Sometimes called pH probes, pH sensors are used to measure the concentration of hydrogen ions in a fluid to determine the acidity or alkalinity. The pH scale ranges from 0 - 14, with a pH measurement of 7 indicating a neutral solution. Any reading below pH 7 is acidic, and any reading above pH 7 is basic. 

pH sensors feature a pH-sensitive electrode and a reference electrode consisting of metal wire suspended in a stable, neutral electrolyte solution. The reference electrode maintains a steady reference voltage and completes the electrical circuit with the pH-sensitive electrode by allowing a small number of ions to flow from the reference solution into the process solution through a small porous opening called a reference junction. The pH-sensitive electrode has a small bulb at its tip that is made of pH-sensitive glass. The electrical voltage across this bulb changes in response to the hydrogen ion concentration of the process fluid around it.

If the hydrogen ion concentration is higher on the outside of the glass bulb than on the inside, a positive voltage occurs, indicating that the process is acidic. If the hydrogen ion concentration is lower on the outside of the glass than on the inside, a negative voltage occurs, indicating the process is basic. If the hydrogen ion concentration is the same on either side of the glass electrode, the voltage is zero, indicating the process is at a neutral pH of 7. 

A two-point buffer calibration is the best way to ensure that a pH sensor provides accurate measurements. During a two-point calibration, the transmitter calculates new values for slope and zero offset. To calibrate, immerse the pH sensor in the first buffer solution. Allow the pH sensor to adjust to the buffer temperature (to avoid errors due to temperature differences between the buffer solution and sensor temperature) and wait for readings to stabilize. Once the transmitter has acknowledged the first buffer, rinse the buffer solution off the sensor by pouring a small amount of the second buffer solution over the tip of the sensor. Then immerse the sensor in the second buffer solution and repeat the process. Once the transmitter has acknowledged both buffer solutions, a new pH sensor slope is established.

Temperature changes within a process fluid can affect the accuracy of pH readings. The pH level of a solution is inversely proportional to the temperature. When temperature increases within a solution, the pH level will decrease, making the solution more acidic. All Rosemount pH sensors include a temperature element to adjust for any temperature fluctuations.

Yes, many pH sensors can also measure oxidation-reduction potential (ORP), sometimes referred to as redox potential. While pH sensors measure the acidity or alkalinity of a solution based on the activity of hydrogen ions, ORP sensors measure the redox potential of a solution based on the activity of oxiders and reducers in the solution.

Emerson pH sensors are used across an array of industries. For example, oil and gas industries use pH measurements for crude distillate refining, tail gas clean-up and sour water stripping. Chemical industries rely on pH measurement for the chlor-alkali process, sulfuric acid production and more. Wastewater industries use pH for desalination and water purification. For more examples and in-depth applications, please see the pH Sensor Applications below.

Most pH sensors are disposable, but some can be rebuilt and reused, such as the Rosemount 3300/3400/3500 pH Sensors. With these pH sensors, the reference electrolyte and reference junction can be replaced, and the sensor can be reused after. Some processes can coat the pH glass. In these situations, retractable probes can allow for easy cleaning and recalibration of the sensor before reinsertion.

Every application is different, each with its own unique needs and challenges. Emerson offers pH sensors that work across various industries like chemical, oil and gas, wastewater and more. If you require a pH sensor for use in high temperatures, dirty process fluids, harsh chemicals, or other unique applications, we have a pH sensor solution.

A pH sensor is connected via cable to a liquid analysis transmitter, sometimes called a liquid analyzer. The transmitter receives raw voltage signals from the pH sensor then converts the raw signals to display the pH readings.

The lifespan of a pH sensor varies based on its application. In general purpose applications like  purifying water, a pH sensor can last up to two years. In high temperatures or harsh chemical applications, a pH sensor may only last for up to six months.