Whether the job involves starting up a brand-new plaster pool or maintaining chemistry in an old fiberglass vessel, pool professionals know that a proper understanding of water balance is essential.
Balanced water is especially crucial during a plaster pool’s
28-day start-up period. In fact, it often makes all the difference
between a smooth, durable surface and a scarred disaster zone.
Though some problems resulting from improper water balance, such as
calcium scaling, usually can be corrected if caught in time,
others, such as etching, can permanently ruin weeks of plaster work
in less than a single day. Once a pool project goes south, millions
of dollars may suddenly be at stake in court, as the customer, and
judge, demand answers about what wrecked the job — sloppy
plastering practices or inaccurate water-balance calculations.
On the other hand, a strong grasp of the physical principles
underlying water balance can help prevent these sorts of disasters
altogether — or at least correct them, and identify their
causes, before they get out of hand. Careful calculation and
diligent record-keeping typically go a long way toward keeping the
pool service technician out of legal hot water.
One popular method for calculating water balance is the Langelier
Saturation Index — often abbreviated as “LSI” or
simply “SI” — which uses five basic factors to
calculate the water’s overall tendency to scale or corrode
the pool’s surface. Though it might seem complex at first
glance, calculating the LSI of a pool’s water essentially
comes down to simple arithmetic.
Here, we’ll walk through the calculation process step by
step, clearing up a few tricky spots and debates along the way.
A Repurposed Index
The LSI was first developed in 1936 by Professor W.F. Langelier, a
chemist working at the University of California, Berkeley.
Professor Langelier’s original goal was to create a formula
that would indicate whether water stored in large boilers would
tend to create etching on the one hand, or calcium scale on the other.
Below is his original formula:
SI = pH + TF + CF + AF - Constant
(TF: TEMPERATURE FACTOR; CF: CALCIUM HARDNESS FACTOR,
AF: ALKALINITY FACTOR, CONSTANT: TDS CONSTANT)
Throughout the 1930s, calcium scale was causing major clogging
problems in municipal water storage, while etching was considered
less of a danger to the surfaces of metal boilers. However, the
situation in swimming pools today is precisely the opposite:
Calcium scaling usually can be treated, but etching often causes
permanent damage to surfaces. Thus, today’s service
professionals and scientists generally recommend that water be
balanced toward the higher (potentially scaling) side of the LSI,
rather than the lower (potentially etching) end.
The LSI is particularly helpful for plaster pools, because those
surfaces tend to be especially vulnerable to aggressive or scaling
water. In chemical terminology, the LSI is used to determine the
degree of calcium carbonate saturation in a pool’s water
— in other words, whether the water is more aggressive
(undersaturated with calcium), and will tend to dissolve calcium
carbonate out of the plaster; or whether it’s more scaling
(oversaturated with calcium), and will tend to form calcium
deposits on the pool’s surfaces.
Speaking in more general terms, balanced water creates a carbonate
alkalinity buffer, which prevents the water’s pH from
drifting too high or too low when alkaline or acidic substances are
added to a pool.
In view of this background, the actual factors used in calculating
LSI will make more sense. Now it’s time to delve into each
factor in detail.
Ideal range: 7.2 to 7.6
As many pool technicians know, pH is a measurement of a
substance’s acidity or basicity — values below 7 are
more acidic, while values above 7 are more basic. Not as well known
is the fact that the pH scale is base-ten logarithmic: A pH of 5,
for example, is ten times more acidic than a pH of 6, which, in
turn, is ten times more acidic than a pH of 7 — making a pH
of 5 one hundred times more acidic than a pH of 7.
This means even small changes in pH can create major changes in
water chemistry — a shift from 7.8 to 7.5 doubles the
concentration of calcium and carbonates needed to balance the
water. Thus, many techs and scientists say that pH should be the
first factor measured for LSI calculations, and it’s the one
most important to measure with precision.
While not everyone agrees on the exact pH range that’s
non-damaging to plaster surfaces, it has been widely agreed that a
range of 7.2 to 7.6 is ideal. Some extend that range to 7.8 —
and others as high as 8.2.
As can be seen in Graph A, when pH rises to 8 and
above, more and more of the water’s carbonate alkalinity
exists in the form of carbonate ions
(CO32-), which tend to bond with
calcium ions (Ca2+) to form calcium carbonate
(CaCO3), the chemical compound that often causes
cloudiness, precipitate and scale in plaster pools. In water with a
pH around 7.4, however, the vast majority of carbonate alkalinity
will be in the form of bicarbonate ions (HCO3-),
which don’t provide any free carbonate ions for this
In short, pH should be tested — and, if possible, balanced
into the ideal range — before any other LSI factors are
2 Temperature (TF)
Ideal range: 0.6 to 0.9 (76° to 104° Fahrenheit)
Water temperature is perhaps the simplest of all the LSI factors to
measure. For several reasons, it’s a significant influence on
water’s tendency to etch or scale. Most importantly, hotter
water accelerates the loss of carbon dioxide (CO2) from
the pool, which causes pH to drift upward. More generally, the
higher the water’s temperature, the faster chemical reactions
Another important aspect of temperature is related to a unique
property of calcium: It’s more soluble at lower temperatures
than at warmer ones. This means colder water is more corrosive to
the surfaces of plaster pools.
