CYA Pool Test Kit

The CYA test is the least precise test in your kit and it is the one the most other numbers depend on. It is not a color match. You are judging when a black dot disappears in a cloudy sample, by eye, outdoors, and two people looking at the same tube can land 20 ppm apart. Run it anyway, because everything else keys off it.

This page tests the water. Its twin, test salt cell, tests the machine, and the two failures look nothing alike once you can separate them.

Why this test carries so much weight

Free chlorine has no correct value on its own. It has a correct value for your CYA. A salt pool’s minimum runs at about CYA times 5%, so the same 3.0 ppm reading is comfortable at CYA 60 and under the floor at CYA 80. The full table is on the CYA chlorine chart.

Shock scales the same way. In The Swim says add 50% more shock above CYA 20 and double it above CYA 40. So one eyeball measurement sets your chlorine floor, your working target, and the cost of every shock you do this season.

How the test works, and why it is different

Every other reagent test in the kit is a color comparison. Add drops, the sample turns a color, hold it against a printed standard. Your eye is good at that.

The CYA test does something else. You mix your pool sample with a reagent that reacts with cyanuric acid to form a fine white precipitate. The water goes cloudy, and how cloudy depends on how much CYA is in it. Then you pour that cloudy water slowly into a tube with a black dot at the bottom, looking down the column the whole time. The instant the dot vanishes you stop pouring and read the level on the scale.

You are measuring turbidity, not color. There is no standard to hold it against. The reading is your own judgement about when a dot went away.

We are describing the method the way kits generally present it. None of the sources we cite publish the procedure, so follow the sheet that came with yours, particularly on sample temperature, which several kits specify because the precipitate forms differently in cold water.

Reading it properly

Outdoors, in daylight, without direct sun on the tube. You need real light on the dot. Glare on the tube wall and a shadow across the top both move the result.

At arm’s length, not up at your eye. This is the instruction people skip and it matters more than it sounds. Hold the tube down near your waist and look down the column from normal reading distance. Up close you see the dot through less water at an angle, so you pour too much and read high.

Stop the first time the dot is gone. Not faint. Gone. Overshoot and you start over rather than pouring some back.

Let the mixed sample stand for the time the kit says. The precipitate needs time to form, and reading early reads low.

Do it twice. Two close readings give you a number. Two readings 30 ppm apart give you a technique problem, not a pool problem.

The dilution trick for high readings

The scale on the tube is not evenly useful. At the low end the graduations are far apart and 30 is distinguishable from 40. At the top the marks crowd together and telling 80 from 100 is close to guesswork, because the dot disappears almost immediately either way.

The way around it is to dilute. Mix your pool sample half and half with distilled water, test that, and double the result. The arithmetic is exact: half the sample is half the concentration, so a mixture reading 45 came from a pool at 90. It moves the measurement into the part of the scale the tube can actually resolve.

Use distilled, not tap. Orenda notes that cyanuric acid “is not naturally occurring and should not be in tap water,” so tap should not skew it in theory, but distilled removes the question and a gallon costs almost nothing.

Why the pool store’s number disagrees with yours

It usually will. Three reasons, and only one is anybody’s fault.

Strips are a different measurement. A CYA strip is a pad that changes color, doing the color comparison the tube deliberately avoids. Strips tell you whether you have CYA. They are not the tool for telling you whether you are at 70 or 100, which is the distinction that moves your chlorine target by a full ppm.

The store is often running the same imprecise test. If they use the turbidity method, their number carries the same eyeball error yours does, just somebody else’s eyeball. A store reading is a second opinion, not a calibration.

Sample handling. A sample that sat in a hot truck, a bottle that held something else last week, water dipped off the surface next to a return. Take it from elbow depth away from returns, in a clean container, and test it the same day.

Ten or twenty ppm of disagreement is the normal spread of this test, not a mystery. Fifty ppm means retest both.

An approximate number is still worth having

This is what gets lost when people decide the test is unreliable and quit doing it. Work out what the imprecision actually costs.

At CYA 70 the salt pool minimum free chlorine is 3.5 ppm. At CYA 80 it is 4.0. Be wrong by 10 ppm and your chlorine floor is wrong by 0.5 ppm, which is inside the noise of an ordinary FC test anyway. Nothing bad happens.

Now compare CYA 40 with CYA 90. Minimums of 2.0 and 4.5. That is the difference between a pool that stays clear and a pool that greens up in July with chlorine showing on the strip the whole time.

So the test only has to be right within about 20 ppm to do its job, and it is reliably that good. What it cannot do is settle whether you are at 78 or 82, and nothing in pool care depends on that. Test it, accept the number is rough, and dose in the direction that is cheap to undo. Going up costs a bag. Coming down costs thousands of gallons.

When to test

CYA is not consumed and does not evaporate. It only moves when you move it, so this is not a weekly test. Fold it into salt water pool maintenance at the points where the number can have changed.

• At opening, because winter rain, snowmelt off the cover and a spring refill all dilute it.
• After any drain and refill, planned or not.
• Midsummer, while there is still season left to correct it.
• After any stretch of running trichlor tablets, because that is the one thing that pushes it up without you deciding to.
• No sooner than 24 to 48 hours after adding stabilizer. HTH says it dissolves slowly and to allow 24 to 48 hours of circulation before retesting. Test early, read low, add more, and you have overdosed a chemical that only leaves with water.

How CYA behaves across a season is on cyanuric acid for pool, and the target to aim at is on best range of CYA for a salt pool.

What this test tells you nothing about

Your salt level. People conflate them because both sit in the water and neither gets consumed, but they are unrelated, and CYA is invisible to your generator.

Worth saying because of how the panel works. A salt cell has no salt sensor. It reads how easily current crosses the plates and infers salinity from it, which Poolside describes as systems that “estimate salt levels using an algorithm.” Stabilizer does not register there at all. So a LOW SALT light is never a CYA problem, and low chlorine with a panel happily reading 3,200 very often is one. Check the salt number against a real test on salt levels and testing, and size any dose with the pool salt calculator.

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