Water Balance, and the Daily Battle to Maintain It

There are two completely different things people mean when they talk about pool water being "good." There's water quality, and there's water balance. They sound similar but they're solving different problems.

Water quality is manmade. It's the disinfection layer: chlorine, salt cells, UV, ozone, whatever's keeping the water free of things that could make you sick. That's the part most pool services obsess over, because it's the part a customer can see (clear water, no algae, no smell). Quality has very little to do with balance.

Water balance is something else entirely. It's whether the water itself is content. Whether it's at peace with its surroundings. The shorthand we use for that is the Langelier Saturation Index, or LSI for short.

The simplest way to think about LSI

LSI is the difference between the pH the water has, and the pH the water wants.

Water wants to sit at a particular point of equilibrium based on its temperature, alkalinity, calcium hardness, and total dissolved solids. When it's right at that point, the LSI is zero. The water is happy. It isn't trying to give anything up to its surroundings, and it isn't trying to take anything from them.

When the water isn't at that point, the LSI goes negative or positive, and the water gets hungry.

Three states of pool water: happy (LSI = 0), hungry (LSI < 0, leaching calcium), scaling (LSI > 0, depositing calcium).

Negative LSI water is undersaturated. It wants more calcium, and it will steal it wherever it can find it. In a pool, the easiest source is the surface itself. Plaster, pebble, exposed grout, tile grout: all of those are calcium-based, and undersaturated water will leach the calcium right out of them. Over time you see the symptoms. Rough plaster. Mottled patches in the shallow end. A finish that wears out years before it should have.

Calcium leach damage on a pool surface: a real-world example of what undersaturated water does over time.
Calcium leach damage on a pool surface, the long-term cost of undersaturated water.

Positive LSI water is supersaturated. It has more calcium than it knows what to do with, and the excess starts coming out of solution as scale. White deposits creeping along the tile line. Crusty buildup on heater elements. The inside of the pipes narrowing year over year.

Scale buildup on heater elements caused by supersaturated pool water.
Scale buildup on heater elements, the long-term cost of supersaturated water.

Both are bad. The entire job of a pool service that knows what it's doing is to keep the LSI as close to zero as possible, week after week, season after season.

−1.0 −0.3 0.0 +0.3 +1.0 CORROSIVE SCALING BALANCED The Langelier Saturation Index
The LSI in one number. We aim for the middle, every visit.

Enter Henry's Law

Here's the catch: outdoor pools refuse to sit still.

That's because of a chemistry principle called Henry's Law. The short version is that gases dissolved in water want to be in equilibrium with the gases in the air above the water. If there's more dissolved gas in the water than there is in the air, the gas escapes from the water until equilibrium is reached. If there's less, gas dissolves into the water from the air.

This matters because pool water carries a lot of dissolved carbon dioxide (CO₂). The CO₂ in the water forms carbonic acid, and the carbonic acid is what holds the pH down. As long as that CO₂ stays in the water, the pH stays where it is.

But the atmosphere only has about 0.04% CO₂ in it. Pool water has far more dissolved CO₂ than that. So Henry's Law is constantly at work, pulling CO₂ out of your water and into the air. Every CO₂ molecule that escapes means a little less carbonic acid, which means the pH creeps a little higher.

Pool water with dissolved CO₂ molecules escaping into the lower-CO₂ atmosphere above, illustrating Henry's Law in action.

A few things make CO₂ escape faster:

So an outdoor pool with a waterfall feature, in summer, on a windy day, is essentially a Henry's Law machine. CO₂ is pouring out of the water all day. The pH is climbing whether you want it to or not.

(Henry's Law is also why heated pools lose chlorine and dissolved oxygen faster, and why agitated pools off-gas volatile chloramines more quickly, which is part of why outdoor pools usually need more frequent chemical attention than indoor ones. But the dominant effect, by far, is the CO₂/pH story above.)

Pool waterfall and aeration features in action, accelerating gas exchange at the water surface.
Aeration features are essentially Henry's Law accelerators. Beautiful to look at. Hard on your chemistry.

The fight

This is where LSI and Henry's Law go to war.

The LSI is asking for a stable pH that holds the water in balance. Henry's Law is constantly raising the pH by stripping CO₂ out of the water. Left alone, that drift takes the LSI positive and you start seeing scale.

The pool industry's standard response is to add muriatic acid every week or so to bring the pH back down. That works in the short term. The acid neutralizes some of the alkalinity, the pH drops, and the LSI comes back toward zero.

But muriatic acid doesn't replace the lost CO₂. It just lowers the pH artificially. The water is still gassing off whatever CO₂ remains, and within days the pH is climbing again. So you add more acid. And again. And again. Over time, all that acid drives the alkalinity down, which means less buffering capacity, which means bigger pH swings, which means even more acid. It's a slow downward spiral that ends with water that's chemically exhausted and a pool service that's just feeding it acid every visit.

There's a better way to do this, but it requires patience instead of just dosing. The most efficient lever isn't aggressive acid every visit, it's keeping the alkalinity buffer healthy. When alkalinity dips below where we want it, we restore it with sodium bicarbonate, which gives the pool the buffering capacity to absorb pH drift without us chasing it down with acid every week. (In rare cases where the alkalinity has gotten too low and the pH needs a nudge back up, we'll aerate, but it's a tool we use occasionally, not routinely.) The result is a chemistry that doesn't get progressively exhausted, and a pool finish that lasts the way it should.

