pH transmitter calibration with standard solutions

What does a pH transmitter do?

• The instrument consists of a pH sensor, which measures the pH of a solution using electrodes or wires, and a controller, which modifies the pH sensor’s raw signal before sending it to the transmitter.
• A pH transmitter can accomplish this conversion of a solution’s pH into an electrical signal.

What is the unit of pH?

• The pH scale indicates the acidity or basicity of water. On the pH scale, which ranges from 0 to 14, neutrality is indicated by the number pH 7.
• pH levels below 7 indicate acidity, whereas pH values above 7 indicate baseness.
• The pH scale really measures the amount of hydrogen and hydroxyl ions that are freely present in water.
• Water that has a higher concentration of free hydrogen ions will have an acidic pH, whereas water that has a higher concentration of free hydroxyl ions would have a basic pH.
• Since chemicals in the water can alter pH, pH is a crucial sign of a chemical change in the water.
• “Logarithmic units” are used to report pH. Each value represents a 10-fold variation in the acidity or basicity of the water.  Water with a pH of 5 is ten times more acidic than pH 6.

How do you calibrate a pH transmitter?

Purpose and Scope:

This procedure provides a thorough description of how to calibrate a pH transmitter using common reference solutions in the process area or analyzer room.

Tools required for pH transmitter calibration:

• Necessary hand tools
• Standard buffer solutions
• Standard temperature sensor
• Standard pH meter
• Leads and probes
• Beaker
• Soft tissue paper for cleaning electrodes
• Soft Cloth for cleaning
• Reference manual

Safety

• Check out the link provided below for details on basic safety, general concepts, and calibration procedures in process industries.

Important Safety and Health Procedures and General Process Industry Considerations for the Calibration Process

• Ask the SCADA/DCS panel operator to set the pH controller to MOS for the ESD loop and manual for the pH transmitter control loop.
• Find the pH transmitter you want to calibrate, double-check that it is the correct pH transmitter, and make a note of any important details, such as the tag number (for example, tag number, the manufacturer, model number, pH range, etc.).
• Switch off the power supply to the pH transmitter. Make sure that any nearby junction boxes in the analyzer panel or marshalling panels close to the control room have their power sources turned off. (For instance, shut off the circuit breaker or unhook the cables from the pH transmitter).
• Keep in mind that depending on the particular pH transmitter and calibration place, this basic method may need to be modified. When working with pH transmitters or other process equipment, always adhere to any manufacturer’s instructions and local safety standards.
• In order to avoid an unauthorized start-up or usage of the pH transmitter, adhere to the necessary lockout/tagout protocols. Take care to keep the pH transmitter out of the way of the operation.
• Always wear the appropriate PPE based on the calibration environment and potential hazards involved. This may include safety glasses, gloves, lab coats, or other protective gear.
• pH calibration often involves working with acidic or alkaline solutions. Handle all chemicals with care, following the manufacturer’s instructions, and wear appropriate PPE when working with hazardous substances. Store and dispose of chemicals properly.
• Before starting the calibration, perform a risk assessment to identify any potential hazards and implement appropriate control measures to mitigate risks. This assessment should consider factors such as electrical hazards, chemical exposure, and potential equipment malfunctions.
• If your organization has specific SOPs for conducting pH transmitter calibration, follow them closely. These procedures are designed to ensure safety and consistency in calibration practices.

Calibration Setup

• Collect all the necessary equipment and materials for the calibration process. This typically includes the pH transmitter, a pH electrode or probe, buffer solutions of known pH values (pH 4, pH 7, and pH 10), distilled water, calibration certificates, gloves, and add any other tools the manufacturer may have recommended.
• Choose a clean and stable work area for the calibration process. Ensure that the area is free from sources of contamination, excessive vibration, or electromagnetic interference. 
• Use an instrument loop diagram and probes to create a series-connected analogue input loop to the multimeter (mA mode) between the junction box and the pH transmitter. Connect the field communicator as well if the pH transmitter is a smart model.
• The connections have been carried out and are currently being prepared for the pH transmitter’s calibration, as shown in the diagram

Calibration principle

• The calibration principle of a pH transmitter involves adjusting the output of the transmitter to correspond accurately to the pH values of known buffer solutions. This calibration process establishes a linear relationship between the mA current output of the pH transmitter and the corresponding pH values.
• The calibration process involves immersing the pH electrode in each buffer solution, allowing it to stabilize, and then adjusting the transmitter’s output to match the expected pH value.

Calibration procedure

• Check that the sample tubing and wire connections on the pH transmitter and sensor electrode are stable.
• Turn on the power supply, and then take off the pH transmitter terminal cover to make sure there is a power source present.
• The instrument data sheet  or HART field communicator  can be used to confirm a number of the parameters. The tag number, the LRV, and the URV are common characteristics of pH transmitters.
•  Remove the pH sensor electrode from the sample line after closing the sample isolation valve.
• Stains on the pH electrode might negatively affect the calibration, making it difficult to measure pH accurately.

• Always use distilled water to rinse the sensor, and then before putting it in a buffer, wipe off any remaining water with a clean tissue. Do not wipe the sensor; only touch it. When you wipe a sensor, static electricity builds up and changes the reading.

