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JJG 975-2002: Verification regulation of chemical oxygen demand (COD) meters
JJG 975-2002
JJG
NATIONAL METROLOGY VERIFICATION
REGULATION OF THE PEOPLE’S REPUBLIC OF CHINA
Chemical Oxygen Demand (COD) Meters
ISSUED ON. NOVEMBER 04, 2002
IMPLEMENTED ON. FEBRUARY 04, 2003
Issued by. General Administration of Quality Supervision, Inspection and
Quarantine
3. No action is required - Full-copy of this standard will be automatically and
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Table of Contents
Foreword ... 3
1 Scope ... 4
2 Overview ... 4
3 Requirements for Metrological Performance ... 5
3.1 Type-A instrument ... 5
3.2 Type B instrument ... 6
4 General Technical Conditions ... 6
4.1 Appearance... 6
4.2 Safety performance ... 6
5 Metrological Apparatus Control ... 6
5.1 Verification conditions ... 6
5.2 Verification items ... 7
5.3 Verification method ... 8
5.4 Treatment of verification result ... 11
5.5 Verification cycle ... 11
Appendix A Verification Record Format (Type-A Instrument) ... 12
Appendix B Verification Record Format (Type-B Instrument) ... 14
Appendix C Internal Page Format of Verification Certificate (Type-A Instrument) ... 15
Appendix D Internal Page Format of Verification Certificate (Type-B Instrument) ... 16
Appendix E Internal Page Format of Verification Result Notice (Type-A Instrument) ... 17
Appendix F Internal Page Format of Verification Result Notice (Type-B Instrument) ... 18
Verification Regulation of
Chemical Oxygen Demand (COD) Meters
1 Scope
This Regulation is applicable to the first-time verification, follow-up verification, and in-
service inspection of the chemical oxygen demand meters; while the calibration of
online chemical oxygen demand meters can refer to this Regulation for implementation.
2 Overview
The chemical oxygen demand (also called COD) indicates the amount of oxidant
consumed by the substance that is easy to be oxidized by the oxidant in the water; it
is the important indicator to evaluate the relative content of the organic substance in
the water; it is mainly used for environmental monitoring in the petroleum, chemical
industry, pharmaceutical-heath areas. The chemical oxygen demand meters can be
divided, according to the technical principles, into 2 types; one type is
spectrophotometric principle. use certain prescribed amount of potassium dichromate,
under certain conditions, to oxidize water; so that the hexavalent chromium
quantification changed into trivalent chromium; use the trivalent chromium absorption
peak at 610nm or hexavalent chromium absorption peak at 420nm to measure the
content of COD. Such meter consists of the digestion furnace part and the
measurement part; the principle drawing of its measurement part can refer to Figure 1
(hereinafter refer to as Type A).
Figure 1 -- Schematic Diagram of
Spectrophotometric Principle Instrument Structure
The other type is electrochemical principle. use quantitative potassium dichromate,
under certain conditions, to heat and reflux the digestion sample; use the ferrous ions
generated by electrolysis to react with the remained hexavalent chromium; when the
hexavalent chromium is completely consumed, the electrolysis comes to end.
Calculate the COD content according to the consumption of the electricity. Its principle
drawing can refer to Figure 2 (hereinafter referred to as Type B).
Data
processing
system
Test system Sample Filter Light source Digestion furnace
3.2 Type B instrument
3.2.1 Indication error
Under the prescribed conditions, the instrument’s indication error shall not exceed
±2.0mg/L.
3.2.2 Repeatability
Under the prescribed conditions, the measurement repeatability shall be no greater
than 2%.
4 General Technical Conditions
4.1 Appearance
The instrument shall have the following signs. instrument name, model, exit-factory
number, manufacturer name, exit-factory date, sign, working voltage and
frequency, etc.; the instrument appearance is exempted from the damage that shall
affect the normal work.
4.2 Safety performance
The insulation resistance of instrument shall be no less than 20MΩ.
5 Metrological Apparatus Control
The metrological apparatus control includes first-time verification, follow-up verification
and in-service inspection.
5.1 Verification conditions
5.1.1 Equipment for verification
5.1.1.1 Thermometer. measuring range is 100~200°C (Grade-0.5).
5.1.1.2 50, 100, 300, 1000 mg/L COD solution standard substance; its uncertainty
shall be no greater than 3% (k=2).
5.1.1.3 0.05mol/L 1/6K2Cr2O7 solution standard substance; its uncertainty shall be no
greater than 1.0% (k=2).
5.3 Verification method
5.3.1 Appearance and routine requirements
It shall be performed through visual examination.
5.3.2 Access the insulation resistance meter to the power incoming line end and the
housing; turn on the instrument switch, measure the insulation resistance.
5.3.3 Type-A instrument verification method
5.3.3.1 Error of temperature indication and temperature filed uniformity
After pre-heating the digestion furnace for 1h, evenly select 6 digestion holes; then
separately insert the thermometer into them; after the temperature is stable; read 1
digit every 1min, totally read 3 times; then calculate their arithmetic average Ti.
Calculate the error of temperature indication (ΔT) and temperature filed uniformity (W)
as per Formula (1) and Formula (3).
Where.
T0 – setting temperature of digestion furnace, °C;
- average temperature of the digestion furnace holes, °C;
Ti – average temperature value for 3 times readings on the ith furnace hole, °C;
n – the number of measured furnace holes.
Where.
Tmax – temperature indication with maximum temperature of furnace hole, °C;
Tmin – temperature indication with minimum temperature of furnace hole, °C.
5.3.3.2 Error of digestion time indication
After the instrument is stable, press the instrument digestion key; stop timing when the
digestion comes to end; record the digestion time; repeat for 3 times. Calculate the
error of digestion time relative indication (Δt) as per Formula (4).
Where.
Δt – error of digestion time relative indication;
x0 – setting value of digestion time;
- average value for 3 times measurements of digestion time.
5.3.3.3 Indication error
After the instrument is stable, calibrate the instrument as per the instructions; on the
0~150mg/L shift of the instrument, separately measure the 50, 100mg/L COD solution
standard substance for 3 times; on the 0~1500mg/L shift of the instrument, separately
measure the 300, 1000mg/L COD solution standard substance for 3 times. Calculate
the indication error ( ) as per the Formula (5).
Where.
- indication error of instrument;
- average value of 3 times measurements;
Cs – standard value of COD solution.
Take the maximum value of indication error as the verification result of the
indication error of instrument.
5.3.3.4 Repeatability
After the instrument is stable, calibrate the instrument as per the instructions; on the
0~150mg/L shift of the instrument, measure the 100mg/L COD solution standard
substance for 6 times; on the 0~1500mg/L shift of the instrument, measure the
300mg/L COD solution standard substance for 6 times. Calculate the repeatability (SA)
Where.
- indication error of instrument;
- average value of 3 times measurements;
V – the number of liter of adding 0.05mol/L 1/6K2Cr2O7 solution;
40.0 – the corresponding COD value for con...