Testing the Pool Chemistry

Many thanks for setting up this group John.

In your article on opening the pool for the new season you emphasise the need for testing the chemistry but caution against test strips. We are about to complete on a property with a swimming pool and the current owner uses a kit based on test strips.

What alternative method do you recommend, or is it more about getting a top quality kit?


Hi Alan,

That test kit is the OTO test, the pH side is phenol red as mentioned above. The chlorine side is OrthoTOlidene (OTO) but really that only measures Total chlorine and not free available chlorine.

Let me explain, in the pool with the OTO test you could measure the chlorine level at 2ppm (total chlorine)but of the total chlorine for this example only 0.5ppm is actually free chlorine the remainder is combined chlorine (that's the part we don't want and gives rise to the "Pool smell") Something has happened or is happening to produce the combined chlorine so the pool can still go cloudy or green despite what originally looked good.

Total chlorine - free chlorine = combined chlorine, in a good pool combined chlorine should be 0 to 0.5 ppm.

Not all bad though as i still carry OTO test with me as it's useful when shocking a pool at high chlorine levels as DPD can bleach out, OTO doesn't but may still be hard to read at high levels (deep yellow)

Sent you a friends request so I can talk about your personal requirements.

Completely understand about the helper, too rare to upset!

Hi John,

Many thanks for the prompt and comprehensive reply. I don't know the precise make they are using; it's a double-tube test kit - actually just realised, it isn't paper strips. I think he filled the two-tube bottle then added separate drops to each side and after a measured time he matched the water colour against colour chart strips. Does this make more sense?

How do we source the tests above? (We anticipate engaging a pool company for periodic support (including seasonal opening up and closing down) but presently the owner does it with their gardener-cum-handyman - so we need to be a little sensitive about this as we envisage keeping on the gentleman.)

Thanks again.


Hello Alan, welcome to the group.

May I ask which bit of kit they are using?

The problem with test strips is they are only really an indication of something, I think people see a colour change and assume that everything is fine but the gaps between the reading scale are too large and the colours from the printer make them very hard to interpret.

When you add that the dots of reagent are all in a line so one chemical colour/reagent runs into another the you really have to question what's being measured. now when you dip them how long, how much of a flick/shake do you give them and do they measure from the minute they hit the surface or some point under the surface assuming they arrive at that point with the 3 or 5 seconds. All to hit and miss so more accurate testing will make looking after your pool easier once you have grasped the basics. The additional cost of the better testers is soon recouped by not running to a pool shop and leaving with arm fulls of things you probably didn't need.

Testing for pH is universally a phenol red reagent but matched with a good colour slide in fine graduations so you can observe what the real pH is likely to be.

Testing for chlorine is an industry standard using a DPD test, this is a requirement for commercial and multi occupancy pools in france for a very good reason, it's proved itself. The only downside with DPD is it can bleach out at shock levels of chlorine.

Other tests for alkalinity, and calcium hardness can be done with titration tests, a tablet count method where the sample changes from one colour to another and the number of tablets is checked against a card which tells you the level, very accurate.

Cyanuric acid, this cannot be tested with dip strips, I have been to pools with owners having 180 and 200ppm where the strips read 0, 30-50ppm (the strips are given to me by the manufacturers but they are rubbish. The conventional test for CYA is a turbidity test where the pool water turns cloudy and you view a dot on a test tube and watch it disappear, at the point it disappears you read the side of the test tube and have the reading. This test is so important to the overall health of the pool.

These test can be purchased individually or as a kit which is invariably cheaper but you must still be able to see colours to use it.

Moving up the scale are the electronic testers. a sample of pool water has a reagent added and then a colour is given which when put into the reader the light source shines and gives a reading digitally so no colour matching errors.

It all comes down to cost and preference the more things you test for the more you generally pay but the clever marketing people also have tricks up their sleeves to catch the unwary. Some manufacturers don't include many reagents so the headline price looks lower against the competition but by the time you have added up the cost of the extra ones you can end up paying more.

Then other companies have pre set parameters so you get annoying "lo" or "hi" when outside of what they think you need but they haven't a setting for different types of pool so how do they know what's right? Vinyl pools need less alkalinity than concrete plaster tiled often lower than some units can measure. sure you can ignore it but then you will use more pH- to control your pool.

To try and make some headway I have looked at the market and made judgements on various companies offerings with cost and features the primary and of course does the company have good after sales should something go wrong. These testers are more than adequate for our use and have been tested over a fair length of time to prove them worthy, if new ones arrive I have a good look at them. One of the dip strip company's latest tester uses individual strips for each test obviously because of interference from one colour to another as I described earlier and the fact the penny finally dropped, as things arrive I let people know but for now I have a good range of testers.