A testing device for testing the level of a selected chemical, for example a corrosion inhibitor, in central heating system water in a central heating system circuit, the device comprising: a sample chamber for holding a sample of central heating system water to be tested, the sample chamber being connectable to the central heating system circuit to allow fluid to pass between the central heating system circuit and the sample chamber ; a piston for controlling filling of the sample chamber with central heating system water from the central heating system circuit, and emptying of the sample chamber , the piston being movable to a retracted position for drawing water out of the central heating system circuit into the sample chamber , and to an extended position for emptying the sample chamber and returning the water to the central heating system circuit; at least one valve for isolating the sample of central heating system water from the central heating system circuit during testing; and optical testing apparatus including a light source and a detector , for measuring an optical property of the sample of central heating system water isolated within the sample chamber , and the testing device being adapted to thereby make a determination as to whether or not the level of the selected chemical in the water is greater than a predetermined threshold level, characterised in that the valve for isolating the sample during testing is integrated into the piston , a sealing element on the piston sealing the entrance to the sample chamber when the piston is in a retracted position, for preventing any further flow between the sample chamber and the central heating system circuit. a sample chamber for holding a sample of central heating system water to be tested, the sample chamber being connectable to the central heating system circuit to allow fluid to pass between the central heating system circuit and the sample chamber ; a piston for controlling filling of the sample chamber with central heating system water from the central heating system circuit, and emptying of the sample chamber , the piston being movable to a retracted position for drawing water out of the central heating system circuit into the sample chamber , and to an extended position for emptying the sample chamber and returning the water to the central heating system circuit; at least one valve for isolating the sample of central heating system water from the central heating system circuit during testing; and optical testing apparatus including a light source and a detector , for measuring an optical property of the sample of central heating system water isolated within the sample chamber , and the testing device being adapted to thereby make a determination as to whether or not the level of the selected chemical in the water is greater than a predetermined threshold level, characterised in that the valve for isolating the sample during testing is integrated into the piston , a sealing element on the piston sealing the entrance to the sample chamber when the piston is in a retracted position, for preventing any further flow between the sample chamber and the central heating system circuit. <EOS>