You’ve just tested the pool water with your test kit or strips and it’s indicating a low level of free chlorine. The pool water is clear and clean looking so what does free chlorine mean on a test strip? Read this article to learn how to control free chlorine in your pool.
Your pool water may still be clear but it won’t be for much longer if you don’t correct the chlorine level in your pool. The pool does not contain enough chlorine to sanitize the water and ensure it’s safe for swimming. If you have a salt water pool you need to check that the chlorinator is functioning correctly and generating sufficient free chlorine. Otherwise, you need to add more chlorine to the pool and check your maintenance routine.
Table of Contents
What is Chlorine?
Chlorine (Cl) is a fundamental chemical element of nature. At room temperature (25C), pure Chlorine is a pale yellow-green gas that is extremely reactive, poisonous and toxic.
However, it is not found as a free element in nature because it is too reactive and always combines with other elements. It is abundantly found in nature in combination with metals to form chloride salts, such as sodium chloride (NaCl) which is common table salt.
The highly reactive and oxidising characteristic of Chlorine led to the development of commercial bleaches and disinfectants to sanitize bathrooms, hospitals and other public areas.
Chlorine is commonly used to sanitize bacteria and microbes in water purification processes in water treatment plants.
Chlorine, in the form of sodium hypochlorite (NaClO), is used to sanitize swimming pool water. Sodium hypochlorite solution is unstable and rapidly decomposes releasing aqueous chlorine which kills bacteria, algae and other microbes in your pool water.
Sodium hypochlorite is strongly alkaline and corrosive so you need to protect your skin by wearing gloves when handling this chemical.
What Does Free Chlorine Mean on a Test Strip?
Free chlorine is simply the chlorine that is available in the water to kill bacteria, viruses and other contaminants. The free chlorine is either added manually, by an automatic dispenser or by way of salt water chlorination.
The recommended free chlorine level for a swimming pool is:
Free Chlorine 1.0 – 3.0 ppm
Combined Chlorine
Chlorine dissolved in your pool water kills bacteria and any other organic contaminants.
Once the free chlorine has done its job it becomes inactive and combined with other elements so it can no longer sanitize the pool water.
Total Chlorine
The total of free chlorine and combined chlorine in the pool water is total chlorine.
Total chlorine = free chlorine + combined chlorine
Stabilized Chlorine
Stabilized chlorine is simply chlorine with cyanuric acid added to it.
When you add plain chlorine, either manually, floater or salt water chlorinator, to an outdoor pool the UV light and heat from the sun will burn off the unstabilized chlorine quickly.
To prevent the chlorine from being burned off cyanuric acid or pool stabilizer is added to the pool water. This chemical binds to the chlorine preventing it evaporating and therefore stabilizes the chlorine making it last three times longer. The stabilized chlorine is much more effective at sanitizing the water especially in the hot summer period when you need it most.
Unstabilized Chlorine
Unstabilized chlorine is simply plain chlorine without added stabilizer.
If you use normal chlorine without cyanuric acid (stabilizer) the sun will evaporate the chlorine quickly and reduce its sanitizing capacity. You will have to add more chlorine or turn up the chlorinator and pump to generate more chlorine costing more money.
You have the choice of using unstabilized chlorine and adding cyanuric acid manually, or use stabilized chlorine that already has cyanuric acid mixed in.
Mostly, if you are manually adding chlorine to a pool you would want stabilized chlorine otherwise you have to add cyanuric acid separately. However, you have to be careful that you are not overdosing the pool with cyanuric acid (stabilizer) because this will reduce the effectiveness of the chlorine.
Most pool shock products are unstabilized chlorine because your pool would already be stabilized and you don’t want to add any more cyanuric acid when you shock the pool.
If you have an indoor swimming pool you will not have to worry about stabilizer because the pool is protected from the harmful UV light and heat of the sun. So only add unstabilized chlorine to an indoor pool.
How to Test for Free Chlorine in Your Pool
1. Pool Test Kits
The process is as follows:
- Take a water sample and place it the provided tube or vial.
- Add the prescribed number of drops of a chemical into the tube.
- Some kits require adding two chemicals to the tube.
- Put a lid on the tube & shake it up.
- Compare the color of the fluid to the test chart.
- Record your reading for free chlorine.
Now repeat the above test for pH, total alkalinity, stabilizer, calcium hardness, bromine and whatever else you need to test.
