As the industry’s sophistication in
understanding pool water
chemistry has grown in
the last few decades, so has popular
acceptance of The Langelier Saturation
Index (LSI) as a tool for maintaining
balance.
There remains some confusion
about the Index both within the pool
water testing community (as in those
who may use it in balancing water,
but don’t really know much more
than that) and in the larger pool and
spa community (as in those who are
vaguely aware it’s got something to do
with pool chemistry, but often get LSI
confused with CSI, a popular television
show).
In simplest terms, LSI is an
equation into which you can plug
your pool water test results that will
spit out a single number that tells
you whether your water is aggressive or scale forming. Aggressive, as in
desiring to suck calcium from plaster.
Scale forming, as in tending to deposit
a layer of calcium scale on pool walls,
heat exchangers, pipes and other
places it’s not welcome.
Calculating LSI is part of balancing
pool water — the part that tells you
if your water is aggressive or scale
forming. But balancing to LSI neutrality
is not a full examination of pool water.
Or put another way, when you’ve
balanced water to LSI, you’re not done.
There are other factors to consider and
perhaps adjust to make sure the water
doesn’t grow algae or irritate bathers.
In the words of Alicia Stephens,
director of training and education for
the BioLab Pro Dealer Division, LSI is
just a snapshot of your pool telling you
something about the overall nature of
your pool water, but it doesn’t give you
details.
“When I’m teaching a class to
people that may not be familiar with
it,” she says, “I always describe LSI
this way: Say you pull up to a drive-up
window and read their health score
posted in the window. And it’s like 93
out of 100. Which is satisfactory but
not perfect. And you’re like, ‘Okay, 93,
they were inspected, and I assume that
most things were fine.’
“You have no idea what was wrong,
why they didn’t get 100. But you’re like,
‘Okay, that’s fine. I’m going to eat the
food.’
“Now, if you see a health score of
72, you still have no idea what’s wrong,
but you know that’s… not so good. It
could have been cockroaches in the
kitchen, it could have been meat stored
at the wrong temperature. You have no
idea. You just know it’s not good.
“And that’s kind of the way LSI
works. It just gives you a ‘not so good’
answer or a ‘you’re fine’ answer, but
it doesn’t really tell you exactly what’s
going on.”
In the same way a restaurant might
have a hygiene problem, but can still
achieve a satisfactory health score by
doing a great job on all other facets,
the LSI can be adjusted to achieve
neutral water even if a parameter is
out of whack. For example, pool water
might have a high pH value of 8.2, but
the LSI equation can still be balanced
to 0.0 by adjusting other factors,
maybe low total alkalinity and calcium.
And that means the water truly
isn’t corrosive or scale-forming.
It’s balanced to the LSI, but there’s
definitely a problem with a pH of 8.2.
That’s going to make chlorine weak
and ineffective, which will prevent
it from killing pathogens and algae,
leading to big problems in the pool. So
techs look beyond LSI to completely
understand a pool’s chemistry.
Dr. Wilfred Langelier, inventor of the Langelier Saturation Index, pictured here in 1912.
JUDGMENT CALLS
As a practical matter, there are only
three factors that can be adjusted by
a poolside tech to balance to LSI —
pH, alkalinity, and calcium hardness
(temperature and TDS cannot be
altered in a short visit). So the pool
tech can adjust those three while
keeping in mind if they get too far out
of whack to satisfy LSI, they will cause
other problems. The tech is really
engineering the water at this point, but
like any engineer, must keep in mind
the projected service life.
Stephens describes a challenging
example: “Someone might say, for
someone closing a pool for winter,
‘Raise your calcium up super high
because cold water is aggressive, and
raising up your calcium will balance LSI and keep your water from being
aggressive. At the same time, in order
to balance LSI, you must drop your
pH.
“So you lower pH to say, 6.8, to
balance the LSI. But just because LSI is
balanced doesn’t mean you don’t get
the impact of what happens with low
pH. And once you roll back into warm
temperatures, well, now LSI is skewed
to the scale building side because of
all the calcium you put in and can’t
take out. Now you’ve got to keep your
pH level low to get anywhere close to
balancing the LSI. So you’ve created a
problem that you didn’t have in cold
water.
“With your pH at 6.8, your chlorine
becomes so active that it burns up
immediately. You use it so quickly you
can’t keep your residual. So your LSI
is balanced, but you’re not sanitizing
your pool because you can’t keep
chlorine in the water. And now things
start to grow.
“So that’s why I call it kind of a
dual approach. We do use the LSI, and
we’re very cognizant of it, and we do
balance to it, but we also don’t let our
balance parameters like pH go outside
of accepted limits.”
RUNNING IN THE
BACKGROUND
While plenty of pool care pros think in
terms of LSI every day, there are many
others in the industry who have been
calculating LSI for years and don’t even
realize it. It’s embedded into water
balancing software and apps, whether
they are being pulled up on a smart
phone poolside, or on the pc at the
water test station at the dealership.
“Those all run LSI in the
background, in the algorithms of
the software,” Stephens says. “In
our [BioGuard’s Alex Water Testing]
software, we don’t print out that LSI
number, but we are balancing to LSI.”
Which, ultimately, is much to
the benefit of the residential pool
customer. In the 88 years since Cal
Berkeley professor Wilfred Langelier
came up with the LSI equation, its
gradual acceptance in the industry
represents a step forward in the care
of swimming pools, providing pros
the ability to determine whether pool
water will cause corrosion or deposit
scale.