• Once the pH electrode has been cleaned, gather the buffer solutions for the calibration.

Which solution is used to calibrate pH?

• Follow the manufacturer’s specific preparation instructions and make sure the buffer solutions are prepared at the proper temperature. Accurate calibration requires maintaining the buffer solutions’ quality and temperature at the predetermined levels.
• Ensure that the temperature of the solution is the same as the sensor’s temperature for optimal results. Permit the temperature of the entire measuring cell, sensor, and solution to stabilize.
• Using buffers at high temperatures increases the risk of evaporation, which affects the buffer’s concentration. An incorrect calibration results from the pH changing as a result of the concentration changes.
• The lifespan of buffers is limited. If the expiration date of a buffer has passed, do not use it. Buffers should be kept in a temperature-controlled room.
• Reusing buffer solutions is prohibited. Don’t expose buffers to air too much. The pH of the buffer solution might change with exposure to air.
• The standard buffer solutions provide a risk of injury if swallowed or inhaled, and they can irritate the skin and eyes. When handling solutions, gloves and safety glasses should always be worn.
• The MATERIAL SAFETY DATA SHEET (MSDS) must be followed when using any standard solutions and copies should be kept in the workstations.
• Start the calibration process by calibrating with the pH buffer solutions in the correct order. The sequence is typically pH 7, followed by pH 4 and then pH 10. This order helps minimize the carryover of buffer residues from one calibration point to another.

How do you calibrate a pH meter with a solution?

Calibration with buffer Solution

• Place the sensor in the first buffer solution with value 7, along with a calibrated thermometer, after thoroughly cleaning it with DI water. Put the sensor tip in the glass beaker at least 3 inches (76 mm) below the liquid level.
• The sensor should not sit on the glass bulb. Swirl the sensor body to release any bubbles that have become trapped.
• Give the pH transmitter and electrode enough time to get into thermal equilibrium with the calibration environment. The temperature usually needs to stabilize for a few minutes.
• Go on with the calibration changes after the temperature and pH levels are essentially stable.
• After measuring the sample temperature with a high precision thermometer (only zero offset calibration), the value of “Measured temperature” should be updated to match the reading by the transmitter.
• For the best accuracy, this should be done as close as feasible to the standard operating temperature.
• Adjust the pH transmitter’s calibration settings to match the mA reading measured by the multimeter. This is typically done by adjusting potentiometers or digital calibration settings on the transmitter display keypad or done through the HART communicator. Follow the manufacturer’s instructions for specific calibration procedures.
• Repeat the calibration process for all the required calibration points (pH 4 and pH 10). After calibrating all the points, verify the accuracy of the readings using additional buffer solutions or reference standards, if available.
• After each calibration point, rinse the pH electrode with distilled water to remove any buffer residues before immersing it into the next buffer solution. This ensures precise measurements and helps prevent contamination

• Repeat the calibration process as necessary to calibrate the pH transmitter to the specified tolerance.

Calibration by verification with standard reference pH meter

• If the electrode slope, which is determined by the transmitter during calibration, is not appropriate, the sensor will not calibrate properly.
• The difference in pH between the two readings is converted into an equivalent voltage by the pH transmitter. This voltage is referred to as the reference offset.
• Clean and reinstall the pH measurement probe in the sampling pipe line, and then open the sample isolation valve.
• After the sample flow has become regular and is clear of air bubbles, take a sample of the process from the output of the pH transmitter.
• Then use a standard reference pH meter to measure the collected sample.

• With a portable standard reference pH meter, compare the value displayed by the pH transmitter.
• Both indicators should have the same value. If not, make adjustments to the pH transmitter.
• Additionally, in order to avoid any potential mistake caused by the temperature compensation circuitry of the instrument, the calibration of the process loop using the portable standard pH meter must be performed at the same temperature.

Recording calibration

• To ensure that the pH transmitter is generating the right results, check its linearity.
• Recalibration is necessary if the pH transmitter output reading value does not fall within an acceptable range. Once more, a pH transmitter electrode has to be fixed or changed if the output values have deviated from the permitted range.
• If every output result (+/-%) falls within acceptable bounds, no additional calibration of the pH transmitter is necessary.
• The blank calibration report with pH electrode data should include the output values from the pH transmitter written in the as found/as left column.

Completion of calibration

• Attach the calibration label on the pH transmitter once the calibration has been completed successfully.
• Clean the pH transmitter, store it securely, and save the calibration data for later use when the calibration is finished.
• Remove all of the pH standard solutions, and then discard any used ones in a responsible manner.
• Make sure the pH transmitter’s calibration area is neat.
• De-isolate the pH transmitter and take off any maintenance tags from it.
• Before resuming usage of the pH transmitter, make sure it is functioning properly.

Sample pH transmitter report

The illustration below shows how the pH transmitter sample report of calibration was performed in the process area or analyzer room using reference standard buffer solutions.

You can obtain the Excel file that was utilized in the process of creating the pH transmitter calibration report by selecting the link that is provided below.