When (not) to Calibrate Auto-Dosing Sensors


It is recommended that a service contract is arranged for the ongoing service and maintenance of automatic dosing systems. As technology improves, these systems are becoming ever-more complicated and sophisticated. The following is a list of items that could be undertaken by a suitably competent member of staff:

Operation, cleaning and calibration of the probes

Automatic monitoring systems vary in their levels of sophistication. Some are fairly simple: once the chemical pumps’ stroke rate and speed have been set, pumping is activated/deactivated according to the discrepancy between the programmed set-point and the readings obtained from the sample. At the other end of the spectrum, there are controllers that are programmed to predict its response to readings in different circumstances and adjust accordingly in order to prevent under or over dosing, i.e. they are ‘self-tuning’. They can also be connected to computer software to enable better communication in addition to being able to monitor and adjust from anywhere.

Systems also vary in their requirements for cleaning and/or calibration. Some systems will need neither as they are self-cleaning. Refer to the manufacturer’s instructions to find out what type of on-going maintenance your system requires. If it does need to be cleaned and/or calibrated by the user, this usually involved isolating the incoming feed, disconnecting the leads from the control unit to the probes, unscrewing the probes and either dipping them into, or wiping them with a probe cleaning solution.

If calibration is being done at the same time, this usually involves navigating into the appropriate menu screen on the control unit and following the step-by-step instructions. For disinfectant calibration, a sample of pool water is taken from the sample line (not the swimming pool itself) and a free chlorine test is carried out using a DPD 1 tablet in the usual way. The control unit reading is then overwritten with the reading obtained from the DPD 1 test using the on-screen menus.

There are two different types of probes for the measurement and control of disinfectant; amperometric and redox. Amperometric probes work by measuring the hypochlorous acid in the sample. Redox probes work by measuring the oxidative power (ability to break down other substances) of the water.

For pH calibration, the probe is dipped into a solution with a known pH value (supplied by the manufacturers of the system) and then dipped into another solution with either a higher or lower pH value than the first solution. As with chlorine calibration, the operator is required to follow the manufacturer’s instructions specific to the type of equipment and follow the on-screen instructions carefully. It is recommended that pool plant operators request that the service engineers provide them with a tutorial on the automatic control unit during one of their service visits. It is worth remembering that most probes require replacement on an annual frequency.

Cleaning/replacing the in-line filter

The in-line filter will get dirty, grimy and clogged up over time. This will then mean that the probes are no longer obtaining accurate chlorine and pH readings of the water. The filter should be isolated from the incoming feed and the assembly unscrewed so that the filter can be taken out and either cleaned or replaced with a fresh one (some in-line filters are only designed to be used once, then discarded). This task should be done as often as is necessary, but once per month is usually sufficient.

Adjusting control unit parameters

Operators may need to change the set-point parameters occasionally. This may be because there has been a contamination issue that requires the disinfectant levels to be at the top of the recommended range for a period of time (as would be the case following a liquid faecal release into the swimming pool). In these situations it is useful for operators to know how to adjust the set-point parameters so that the system will adjust the dosing rate to achieve 5.00mg/l rather than the usual operating level of 2.00mg/l for example.