Unlike pH, the actual temperature measurement is not the factor
used when calculating LSI. Instead, the LSI uses a temperature
factor (TF), which is a number corresponding to a particular
temperature. Table B shows the TFs for a wide range of water temperatures.
It’s important not to try to estimate a decimal number
between two TF values — instead, find the temperature on the
chart that’s closest to the actual temperature measured in
the water, and choose the TF that lines up with that temperature.
Then, simply plug that TF value into the LSI equation.
3 Carbonate Alkalinity (AF)
Ideal range: 1.9 to 2.0(80 to 120 ppm carbonate alkalinity)
This measurement indicates the water’s ability to resist
shifts in pH — higher total alkalinity readings indicate a
greater resistance to pH shifts. Though Langelier’s original
formula only called for a measurement of the water’s overall
alkalinity, the complexity of modern pool chemistry requires that
two different types of alkalinity be taken into account when
performing total alkalinity calculations.
Most test kits measure total alkalinity, which indicates the
overall alkalinity of the water. Much of this is carbonate
alkalinity, which is contributed by bicarbonate
(HCO3-) or carbonate ions
(CO32-), depending on the water’s
pH (see “pH” above). However, many pools today use
cyanuric acid as a stabilizer — both in pure form, and as a
component of dichlor or trichlor tablets — and this chemical
contributes another type of alkalinity: Cyanurate alkalinity.
However, the LSI is focused on alkalinity’s ability to keep
calcium carbonate in solution — and thus, only carbonate
alkalinity should be taken into account for the calculation.
“Since the alkalinity factor used in the LSI calculation is
based on bicarbonate alkalinity only, the titratable cyanurate
alkalinity requires correction,” says Tom Metzbower, vice
president of sales at Taylor Technologies in Sparks, Md.
“Generally, this correction equates to one third of the
cyanuric acid concentration.”
The percentage of alkalinity as carbonate can be separated from
percentage as cyanurate through a simple process of multiplication
and subtraction. In fact, cyanuric acid percentage varies quite
predictably as a function of pH. Table C lists
conversion factors for cyanurate alkalinity at a range of common pH
After finding the appropriate conversion factor on the table above,
multiply this factor by the test kit’s actual cyanuric acid
reading (in ppm). Then, subtract this product from the test
kit’s total alkalinity reading (in ppm) to reach the correct
carbonate alkalinity reading:
Carbonate Alkalinity = Total Alkalinity – (Cyanuric Acid x F)
Now comes the final step in applying alkalinity to LSI
calculations. On Table D, simply find the
alkalinity ppm value that most closely matches the carbonate (not
total) alkalinity value calculated.
As with the temperature factor, don’t try to estimate an
alkalinity factor between two AF values on the chart — just
find the ppm value closest to the calculated carbonate alkalinity,
then apply the alkalinity factor that matches it.
4 Calcium hardness (CF)
Ideal range: 1.9 to 2.2 (200 to 400 ppm calcium hardness)
This measurement simply reflects the amount of calcium dissolved in
the water. However, some kits test for total hardness, which
measures other chemicals in the water as well. Thus, it’s
important to check that the test is measuring calcium hardness
Though the level of calcium is an important contributor to water
balance, a calcium hardness reading alone does not predict how
likely calcium is to precipitate (fall) out of solution — pH
is actually the main factor in determining this likelihood, with
alkalinity also playing a major role. Thus, these three factors are
all closely intertwined.
Converting calcium hardness for use in the LSI is straightforward.
Find the ppm reading on Table E to the right that
most closely matches the test kit’s calcium hardness reading,
and apply that calcium hardness factor (CF) to the equation.
As with the temperature and alkalinity conversions, just choose the
number on the chart that falls closest to the test kit’s
calcium hardness ppm value, and plug in the matching CF value.
