Pyxis ST-765SS-FCL Free Chlorine + pH Sensor

The Pyxis ST-765SS-FCL measures free residual chlorine (0.00-5.00 ppm, ±0.01 mg/L or 1%), pH (0-14, ±0.01) and temperature at the same time in one 304 stainless steel body (IP67, 6.9 bar / 100 psi), using a membrane-less bare-gold electrode, with automatic pH and temperature compensation up to pH 9.0. It is compliant with USEPA 334.0 and ISO 7393 guidelines and has 2 × isolated 4-20 mA and RS-485 Modbus outputs. In Turkey, Mikron Proses (Üsküdar, Istanbul) provides application consulting on selecting the model and flow assembly.

Model selection · Flow assembly · Controller integration → Send us your requirement

Free residual chlorine is the part of the chlorine added to water that has not yet reacted with microorganisms. Unlike conventional amperometric sensors that need a membrane or gel, the ST-765SS-FCL is based on the bare-gold electrochemical detection technology developed by Pyxis. For more Pyxis inline sensors, see Online Analyzers. For the other models of this platform, see the ST-765SS oxidizer and pH sensors.

Measuring principle and accuracy

With its four-electrode composite design, the sensor produces three parameters at the same time: free chlorine (0.00-5.00 ppm, auto-range), pH (0-14) and temperature. Internal temperature and pH compensation corrects the measurement in real time in water up to pH 9.0, which gives a result consistent with the classic DPD wet chemistry method (precision ±0.01 mg/L or 1% of the value). The manufacturer states that the device is compliant with USEPA 334.0 and ISO 7393 guidelines. The sensor is non-selective: it is cross-sensitive to other oxidizing species in the water and, where several oxidizers are present, gives a combined result without distinguishing the species.

Mounting: FR-50 flow reservoir

The ST-765SS-FCL is supplied as standard with a self-regulating FR-50 flow reservoir that includes a rotameter and a pressure reducing valve (PRV). The inlet pressure of the reservoir must be 0.05-0.2 MPa (7.25-30 psi) and the flow rate is kept between 600 and 1,800 mL/min. The sample inlet is ¼ in OD, and the sample and overflow outlets are routed to drain through 20 mm lines. For dirty water applications, the manufacturer offers the optional FR-300-PLUS auto-brushing flow assembly.

Outputs and wireless configuration

The sensor gives 2 isolated 4-20 mA analog outputs (separately for chlorine and pH) and an isolated RS-485 Modbus digital output at the same time, and draws 2 W from a 22-26 VDC supply. With the MA-CR Bluetooth adapter, wireless calibration and diagnostics are possible through the uPyxis app over Bluetooth 5.0, so calibration is done directly on the sensor rather than through the receiving controller.

Body, protection class and electrode life

The sensor has a 304 stainless steel body, a UPVC wet material and IP67 protection (fully dustproof and waterproof). It works up to 6.9 bar (100 psi), is 210.8 mm long and 35.6 mm in diameter, and weighs 530 g. The replaceable, front-loading EH-765 reference electrode head needs no membrane or gel replacement, and the manufacturer states a typical electrode service life of 2 years.

Controller requirement

The ST-765SS-FCL is a bare sensor with no display or relay of its own; its 4-20 mA or RS-485 output must be connected to an existing controller, PLC or DCS. For users who want a fixed display and data logging as a stand-alone monitoring panel, the manufacturer offers the OxiPanel IK-765SS-FCL package (P/N 42082), which pre-assembles the sensor with the FR-50 flow reservoir and the UC-50 display and data logger terminal.

Ordering formats

The sensor can be ordered with the FR-50 flow reservoir (P/N 53607) or as a sensor only without it (P/N 53607-NFR). Which format and which flow assembly option (FR-50 or FR-300-PLUS) suits the cleanliness of your process water and your automation infrastructure is reviewed together with you. Tell us your requirement and we will select the right model and mounting combination.