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Odd Chlorine Readings After MPS Use

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I was looking for a way to shock a 38,000 gal indoor pool.  We usually use Liquid Chlorine, but I was hoping to use MPS instead.

Recently, we drained and filled the pool, then got the numbers set:

Fc- 3.2, Tc- 3.4, pH- 7.4, Alk- 80, Hard- 205

In the past, I've had issues juggling the Combined Chlorine between 0.0-0.4, so after a swim session, I tossed in 4lbs of MPS.  The next day, the Total was unreadable as it exceed the value on my PalinTest PoolTest 9 device.  Over the course of a week, one of my pool maintenance workers noticed the free chlorine would start clear and slowly build in pink color over time.  Not exactly sure when this started happening.  After about 20 mins, the color starts to stabilize.  We diluted the test in half and were able to get a reading.  It would be about 4.1 ppm.  I assume multiply that by two with the dilution.  So, we are looking at 8.2 ppm.  We stopped adding chlorine once this happened, and the same values seem to keep coming up.

I attempted to get a K-1518 kit to test with MPS present.  The chlorine acted the same way.  Usually, when we test and drop the DPD1 tab in with the PalinTest, the water turns pink instantly and the pink color remains until we throw it out.  While working with the K-1518 kit, even with the deox reagent, the color is still changing while I'm performing the test.  I'm unsure if this is still ok to do if this happens.  I did try waiting out the 20 mins until stable and the tests usually line up with one another... Palin vs. Taylor.

Any insight?

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Today I did the test using the K-1518 kit.  I didn't bother waiting for the color to stabilize.  I had an initial reading of 1.4 Free (7 drops) and 0.6 Combined (3 drops).  Added some chlorine and got it to measure out to 3.0 Free and same 0.6 Combined.  I'll check it tomorrow and see what happens.

I tried to use the PalinTest with the deox.  I used a DPD1 tablet with 5ml deox since the PalinTest uses 10ml vials for testing instead of 25ml.  I got a matching reading the first time, but after adding chlorine, it didn't line up with the drop test.  Also tried adding DPD3 tab, waited 1 minute like the Taylor test and it didn't line up, then tried 2 minutes and it deviated even more.  Was hoping I could do this to get a more accurate digital reading, but it didn't pan out.

I've asked numerous outlets if anyone has experienced what I'm experiencing, but no one seems to know.  Seems like not a lot of people care about Combined Chlorine that much or just don't use MPS.  Only response I received was from a Taylor Tech rep that at my chlorine level, the MPS should've oxidized out and I shouldn't get interference.

Anyway, I believe at this point this is how the product should be used... Again, if anyone has any better insight on how this product works, I'd love to hear about it.

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