Testing is the cheapest thing you do all season and the thing everything else depends on — every dose this site calculates is only as good as the numbers you feed it. The good news is that getting it right is mostly technique, not equipment. The bad news is that the most common testing mistakes produce readings that look perfectly normal.
Take a sample that represents the pool
Water isn't uniform. The surface has been baking in UV, water beside a return is whatever you added most recently, and water near a puck floater is a pocket of concentrated chemistry that exists nowhere else. Any of those will happily give you a confident, wrong number.
- Go elbow-deep — about 18 inches down. Turn the bottle upside down, push it under, then turn it upright to fill.
- Away from returns, the skimmer and any dispenser. The middle of the deep end or by a side wall is fine.
- After the pump has run long enough to mix the pool — an hour is plenty.
- Before you dose that day, not after. Testing 20 minutes after pouring acid tells you about the acid.
- Rinse the vial with pool water first, and test promptly — chlorine readings drift as the sample sits.
The three kinds of test
"Strips versus drops" is the usual framing, but there are really three things on the shelf, and the third is the one that matters most:
| Method | How it reads | Good for | Falls down when |
|---|---|---|---|
| Test strips | Color pads, matched by eye | Quick "is anything obviously wrong" checks; pH and TA trends | Precision matters, or chlorine is above roughly 10 ppm |
| OTO / DPD drop kits | Sample changes color, matched to a printed scale | Everyday chlorine and pH, more reliably than strips | High chlorine — and DPD can mislead badly there (below) |
| FAS-DPD titration | You count drops until the color changes; the count is the number | Any decision that depends on knowing FC precisely — especially shocking | Nothing much, beyond costing more and taking a few minutes |
The important difference isn't accuracy in the abstract — it's range. Strips and standard DPD comparators top out around 5–10 ppm of free chlorine. FAS-DPD counts drops, so its ceiling is however long you keep counting, typically to 50 ppm in 0.5 ppm steps.
Which test does each job
Rather than crowning a winner, match the tool to the decision. Most people need two of these and only one of them regularly:
| What you're deciding | Precision needed | What will do it |
|---|---|---|
| "Is the pool roughly OK today?" | Low | Strips |
| Routine weekly chlorine and pH | Moderate | Drop kit, or strips if levels are steady |
| How much acid to add | Moderate — pH to 0.2 | Drop kit (phenol red) |
| Setting your CYA target and FC target | Moderate — CYA to ±10 | Drop kit CYA test, or a pool-store check |
| Whether an algae bloom is actually clearing | High — FC to 0.5 ppm, up to 40 ppm | FAS-DPD only |
| The overnight chlorine loss test | High — differences under 1 ppm matter | FAS-DPD only |
Clearing algae means holding chlorine at shock level until the water passes the overnight test — and shock level scales with your stabilizer: 16 ppm at CYA 40, 24 ppm at CYA 60, 32 ppm at CYA 80. A test strip that stops reading at 10 ppm cannot tell you whether you're at 16 or 40, or whether you held it overnight. You would be running the process blind, which is the single most common reason a SLAM "doesn't work." This is why the shock calculator treats the kit as part of the method rather than an accessory — it's a genuine prerequisite, not an upsell.
What each option costs
| Option | Rough cost | Ongoing |
|---|---|---|
| Test strips | $15–40 a year | New bottle each season; they degrade once opened |
| Basic OTO/DPD drop kit | $20–40 up front | Replacement reagents, a few dollars a year |
| FAS-DPD kit (Taylor K-2006 class) | $90–140 up front | Refill reagents, roughly $30 every year or two |
| Digital / photometer | $100–250 | Reagent packs, plus calibration and batteries |
Prices move, so treat these as orientation rather than quotes. The honest arithmetic: a full FAS-DPD kit costs about what you'd spend on chlorine clearing one algae bloom, and it's the difference between fixing that bloom deliberately and guessing at it for a fortnight. If you own an outdoor pool and only buy one testing thing, buy that. If your pool is reliably boring and you just want to confirm it's still boring, strips are genuinely fine, and there's no virtue in over-equipping.
A note on digital testers: they remove the color-matching judgment, which is real, but they don't add range — a photometer that reads FC to 10 ppm is no more able to run a SLAM than a strip is. Check the stated range before assuming that "digital" means "better."
The misreadings that fool everyone
DPD bleach-out. This one is genuinely dangerous. Add DPD reagent to a sample with very high chlorine and the pink color flashes and then disappears, leaving the sample looking clear — which reads as zero chlorine. People see zero, add more chlorine, and make an already over-chlorinated pool worse. If a sample goes clear instantly, or you see a flash of color before it fades, suspect high FC rather than none. Dilute the sample with distilled water and retest, or use FAS-DPD, which doesn't behave this way.
Tired reagents. Test chemicals expire, and heat kills them faster than time does. A kit that spent last summer in a hot shed will read low and quietly push you into overdosing. Replace reagents yearly, keep them somewhere cool and dark, and treat a surprising result from an old kit as a question rather than an answer.
Reading colors badly. Match against a white background in open shade, not in direct sun and not indoors under warm bulbs. Sunglasses off. If you're comparing a borderline pH between 7.6 and 7.8, that's a lighting problem as much as a chemistry one.
The CYA test is the least precise one you own. The black-dot turbidity test is read by eye and ±10 ppm is normal. Treat the result as a neighborhood rather than a number. If your reading is off the top of the scale, mix equal parts pool water and distilled water, test that, and double the answer — "off the scale" and "twice the maximum" call for very different amounts of draining.
What about free testing at the pool store?
It's genuinely useful, with one thing to keep in mind: it's free because it's the front end of a sales conversation. The test itself is usually fine — often a decent photometer — but the printout ends with a recommended shopping list, generated by software that assumes you'll buy the products stocked in that aisle.
Use it for what it's good at: a second opinion on the tests that are hard to do well at home, particularly CYA and calcium hardness, and a sanity check when your own numbers look impossible. Don't use it as your weekly routine — driving a sample across town is slower and less accurate than testing at the pool, and a sample that has been warming in a car for an hour isn't the water in your pool any more. And run any recommended purchase through the dashboard first; more often than not the dose you actually need is a fraction of the one on the printout, or it's a product you already own.
A routine that actually gets done
The best testing schedule is the one you'll keep. In season, that usually means a quick FC and pH check two or three times a week — 30 seconds with strips or a couple of minutes with a drop kit — and a fuller test including TA, CYA and calcium every few weeks. Test after heavy rain, after a party, after topping up a lot of water, and any time the water looks or feels different.
Write the numbers down, or put them into the dashboard, which remembers your last test on your own device. The trend matters more than any single reading: chlorine drifting down over a week tells you something a single measurement never can, and it's the earliest warning you'll get that something is starting to go wrong.