A complete guide to pH controllers — how pH is measured, what auto temperature compensation means, the difference between pH indicators and controllers, and which model suits your application.
pH is a measure of the acidity or alkalinity of a liquid, expressed on a scale from 0 to 14. A pH of 7 is neutral (pure water at 25°C). Values below 7 are acidic — the lower the number, the stronger the acid. Values above 7 are alkaline — the higher the number, the stronger the alkali.
pH is measured using a glass combination electrode which generates a millivolt signal proportional to the hydrogen ion activity of the solution. This signal — typically ranging from about +414mV at pH 0 to −414mV at pH 14 at 25°C — is amplified and converted to a pH reading by the controller electronics.
A pH controller system has four main components. The glass pH electrode is immersed in the process liquid and generates a millivolt signal proportional to pH. A PT100 temperature sensor measures the liquid temperature for auto compensation. The pH controller instrument converts the electrode signal to a pH reading, compares it to the set point, and activates the appropriate relay output. The dosing pump or valve connected to the relay output adds acid or alkali to the process liquid to bring pH back to the set point.
| Feature | pH Indicator | pH Controller |
|---|---|---|
| Displays pH | ✅ Yes | ✅ Yes |
| Relay output | ❌ No | ✅ Yes |
| 4-20mA output | Selected models | ✅ Yes |
| Automatic dosing | ❌ No | ✅ Yes |
| Set point control | ❌ No | ✅ Yes |
| Use case | Monitoring only | Monitoring + automatic control |
The millivolt output of a pH electrode changes with temperature — the same solution at 20°C and at 40°C will give different electrode readings even though the true pH has not changed. Auto temperature compensation uses a PT100 RTD sensor immersed in the same liquid to measure its temperature continuously and correct the pH reading mathematically. Without ATC, pH readings in processes where temperature varies will be inaccurate.
A dual-limit pH controller has two independently settable set points, each with its own relay output. This allows two-stage automatic control — for example, Relay 1 activates an acid dosing pump when pH rises above the high set point, and Relay 2 activates an alkali dosing pump when pH falls below the low set point. This is the most common configuration for water treatment, ETP, and STP applications where pH must be maintained within a band using two dosing chemicals.
A pH controller measures the pH of a liquid continuously and activates dosing pumps or alarms when pH crosses set limits. Used in water treatment, ETP, STP, swimming pools, pharma, and food processing for automatic pH maintenance.
A pH indicator displays pH — no control output. A pH controller has relay outputs to activate dosing pumps automatically. Choose an indicator for monitoring only; a controller when automatic dosing is needed.
Two independently settable set points, each with its own relay. One relay activates acid dosing when pH is too high; the other activates alkali dosing when pH is too low — enabling automatic two-stage pH control.
Standard glass combination pH electrodes with BNC connector. Countronics recommends the CAPJ pH sensor. Contact +91-9810536887 for sensor selection guidance specific to your application and process conditions.
pH electrode output changes with temperature. ATC uses a PT100 sensor to continuously correct the pH reading for temperature effects — ensuring accurate pH readings even when process liquid temperature varies.
Countronics manufactures online pH indicators and controllers in Sahibabad, Ghaziabad. ISO 9001:2015 certified, established 1978. Models: 121pH, 122pH, 112pH. Contact: +91-9810536887.
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