Salt chlorinators are probably the easiest and most popular product for swimming pools. This makes perfect sense after all why manually dose the pool every day when the salt chlorinator can do it for you.

However there is a tendency to oversize and undersize chlorinators which can lead to both operating the chlorinator cell at very low output in which case you feel like you have paid too much or the chlorine level in your pool dropping away in peak use or full sun and the like.

There is a formula I have used for many years and it is very simple to apply to almost any pool. Using the formula the amp draw of the cell can be calculated and once the amp draw is known the manufacturers data can be used to correctly size the cell.

The formula works by applying the following data:

PV = Pool Volume in cubic metres

R = Free available chlorine residual you are targeting

T = time the filter and chlorinator will be running in hours

The formula looks like this:

R * PV = cell amps

T

Or in English residual multiplied by cubic metres all divided by running time.

Lets work through some examples to show you how it works. Well start with an average backyard pool of say 50 000 litres where the targeted residual free available chlorine level is 3 mg/l and the filter system normally runs for 8 hours a day.

PV = 50 cubic metres

R = 3

T = 8

So applying our formula we get

3 * 50 = 150 / 8 = 18.75

So the correct sized chlorinator cell for this pool would be 18.75 amps.

As another example work through this, the pool volume is 120 000 litres, the targeted free available chlorine level is 2 mg/l and the pump and filter run time is 12 hours.

PV = 120 cubic metres

R = 2

T = 12

So again applying our formula we get

2 * 120 = 240 / 12 = 20

So the correct size cell for this pool is 20 amps.

In reality both these pools chlorinator cells would probably be the same cell as manufacturer tolerances would probably mean there is a cell drawing around 20 amps.

Even though this formula works and works all the time you still need to add cyanuric acid or stabiliser as it is often called and run the pump filter system whenever the pool is in use.

Where chlorinators come in a one size fits all set up it is even more important to understand and apply the formula. By working the formula backwards you can get the run time for the chlorinator. This formula looks like:

R * PV = T

cell amps

Using our examples from above see below some equation run throughs.

PV = 50 cubic metres

R = 3

cell amps=18.75

So applying our formula we get

3 * 50 = 150 / 18.75 = 8

So to maintain a residual of 3 mg/l using a cell that draws 18.75 amps at 12 volts the calculated run time for the plant is 8 hours.

PV = 120 cubic metres

R = 2

cell amps = 20

So again applying our formula we get

2 * 120 = 240 / 20 = 12

And similar in this pool using our formula in reverse to get 2 mg/l or ppm using a 20 amp cell the run time would be 12 hours.

One common error a pool owner is likely to make is to use the dial control to reduce the chlorine output by reducing the current going to the cell. Sure I know manufacturers add this control for this purpose, however a very common occurrence is for this to be turned down when the pool is first charged with salt and slowly turned up over time as the salt depletes. This practice is not the best idea. By doing this as the salt level falls the salt cell is being asked to increase its chlorine output a sort of two way pincer.

A better practice is to adjust the salt chlorinator run time to the calculated run time and run the salt chlorinator for that time at 80% or thereabouts, and top up the salt if the chlorine level is found to be dropping.

Monitoring the salt level in the pool is critical to the longevity of the cell. Combined with correctly sizing the salt cell and adequate pool maintenance the cell should last a reasonable period before requiring replacement.

In this article I have shown how to calculate the correct salt cell size for your pool. I have demonstrated applying this formula in reverse to obtain a correct operational run time for a given chlorine residual and chatted generally about salt levels and stabiliser or cyanuric acid levels. If you want to find out more information on this or any other pool equipment topic you can use www.poolindustrysecrets.com site. For those people who use non metric systems here is a quick conversion guide.

1 cubic metre = 1000 litres

1000 litres = 264 us gallons

1000 litres = 219 uk gallons

ppm = mg/l

1000 us gallons = 3785 litres

1000 uk gallons = 4546 litres