daviteq A420-FCL User Manual page 6

3.3. Installation into a flow cell or similar system
To achieve accurate measurements, an appropriate flow rate in the range of 30 to 60 L/hr is required to reach
equilibrium between the electrochemical consumption and the diffusion of chlorine. A beaker with magnetic stirrer can
be used for electrode evaluation or polarization in a lab environment, but for real continuous measurement
applications, a flow cell system is required. A diagram of a flow cell system for the A420-FCL electrode, a pH electrode,
and a flow regulator is shown in picture below.
NOTE:
Make sure the stainless-steel ring on the A420-FCL chlorine electrode is completely submersed in the water
(above the stainless-steel counter electrode).
The flow regulator and 90o elbow tubing is important for the devices to maintain a constant flow rate. An
unstable flow rate will lead to an inaccurate chlorine measurement.
The flow regulator can be installed at INLET side as well.
Avoid bubbles adhering to the center hole of membrane cap. Bubbles in other areas of membrane cap are
acceptable.
Make sure to maintain a clear space of ~25mm (about 1") between the tip of the sensor cap and the bottom of
the flow cell tube.
4. Calibration
The output of A420-FCL is linear with chlorine concentration. As a result, a 2-point calibration determine the final
calculation equation. Usually, the zero and the half-scale points are recommended for calibrating A420-FCL.
For the zero point (1st point), it can be obtained in zero chlorine water during the conditioning processing. Generally,
the zero point of A420-FCL should be within the specification ranges provided in Tables 1 and 2. The zero point drift is
very slow and negligible in most cases. If the zero point of A420-FCL is not within the specification ranges, please
change the internal filled gel, clean the working electrode and polarize the probe in the zero-chlorine water again.
Please check the zero point every quarter or half a year at least for more precision.
For the half-scale point (2nd point), it is usually around the middle point of the measurement range and it is the most
important point for the accuracy of A420-FCL. In most cases, the calibration of A420-FCL is actually a 1-point
calibration while zero point drift can be ignored. Thus, the colorimetric DPD method (colorimeter, photometer or
spectrophotometer) is strongly recommended to standardize the exact concentration of chlorine at this point. There
are primarily two types of DPD Reagents commercially available:
1. DPD Free Chlorine Reagent (DPD Method #1) for measuring free chlorine.