5 Total dissolved solids (TDS)
Constant Ideal range: Non-salt pools, 12.1 to 12.19 (300 to 1,800 ppm); salt pools, 12.29 to 12.35 (2,500 to 3,500 ppm)
The final factor in an LSI calculation depends on the water’s
dissolved solids. The TDS value is not plugged directly into the
equation at all; rather, it’s used to determine whether
Langelier’s original constant of 12.1 is suitable, or if a
slightly higher constant should be used. This is because some
modern pool systems — especially those that generate chlorine
from salt — carry much higher levels of TDS than the boilers
Langelier was studying back in the 1930s.
As Table F shows, most non-salt pools will still
use Langelier’s original constant of 12.1. However, saline
pools, or others with unusually high TDS, may need to apply a
slight adjustment factor.
“If the TDS falls on the low side, you don’t need to
change the constant,” says Kim Skinner, co-owner of Pool
Chlor in Livermore, Calif. “But in reality, the TDS in some
pools is going to be higher than 1,000 — and in those cases,
I think service techs are wise to adjust it a little higher.”
Putting it all together
Once all the values have been converted into usable factors, the
LSI equation itself is an easy arithmetic problem:
SI = pH + TF + CF + (TA – (CYA x F)) – TDS constant
The next step is to determine whether the calculated LSI represents
in-balance water, or water whose chemistry needs to be adjusted. A
great deal of debate has arisen around the question of exactly what
LSI range is safe for surfaces — especially in plaster pools.
As described in the first section of this article, most agree that
it’s better to err toward potentially scaling values than
potentially etching ones — thus, a range of -0.3 to +0.5 is
generally the widest acceptable range recommended.
However, many have narrowed this recommendation further, specifying
a range of -0.3 to +0.3. Others state that no negative indices are
ideal, and specify a range of 0.0 to +0.3, or 0.0 to +0.5. As with
any range recommended for LSI calculations, debate continues about
whether an acceptable range or an ideal range should be taught
— and just what such a range should be. Still, it’s
widely agreed that values between 0.0 and +0.3 are safe for the
majority of pools.
A second common source of disagreement is the question of whether
any LSI value outside of the acceptable range should always be
addressed, or if any LSI that balances into the acceptable range
overall should be considered safe for surfaces.
“You might have a calcium hardness of 400 ppm in your tap
water, so to limit it to [the ideal minimum of] 200 isn’t
really practical in that case,” says Dr. Ellen Meyer,
technology manager at Arch Chemicals’ Water Products in
Smyrna, Ga. “You need to adapt some of the other index
values, like pH or alkalinity, to compensate for that.”
Others, however, warn that compensating for out-of-range values
with other out-of-range values can result in corrosive or scaling
water, even if the LSI is balanced overall. For example, the
Association of Pool & Spa Professionals’ Basic Pool &
Spa Technology manual explains, “If any one factor is outside
the recommended concentration range, the balanced condition will be
difficult to maintain.”
One thing on which all service professionals agree is that no two
pools are chemically identical, and ideal ranges can be adjusted to
account for local conditions. It may also be necessary to adjust
the LSI for a vinyl or fiberglass shell, which doesn’t
generate carbonate alkalinity, and also isn’t as prone to
etching as plaster.
However, such adjustments are safest when made with the guidance of
an LSI expert. And in the end, it’s easiest to prove a job
was performed properly when all LSI values were regularly recorded
within ideal — or at least acceptable — ranges.
Point of Debate
Acceptable Ranges vs. Ideal Ranges
A common cause of debate in LSI calculations is whether acceptable ranges or ideal ranges should be emphasized. For example, some have found that a pH as high as 8.2 fails to cause scaling in certain
pools, and should thus be considered part of the acceptable (i.e., non-damaging) range. Others, however, say that students of the LSI should be taught a narrower ideal range, which is highly unlikely to lead to scaling or etching in the majority of plaster pools. The debate continues — and not just for pH, but for all LSI factors. When a document specifies a range for any factor, it’s helpful to check whether that range is ideal, or merely acceptable.
Point of Debate
Ranges and Overall Balance
Many service professionals say that keeping all individual SI values within their ideal ranges —
or at least the acceptable ranges — is just as crucial as having a balanced LSI overall. However, others claim the purpose of the LSI is to determine what chemical adjustments are necessary to balance others. This, they say, means if, for example, the calcium hardness out of the tap is 75 ppm, a total alkalinity of 1,100 ppm would be necessary to balance the LSI — and therefore, this would balance the water as well. However, the Association of Pool & Spa Professionals’ Basic Pool & Spa Technology manual states, “Although [such] water is technically balanced, the low pH and calcium hardness plus the high TA would cause the water to act corrosive. It will also be difficult to lower the pH because of the buffering effect of the high TA.” In short, emphasizing overall LSI balance over the ranges of individual factors can be a dangerous game to play.
Swimming Pool Temperature
Carbonate Alkalinity Expressed as ppm
Calcium Hardness Expressed as ppm
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