If you are exposed to a company that is using a water on one of its main processes, then surely you encounter this kind of meter, the pH/ORP meter. Just like a conductivity meter, a pH meter or ORP meter also has a probe that is soaked in the water (or some other liquid) to monitor its pH or ORP.
Calibrating pH meters can be accomplished in 2 methods (the most accurate), 1st method is Wet calibration where the pH probe is soaked in a liquid solution usually in the range of 4, 7 and 10 pH, then comparing the result, and the 2nd method is a dry calibration which is a simulation of electrical signal specifically, a mV signal with a corresponding pH value.
Dry Calibration Means there is no liquid solution involved in the process of calibration. The most familiar term is the electrical simulation, where a mV signal with a corresponding pH or ORP value is used. And this is the method that I will present here in addition to the calibration set up and adjustment. I have included the ORP meter verification since the 2 are related.
I will use a Fluke 754 process calibrator to simulate and calibrate a GF Signet pH/ORP meter. GF Signet pH/ORP meter is also a controller and a transmitter because it can be connected to a PLC using a 4-20 mA signal.
What is a pH?
A pH (potential hydrogen) is a scale or range (from 0 to 14) use to describe how high or low the acidity or basicity of a certain aqueous solution like water used in pharmaceuticals or food industry. A lower pH value (approaching zero) means very acidic while a high value (approaching 14) means very basic or alkaline. 7 which is in the middle is the neutral value.
pH to mV conversion
You can read more about pH in this link.. WIKIPEDIA
What is ORP
ORP stands for Oxidation Reduction Potential which is measured in millivolts. ORP meter measures how much electrons an atom, molecules or an ion loses or gain. A higher positive reading indicates a more oxidizing while a lower negative reading indicates a more antioxidizing.
Read here for more info about ORP…Fundamentals of ORP
How to calibrate pH meter using Fluke 754 as the reference standard.
- Warm-up time ( Unit Under Calibration): At least 1 hour for proper stabilization
- Temperature: 23 +/- 5 deg C
- Humidity: 50 +/- 30%
- Perform 3 trials for each point or range
- Measurement Data Sheet (MDS)- for data recording
pH/ORP Meter Calibration Setup
Procedure for Calibration pH Meter
- Check the pH meter for any visual defects that can affect its accuracy. Discontinue calibration if any defect is noted.
- Observe proper stabilization time of at least 1 hour.
- Prepare the measurement data sheet (MDS) and record all necessary details or information (Brand, Model, serial #, etc).
- Determine the test point to be calibrated (we will choose 4, 7, and 10 pH).
- Connect the Fluke 754 as shown on the calibration set up above
- Set Fluke 754 to the voltage source.
- Based on the table presented above, simulate the desired voltage with equivalent pH value.
- 4 pH = +177
- 7 pH = 0
- 10 pH = -177
- Wait for the display to stabilize and record to it on your MDS
- Check readings if within the accuracy defined by the manufacturer, in this brand of GF Signet, accuracy = +/- 03 pH and 2 mV for ORP. If the readings are already within limits update the corresponding record, do labeling and sealing and issue to the owner, otherwise, do necessary repair or adjustment. ( see adjustment procedure below)
- End of Verification
ORP Meter calibration procedure
The set up and calibration procedure is almost the same with the pH meter, the only difference is the range of simulated input. The ORP meter have a range of -1000 to +2000 mV.
Below are the calibration points that we can use:
- -1000 mV
- -500 mV
- 0 mV
- +500 mV
- +1000 mV
GF Signet Adjustment procedure
During the verification process where the result are not within their tolerable range. We can adjust the meter by performing below procedure. This is applicable to both ORP and pH settings.
Check the video below on how to adjust. It is presented using a buffer solution, a wet calibration, but the procedure for accessing the EASY CAL is the same.
For the complete details of adjustment procedure including the use of pH buffer solution, you may check the GF Signet pH/ORP meter manufacturer’s manual.
I have shared to you the basics regarding what is a pH and ORP meter. I also presented on how to set up and calibrate a pH/ORP meter using a dry calibration method also known as electrical simulation method. In this method, we used the mV source function of Fluke 754 to simulate the required signal used by the GF Signet pH/ORP meter.
This method provides a fast and accurate result, but the disadvantage is that we are only performing the calibration only on the meter itself, the probe is not included. But for accuracy verification of the meter, this is a good method to follow.
Looking for a pH meter to use? Check this link ph meter.
Do you need a complete set of a buffer solution which is NIST traceable? visit below link.
Biopharm Buffer Calibration Solution Kit 3-Pack of 500 ml each pH 4, 7 and 10 Calibration Standards Color coded NIST Traceable For All pH meters
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