Quote Originally Posted by Gracy101
Geez - I'm lost....I get why he had/has to drain but I got lost when you and hulla started mixing water samples....Please explain in newbie terms
The measurement test for CYA may max out at 100, so if you have more than 100 ppm CYA in your water the test will still show 100. That's what the "100+" means.

If you dilute your water by making a new mixture with half having your pool water (which has the 100+ ppm CYA) with half tap water (which has 0 ppm CYA) you end up with water that is a 50/50 mixture of the two so the amount of CYA in this mixture is the average between the "100+" unknown amount and 0.

So let's say you take this mixed (half diluted) sample to the pool store to be measured and now they say you have 70 ppm CYA. This means that your original undiluted pool water has double this amount, or 140 ppm CYA in it, because the average of 140 and 0 is 70 (that is, (140+0)/2 = 70).

Does that make sense?

Richard