How to Know When to Add Chlorine to Your Pool or Spa,…and How Much

There is a formula for anything that you need to accomplish with regards to swimming pools, spas and hot tubs. Whether it’s a chemical adjustment or a piece of equipment that you need to replace and you want to be sure it’s the right size.

This article, as is stated in the title, is about when to add chlorine and how much you should be using. The formula we will be using is the same one that is used to calculate any chemical change in your pool or spa but we are looking at chlorine separately because it serves a different purpose in your pool than the chemicals, such as acid, do. It is used for sanitation and oxidation of organic matter.  

Formula:

Amount of Chlorine (oz, lb, fl. oz, gal) x Pool Volume (gal) / 10 x Adjustment (PPM) / 1 = Total Amount needed

To get the first value, you must decide which type of chlorine you will be using and at which concentration levels.

 Each has it’s own number to raise Free Chlorine (FC) by 1PPM, as follows:

Calcium Hypochlorite (Cal Hypo) 65% AC = 2.0 oz

Cal Hypo 70% AC = 1.9 oz

Cal Hypo 75% AC = 1.8 oz

Sodium Hypochlorite (Liquid Chlorine)

12% AC = 10.7 fl oz

10% AC = 12.8 fl oz

Sodium Dichloro-s-triazinetrione (Dichlor)

56% AC = 2.38 oz

62% AC = 2.15 oz

Trichloro-s-triazinetrione (Trichlor)

90% AC = 1.5 oz

Lithium Hypochlorite (Lithium Hypo)

28% AC = 4.7 oz

35% AC = 3.8 oz

Chlorine Gas

100% AC = 1.3oz

For our example, we’ll just use liquid chlorine, or bleach at an AC level of 12%. Let’s say we’re standing over our 40,000 gal pool which has a chlorine reading of 1.0 PPM and we would like that number to be a 3 PPM.

Ex: 10.7 fl oz x 4 x 2 = 85.6 fl oz of liquid chlorine would be needed.

There are times when you will need to Super-chlorinate or Shock your pool or spa water. To calculate how much chlorine you will need to reach, what is called, “Breakpoint Chlorination”, you will need to measure, both, Free and Total Available Chlorine (FAC and TAC). If these numbers are not within 0.3 of each other, you need to shock.

Formula:

Step 1. TAC – FAC = Combined Active Chlorine (CAC)

Step 2. CAC x 10 = Adjustment

Step 3. Use the Chemical Adjustment Formula

Ex: Same pool as above but the FAC = 2.0 and TAC = 4.0

4.0 – 2.0 = 2.0

2.0 x 10 = 20.0

10.7 x 4 x 20 = 856 fl oz or 6.69 gallons of chlorine. Obviously, we can just use 7 gallons.

Article by: Jef Kirchmaier, CPO, CPT, AEA, WQM

                 

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