pH control for chemical processes

Description
The FB_pH_Control function block performs a 2-point pH control with hysteresis and controls valves for acid and alkali. It integrates emergency stop logic and scales the analog input (4–20 mA) linearly to 0–14 pH. The code is implemented in SCL for Siemens TIA Portal.

Note: All modules are designed exclusively as a learning and optimization basis and are not approved for live operation. System modules, in particular, must be finally tested and commissioned by qualified personnel before productive use.

How it works

• Emergency stop ( Stop=TRUE ): Stop dosing, close both valves, Fehler=TRUE , StatusText=“System gestoppt” , RETURN
• Scaling : pH = pH_RAW × 14 / 27648
• Safety check : pH < 3.0 or pH > 11.0 → Alarm=TRUE , valves closed, StatusText=“Warnung: pH außerhalb Toleranzbereich”
• Control ( Start=TRUE ∧ no error):
  • pH ≥ Setpoint_pH + Hysteresis → VentilSaeure=TRUE , StatusText=“Dosiert: Säure”
  • pH ≤ Setpoint_pH − Hysteresis → VentilLauge=TRUE , StatusText=“Dosiert: Lauge”
  • otherwise → both valves closed, StatusText=“pH-Wert im Sollbereich”
• Acknowledgement ( Quittieren=TRUE ): Alarm=FALSE , Fehler=FALSE , StatusText=“Fehler quittiert”

Technical details

• Inputs: Stop, pH_RAW (INT), Start, Setpoint_pH (REAL), Hysteresis (REAL), Acknowledge
• Outputs: Valve Acid, Valve Alkali, Alarm, Error (each BOOL), Status Text (STRING)
• Calculation: pH = pH_RAW × 14 / 27648
• Logic: IF/ELSIF with RETURN on stop or safety violation, simple hysteresis switching points

Possible applications
pH control in water treatment, process chemistry, bioreactors, and laboratory systems; batch and inline dosing; training and demonstration projects for PLC control technology.

compatibility
Siemens S7-1200/S7-1500, TIA Portal ≥ V13 (recommended for HMI/PLCSIM V16+).

Scope of delivery
.scl file with declaration of all inputs/outputs, variables, and parameters; header comment with customization instructions; download link after purchase.

Support
Adjustment of setpoint and hysteresis, integration of additional sensors or HMI layouts – advice via shop message or email.