MANUALPC-2010 CHAPTER VI – MAINTENANCE Page 56
CHEMICAL MAINTENANCE
Overview
In addition, it is recommended to check the calibration of the
controller at the same time of the day, preferably in the
morning after a couple of hours of operation, but before full
sun. This is especially important for pools stabilized with
cyanuric acid as the effects of sunlight on chlorine activity are
not detected by the test kits and may lead to false and
unnecessary readjustments.
Finally, the pool operator should become familiar with ORP
technology (see below) and learn to trust the information it
provides rather than less reliable test kits.
pH Control
T
he importance of proper pH control cannot be over
emphasized, as it affects every aspect of water chemistry.
For pools and spas, the recommended pH setpoint is between
7.4 and 7.8. Below 7.4, the water becomes increasingly
corrosive and causes stains, etching of plaster and eye
irritation. Above 7.5, the efficiency of the sanitizer decreases
rapidly and the water becomes too alkaline - which causes
cloudiness, stains and scaling.
pH control is also affected by Total Alkalinity (TA). If it is too
high (above 150 ppm), pH response is slow and requires more
acid or base feed. If it is too low (under 100 ppm), pH control
becomes very sensitive.
Because of the Time Lag for mixing of the chemicals in the
water, there is always a fluctuation (0.1 to 0.2 pH units) above
or below the setpoint, depending on the chemical feed rate.
If the pH tends to overshoot the setpoint, the Control Mode
should be set to Proportional. Alternatively, the feed rate of the
acid or soda feed pump can be reduced or a more dilute
solution can be used (especially in a small body of water, like a
spa). DO NOT CHANGE THE SETPOINT.
In an ACID FEED system, if the pH display consistently reads
too high (not enough acid), the feed rate of the acid feed pump
should be increased, or a stronger solution should be used.
DO NOT CHANGE THE SETPOINT.
In a SODA FEED system, if the pH display consistently reads
too low (not enough soda), the feed rate of the soda feed pump
should be increased, or a stronger solution should be used.
DO NOT CHANGE THE SETPOINT.
ORP and Sanitizer Control
The recommended control level is 1.5 to 2.0 ppm of chlorine or
3.0 to 4.0 ppm of bromine at a pH of 7.5. To be sure of proper
sanitation, the ORP should always be above 650 mV.
Even if using additional purification systems, such as ozone,
UV systems or metal ion systems, THE ORP READING MUST
ALWAYS BE MAINTAINED ABOVE 650 mV.
Because of the Time Lag between injection of chemicals,
mixing in pool water, and return to the sensors, it is normal to
see a variation of a few tenths of a PPM around the setpoint,
depending on the feed rate of the chlorinator or brominator.
If the display shows too much overshoot, the Control Mode
should be set to Proportional to reduce the feed rate. DO NOT
CHANGE THE SETPOINT.
If the display consistently reads below the set point, reduce the
width of the Progressive Zone or set the control mode to
ON/OFF to increase the feed rate. DO NOT CHANGE THE
SETPOINT.
The sensor reads ORP (Oxidation-Reduction Potential) which
is closely related to the FAST ACTING FREE CHLORINE
(HOCl), the most effective sanitizer. The DPD and FACTS test
kits - and most other controllers - however read only the
combination of FAST ACTING and SLOW ACTING FREE
CHLORINE (HOCl and OCl
-
). This is not very meaningful
because the slow acting form of chlorine is about 80 to 100
times slower then HOCl in killing bacteria.
With proper automatic pH control, the reading on the sanitizer
display is very close to test kit readings. If the pH varies too
much however, the CHEMTROL® controller will show the
variations in HOCl - which are not shown by normal test kits. It
is normal therefore to see small differences in readings
between display and test kit if the pH varies.
If the ORP reading is maintained above the recommended
minimum of 650 to 750 mV, the water should be free of germs
and bacteria. Below 650 mV, germs and bacteria will develop
rapidly.
ORP readings are closely tied to the concentration of Fast
Acting Free Chlorine (HOCl), which is affected by pH and by
the cyanuric acid level. If the pH and/or cyanuric acid level is
too high, the ORP will be reduced even with high levels of
chlorine.
With stabilized forms of chlorine (dichlor powder or trichlor
tablets), it is important to test the cyanuric acid level in the
water regularly and to dump or replace part of the water when
it gets over 40 ppm - especially in spas.
If other purification systems are used (ozone, UV or metal ions
systems), it is very important to maintain the proper ORP level
at all times with chlorine or bromine residuals.
NOTE: Make sure to shut off the Bypass Line when adding
sequestering agents as they will coat the platinum
ring of the ORP sensor, resulting in false readings.
For best results, it is strongly recommended to have
the same operator in charge of water maintenance and
testing, as different people read test kits differently.
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