Sodium thiosulfate stays in the water until it has fully reacted with the chlorine. This means you can actually get into a situation where you have to add chlorine to reach 0 ppm Cl.
I once had a pool that had to superchlorinate and then used thiosulfate to decrease the Cl levels. They were a couple days away from their chemical supplier. They overdid it, and had to buy their entire small community's supply of household bleach. After adding 70 gallons of bleach, they finally reached 0 ppm.
Sodium thiosulfate should be used carefully. Calculate the addition several times to make usre you have the right amount. Add half, or less, of the calculated amount, mix and retest. If necessary repeat. Approach your desired Cl level slowly and carefully.
"make usre you have the right amount. Add half, or less, of the calculated amount,"
Words to live by. Any time I'm making additions to an new pool or one I haven't made additions to in a long time I figure out how much I need and add half. Once that's mixed in I check to see where that got me and if it's not half the desired effect I do the calculations to see what the pool volume really is and go from there. I've saved my butt several times over the years doing this thanks to published volumes that were just wrong.
Agreed. One of our maintenance people took it upon himself to add sodium thiosulfate the other day when the levels were high. He added 16 oz, which should be more than enough in our therapy pool, and then retested the pool 10 minutes later. Having not seen a significant change in 10 minutes, he added another 24! In our smaller pool this was disastrous. When the lifeguard arrived to open the pool a couple hours later there was no chlorine and little we could do to change it. It cost us an entire morning of open swim and lessons.
Definitely add half as much as you think you need and retest (after a reasonable amount of time). My usual shocking procedure is to shock it early in the morning or the night before, come in mid afternoon and add my first round of reducer, then come back in that night, retest and add reducer.