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pH Sensors

pH sensors or probes are widely used across many industries to measure the acidity or alkalinity of a solution, which is represented by the pH value. Their functionality is based on the ion-selective potentiometric principle. These sensors typically contain a sensing part and a reference part. The sensing part, usually a glass electrode, responds to the H+ ion concentration (or activity) in the solution, while the reference part provides a stable reference potential.

As illustrated by the two sensors described above, pH probes can be specialized to handle different types of environments. Some sensors are specifically designed for ultra-pure water, making use of sensitive, low resistance glass bulbs and rugged polypropylene housing. These sensors may use gel electrolyte and solid electrolyte salt bridges to increase life span and stability. Features such as large PTFE liquid junctions and advanced solid dielectrics reduce fouling and maintenance needs. In contrast, other sensors are designed to resist high-velocity solids and high levels of abrasion, demonstrating unprecedented periods of survival in applications where other sensors may fail.


Application Features:


  • Measurement Range: Different probes are designed to cover different pH ranges. Some can measure from 0 to 14, covering the entire pH scale, while others may have a narrower range, such as 2 to 12.
  • Material: The materials used in the construction of the sensor can vary. Some sensors have a rugged polypropylene housing while others may use RADEL or 316 stainless steel. The choice of material is usually influenced by the environment in which the sensor will operate.
  • Temperature Compensation: Most pH sensors have an inbuilt temperature sensor (e.g., PT1000 or NTC10K) for automatic temperature compensation, given that pH measurement can be affected by temperature.
  • Calibration: The calibration process can differ, with some sensors using sample calibration or standard liquid calibration methods.
  • Installation: Sensors can have different installation types, including immersion, submersion, or retractable installation types. The choice of installation type usually depends on the application and environment.
  • Communication: Many modern pH probes employ digital communication methods, such as MODBUS RTU via RS-485 interface, allowing for direct and precise digital readings.
  • Protection: Many probes are built to resist harsh conditions. Some models offer features like resistance to high velocity solids, abrasion, and fouling. Some also offer high waterproof grades (e.g., IP68) to withstand submersion in liquids.
  • Durability: Durability and lifespan of the sensors can be enhanced by using a gel electrolyte and solid electrolyte salt bridge, or using thick-wall TEFLON membranes for high stability and low drift. This results in lower maintenance and replacement costs over time.
  • Applications: pH sensors are used in a wide range of applications, from water and wastewater treatment to food and beverage production, chemical processing, and more. Certain types of sensors may be better suited for specific applications, such as ultra-pure water monitoring.
  • Pressure Resistance: The ability of the sensor to withstand pressure varies between models. Some models can resist pressure up to 87 psig (0.6Mpa), while others may be capable of withstanding higher pressures.
  • Connectivity: Sensors may come with quick connect plugs or fixed cables of varying lengths. Some are designed for easy hot-swap, eliminating the need for hard-wiring and allowing for instant sensor change-out.
  • Output: The sensor’s output is typically a voltage or current signal proportional to the pH of the solution. Some modern sensors have on-board data processing, which allows them to output digital signals directly.
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Karma Instrumentation & Control is a specialist supplier of process control and automation equipment and SI systems, as well as a manufacturer and designer of process and electrical control panels.

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