1. Shock Qty Help

Looking at the numbers in Pool School it looks like there is a big difference between what is classified as a shock based on CYA and what the chem makers say a bag can do.

For example in Intheswim it says I can use 1bag per 10,000 gallons, thus for my pool I need 2 bags.

With 2 bags its a 7.7FC raise for my 20k, pool calculator/cya/FC says I need 58oz for my 14FC raise (50CYA) --- 3.5bags.

I figured out their math only works with a CYA of 20. Am I just wasting money if I dont shock to the listed level in Pool School? Across a summer that adds up a lot 2bags vs 3.5bags each time. Any risks shocking at a lower level?

2. Re: Shock Qty Help

Welcome to TFP!

The short answer is that by following the principles in the Pool School, you don't need to shock your pool regularly so you'll save money that way. Basically, if you maintain an appropriate FC level for your CYA level, you are continually shocking your pool (that is, getting rid of the ammonia/urea from sweat/urine). You only need to shock your pool if you're fighting algae or if there is unusual chlorine demand or measurable Combined Chlorine (more than one drop in the test to clear it), or there is some sort of accident (dead animal, fecal release, etc.).

Also, the shock levels in the chlorine/CYA table are for quickly getting rid of algae. The amount needed for other purposes may not be as high, but for simplicity just using the shock levels in the table is easiest. Since you won't be doing it often, it should be economical. I haven't had to shock my pool even once this season.

Richard

3. Re: Shock Qty Help

Are you talking about the bags of shock that contain CYA, also?
Be very careful with those, since you up your CYA every time you use them, thus upping the amount of chlorine needed to kill algea.
Use bleach only to fight stuff in your pool.
Use up "Shock bags" if you need to raise CYA levels anyway. Just like the pucks.

4. Re: Shock Qty Help

Our recommendations are designed to be trouble free. If you do what we suggest, it will reliably solve the problem. Chemical manufacturers tend to give advice that, while it may work 50% of the time, will tend to bring you back to buy more chemicals. It is true that smaller quantities of chlorine will sometimes kill all of the algae, but when it doesn't work the algae comes right back and any chlorine used up to that point is wasted.

5. Re: Shock Qty Help

Bottom line is this, chemical manufacturers want you buying a bag or two of 'shock' weekly and using it. This is entirely unnecessary if you are testing your water. If your CC is .5 ppm or higher then shock. If your water is cloudy then shock. If you have had heavy rains then shock. If you see algae then shock. If the water is clear and there is less that .5 ppm CC then you don't need to shock.

Also realize that shock is not a special product. It is just chlorine. You can shock with ANY unstabilized chlorine. Liquid chlorine (or bleach) is usually your cheapest and has the fewest side effects. What is normally sold as 'shock' is calcium hypochorite. It will cause your calcium levels to rise over time. It also comes in different strengths, the most common these days being the weak 48%. A 1 lb bag will raise 10000 gallons water approx 5 ppm FC, hardly 'shock' level!. It can also be found in stronger strengths. If you are going to use it you want to look for 68-72% calcium hypochlorite. One lb will raise 10000 gal 10 ppm FC so it's essentially twice as strong as the weaker stuff that is commonly found. there is also 57-62% cal hypo which will raise 7,500 gal about 10 ppm FC.

There is a reason why the weaker stuff is most common and it's not what you probably think. 68% and above cal hypo is classified as a class III oxidizer and has been responsible for several warehouse fires in the industry (including such places as Walmart!). The weaker stuff is classified as a class II oxidizer and is 'safer' to store and has fewer restrictions with shipping and storage. This directly translates at higher prices and less performance from the product for the consumer since you are paying for and putting more inert ingredients into your pool.

My main concern with your pool is your low CYA levels. Are you adding any CYA to raise it? What is your main form of chlorination? Are you using trichlor or an unstabilized chlorine.
If you are using trichlor do not pass go, do not collect \$200, and return to Pool School and re read the entire section!

6. Re: Shock Qty Help

Originally Posted by waterbear
My main concern with your pool is your low CYA levels. Are you adding any CYA to raise it?
I don't think he was saying that his CYA level was 20 ppm but rather that the FC level attained from the amount of chlorine recommended (about 8 ppm) for shocking (from instructions on the bag) only corresponded to a CYA level of 20 ppm on the chlorine/CYA chart.

7. Re: Shock Qty Help

Originally Posted by chem geek
Originally Posted by waterbear
My main concern with your pool is your low CYA levels. Are you adding any CYA to raise it?
I don't think he was saying that his CYA level was 20 ppm but rather that the FC level attained from the amount of chlorine recommended (about 8 ppm) for shocking (from instructions on the bag) only corresponded to a CYA level of 20 ppm on the chlorine/CYA chart.