Water
Balance
pH..................................................
(7.2 - 8.0)
Total
Alkalinity (TA)....................... (100-150)
ppm
(Fiberglass,
vinyl-lined and others)... (125-175)
ppm
Calcium
Harness (CH)................. (175-300)
ppm
Total
Dissolved Solids (TDS) ..... <2,000 ppm (max
permissible range)
Langelier
Saturation Index ........ ( 0 ) (+/- 0.5)
Water Balance is a figurative term representing
the dynamic interaction between pH, total alkalinity, calcium
hardness, dissolved solids, and temperature. The importance
of water balance is oftentimes oversimplified. This oversimplification
is due, in part, to a misunderstanding of each water balance
parameter's influence on one another.
pH
Recommended
Levels of pH in pools and spas
pH
Levels should always be maintained between 7.2 to 8.0
Before pH can be convincingly discussed, one must first
survey the fundamental properties of water. We fill
our pool with it; we wash our clothes with it; we even power
generators with it; but many uninformed pool and spa owners/operators
underestimate its solvency potential.
Water is composed of two (2) hydrogen atoms covalently bonded
to one (1) oxygen atom. The result is water H2O. But there's
much more than just water in water. Water dissociated similar
to hypochlorous and hypobromous acid. This dissociation
generates hydrogen and hydroxide ions. The hydrogen ion
concentration determines the pH. The greater the hydrogen
ion concentration, the lower the pH, and the more acidic
is the water. Decreasing the hydrogen ion concentration
increases the pH and reduces the water's acidity. A Neutral
pH exist when the hydrogen ion (H+) concentration is equal
to the hydroxide (OH-) ion concentration. A pH of 7 is termed
neutral; below 7 is acidic; above 7 is basic.
pH is a logarithmic function permitting the relative
acidity or basicity to be expressed in simple mathematical
terms: 0-14. It is this simplification that also
causes misunderstandings. Each unit division on the pH scale
represents a ten-fold increase or decrease in acidity. For
example, a pH of 5 is ten times more acidic than a pH of
6. A pH of 3 is one thousand times more acidic than a pH
of 6. Each division is a multiple of ten. Frequent
water test will enable the pool or spa owner/operator to
monitor pH and make necessary corrections.
Total
Alkalinity
Recommended
Ranges for Total Alkalinity
Plaster
and marsite pools and spas ...........100-150ppm
Fiberglass,
vinyl-lined and others ..............125-175ppm
Note:
A minimum level of 80 ppm
is permissible in hard water (high calcium hardness) areas
with hardness exceeding 500 ppm.
Total alkalinity (TA) is the quantitive measurement
of alkaline components present in water to act as a buffer
against rapid pH changes. Proper total alkalinity
levels are important to ensure optimum chemical balance
in pools, spas and other aquatic systems.
For the most part, total alkalinity is a contribution of
carbonate (CO3-2), bicarbonate (HCO3-5), and hydroxide (OH-1),
with borates, silicates and phosphates possibly making a
minor contribution in certain geographic areas.
Low Total Alkalinity is a highly corrosive
condition resulting in damage to copper heat sinks and heat
exchangers, light rings, stainless steel ladders, and pool
surfaces. Low total alkalinity is frequently coincidental
with green water, plaster etching and accelerated corrosion
rates.
High Total Alkalinity and its associated
increases buffering capacity will make it difficult to adjust
pH with the addition of acid or base (alkali). In addition,
high total alkalinity is frequently concurrent with high
pH and/or cloudy water.
Calcium
Hardness (CH)
Recommended
ranges of Calcium Hardness :
175-300ppm
(parts per million)
10-18gpg
(grains per gallon)
Calcium Hardness (CH) indicates the calcium content
of water usually expressed as milligrams/liter (mg/l) or
ppm. Not to be confused with Total Hardness, which is the
sum of calcium and magnesium, calcium hardness is the primary
parameter that dictates water balance.
1. Calcium hardness determines subsequent adjustment
of pH and total alkalinity. This is true because the reduction
of calcium hardness is very difficult in pool and spa applications.
Therefore, in geographical areas with high calcium hardness,
pH and total alkalinity must be adjusted to compensate for
these elevated CH levels.
2. Plaster and marsite surfaces frequently found in pool
and spa systems are primarily composed of calcium carbonate,
commonly known as calcite. Because of its reduced solubility
(approximately 9.32 mg/l) over its magnesium counterpart
(265 mg/l), calcium hardness dictates water balance.
Of the previously listed problems that may occur with improper
calcium hardness, low calcium hardness levels are most critical.
Low calcium hardness in conjunction with low pH and alkalinity
will disrupt the solubility equilibrium of calcium carbonate
and increase the aggressiveness of water. The result:
1. Surface
etching and pitting. An aggressive condition dramatically
increases the solubility of calcium carbonate, the principle
building block of plaster and marsite surfaces associated
with pools and spas. Etching can occur very quickly, resulting
in expensive replastering.
2. Corrosion
of metal components, especially heat exchangers and copper
heat sinks. This occurs because the insoluble nature
of calcium corbonate provides a thin protective film on
the surface of copper heat exchangers, thereby reducing
direct metal-water flow interaction, but not disturbing
the heat exchange process. If this invisible protective
film is destroyed because of improper water balance, heat
exchangers and associated components will suffer premature
deterioration. This unique protective feature of calcium
carbonate is due to calcium carbonate's inverse solubility-temperature
relationship. As the water temperature increases, the solubility
of calcium carbonate decreases. It is one of the few inorganic
compounds that displays this inverse relationship.
