction side is the
same, and though the platinum surface will not break like the glass pH
surface, its protective glass sleeve can be broken. A surface film will
slow the response time and diminish sensitivity. It can be cleaned off
with detergent or acid, as with the pH glass.
C. Free Chlorine
1. FC
E
as an Indicator of Sanitizing Strength
Chlorine, which kills bacteria by way of its power as an oxidizing agent,
is the most popular germicide used in water treatment. Chlorine is not
only used as a primary disinfectant, but also to establish a sufficient
residual level of Free Available Chlorine (FAC) for ongoing disinfection.
FAC is the chlorine that remains after a certain amount is consumed by
killing bacteria or reacting with other organic (ammonia, fecal matter)
or inorganic (metals, dissolved CO
2
, Carbonates, etc) chemicals in
solution. Measuring the amount of residual free chlorine in treated
water is a well accepted method for determining its effectiveness in
microbial control.
The Myron L Company FC
E
method for measuring residual disinfecting
power is based on ORP, the specific chemical attribute of chlorine (and
other oxidizing germicides) that kills bacteria and microbes.
2. FC
E
Units
The PS9 is the first handheld device to detect free chlorine directly,
by measuring ORP. The ORP value is converted to a concentration
reading (ppm) using a conversion table developed by Myron L Company
through a series of experiments that precisely controlled chlorine levels
and excluded interferants.
Other test methods typically rely on the user visually or digitally
interpreting a color change resulting from an added reagent-dye. The
reagent used radically alters the samples pH and converts the various
chlorine species present into a single, easily measured species. This
ignores the effect of changing pH on free chlorine effectiveness and