What is a salt water swimming pool?
Swimming pool chlorine generators (also referred to as salt water pool chlorinators) create chlorine in the pool water directly, removing the need for users to manually add chlorine- based chemicals to the swimming pool. Free available chlorine is continuously produced at a level which is automatically controlled. This also has the benefit of removing the need for storage and transport of chlorine-based chemicals.
However, as with any equipment, the chlorine generator must be maintained regularly to prevent fouling.
Most chlorine generators require a salt content of 1,500 – 6,000 parts per million ("ppm") as sodium chloride (NaCl) in the swimming pool. In comparison, sea water has a salt content of around 35,000 ppm and humans have a salt taste threshold of around 3,500 ppm.
There are two types of salt water systems, one ‘in-line’ and one ‘off-line’.
In-line Chlorine generators
Sodium chloride (or sometimes magnesium chloride) are added to the pool and used to create salt water. The water passes through an electrolytic cell installed in the swimming pool's circulation system which is subject to low voltage electrical current between a pair of cells with opposing charges and converts chloride into chlorine by electrolysis.
Hydrogen gas, chlorine gas and a solution of sodium hydroxide are also generated.
Off-line Chlorine generators
Sodium chloride is added to a bulk storage tank to produce a saltwater brine solution which in turn is fed into an enclosed electrolytic cell. The liquid chlorine that the cell produces is then injected into pool water as needed. Sodium hydroxide and hydrogen gas are generated and removed from the system and don’t get added to the pool water.
Salt Water Pool Maintenance
- Salt Concentration or Salinity - An efficient salt water chlorine generator requires salinity to be maintained at a constant level as defined by the manufacturer (typically 1500 - 4500 ppm). It is often recommended to test salinity in the pool weekly. Most salt chlorinators will display a salt reading. However, it is a good practice to test the salt in case the system needs to be recalibrated. If the salt concentration is too high, it can cause damage to the cell and other pool hardware. This can cause serious issues and the only way to adjust the salinity down is to add fresh water. If the salt levels are low, it could also lead to an unsanitary swimming environment because there won't be enough chlorine produced.
- Chlorine levels - The free available and total chlorine levels should be monitored and tested frequently. If free available chlorine levels are low, the salinity levels may not be sufficient and the generator performance should be checked. Low levels of free chlorine can also be caused by calcium hardness build up on the salt cell, an expired cell or inadequate pool circulation.
- pH - As with non-salt water pools, maintaining an appropriate pH level is important for enabling the chlorine to work effectively and remove germs and bacteria.
- Total Alkalinity - Total alkalinity is important to keeping the pH stabilised and prevent ‘pH bounce’. Low alkalinity can cause corrosion, staining or green water while high alkalinity can cause cloudy water and fluctuating pH.
- Stabiliser or Cyanuric Acid - Stabilisers are used for outdoor pools as UV light breaks down the chlorine residual. An outdoor salt water pool requires higher levels of cyanuric acid than a regular pool and should be in the range of 60 - 80 ppm. This is to ensure the chlorine that is generated by the cell is not immediately degraded by UV from sunlight.
- Calcium Hardness - If calcium levels are too low, the water can become corrosive and can leach calcium from plaster, stone and concrete surfaces in contact with the pool water. High chloride levels such as that seen in saline pools have been associated with a higher solubility of calcium carbonate. High levels of calcium can result in scale formation or a clogged filter and pipes resulting in poor filtration and circulation that can lead to cloudy water and irritation to swimmers.
- Total Dissolved Solids (TDS) - The Total Dissolved Solids for a salt water pool will be higher than a regular pool because of the way TDS is measured. In salt pools, a TDS value of 4000ppm is more common and this must be measured in order to ensure the Langelier Water Balance Calculation can be carried out accurately to prevent scaling and corrosion.
- Water Balance - To check the overall chemistry in the salt water pool, the Langelier Saturation Index (LSI) should be tested. This formula uses TDS, total alkalinity, calcium hardness, water temperature and pH to help control the risks of scaling or corrosion. It is common to calculate the LSI and subtract 0.1 from the answer in saline pools, this is to take account of the corrosiveness of the high chloride level.
Measuring Conductivity, TDS and Salinity
Most chlorine generators have sensors built in that will monitor and display the salinity reading based on the conductivity of the pool water. Portable salinity meters work in the same way.
- Conductivity and TDS - The more dissolved ionic solids a body of water contains, the more conductive the water becomes. Therefore, by measuring the conductivity of a body of water, it is possible to estimate the total dissolved solids content of that water. The conversion of conductivity into TDS is not the same for all water sources as different kinds of salts conduct differently. If the exact salt composition and concentration of a sample is known, then a highly accurate TDS factor can be used. However, in most cases, the exact composition is not known. Therefore, a set of “standard” TDS factors were devised based on typical compositions.
0.67 is used for normal swimming pools and 0.55 is used for salt pools. These standard factors are not a perfect solution but are considered to be a tolerable compromise to allow for quicker measurement. It is important to make sure the right TDS factor is chosen for the body of water being measured in order to obtain accurate results.
- Salinity - Salinity is related to TDS but focusses only on the sodium chloride aspect. It is also estimated from conductivity and the conversion factor is based upon oceanographic research tables. Concentration is the amount (by weight) of salt in water and is usually expressed in parts per million (ppm) or parts per thousand (ppt) but sometimes as a percentage (%).
1% is 10000 ppm
1 part per thousand (ppt) is 1000 ppm
1 ppm = 1 mg/l
Salinity range: 0.0 to 10.00 ppt is equivalent to 0.0 to 10,000 mg/l (ppm) or 0 – 1%
Palintest Pocket Sensors
The Palintest Pocket Sensors are the ideal solution for accurate testing in a saline pool.
- The Multiparameter Pocket Sensor enables up to 5 measurements (pH, conductivity, TDS, salinity and temperature) to be made simultaneously.
- ATC: Automatic Temperature Compensation with temperature displayed on screen.
What is ATC?
All pH and conductivity measurements must take account of the temperature of the sample. Using a sensor with ATC measures and compensates the instrument response for the sample temperature automatically, removing complication for the operator.
- TDS calibration solutions
Selecting the correct calibration solution is crucial to ensuring accurate results with your Palintest Pocket Sensor. Palintest offers different types of standards for TDS measurements in saline pools. For greater accuracy across the entire test range, it is recommended that all three calibration solutions are used