If you have a chlorinated or brominated swimming pool, you probably know that maintaining a constant and adequate level of chlorine or bromine is the single most important element of proper pool care. And, you probably know you have to test your pool regularly water to check your chlorine level.
What you probably don't know that often, the best way to test for chlorine is also the cheapest and easiest.
Currently, there are three practical methods of testing for chlorine in swimming pools:
OTO (orthotolidine) drops which turn yellow in the presence of chlorine, bromine or other oxidizers.
DPD (diethyl-p-phenyleneldiamine) tablets or powder which turns pink in the presence of chlorine or bromine, as long as there's not too much of it.
SYD (syringaldazine) test pads on test strips which turns bluish purple in the presence of chlorine or bromine.
(There are other -- impractical -- methods of testing that are sometimes advertised. Stay away from them! In particular, the use of an electronic meter with an ORP electrode is probably the most common, one of the most expensive, and one of the worst! Do NOT be suckered into thinking that a home swimming pool ORP meter is good because it's expensive. ORP testing is slow and is thrown off by the presence of many other chemicals. Worse, it doesn't even measure chlorine levels: instead, it measures something called "redox potential", which is only loosely related to pool chlorine levels.)
Hands down, your everyday pool test kit should be OTO based. Simply put, it's the most trustworthy way to test!
If your chlorine is too missing, OTO will tell you by staying clear or turing pale yellow. DPD can be picky, and if you've added a granular mustard algae treatment, may tell you there's no chlorine, which in fact it's still there, just not in the best form.
If you chlorine is too high, OTO will tell you by turning dark orange to brown. At those levels of chlorine, DPD will bleach out, and tell you there's NO chlorine, when it's really way too high. SYD just can't tell the difference between high (but OK) and WAY too high: it reads the same at 100 ppm as it does at 10 ppm!
The other methods do have benefits.
If you planning to use a hotel pool or hot tub, it's much easier to carry a baggie with a few test strips (SYD) with you, than to care an OTO or DPD kit.
And if you are ready to get a bit more advanced in your pool testing, DPD can tell you -- IF you have the right kit and IF you use it correctly -- more precisely what KIND of chlorine is in your pool and just exactly how much is there.
But, for everyday testing: "Is my pool reasonably safe and likely to stay clear?", OTO is best.
But, while OTO is good, it's still possible to screw it up!
Don't take your water sample from the very top surface of your pool or spa! There's oil from swimmers, debris from the air, and (often) no chlorine from the sunlight.
When you collect your water sample, usually the easiest way is to hold the kit upside down, submerge it at least 8" under the surface, and then turn it right side up, allowing it to fill. If you collect your sample from the surface, you will not get reliable readings. Also, you generally should collect near or in front of a skimmer. Otherwise, you might end up to near the inlet carrying water back to your pool, water which may be untypically high or low in chlorine.
One more thing. Our experience with lifeguards suggests that about 1 in 10 guys cannot read the colormetric kits accurately. Although this can result from color blindness, our experience suggests that a lack of color 'awareness' may be more the issue. To put it another, impolite, way: it's not so much that lots of guys are "color blind", though a few are, as that lots of guys are "color dumb"!
You can eliminate this problem by having several people take readings from the same test kit: if one person gets a distinctly different result checking the sample, they may need to let someone else do the testing.
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