Maintaining proper calcium hardness is very critical in the care
of you pool & spa!
Total
Dissolved Solids (TDS)
Total dissolved solids is an electrochemical measurement
of water's ability to conduct an electrical current. The
electrical conductivity increases with increasing TDS
or the presence of dissolved ("charged") chemical species
in the water.
To better understand TDS, let's consider "pure" water;
water that is free of any chemical or microbiological
impurities. "distilled" or "deionized" water is considered
"pure" water and its quality is determined by measuring
its electrical conductance. The lower the conductance,
the purer the water.
A similar concept has been adopted for pool and spa water,
but it is expressed as Total Dissolved Solids. TDS is
a semiquantitative measurement of a water's dissolved
ion content. The conductance (measured in micromhos; a
micromho is the practical unit of conductance equal to
the reciprocal of the ohm) increases with increasing salt
or dissolved ion content. It is important to remember
that TDS is a measurement of "charged" chemical species
(ions) whether they carry a net positive (+) or negative
(-) charge; "uncharged" or neutral chemical species are
not measured as TDS.
Virtually all chemicals added to a pool or spa will have
a positive influence on TDS. TDS of typical fill water
ranges from 100 to 400 ppm, but exceptions do exist in
various geographical areas depending on the water supply
system.
High TDS (above 2,000 ppm) can: (1) enhance algae growth rates. In
addition to supplying essential nutrients, high TDS may
offer ideal harboring sited for algae spores to gain a
foothold in the pool or spa; (2)
increase "natural" (galvanic) corrosion. Each reader is certainly
familiar with metal deterioration (corrosion) that
occurs near the seashore or to automobiles exposed to
"salted" roadways during the winter. These accelerated
corrosion rated are associated with high salt content
or increased TDS. The resulting increase in electrical
conductance promotes "galvanic" corrosion or electrolysis.
This is of course followed by premature heater failure,
corroded light rings or damaged circulation systems. A
good example is heated seawater pools where heater manufacturers
use copper/nickel heater exchangers and heat sinks in
the place of pure copper. The reason: Copper/nickel is
much more resistant to corrosion; (3) accelerate staining
and scaling of pool and spa surfaces.
In conclusion, TDS is a general indicator of pool
or spa water acceptability. If too high, the pool should
be partially or completely drained to reestablish acceptable
levels. Many pool/spa codes have adopted a policy
of daily water replacement based on the average daily
bather use. In addition to maintaining proper levels,
a daily water replacement policy optimizes the potential
for good water quality resulting in a safer, more enjoyable
swimming or soaking experience.
Temperature
and the Langelier Saturation Index
The effect of temperature on water balance cannot be fully
understood without discussing Langelier's Saturation
Index.
Langelier's Saturation Index is frequently
employed as a method of determining whether a water will
support or deposit calcium carbonate. It was primarily
developed for the boiler industry, but other direct applications
soon followed to include pools and spas. The correctness
of the Langelier Index has frequently been questioned,
but to date, it has withstood criticism and remains the
principle water balance index because of its ease and
convenience.
According to the Langelier Saturation Index,
five (5) primary factors effect water balance and are
therefore incorporated into the saturation index formula.
They include: pH, calcium hardness, total alkalinity,
temperature, and total dissolved solids.
Of these water balance parameters,
calcium hardness, total alkalinity, and temperature are
expressed as factors: CF, AF, and TF, respectively. The
resulting Langelier Saturation Index formula is as follows:
Saturation
Index = pH + CF + AF + TF - 12.1
The waters pH is substituted directly into the formula.
An average factor of -12.1 is the representative constant
for the total dissolved solids when less than 1,000 ppm.
With this information and the appropriate factors listed
in Table 9-4,
the pool or spa owner/operator can quickly calculate the
saturation index for any given system.
Table 9-4
Calcium Hardness
Total Alkalinity Temperature
ppm =
CF
ppm
=
AF
'F
= TF
5
0.3
5
0.7
32 0.0
25
1.0 25 1.4
37
0.1
50
1.3
50 1.7
46
0.2
75 1.5
75 1.9
53
0.3
100
1.6
100 2.0
60
0.4
150 1.8
150
2.2
66
0.5
200
1.9
200
2.3
76
0.6
300
2.1
300
2.5 84
0.7
400
2.2
400
2.6
94
0.8
800
2.5
800 2.9 105
0.9
1000
2.6
1000 3.0
128 1.0
*If the Saturation
Index is 0, The water is perfectly balanced.
*If the Saturation Index is a negative value, corrosion
tendencies are indicated.
*If the Saturation Index is a positive value, scale forming
tendencies are predicted.
Values for the Saturation Index are
considered acceptable if they fall between +0.5 and -0.5.
A positive value is considered more acceptable than a
negative index in pool or spa applications.
Example:
A 20,000 gallon pool has a temperature of 84F, pH of
7.4, total alkalinity of 150 ppm, and calcium hardness
of 200 ppm. What is the Saturation index
of this water? Does it exhibit corrosive or scaling
tendencies? Are adjustments necessary?
Saturation
Index = 7.4 + 1.9 + 2.2 + 0.7 - 12.1 = +0.1
With an index of +0.1, this pool falls slightly on the
scaling side of the index no water balance adjustments
are necessary.
