On the calibration mode of PH meter
There are many types because of the different design of electric meters, and their operating steps are different, so the operation of pH meters should be carried out correctly in strict accordance with their operating instructions. In specific operation, calibration is an important step in the application of pH meter.
JJG119-2005 《 Verification regulation of pH (acidity) meter in laboratory 》 clearly requires the verification of pH indication error: connect the circuit, high humidity sensor probe, stainless steel electric heating tube PT100 sensor, cast aluminum heater, the resistance R of heating coil fluid solenoid valve is not connected, and the instrument temperature compensator is adjusted to 25℃ (or some intermediate temperature point of the temperature compensator). Calibrate the instrument according to the instrument manual, then use the calibrator to input the standard signal pH to the ammeter, write down the pH indication of the ammeter respectively, measure it once by increasing or decreasing the input, and take the average value to calculate the error of the ammeter indication.
During the verification, it is found that for intelligent calibration instruments, the calibration methods proposed in the instrument specifications of various manufacturers are varied. In order to meet different measurement needs, the pH scales adopted in the instruments are different, and some instruments combine different pH scales into the instruments at the same time, so they can be selected according to the measurement needs. In order to improve the accuracy of measurement, the optional calibration points of instruments are generally set to 2 ~ 3 points, and some instruments even reach 7 points; Permitted simultaneous calibration points Most instruments are 1 ~ 2 points, some allow 3 points, and some even allow 5 or more points to be calibrated at the same time. The calibration schemes of instruments are different. However, no matter how it changes, in fact, the core of calibration is to solve E0 and K in Nernt formula (E=E0-k×pH) by segments, so as to convert the potential value measured by the electrode into the corresponding pH value.
It can be seen that the requirement of the calibration mode of the instrument in the verification regulation is "calibrate the instrument according to the instrument manual", which is different from the calibration mode of the instrument in the manual. What is required in the verification regulation is to calibrate the secondary ammeter with reference to the calibration method in the manual, and the calibration method of instrument design is to fully consider the electrode characteristics of pH sensor. Due to the influence of electrode linearity and acid-base error, different calibration points are required for the acidic and alkaline intervals of the solution, even for different segments of the same interval. In order to fit the linearity of the sensor well, the corresponding calibration points will be selected accordingly. This requires that we must correctly understand the meaning of calibration in the regulations, grasp the characteristics of some secondary meters of pH meter and electricity meter, and adopt reasonable methods to calibrate the instrument.
