The only way to measure overnight loss accurately, except at very low chlorine levels, is to use an FAS-DPD test kit, like the K2006. I've put links below.
So, for you the only way to check is to let the chlorine levels drop enough so that you can check and see if you are holding chlorine overnight. However, I'd recommend that if you are NOT changing your sand right away, you simply assume that you are still losing chlorine -- you probably are, in your filter if nowhere else -- and keep your chlorine high over the next 2 week, if not quite as high as you have done.
I'd recommend adding fairly large doses IN THE EVENING, and VIA THE SKIMMER. That will let the chlorine work at night, and also will help clean the filter.
Will you still need to replace the sand? I don't know. You can try. I'd STRONGLY recommend check the sand when you close the pool, if not sooner, and making sure it's in GOOD condition before you open next year.
By the way, I've only begun to try to digest the data we've collected through the pool chart. So far we've mostly used it just for individual pool info. But, I've seen enough to realize that the correlation between "no K2006" and recurrent pool problems is even greater than we realized. And I realized we've also failed to make clear that this is not a kit you replace every year; rather you refill a couple of items. In between you can -- I even recommend -- that you use an OTO kit, like the HTH 6-way drops kit, or even something cheaper.
Ben
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+ Get a cheap OTO (yellow drops) / phenol test kit, or if available at YOUR Walmart (check availability), get the HTH 6-way DROPS test kit, which is compatible with the Taylor K2006. Test the pool as soon and you can, and post the results. If you get the 6-way kit, ALSO test the water you FILL the pool with, especially if it's a well, and post THOSE results as well. (The HTH is the best available kit you're likely to find locally, but it's not the K-2006. It can only provide rough measurements chlorine levels above 5 ppm, and it measures "TOTAL" hardness, rather than "CALCIUM" hardness, which is not ideal.)
+ Having a good test kit makes pool care easier for EVERYONE, but is an ESSENTIAL tool for pools with problems. A good test kit means a kit that can test chlorine from 0 - 25 ppm, pH, alkalinity, calcium hardness, and stabilizer with reasonable accuracy. Test strips (AKA 'guess-strips' ) do NOT meet this standard. Some pool store testing is accurate; most is not. The ONLY way you'll know whether your pool store is accurate or bogus, is by testing accurately your own self. On the other hand, pool store 'computer' dosing recommendations are NEVER trustworthy -- ignore them. They are designed to sell more chemicals than you need, and WILL cause many pool problems.
+ We recommend the Taylor K-2006 test kit, which meets the requirements above, for many reasons. The HTH 6-way drops kit is a great starter kit, and is compatible with the K2006 (it's made by Taylor). There are a few alternatives; for example Lamotte makes an FAS-DPD kit that's OK -- but it costs 3x as much. But, we're not aware of any test that is better, and since we are all familiar with the K-2006 (and can help you with it) we recommend it exclusively ( Test kit info page )
One caution for the 2012 season: Amazon does not stock the kits directly. So when buying at Amazon, Amato is our current preferred seller. However, they often don't list enough stock to last the whole day, so try order mid-morning. You should expect a delivered cost under $60 for the K2006A and under $95 for the K2006C. If you can't find that, wait a day.
+ Here are links to the kits we recommend (you can check local availability on the HTH kit, using the Walmart link):HTH 6-Way Test Kit @ Walmart
Taylor K2006A (3/4 oz bottles) @ Amazon
Taylor K2006C (2 oz bottles) @ Amazon
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