Langelier Saturation Index (LSI) is an important hydrochemistry indicator that gives an overall health report based on the four critical components of your pool water chemistry.
The index is calculated on pH, temperature, calcium hardness and total alkalinity.
When the LSI is too low the pool water is corrosive, when too high the water is scale forming where you get clear or white rough calcium build-up on pool surfaces.
If your pool’s LSI is within the acceptable range of -0.3 to +0.3 your pool is in good health.
An LSI reading of 0.0 is the ideal result for swimming pool water.
Table of Contents
What is Langelier Saturation Index?
Langelier saturation index is a chemical indicator developed by the chemist Dr Wilfred Langelier.
In chemistry, LSI is the difference between the actual pH and the saturation pH.
LSI = pH – pHs
The LSI is calculated from pH, temperature, calcium hardness and total alkalinity.
However, it’s mostly affected by pH and total alkalinity levels.
LSI Index must be between -0.3 and +0.3 for Balanced Water
An LSI reading of 0.0 is the ideal result for swimming pool water.
LSI less than -0.3, water is corrosive
The pool water is unbalanced and may cause pitting, etching, dissolving and staining of pool surfaces, tile grout and plumbing pipes.
LSI greater than 0.3, water is scale forming
Scale and other deposits may form on the pool surfaces, equipment and inside pipes. Scale appears as rough, white deposits on the walls. If scale builds up inside the plumbing and filtration system it will reduce the water flow causing higher pressure and more strain on the pump motor. The filter will not perform effectively leaving your pool unclean.
How to Calculate Langelier Saturation Index (LSI)
If you take your water sample to a pool shop for analysis their software will usually calculate the LSI. It’s computed from indices of temperature, calcium and alkalinity using the following formula:
Langelier Saturation Index (LSI) = pH + Ti + Ci + Ai -12.1
Ti = temp index, 60F/16C = 0.4, 66F/19C = 0.5, 76F/24C = 0.6, 84F/29C = 0.7
Ci = calcium index, 100ppm = 1.6, 150ppm = 1.8, 200ppm = 1.9, 250ppm = 2.0, 300ppm = 2.1 etc.
Ai = alkalinity index, 75ppm = 1.9, 100ppm = 2.0, 125ppm = 2.1, 150ppm = 2.2, 200ppm = 2.3, 250ppm = 2.6
Example
pH = 7.6
Ti = 0.6 (76F or 24C)
Ci = 1.9 (200ppm)
Ai = 2.0 (100ppm)
LSI = 7.6 + 0.6 + 1.9 + 2.0 – 12.1 = 0.0
The pool water is neutral and ideal for swimming.
How to Decrease Langelier Saturation Index
Add muriatic acid to decrease LSI
Other factors that decrease LSI:
- Adding stabilizer (cyanuric acid)
- Increased total dissolved solids (TDS)
How to Increase Langelier Saturation Index
Add sodium bicarbonate (baking soda) to increase LSI
Other factors that increase LSI:
- Increases in temperature
- Total Alkalinity increase
- pH increase
- Increase Calcium Hardness
How to Measure pH in a Swimming Pool?
pH is a scale in chemistry to indicate the level of acidity or alkalinity of an aqueous (water-based) solution. The symbol pH means “power or potential of hydrogen”.
The pH scale is an inverse logarithmic scale indicating the concentration of hydrogen ions in solution. The lower the pH the higher the concentration of hydrogen ions and the more acid the solution. The higher the pH the lower the concentration of hydrogen ions and the more alkaline the solution.
Strong acid is pH 0
Pure water, at 25C, is neutral at pH 7.
Strong base (or alkaline) is pH 14.
However, pH fluctuates depending on temperature. Pure water at 0C is pH 7.4.
Sea water is normally between pH 7.5 and 8.4.
Swimming pool water should normally be between pH 7.2 and 7.6 which is slightly alkaline and never acidic.
Pool water must be at the correct pH to ensure the effective operation of your salt water chlorinator to produce adequate chlorine levels to kill algae and bacteria. The correct pH protects the pool surfaces and metal finishes from deterioration.
If you don’t have a saltwater pool you must administer chlorine by a floating chlorine dispenser or manually add it to the pool every day.
What is Total Alkalinity?
Total Alkalinity is the measure of the water’s acid neutralizing capacity which indicates its ability to buffer against changes in pH.
Alkalinity and pH are not the same thing but they correlate together.
pH is a scale in chemistry to indicate the level of acidity or alkalinity in a solution.
Swimming pool water should normally be between pH 7.2 and 7.8 which is slightly alkaline and never acidic.
The correct level of alkalinity depends on many other factors such as water hardness, pH and temperature
In the swimming pool industry, total alkalinity is a measure of the dissolved carbonates and bicarbonates in water in parts per million (ppm).
The Total Alkalinity in pool water should be between 80 – 150 ppm.
Please read our article on How to Lower the Pool Alkalinity Level
Low Alkalinity Level in Pool Water
A low total alkalinity can occur after heavy rainfall especially acid rain.
