Free Chlorine
Free chlorine is usually measured with the DPD1 test. It indicates how much of the total chlorine in the pool has not yet reacted with any pollution (i.e., combined) and is therefore free and available to carry out its purpose as a disinfectant.

There should always be enough free chlorine in the pool to minimise the risk of cross-contamination. The recommended range is 1.00 – 3.00mg/l for most types of pool, but spa pools, because of their increased risk of legionella contamination, have a higher recommended range of 3.00 – 5.00mg/l.

Combined Chlorine
When the disinfectant gets into the pool water, the free chlorine contained within in immediately gets to work and starts combining with pollution. Once chlorine combines it hangs around in the pool water and is no longer effective as a disinfectant and is now actually more of a pollutant itself. It needs to be removed from the pool by a combination of dilution and filtration.
Combined chlorine is measured by calculating the difference between the total chlorine and the free chlorine.

Free Chlorine (DPD1) + Combined Chlorine = Total Chlorine (DPD3)

Much of the chemical pollution in swimming pools is in the firm of ammonia, which is a decomposition by-product of urea (which comes from sweat and urine etc. from bathers). This ammonia reacts with chlorine to form what are known as ‘chloramines’. These are measured as combined chlorine by subtracting the free chlorine reading from the total chlorine reading.

Combined chlorine levels should be kept as low as possible, and certainly no more than 50% of the free chlorine level. 

There are four main categories of chloramines to be aware of:

• Monochloramine
• Dichloramine
• Trichloramines
• Organic Chloramines

Monochloramine is one of the chloramines which contribute to the level of combined chlorine in the water. It is produced when chlorine reacts with ammonia. In simple terms, the reaction is:

Chlorine + Ammonia > Monochloramine

Monochloramine isn't really that much of a problem, in fact, it acts as a disinfectant itself, although it is nowhere near as effective as free chlorine. Things don't stop there though and further chemical reactions will take place to produce dichloramine and trichloramine (these are the chloramines that are the cause of problems and a pool plant operator needs to know how to get rid of them and minimise their production in the first place).

Dichloramine is one of the chloramines that contribute to combined chlorine levels in the pool water. It is the second stage of the chemical reaction that takes place between chlorine and ammonia. It is produced when chlorine (or, to be more specific; hypochlorous acid) reacts with monochloramine (which are produced during the first stage of the reaction):

Hypochlorous Acid + Monochloramine > Dichloramine

You don't want high levels of dichloramine in your pool as it can go on to form further chemical reaction by-products such as trichloramine. It's an unstable chemical though and as long as your pH is at the correct level it will break down fairly easily. At this point, your combined chlorine readings will reduce because there are no chloramines left to react with. This is known as 'breakpoint chlorination'.

Typically, in practice, you won’t know how much monochloramine and dichloramine you have in your pool as normal testing procedures don't distinguish between the two. As long as you keep combined chlorine levels under control (i.e., less than half of the free chlorine), you won’t really need to know, but if you start having difficulties keeping combined chlorine levels low enough, you may need to carry out a DPD2 test in order to find out which of the chloramines is contributing most to the combined chlorine levels.