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JJG 119-2018 (JJG119-2018)

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JJG 119-2018: PDF in English
JJG 119-2018
JJG
NATIONAL METEROLOGICAL VERIFICATION
SPECIFICATION OF THE PEOPLE’S REPUBLIC OF CHINA
Laboratory pH meters
ISSUED ON: DECEMBER 25, 2018
IMPLEMENTED ON: JUNE 25, 2019
Issued by: State Administration for Market Regulation
Table of Contents
Introduction ... 4
1 Scope ... 6
2 Normative references ... 6
3 Overview ... 6
4 Metering performance requirements... 7
5 General technical requirements ... 8
5.1 Legal metrology management signs and marking inspection ... 8
5.2 Appearance and power-on inspection ... 8
5.3 Electrode inspection ... 8
6 Control of measuring instrument ... 8
6.1 Verification conditions ... 8
6.2 Standards and equipment for verification ... 9
6.3 Verification items ... 9
6.4 Verification method ... 10
6.5 Processing of verification results ... 16
6.6 Verification period ... 17
Appendix A pH standard solution ... 18
Appendix B k value at (0 ~ 100) °C ... 20
Appendix C Original record format for verification ... 21
Appendix D Format of the inner page of the verification certificate/verification result
notice ... 25
Verification regulation of laboratory pH meters
1 Scope
This Regulation applies to the initial verification, subsequent verification, in-use
inspection, for laboratory pH (acidity) meters AND laboratory general-purpose ion
meters that can be used as pH (acidity) meters.
2 Normative references
This Regulation cites the following documents:
GB/T 11165-2005 pH Meter
GB/T 20245.2-2013 Expression of performance of electrochemical analyzers - Part
2: pH value
IUPAC: 2002 Measurement of pH. Definition, Standards and Procedures
OIML R54:1981 (E) pH Scale for Aqueous Solutions
For dated references, only the dated version applies to this Regulation; for undated
references, the latest edition (including all amendments) applies to this Regulation.
3 Overview
A laboratory pH (acidity) meter (hereinafter referred to as a pH meter) is an
electrochemical analytical instrument, which is used to measure the pH of an aqueous
solution. The pH meter is mainly composed of an electric meter AND a measuring
electrode. The electric meter part includes impedance converter, amplifier, positioning
regulator, slope regulator, temperature compensator, display, etc. The measuring
electrode includes indicating electrode and reference electrode. The indicator electrode
of the composite electrode is mostly a glass electrode; the reference electrode is often
called the internal reference electrode.
A pH meter measures the pH of a solution, using a comparative method. The
measurement principle is as follows: a battery is composed of an indicator electrode, a
reference electrode, a pH standard buffer solution; the electrometer measures the
battery's electromotive force ES; the pH standard value pHS is input. After calibrating
the pH meter, replace it with the solution to be tested and the same pair of electrodes,
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 GΩ high resistance; for other resolutions,
connect to 3 GΩ 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, ΔpHR, 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,
ΔpH'R, which is caused by the input impedance, takes the larger of ΔpHR and ΔpH'R
as the verification result of this item.
6.4.9 Approximate equivalent input impedance
Take the larger median value of ΔpHR and ΔpH'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
ΔpHR and ΔpH'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, Ω.
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 ΔpHT, 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, ΔpHT, which is caused
by temperature compensation, according to formula (10).
6.4.11 Temperature measurement error of temperature probe
Calculate the temperature measurement error, ΔT, 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 ΔT, 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 GΩ 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 ΔpHS, 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.
......
(Above excerpt was released on 2022-03-06, modified on 2022-03-06, translated/reviewed by: Wayne Zheng et al.)
Source: https://www.chinesestandard.net/PDF.aspx/JJG119-2018