The bigger point is this: managing balance in an outdoor pool isn't a one-time fix. It's a daily negotiation between two forces of physics, and our job is to keep that negotiation from costing you a finish.

Hydraulics in Pool Systems

A short history detour

Today's pools aren't like the ones you remember from childhood. Older pools were typically plumbed with copper pipes, and copper turns out to be one of the most powerful algaecides ever discovered. The old pools we grew up with had a slow, constant leach of copper ions into the water, just enough to keep algae from ever getting a foothold. That's why algae problems weren't the headache then that they are now.

The most visible side effect of all that copper was something every blond kid of the era dealt with at the end of summer: green hair. Everyone blamed "too much chlorine," and they had it half right. The chlorine wasn't the problem on its own. What was actually happening is that copper dissolved in the water was getting oxidized by the chlorine and falling out of solution, then binding to hair. (It bound to everyone's hair, but you could only see it on the blond ones, because oxidized copper is green.)

A copper roof showing the green oxidation patina that develops over years of weathering.
Same chemistry, different scale: oxidized copper turns green whether it's on a roof or in a swimmer's hair.

Copper does the same thing on a roof. A copper roof starts out shiny and golden, then oxidizes over years into the soft green patina you see on the Statue of Liberty. Same chemistry, different scale.

Modern plumbing, and why algae control got harder

Today's pools are plumbed with PVC, usually 2 inches in diameter. PVC is cheaper, easier to install, and doesn't corrode. But it also doesn't leach anything antimicrobial into the water. Modern pools are essentially sterile from a plumbing standpoint, which means the algae control we got for free in the copper era now has to be done deliberately, every week, with chemistry.

The shift from copper to PVC also changed the hydraulics of the pool. That's the part most people don't think about, and it matters more than they realize.

What "Total Dynamic Head" actually means

When water is pushed through pipe, there's a certain amount of resistance. The water doesn't slide through frictionlessly. It rubs against the pipe walls, gets jostled by every turn, and the pump has to fight all of that to keep it moving. The total amount of fight required is called total dynamic head, or TDH. It's measured in feet (the same units you'd use to lift water vertically, because the math works out the same way).

Two things contribute most to TDH in a pool system:

Pipe length. Longer plumbing means more friction. There's only so much water per minute you can force through a 2-inch pipe before the resistance climbs into territory that costs you.

Fittings. Every time the water has to make a turn, friction goes up. A standard 90-degree elbow adds roughly the friction of an extra 8 feet of straight pipe. If the plumbing has six 90-degree elbows in it (which is normal), that's 48 feet of equivalent pipe friction tacked onto the actual pipe length.

Sharp 90 degree elbow versus sweep elbow: the sharp version produces turbulent flow with eddies and bubbles; the sweep produces smooth laminar flow.
TDH friction budget for a typical pool: 30 ft straight pipe, 48 ft from six 90° elbows, 12 ft filter, 8 ft heater, totaling 98 ft of equivalent friction.

This is why we try to bend the water as little as possible. A pool plumbed thoughtfully (fewer turns, sweep elbows instead of sharp 90s, runs that don't take the long way around) has dramatically lower TDH than a pool plumbed without thought.

Why TDH matters in practice

Higher total dynamic head means the pump has to work harder for every gallon of water it moves. Think of it like driving. Coasting on a flat road costs almost no fuel. Climbing a hill costs a lot. The pump motor doesn't care whether it's pushing water through a clean 30-foot run or fighting six elbows and an undersized filter. It just runs harder, runs hotter, draws more electricity, and wears out sooner.

In real terms, lower TDH means:

This is one of the things we audit on every pool we take on. If the hydraulics are working against you, no amount of chemistry will fix what bad flow leaves behind.

The bubble problem

Here's the part of hydraulics that surprises people. Water carries dissolved gases in it (oxygen, nitrogen, carbon dioxide), the same way a sealed soda carries dissolved CO₂. As long as the water is under steady pressure, the gases stay in solution and you don't see them.

But when water is forced through tight bends, or when there's a sudden pressure drop (which happens at every sharp turn), some of that dissolved gas comes out of solution and forms small bubbles. Same thing that happens when you crack open a soda: release the pressure, and the bubbles appear.

In a pool plumbing system, those bubbles get carried along with the water. Most of the time they cause no real harm. But there's one place where they cause big problems: the flow switch on a salt cell.

A salt cell plumbed incorrectly: a 90-degree elbow goes straight into the inlet, creating a perfect place for air to pocket on the flow switch.
Plumbed wrong. A 90 right at the inlet, and another at the outlet.
A salt cell plumbed correctly: a straight pipe approach gives bubbles room to clear before reaching the flow switch.
Plumbed right. Straight run before the cell.

A salt-chlorine generator only runs when its flow switch detects moving water. If a 90-degree elbow is plumbed right before the cell, you can end up with a pocket of air sitting on top of the flow switch. The switch is essentially in a vacuum, doesn't engage, and the cell never turns on, even though water is moving fine through the rest of the system.

This is why the universal recommendation is never plumb a salt cell with a 90-degree elbow going straight into it. There should be at least a foot or so of straight pipe before the cell, to let any bubbles travel past the switch. If space is tight and a turn before the cell is unavoidable, use a sweep elbow (a long-radius bend) instead of a sharp 90. Sweep elbows generate far less turbulence, and far fewer bubbles.

It's a small detail. It's also the difference between a salt cell that works for six years, and one that gets blamed (incorrectly) for failing in one.