The most important chemical when maintaining a swimming pool is chlorine. Maintaining the correct level of chlorine in pool water will kill almost all bacteria and viruses introduced into the water by bathers within a few seconds. As a result, the water will remain clear and safe to swim in.
Chlorine is the most commonly used sanitiser in swimming pools and it comes in many forms. The most common types used in pools are:
|Trade Name||Chemical Name||Strength (Available Chlorine %)|
|Stabilised Chlorine Granules or “Dichlor”||Sodium Dichloro-iso-cyanurate||56%|
|Stabilised Chlorine Tablets or “Tritabs”||Trichioroiso cyanuric acid||90%|
|Shock Chlorine Granules or “Shock”||Calcium Hypochlorite||65 – 70%|
|Unstabilised Chlorine Tablets||Calcium Hypochlorite||65 – 70%|
|Liquid Chlorine or “Liquid Shock”||Sodium Hypochlorite||11 – 15%|
Chlorine is also generated electrolytically from salt water. The type of chlorine you buy makes very little difference. The key point which people do not always appreciate is that once the chlorine is dissolved in water it is all the same. Any form of chlorine, when added to water produces hypochlorous acid (HOCl) and it is the hypochlorous acid that kills bacteria, viruses algae in the pool water. This can be referred to as ‘Free Chlorine’.
Free chlorine is used up by:
- Pollution from sweat and all the other dirt that washes off bathers – this forms chloramines which cause irritation to bathers and the classic chlorine smell onindoor pools.
- Contaminants getting into the pool such as leaves, dust, dirt and even rainwater.
- Sunlight – this breaks up free chlorine in the water in outdoor pools.
So the more people that swim in your pool and the sunnier the weather, the more chlorine your pool requires.
Recommended free chlorine levels
Public indoor pools in the UK aim to keep their free chlorine level between 1.0 and 1.5, however they have sophisticated chemical dosing and control systems. It is normal to keep the free chlorine level in domestic pools (especially outdoor pools) a little higher to give a better margin of safety. Please see the table below for recommended chlorine levels for pools and spas.
|Public Pools with chemical controllers||1.0 – 1.5 ppm|
|Indoor domestic pools||1.0 – 2.0 ppm|
|Outdoor pools||2.0 – 3.0 ppm|
|Hot Tubs and Spas||2.0 – 3.0 ppm|
Testing for chlorine
Chlorine is the most important chemical to control and is at least 10 times more important than any other test. Therefore it is vital that you test your chlorine level on a regular basis. You can test for chlorine using DPD1 tablets and a chlorine colour comparator or chlorine test strips.
- When the chlorine level is very high it can bleach the DPD1 tablet causing you to get a false low reading (see our troubleshooting blog for more information). Usually the water near the tablet goes pink for a short time and then the colour disappears leaving a slight yellow tinge to the sample.
- This applies to tests on photometers as well as comparators.
- If you suspect the chlorine level is high carry out a test using a mixture of 1 part pool water and 4 parts tap (or distilled if you have it) water. Multiply the answer by 5.
How do we increase chlorine level in a domestic pool?
To increase free chlorine quickly
NEVER MIX STABILISED AND UNSTABILISED CHLORINE IN THE SAME CONTAINER OR SKIMMER AS THEY CAN EXPLODE
- Add the granules to a bucket of pool water and then add the solution to a deck level drain, strainer basket or direct to the pool.
- Look at our dosing calculator to work out how much to add to your pool.
- Be patient – always give it time to mix fully.
To increase free chlorine slowly
To increase chlorine slowly put stabilised chlorine tablets (normally one or two 200gram tablets in a 10,000 gallon pool) in the skimmer or a floating dispenser.
What is the maximum level of chlorine it is safe to swim in?
- The pool dosing should be turned off if the free chlorine level reaches 5 ppm.
- The pool should be cleared of bathers if the free chlorine level reaches 10 ppm.
How do we decrease chlorine level?
By adding sodium thiosulphate solution. This must be done extremely carefully and will be dealt with in a further blog