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Ban The Hach meter and many others work in the same way and that is that when chlorine reacts with ammonia it initially forms 'combined chlorine' compounds such monochloramine. As more chlorine is added these are destroyed which is the breakpoint. After this the chlorine residual measured is the free chlorine residual. So if you use the tester with reagents for free chlorine you will only find this once you have passed the breakpoint.

Using total chlorine reagents will show the total of free and combined residuals and the difference between this value and the free reading is the combined chlorine. This is sometimes used where ammonia is added after free chlorine disinfection because whilst the chloramines are not powerful disinfectants they are long lasting and so after free chlorine rapid disinfection creating chloramines can protect water in long distribution systems. The Hach colourimeter is widely used and I would recommend it as long as spares standards and reagents are readily available