I know this is a bit dated, but i happened to see this thread, and no one really responded with any solid answers. One guy was giving some crazy numbers, so i felt it pertinent to weigh in here. OP, you were on the right track in the first place. What follows is how the calculations should go for pool shock @65% calcium hypochlorite :
1ppm=1mg/L
Target of 5 ppm = 5mg/L
Pool shock @ 65% calcium hypochlorite is 60% free chlorine if you read the fine print on the bag, so 1mg shock actually contains 0.6 mg free chlorine, which means you have to add 40% more shock to make up for the difference.
5mg / 0.6 = 8.33 mg shock to achieve 5 ppm/L
100L res x 8.33mg = 833.33mg
833.33mg/1000mg per g = 0.83g
You can also do it this way:
5mg x 0.6 = 3mg
5mg + 3mg = 8mg/L to acheive approx 5ppm
8mg/L x 100L = 800mg
800mg / 1000mg per g = 0.8g
You technically get the same answer, but doing it the first way is more accurate because it gives you those extra decimal places. This is due to the fact that the extra shock you have to add is also only 60%, and doing it the first way takes that into account and adds the difference to the total. The second way is often easier to do in your head but is less accurate.
So 0.83g shock in 100L water will give you 5ppm free chlorine by the numbers. If your scale is only accurate to 1 gram, add shock until it just barely registers 1g and thats plenty accurate enough. Just dissolve it in a cup of warm water and dump it in at res fill or whenever you need some defunking.
Keep in mind that tap water already contains 1-4 ppm chlorine depending on your water source, so your final chlorine level will actually be higher than 5ppm. We are simply adding 5 ppm to whatever came out of the tap by this method. This fact is often overlooked in these discussions.
There is a lot of different info available out there for this, but i would avoid looking at cannabis oriented sites if you want accurate info honestly. As you can see, you will get a lot of different, sometimes vague, sometimes nonsensical answers, and respondents will likely argue with eachother about what the correct answer is even though they're both wrong. Google ppm calculations, dilution calculations, batch proportioning etc. Also calcium hypochlorite for horticulture or pool shock calculations for hydroponics, or chlorine water disinfection calculations. Google chlorine use in horticulture or greenhouses if you want to settle your mind about what is or isnt a safe level. Studies have found that damage commonly attributed to chlorine was usually caused by other factors, and they have also found that container plants can tolerate chlorine levels as high as 150ppm. Additionally they found some strains of pythium required 20ppm for 15 minutes contact time in order to be killed.
Any other questions feel free to hit back or pm.
will run 3ppm and go from there.