2. Pool Test Strips
The process is as follows:
- Take a test strip and dip it into the pool water for 5 seconds.
- Remove the strip and wait 10-20 seconds.
- Compare the colors on the strip to the label on the container.
- Record your readings
Most test strips test 4 or more tests at once.
The common swimming pool strips test:
Free Chlorine, pH, Total alkalinity & Stabilizer.
The common spa pool / hot tub strips test:
Free Bromine, pH, Total alkalinity & Calcium hardness.
There are other test strips available, such as:
Salinity
Phosphate
Baquacil
TDS
3. Take a Sample of Water to a Pool Shop
If you are lucky and there is a local pool shop nearby you can take a water sample to the pool shop for testing. In recent times, many pool shops have closed, converted to mobile businesses or gone online.
Pool shops are very expensive and you can source everything online for less money. As a result, I haven’t been to a pool shop in years.
Advantages of Pool Shop Test
- The result will be accurate
Disadvantages of Pool Shop Test
- Takes time.
- There may be a fee for a test
- The shop will try to sell you overpriced chemicals
- The shop will try to sell you other services
Pool Test Strips vs Pool Test Kits
I started out years ago with the old style pool test kit and these are still available and many people still use them.
It takes time to work through each test and record the results and you do need to record the results because you need to empty the tube after each test to perform the next test. The tests are more prone to error because if you count the number of drops incorrectly you get an incorrect result.
However, rather than using a test kit I recommend pool test strips. They are far more convenient, simple to use and the result will be as accurate as a test kit. Also you really need to test your pool water twice a week so strips are more convenient.
Pool Test Strip Advantages
- Very accurate if used correctly
- Easy to use
- Fast result (20 seconds maximum)
- Multiple results on the one test strip
- No need to record the test. You have a strip with the results.
- Specialized strips for outdoor pools, hot tubs, salinity, etc.
Pool Test Strip Disadvantages
- Strips have an expiry date but so do the chemicals in test kits.
- Touching the chemical pads will give an incorrect result.
What is Shocking Your Pool?
The term “shocking your pool” is pool industry slang language for super chlorinating your pool. All it means is giving your pool an extra dose of chlorine to ensure the pool is adequately sanitized by killing bacteria, viruses and algae that may exist in the water.
Please read our post on How to Shock Your Pool
What are Chloramines?
Chloramines are the chemical residue after chlorine has reacted and bound with dirt, sweat, sunscreen, urine, bird droppings, decaying insects, bacteria, algae and anything else that invades your pool water. If you pool water smells like bleach then you have too many chloramines in the water and it’s time to shock the water.
Chloramines build up in the water making it easier for bacteria and algae to multiply and take over the pool. Shocking the pool water (super chlorinating) eliminates chloramines and keeps your pool water sanitized.
Best Way to Shock Your Pool
Before you handle any chemicals make sure that you are using protective clothing and gloves. I recommend a mask and eye protection as well. Try not to add chemicals to your pool when there is a breeze as you might end up wearing the chemicals.
Most shock treatments are a powder or granular chemical that comes packaged in a one dose pack which is the correct size for the average backyard pool. Normally, you can spread the shock around over the pool surface. Carefully read the manufacturer’s instructions to ensure you are applying the shock correctly for your pool.
Test the water the next day to make sure the pool is still balanced, otherwise adjust levels as required.
Shock treatments can cause the pool water to look cloudy for a few days. This is normal and will dissipate over the next few days as the free chlorine reacts to other compounds in the pool.
Which Shock Treatment to Control Free Chlorine in Your Pool?
The most widely used shock treatment is calcium hypochlorite.
If your calcium level is already high don’t use shock containing calcium hypochlorite as this will make it worse. Use lithium hypochlorite or sodium hypochlorite instead.
Some products can be directly added to the pool water whereas others are best mixed into a bucket of water first. Always follow the instructions on the packet.
Final Thoughts
You must have an adequate level of free chlorine in your pool. Chlorine is the chemical that sanitizes the pool and keeps you safe from bacteria and other organic contaminants. If the chlorine level drops the pool water will turn green and be unsafe for swimming.
Testing for free chlorine in your pool water is probably the most important test that you need to perform. You need to test frequently, I recommend twice weekly because chlorine levels can drop suddenly especially in the heat of summer when it’s needed most. You need to check and maintain your chlorinator regularly too.
