Myron L 9PTK - Manual (Page 69)

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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 Ultrameter III
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. 62, 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. Free Chlorine as an Indicator
Chlorine is a germicide used in the disinfection of drinking water,
wastewater, and recreational waters. It is dissolved in water and reacts
with water to form the hypochlorite ion and hypochlorous acid, the
concentrations of which are directly dependent on pH.
Some chlorine is initially consumed by organic matter and metal ions
present in solution. The amount of chlorine a particular solution consumes
is called chlorine demand. The amount of chlorine that remains beyond
this initial consumption is called free chlorine. Enough chlorine must be
added to solution so that residual levels of free chlorine have ongoing
germicidal properties. The 9P is designed to measure these residuals.
The 9P does NOT measure combined chlorine (compounds of chlorine
and nitrates that have no germicidal benefits), total chlorine (the sum of
free and combined chlorine), and chloramines (compounds of chlorine
and ammonia).
2. Free Chlorine Units
The 9P converts Oxidation Reduction Potential measured in mV to ppm
free chlorine using a proprietary algorithm developed by the Myron L
Company. This algorithm is based on a conversion curve that assumes
a pH of 5-9 in water sanitized by chlorine only.
The Myron L Company has been able to increase the accuracy of low-
range ppm free chlorine readings through extensive bench testing.
This feature allows the 9P to measure a dynamic range of sanitizer
concentrations that is wider than the range of a colorimetric test kit.