Test Salt Cell

You can tell whether a cell is producing chlorine in about ten minutes without buying anything. Look for gas bubbles at the return while the generator is on, then measure free chlorine before and after a known run time and see whether the number moved. Everything else here is confirmation and diagnosis.

Its twin page, the CYA pool test kit, tests the water. This one tests the hardware. Keeping them apart is most of the diagnostic, because a dead cell and CYA at 110 present identically. No chlorine, and a panel that looks fine.

The panel is evidence about the cell, not about the water

A salt cell for pool use has no salt sensor. It passes current through the plates, measures how easily that current crosses, and infers a salinity number from the conductivity, temperature compensated. Poolside’s troubleshooting guide says it directly: most systems “estimate salt levels using an algorithm” rather than reading salinity. Orenda adds that a faulty sensor can read lower than actual and says to verify with a separate salinity meter.

So scale, plate wear, cold water, low flow and the wrong cell model selected in the menu all move the displayed salt number while not one grain of salt goes anywhere. When the panel and a real salt test disagree, the panel is not lying about the pool. It is telling you something true about the cell, in the wrong units.

One owner in the comment thread on Inyo’s guide to reducing salt ran the experiment by accident: “Turned on old cell for one last reading. It was 2500. Put in new cell and it said 3600.” Same water, same afternoon, 1,100 ppm of difference that existed only in the cell. Another in that thread: “I added to much salt to my 15000 gallon pool because I did not realize my T-cell was going bad.”

Test the cell before you trust the panel. If you do end up dosing, size it with the pool salt calculator against a titration test rather than the display, using salt levels and testing.

Test 1: look for gas

Electrolysis splits water as well as salt. Orenda describes hydrogen gas bubbling off the cell, creating the aeration that off-gasses carbon dioxide and drives the pH climb in every salt pool. The useful part here is that the gas is real and it comes out of the return.

With the pump running and the generator calling for chlorine, stand at the return and look. A producing cell sends a fine steady fizz of very small bubbles, not the big irregular slugs of a suction side air leak, which arrive in bursts alongside a spitting pump basket. A clear housing lets you watch bubbles form on the plates directly.

No bubbles at all, with the panel showing it is generating, means no electrolysis is happening. Two cautions. At low output many systems generate for part of each hour only, so set output to 100% or you may be watching an off cycle. And weak cells still bubble a little, so absence is conclusive and presence is not.

Test 2: the free chlorine rise test

This is the one that answers the question, because it measures output rather than activity.

1. Test free chlorine and note the time. Evening or first light, not noon, so sunlight is not eating the result while you measure.
2. Set output to a known percentage. 100% is easiest to reason about.
3. Run the pump a known number of hours.
4. Test free chlorine again at the same spot, same depth, same kit.

A healthy cell moves the number. A dead one does not.

For a sense of scale, Hayward’s super chlorinate mode runs the cell at full output for 24 hours, and In The Swim says most salt systems in that mode “will only raise the FC level by a few clicks - up to 5-7 ppm”. They mean it as criticism of using boost as a shock, which it is, but it doubles as a yardstick. A full day at full output is worth single digit ppm on a typical pool, so an ordinary 8 hour run at half output should move free chlorine a fraction of a ppm, not 3 ppm. That is an order of magnitude, not a spec, and gallons, temperature and bather load all move it.

So the number to care about is your own history. Run it once on a cell you know is healthy, write the result inside the equipment pad door, and the comparison two years later beats any absolute figure.

No rise but you had bubbles means chemistry, not hardware. High CYA does not stop production, it changes what counts as enough, and an algae bloom eats chlorine as fast as a cell makes it. Check CYA against the CYA chlorine chart, then work salt cell not producing chlorine in order.

Test 3: the panel’s own instruments

Most control panels have a diagnostic display that cycles through more than the salt number. The two readings worth finding are cell voltage and cell current, usually in amps. Together they show what the electronics are doing to get the output they get. Plates worn thin, or insulated by scale, present differently to the power supply than healthy ones, and voltage high with current low is the classic profile of a cell not conducting properly.

We are not printing expected values. They are specific to the model and the water temperature, and a made-up range would be worse than none. Yours are in your manual.

Note the water temperature first. Hayward states that “the Aqua Rite automatically stops generating when the pool water temperatures drops below 50ºF.” Orenda puts the general case higher, at most cells alarming or shutting off between 55 and 59°F, since cold water takes more energy. Not the same threshold, so read your own manual. In shoulder season a healthy cell produces nothing and reports a problem, and owners who add salt to chase it spend June draining.

Test 4: look at the plates

Hayward’s manual says to inspect the cell every three months or when the filter is cleaned. Most people never do it once. Shut the pump off, pull the cell, look through it with a flashlight.

• Dark grey, smooth, evenly coated plates. Healthy.
• White crusty deposits bridging the gaps. Scale, which insulates the plates, cuts output and drags the inferred salt reading down with it. Hayward’s fix is a hose flush for mild deposits and a 4:1 solution of water to muriatic acid for severe scale, soaked “for a few minutes,” with the safety line in capitals: “ALWAYS ADD ACID TO WATER - NEVER ADD WATER TO ACID.” No metal tools, because they scratch the coating and the coating is the cell. Procedure on how to clean a salt cell.
• Plates that look clean but thin, patchy, or bare in places. Wear, and it is not fixable. Scale you can remove. Coating you cannot put back.

Scale is also preventable, and Orenda notes it forms on the cathode side from the high pH sodium hydroxide produced during electrolysis. Letting pH sit high in a salt pool is not cosmetic. It is a cell-life expense.

Test 5: audit the cell type setting

The one nobody checks, and it costs nothing.

Control panels work with several cell sizes and the correct model has to be selected in the menu. The conductivity to salinity conversion depends on that setting, because a bigger cell has more plate area and conducts more at the same salinity. Set the wrong model and every salt reading is converted with the wrong constant. Cell fine, water fine, number wrong all season.

Read the label on the cell you own, then read the cell type configured in the panel. They should match. This goes wrong after a cell swap, a board replacement or a service call, so it lands on people who just spent money and are least likely to suspect a free explanation.

Putting a result together

A failed cell is not necessarily a surprise. River Pools puts a well maintained cell at three to seven years and $700 to $1,100 to replace, and Leslie’s lists a Hayward T-CELL-9 at $899 and a Jandy AquaPure 1400 at $1,949. Where you land in that window is mostly chemistry and whether anyone ever looked at the plates, the argument on how long does a salt cell last. If the tests point at the cell, the buying decision is on salt cell replacement. Do the cell type audit first though. It is free.

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