A low total alkalinity in pool water may lead to the following issues:
- Stains forming on the pool surfaces
- Pitting, etching or erosion of the pool surfaces
- Irritated eyes and itchy skin for swimmers
- Green water
- Rapid changes in pH
- Corrosion of metal equipment parts (ladder, vacuum pole etc.)
High Alkalinity Level in Pool Water
A high total alkalinity can occur when too much chemical, such as sodium bicarbonate or chlorine shock, has been added to the water too quickly. High total alkalinity can be caused by the water supply to your pool.
If your pool water test is indicating total alkalinity is too high you will need to lower alkalinity in your pool water. Otherwise, you will encounter the following problems:
- High and unstable pH level.
- High calcium level and scaling
- Cloudy water
- Chlorine becomes less effective resulting in algae growth
- Filter frequently clogging up
- Irritated eyes and itchy skin for swimmers
Testing for Total Alkalinity in Pool Water
Whenever you normally test your pool water you should test for total alkalinity.
I highly recommend using pool water test strips because they are simple to use, accurate and most include a test for total alkalinity.
What is Calcium Hardness
Calcium Hardness is the measure of all calcium compounds dissolved in the pool water. If you have a salt water chlorinated pool do not use calcium chloride to raise hardness or calcium hypochlorite (dry granular chlorine) to raise the free chlorine level as these chemicals will change the calcium hardness of the water.
The recommended level of calcium is as low as possible to minimize any problems with calcium type deposits forming on the pool surfaces or in the salt water chlorinator cell.
Calcium Hardness 200 – 400 ppm
Warning: Water sources from bores, dams and rivers can be high in mineral and organic contaminants resulting in faster mineral deposit formation and poor chlorine production. These water sources will require additional chemical treatment before the water is suitable for a swimming pool.
What is Scaling in a Pool?
We get asked this question all the time.
Scale is a rough, white, crystalline deposit that builds up on the walls, bottom, pool equipment and inside the plumbing and filtration system.
If scaling builds up inside the plumbing and filtration system it will restrict the water flow causing higher pressure and more strain on the pump’s electric motor. The filter will receive a weak water flow and not effectively clean the water leaving your pool dirty. If scale is in your plumbing you will need to call a pool professional or plumber to remove it with special pipe cleaning equipment.
Brushing your pool surfaces helps to remove or reduce the build-up, however, if your pool water is not balanced you won’t be able to control it.
Please read our article How to Clean Calcium Scale from a Pool
How to Add Chemicals to a Swimming Pool
Pool chemicals are dangerous and reactive. Please ensure that all pool chemicals are stored in a cool, dry location and kept out of the reach of children. Wear gloves and protective clothing when handling chemicals.
Always read the label on the product and follow the instructions for adding the chemicals to pool water. Some products are poured or spread directly into the pool. Other products recommend you add the chemical to a bucket of water first, mix it and spread around the pool. Make sure the bucket is clean and doesn’t contain any chemical residue as it may react with the product you are mixing in the bucket.
Always run the pump for a few hours after adding chemicals to pool water to ensure the product is thoroughly mixed in. The best time to add chemicals is right before the pump is due to operate. So look at the timer and add chemicals when the pump comes on.
NEVER MIX CHEMICALS TOGETHER
Many people have suffered chemical burns, explosive reactions or breathed in toxic chlorine and burnt their lungs when mixing pool chemicals. You should never mix pool chemicals together as they may react leading to fire, explosion or release of toxic chlorine gas. Even mixing dry chlorine with liquid chlorine may cause an explosion.
NEVER ADD WATER TO CHEMICALS, ALWAYS ADD CHEMICALS TO WATER.
You must never add water to chemicals because the chemical may react with the water causing an explosion or the release of toxic chlorine gas that has the potential to damage your lungs. Always add a small amount of the chemical to a large amount of water to minimize any unwanted reaction. For example, mix the chemical into a clean full bucket of water or pour the chemical directly into the pool.
When to Add Chemicals to a Pool
The best time to add chemicals to a pool is when your family has finished using the pool for the day. So late afternoon or early evening is the ideal time because you cannot swim in a pool when you have just added chemicals. Also, the pump takes time to mix in the chemicals.
Adding the chemicals in the evening prevents UV sunlight from inactivating the chemicals and ensures the chemicals have the longest amount of time to work before the UV light returns the next day. Adding chemicals, especially chlorine, in the middle of a hot summer day is least effective as it will evaporate quickly and become ineffective by UV radiation.
Ensure that your pump is going to operate for at least 3 or 4 hours after you have added the chemicals to mix and circulate the product thoroughly around the pool.
Please read The Ultimate Guide to Pool Care
Conclusion
Langelier Saturation Index (LSI) is a universal indicator that gives an overall health report on your pool. The index is based on the four critical components of water chemistry – pH, temperature, calcium hardness and total alkalinity. If your pool’s LSI is within the acceptable range of -0.3 to +0.3 your pool is in good health. An reading of 0.0 is the ideal result for swimming pool water.