JJG 119-2018Verification Regulation of Laboratory pH Meters (English PDF)

6.4.8 Indication error caused by input impedance

Connect the pH meter and the calibrator, according to the circuit in Figure 1. Disconnect the calibrator's high resistance. The pH meter selects the pH measurement mode. Adjust the temperature compensation to 25 °C (or a certain intermediate temperature point).

Adjust the calibrator, so that it outputs a signal, which is equivalent to (pHD + 6). Record the pH meter's indication value, pH1. Connect to high resistance (when the resolution of the display unit is 0.001, connect to 1 Gohm high resistance; for other resolutions, connect to 3 Gohm high resistance). Adjust the calibrator, so that its output signal is pHD.

Adjust the pH meter, so that the pH indication value is 7 (or the pH meter's equipotential pH value). Adjust the calibrator, to make it output the signal of (pHD + 6) again. Record the pH meter's indication value pH2. Repeat the above operation three times. Use the formula (7), to calculate the indication error, deltapHR, which is caused by the input impedance.

Where:

- Respectively, the average pH value 3 measurements, before and after high resistance access.

When testing the input signal (pHD - 6), by the same method, the indication error,

deltapH'R, which is caused by the input impedance, takes the larger of deltapHR and deltapH'R as the verification result of this item.

6.4.9 Approximate equivalent input impedance

Take the larger median value of deltapHR and deltapH'R in 6.4.8. Use the formula (8) or formula (9), to calculate the equivalent input impedance r or r' of the electric meter. If

deltapHR and deltapH'R are both zero, substitute half of the resolution of the display unit of pH meter into the formula, for calculation.

Where:

R - The high resistance of the calibrator, ohm.

6.4.10 Indication error caused by temperature compensation

6.4.10.1 Indication error caused by manual temperature compensation

Connect the pH meter and the calibrator, according to the circuit in Figure 1. Disconnect the calibrator's high resistance. The pH meter selects the pH measurement mode. Adjust the temperature compensation, to a temperature other than 25 °C. Including the upper and lower limit temperatures of the temperature compensation function, there are no less than 5 temperature points to be verified. At each verification point, adjust the calibrator, so that it outputs a signal equivalent to (pHD + 6) at the temperature. Record the value of the pH meter. Repeat the measurement 2 times. Calculate the indication error deltapHT, which is caused by temperature compensation, according to formula (10).

Where:

- The average pH value of pH meter in 2 measurements;

pHD - Equipotential pH value of the calibrator;

pHS - The standard value of the calibrator.

6.4.10.2 Indication error caused by automatic temperature compensation

Connect the pH meter and the calibrator, according to the circuit in Figure 1. Disconnect the calibrator's high resistance. The pH meter selects the pH measurement mode. Put the thermometer and pH meter temperature probe, into the constant temperature water tank. Adjust the temperature of the constant temperature water tank, to a temperature other than 25 °C. Select not less than 5 temperature points uniformly, in the range of (5

~ 60) °C. At each verification point, adjust the calibrator, to output a signal equivalent to (pHD + 6), at the temperature (standard temperature of the thermometer). Record the indication value of pH, temperature, temperature indications of the electric meter.

Repeat the measurement 2 times. Calculate the indication error, deltapHT, which is caused by temperature compensation, according to formula (10).

6.4.11 Temperature measurement error of temperature probe

Calculate the temperature measurement error, deltaT, of the pH meter's temperature probe at different temperature points, according to the pH meter's temperature indication value and the thermometer temperature's indication value, which are recorded in 6.4.10.2, according to formula (11). Take the largest absolute value of deltaT, as the verification result of this item.

Where:

- The average value of the two temperature measurements of the pH meter's temperature probe, °C;

- The average value of 2 temperature measurements of the thermometer, °C.

6.4.12 Repeatability of indication value of electric meter

Connect the pH meter and the calibrator, according to the circuit in Figure 1. Disconnect the calibrator's high resistance. The pH meter selects the pH measurement mode. Adjust the temperature compensation to 25 °C (or some intermediate temperature point).

Adjust the calibrator, to output a signal equivalent to (pHD + 3) at this temperature.

Connect a 1 Gohm high resistance. Record the pHi of the pH meter, after the pH meter is stable. Repeat the above operation 6 times. Calculate the repeatability spH of the electric meter's indication value, according to the formula (12).

where:

- The average pH value of 6 measurements;

n - The number of measurements, n = 6.

6.4.13 Indication error of instrument

Select (3 ~ 5) kinds of standard solutions in Table A.1 of Appendix A. Place them in a constant temperature water tank, at a constant temperature. Under normal operating conditions of the pH meter, use a standard solution to calibrate it. THEN, measure another standard solution, which is not used in the verification. Repeat the above operation 6 times. Calculate the pH meter's indication error deltapHS, according to formula (13). The pH meter's verification shall try to choose a method with high accuracy; the pH difference, between the verification solution and the solution to be measured, should not exceed 3.

Where:

- The average value of six measurements of the standard solution to be tested;

pHSS - The pH value of the standard solution.

6.4.14 Repeatability of instrument's indication value

Take the 6 measurement data in 6.4.13. Calculate the measurement repeatability of the pH meter, s'pH, according to formula (14).

Where:

pHi - The measured value of the standard solution to be tested;

- The average value of six measurements of the standard solution to be tested;

n - The number of measurements, n = 6.

6.5 Processing of verification results

6.5.1 A pH meter, that meets both general technical requirements and measurement

performance requirements, is a qualified instrument. The pH meter, that has passed the verification, is issued with a verification certificate. The verification results and instrument level of each item shall be given on the verification certificate.

6.5.2 For a pH meter, that is in use AND after repair, if the electrical meter's verification meets the requirements of the Regulations, BUT when the pH meter with original electrode is beyond the requirements of the Regulation, in case of the verification of the whole machine, the inspection unit can choose a new electrode for re-verification. After the electrode is replaced, if the whole machine is verified as qualified, it is still a qualified instrument, AND a verification certificate is issued.

6.5.3 When the pH meter cannot meet the requirements of this level, it is allowed to downgrade for use. Descending to the next level must meet the requirements of that level of pH meter. If the minimum requirements of the verification Regulations are still not met, after the downgrade, THEN the instrument is unqualified, a notice of verification results shall be issued, AND the unqualified items shall be indicated.

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