Testing Discrepancy

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savdoc2

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Jun 1, 2008
185
NW MO
Up until this year, I have been using the test kit from Pool Solutions. This year I ordered new reagents and have been fighting a cloudy/algae pool. I ran out of R-0871 and ordered the FAS/DPD refill kit. This kit came with a graduated cylinder. When testing this evening, I used the "old" graduated cylinder and got an FC = 13.5. For some reason I decided to repeat the test with the "new" graduated cylinder and got an FC = 11. Both cylinders have the 10ml level marked clearly on the cylinder (not stickers). So I filled up the "new" one to the 10ml level and dumped it into the old and when I checked the measurement, it only measured barely over 8ml. Obviously, this has created the discrepancy in my test results. The odd thing is that when using the "old" one the pink color was a darker pink and the "new" one was a lighter pink. I am very confused why I would get higher FC levels with more pool water.

How can I tell which cylinder has the correct 10ml level?
 
More pool water in the sample, means more total chlorine in the sample, means it takes more drops to neutralize the chlorine, means you read a higher FC level in the test.

There are several ways to test the 10 ml accuracy, but they all involve either getting another measuring device that you trust, or making some assumptions. Most drug stores sell 10 ml syringes designed for measuring children's medications. They are inexpensive and make a reasonable candidate for a measuring device you can trust.

Or, if you have a precision kitchen scale that reads in grams you can weight 10 ml of water. It ought to weight 10 grams. You can improve the precision by zeroing the scale with a cup on it, and then filling the vial with 10 ml of water twice or three times and pouring each one into the cup. That will amplify the difference in weight. The vial that gives you a reading closest to 10 grams per 10 ml is right.

Another approach that you can do with what you have, but which takes many more steps is to compare your two existing vials based on their other markings. My guess is that the scale on the side is correct, but it is offset from the bottom of the vial. That would mean that both cylinders will require the same amount of water to go from 10 ml to 20 ml, and if that is true then that amount of water is the true 10 ml. It will take several steps, and another container, to verify that however.
 
OK, so I have a child's medicine dropper (it has never been used for medicine) and I actually use it all the time for putting pool water into the vials. It measures 5ml of liquid. When I put 5ml from the dropper into the Pool Solutions cylinder, it measured exactly 5ml. Adding an additional 5ml measures exactly 10ml. When I put 5ml from the dropper into the TF Testkits cylinder, it measured 6ml. Adding an additional 5ml measured 12ml. So from this test I am going to continue to use the Pool Solutions cylinder for my CL tests as I would prefer my results to be as accurate as possible. Maybe 2ml less water isn't significant, but 20% seems significant to me.

Now I am curious if I got a flawed cylinder or if the batch of cylinders is flawed?
 
This came up once before, and it does seem like there was a small batch of bad cylinders at some point. Several of us tested our cylinders at that point and nearly all of them were fine, so there haven't been all that many bad ones. However, we never found enough bad ones to really be able to pin down what was going on for sure.

It is also important to realize that consistent results are more important than absolute accuracy. If you are off by the same amount every time you can still learn what levels work for your pool using your test technique and never have a problem.
 
I just compared my TF100 tubes to one from a Taylor DPD kit from the previous owner, and to a medicine dropper. The Taylor tube was very close; the TF100 tubes are off by about 10% (25ml in the Taylor tube is about 27.5 in the TF100 tubes).

All this means is that slightly overshooting in the TF100 tubes is okay.
--paulr
 
Savdoc,

In my kit, both cylinders are off by just a little but it's not quite 10%. (one's short, the other is long) Those gradations are put on there by a screen print process so I would imagine there can be a little movement in the positioning of the cylinder as they are applied.

The solution would be injection molded gradations which would add a noticable cost to the kit. I would be happy to replace yours if you feel you'd like to try another one.

Simply PM me and let me know who you are and we'll get one out to you. :-D
 
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