There are two reasons why you would need to add more salt to your pool. If you have drained the pool completely/partially OR your chlorinator control box indicates your levels are low.
The salt level required to maintain a safe, chlorinated pool is about 2500 to 4000 PPM (parts per million). The human body cannot taste salt until the PPM is around 5000. The amount of salt in the pool is not noticeable. Once the salt is added to the pool, there is no need to add salt on a yearly basis unless the pool is drained or a significant amount of water is removed. In order to determine the amount of salt to add to a pool you must know how many gallons of water your pool/spa holds.
Plug in your dimensions to the following equations:
Gallons = area x average depth x 7.5
Average depth = (shallow end + deep end) / 2
Example: Pool is 50 feet long x 24 feet wide. The shallow end is 3.5 feet and the deep end is 8.5 feet.
Average depth = (3.5 feet + 8.5 feet) / 2 = 6 feet
Gallons = (50 feet x 24 feet) x 6 feet x 7.5
Gallons = 54,000 gallons
Where do I add the salt?
What is most important when adding salt is to brush it around until it is dissolved. This is accelerated by turning your pump on, opening the bottom drain and adding the salt over the drain, rather than walking around the perimeter while adding the salt. It is recommended that you continue to run the pump for 24 hours so the salt can spread evenly throughout the pool. With Granular salt, 60 – 70% will have dissolved before hitting bottom. The remaining salt can simply be brushed into the drain which will then complete dissolution. With all other forms of salt, it will take longer to dissolve but the same process will accelerate the dissolve rate. Simply brush the salt in a pyramid over the drain to increase dissolution.
How much salt do I add?
Enough for 4000 ppm (parts per million) as a starting point. So depending on the initial salt level of your water, you only add the amount needed to establish 4000 ppm. For new pools or freshly filled pools, the salt level will most likely be zero. In this case, 50lbs of salt per 2,000 gallons of water will establish 4000 ppm. For existing pools, the previous usage of chlorine bleach or tablets will have already introduced a level of salt into the water. Have the water tested first then add the appropriate amount to establish 4000 ppm.
What happens if I add too much salt?
Over-salting will not harm your chlorine generating system, but will lead to a salty tasting water. For some, this is not undesirable as it will more closely match our bodies natural salinity level, making it more comfortable to swim in. If it is too excessive (over 6000 ppm), you can sustain corrosion damage to metallic equipment such as stainless steel handrails, ladders and filters, light rings, or copper heat exchangers. To reduce the salt level, dilution is the solution. Drain some water and refill with fresh water.
How often will I need to add salt?
After the initial dosage of salt, you will only need to add salt when necessary. The most common ways salt is lost is through leaks, rainwater overflow, filter backwashing, and bather splash-out/carryout. Normal water evaporation does not lose salt, it increases the concentration. The make up water added to bring the water level back to normal will then reduce the salt concentration back to 4000 ppm. Most chlorine generator units have low salt indicators, with the Digital going further to provide the proper salt amount needed to reestablish 4000 ppm.
Vaughn Berger (Owner, Red Square Pools)