Measuring Higher Levels of Chlorine with Test Kit Question

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brettdawnj

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Jun 22, 2008
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I am having challenges with my pool. Water is slightly green and I am getting green deposits. Here is my latest water numbers from the pool store:

CYA 70
FC 4
TC 4
PH 7.2
TA 92

I used some copper based algaecide to kill off the algae, and I am not seeing the results I would expect. I am also trying to keep the chlorine level high, but have no test kit that will go over 10. My strips go up to 10, and the OTC dropper goes to 5.

After seeing what the CYA does in requiring high levels of chlorine to impact algea, I want to keep my chlorine level high for some days until the bad stuff goes away.

Since it takes 5 drops of OTC into the tube, my thoughts are that if I only add one drop to the tube instead, then my readings should roughly end up as 5 times the amount I find on the color comparator. IS THIS A CORRECT ASSUMPTION?

Because when I did this, I think my chlorine level is greater than 25 as with one drop it is darker than the 5ppm on the tester (1 drop x 5ppm color max x 5 times more water). But my pool store's test indicated that it is only at a level of 4. (my test strips show it at 10 or max level).

Because of the green, and the CYA charts for FC levels to kill the worst stuff, I think I need to run the pool at a shock level of 25 and a mustard kill of 40. How do I know what level I am really at if my tester only goes to 10 for strips and 5 for OTC drops?

Also, am I better off lowering my CYA to 30 or so to get chlorine to be more effective? I don't know how it got this high, but I live in Savannah, Ga with some pretty hot and direct sun. Does anyone have a CYA level that is best for this area (would it be different for a sunny pool and a partially shady pool)?

Any help and insight would greatly be appreciated. This green stuff is kicking my behind. Thanks
 
brettdawnj said:
I

Since it takes 5 drops of OTC into the tube, my thoughts are that if I only add one drop to the tube instead, then my readings should roughly end up as 5 times the amount I find on the color comparator. IS THIS A CORRECT ASSUMPTION?
No

Because when I did this, I think my chlorine level is greater than 25 as with one drop it is darker than the 5ppm on the tester (1 drop x 5ppm color max x 5 times more water). But my pool store's test indicated that it is only at a level of 4. (my test strips show it at 10 or max level).
Their dpd test is possibly bleaching out.

Because of the green, and the CYA charts for FC levels to kill the worst stuff, I think I need to run the pool at a shock level of 25 and a mustard kill of 40.
If your water is cloudy and green from algae than your CYA might be lower than you think because the CYA test is a turbidity test (cloudiness test).

How do I know what level I am really at if my tester only goes to 10 for strips and 5 for OTC drops?
Get yourself an FAS-DPD chlorine test kit. It can test FC up to 50 ppm with a precision as great as .2 ppm and does not rely on color matching. That is what we recommend. category/pool-school/pool_test_kit_comparison

Also, am I better off lowering my CYA to 30 or so to get chlo€rine to be more effective? I don't know how it got this high, but I live in Savannah, Ga with some pretty hot and direct sun. Does anyone have a CYA level that is best for this area (would it be different for a sunny pool and a partially shady pool)?

Any help and insight would greatly be appreciated. This green stuff is kicking my behind. Thanks
Click to expand...
Until you have a test kit that can accurately test high FC levels you are just guessing what to do. Get a good test kit, kill the algae, then worry about the rest of your water balance.
 
The Mod Squad replied:

Since it takes 5 drops of OTC into the tube, my thoughts are that if I only add one drop to the tube instead, then my readings should roughly end up as 5 times the amount I find on the color comparator. IS THIS A CORRECT ASSUMPTION?
No

Can you help me understand this better? My simple sense is that the tube doesn't know how much water or how much reagent is in it. So if one puts in 1 drop instead of 5 drops, shouldn't the color comparator give us 5 times the level of chlorine in the water? I will try to make some basic reasoning with this thought. Take for example a pool with 5 ppm (parts per million) chlorine. If we put in 5 drops, our tester will show 5ppm. Now if I add pure non chlorinated water to the tube water in a quantity of 5 times the tube volume, the amount of chlorine in the tube should be 1/5 of the original amount. So when I pour in this new mixed quantity back into the tube, it had better then show the water to be at 1ppm because I reduced the chlorine in the tube to 1/5 of the original ppm. Hence a test at this point should give us 1 ppm.

A similar thing happens with the water after the sunlight or the organics use up the free chlorine... it goes lower in ppm or parts per million and our OTC tester shows it dropping from 5 ppm to 1 ppm because of the ratios of the chlorine (in ppm) to the pure water. SO CAN SOMEONE EXPLAIN WHY MY ROUGH CONCEPT OF USING 1 DROP IN THE TUBE VS THE 5 DROPS IN THE TUBE WILL NOT GIVE US ROUGHLY A PPM VALUE THAT IS 5 TIMES THE SCALE OF THE TESTER? :?: As an engineer, I am always one that can't sleep until I know the WHY behind the issue. :hammer:

... also thanks to The Mod Squad for your suggestions and pointing out a good tester for this application.
 
Brett,

It does sound good on paper but it doesn't work well at all due to the imprecision of the test.

Let's say the imprecision is normally + or - 2ppm (that's about right, too)

Now if you cut the mixture in half with distilled water the imprecision doubles to 4ppm....making the results somewhat troublesome.

You can see how badly the results get skewed if you dilute further.

That method is occaisionally recommended here as a stop-gap measure but the results are almost always so far off from what you need to know that I never even suggest trying it.

An FAS/DPD test for chlorine is such an eye-opener with it's precision and is integral with the practice of accurate testing we use here on the forum.
 
Your assumption in using less reagent will give you a wider range is wrong because the concentration of Cl in the water is still the same and the color produced by the OTO reagent is not THAT dependant on the number of drops added. Try it yourself with the same water and test with 1-5 drops. The colors will not correspond to 1/5,1/4, 1/3, 1/1, and 1/1 concentrations. Now, if you dilute your sample with 4 parts distilled water you have REDUCED the amount of chlorine in the water and if you test this sample in the normal way you will have a ballpark figure if you multiply the testing results by 5. This is because you have actually lowered the amount of Cl in your sample.

There is a well known method of dilution to extend the range of an OTO test but it will only give you a ballpark and OTO is not that accurate to begin with.
Pool water distilled water Multiplier
1 part 1 part X2
1 part 2 parts X3
1 part 3 parts X4
1 part 4 parts X5

Don't try and go over a 1/4 dilution, you will just not have enough accuracy. remember this method is really just for a ballpark figure. If you want accurate test results at high chlorine levels you must us an FAS-DPD test.
 
I had the same thought early on while I waited for the better test kit (FAS/DPD) to arrive. The guys here told me the same -- it just doesn't work that way.

The FAS/DPD test kit can be had by itself. I don't see how you'd get through the shocking part of cleaning up a pool without it. I used up my two reagent bottles in a week from testing multiple times per day. My pool is pretty close to cleared up now so I won't be using them at that rate going forward. But you just don't know where you really are with the chlorine levels until you get that kind of kit. Also, the test for combined chlorine with the drops gives you so much simpler of a result. Takes all the guesswork away.

I got this test kit within 2-3 days of ordering:

http://www.tftestkits.com/index.php?act ... oductId=23

Steve
 
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