1 
  Technical Specification for 
Verification and Inspection of 
Diaphragm Gas Meter s S/N CNMV 31  
Rev. 5 
1. This Technical Specification is developed  pursuant to the stipulations in Paragraph 2 of 
Articles  14 and Paragraph 2 of Articles  16 in the Weights a nd Measures Act.  
2. The date of promulgation, document number, date of enforcement and content of 
amendment are listed as follows:  
 Rev. Date of 
Promulgation  Document N o. 
(Ching -Piao-Szu-Tsu ) Date of 
Enforcement  Content of Amendment  
1 Jun. 2, .2003  No. 0 9240005160  Jul. 1, 2003   
2 May 1, 2006  No. 0954000 1570  Jul. 1, 2007  Revised according to related OIML 
requirements . 
3 Nov. 18, 2010   No. 09940005810   Jan. 1, 2011  1. When implements re -verification 
against meters that passed initial  
verification prior to subject to type 
approval, meters shall be sampled 
and tested for their errors at flow rate 
point 3 Q min. as required against 
meters that has been subject to type 
approval .  
2. For the verified flow rate  between 
0.1 Q max and Q max and when  the 
symbols “±”  are the same, the 
stipulations regarding the errors are 
revised.  
4 Sep. 2, 2014  No. 10340007390  Sep. 2, 2014  1. For gas meters with Q max more  
than 16 m3/h that are not subject to 
type approval , the test  for errors at 
flow rate 3 Qmin is no longer 
conduct ed. 
2. For gas meters with Q max equal to 
or less  than 16 m3/h that passed 
initial verification before the 
implement of type approval, the test  
for errors at flow rate 3 Qmin is 
maintained. For gas meters with Q max 
more than 16 m3/h, the test  for errors 
at flow rate 3 Qmin is no longer 
conducted with reference to those 
procedures for gas  meters that are not 
subject to type approval.  
5 
 .
, 2017   No. 
  July 1,2018  1. For effective management , year of 
manufacture and history of repair are 
added  as mandatory  marking items.  
2. To regulate the external airtight 
test in accordance with the claimed 
maximum working pressure  of gas 
meters , the leakage rate of external 
airtight and the submitted documents 
required for verification are revised.  
3. For effective use o f verification 
resources, the determination of pass 
or fail for errors at flow rate 3 Qmin is 
revised.  
CNMV 3 1 
2 
  
4. In consideration  of the material of 
seal may change with the progress of 
technology and the requirement of 
environmental protection, the 
material of s eal is widened and no 
longer limited to lead . 
3. This specification  is formulated with reference to the following international 
specifications  and National Standards  of the Republic of China  (CNS)
  
OIML R6  General provisions for gas volume meters  
 1989
 E
  
OIML R31  Diaphragm gas meters  
 1995
 E
  
CNS 14741  Microcomputer diaphragm gas meters for natural gas  
2003
 (E)
 
OIML R 137-1&2  G as meters                              
 2012
 E
  
 
 
Date of Promulgation   
.
, 201 7  Bureau of Standards, Metrology and 
Inspection, Ministry of Economic Affairs  Date of Enforcement  
July 1, 201 8 
 NO GUARANTEE ON THE TRANSLATION  
In case of discrepancies  between the English translation and Chinese text, the Chinese text shall govern.  
 
 CNMV 3 1 
3 
  
1. Scope: This technical  specification s applies to  diaphragm gas meters (hereinafter 
referred to as "gas meters") subject to verification and inspection, that are the gas 
volume  meters in which the gas flow is measured by means of measuring 
chamber s with deformable thin wall s, including the gas meters with a built -in 
temperature conversion device.  
 
2. Definition  
2.1 Flow rate (Q)   
- Quotient of the actual quantity of gas passing thr ough the gas meter and the time 
taken for this quantity to pass through the gas meter . 
 
2.2 Maximum  flow rate (Q max) 
- Highest flow rate at which a gas meter is required to operate within the limits of 
its maximum permissible error . 
 
2.3 Minimum  flow rate  (Q min) 
- Lowest flow rate at which a gas meter is required to operate within the limits of 
its maximum permissible error . 
 
2.4 Flow rate range  
- The range of the flow rate of gas that is limited by the maximum flow rate Q max 
and the minimum flow rate Q min. 
 
2.5 Maximum  permissible error  
- The extreme value of the error that is permitted by the legal requirements.  
 
2.6 Maximum  working pressure (P max) 
- Maximum internal pressure that a gas meter can withstand, within the limits of 
its maximum permissible er ror, without deterioration of its metrological 
performance . 
 
2.7 Minimum  working  pressure (P min) 
- Minimum internal pressure that a gas meter can withstand, within the limits of its 
maximum permissible error , without deterioration of its metrological 
perfo rmance . 
 
2.8 Working p ressure range  (Pm) 
- The range that is between the maximum working pressure and the minimum 
working pressure.  
 CNMV 3 1 
4 
  
2.9 Working temperature  range  (tm) 
- The temperature range that is allowed when a gas meter works within the 
maximum permis sible error . 
 
2.10 Pressure  loss 
- The pressure difference between the inlet and the outlet of a gas meter  when gas 
is flowing.  
 
2.11 Value of a given air volume quantity  
- The specified air volume that has been passing through the gas meter under test 
to ensure correct error of the meter.  
 
2.12 Cyclic  volume of a gas volume meter  
- The volume  of gas corresponding to the working cycle of the gas volume meter, 
i.e. to all the movements of the moving components which, except for the 
indicating device and the intermediate transmissions, resume for the first time the 
position they occupied at the beginning of the cycle .   
 
2.13 Built -in temperature conversion device  
- A device  which converts the volume measured at the metering conditions to the 
volume at base co nditions . 
 
2.14 Pressure  absorption  
- The difference between the pressures at the inlet and outlet of the gas volume 
meter while the gas is flowing . Using air under ambient pressure and ambient 
temperature as the medium , the average pressure loss in one me asurement cycle at 
flow rate Q max is taken as the total pressure absorption.  
 
3. Constitution   
3.1 Gas meter s shall be clear ly marked with the following items on easy scrutiny  
spot: 
(1) Model and serial  number.  
(2) Name of measured gas.  
(3) Flow rate range : mark the maximum flow rate and the minimum flow rate  as 
Table 1,  in cubic meters per hour (m3/h). 
(4) The nominal diameter (inside diameter of inlet and outlet, in mm)  
(5) Cyclic volume: V = …m3 (or dm3). 
(6) Directions of gas inlet and outlet.  
(7) Manufacturer ’s name or trademark.  
(8) Working pressure  range:  Pm =... - ... kPa ( or Pa). CNMV 3 1 
5 
  
(9) Maximum permissible pressure difference between inlet and outlet : ΔP 
max=...kPa ( or Pa). 
(10) Working  temperature range: t m= … - … ℃ 
(11) Type approval number . 
(12) Year of manufacture.  
(13) History of r epair: including m anufacturer ’s trademark and year of repair.  
(14) For a gas meter with a built-in temperature conversi on device, the base 
temperature and the range of convertible  temperature shall be marked.  
 
For meters applied for re -verification, t he aforementioned Items 8 to 1 4 do not 
apply to meters not subject to type approval or meters which passed initial 
verificat ion prior to type approval has come into force.  
 
For meters applied for re -verification, t he aforementioned Item 12 regarding the 
marking for year of manufacture does not apply to meters which pass initial 
verification before  June 30, 2018 . 
 
3.2 The indica ting device of a gas meter must be provided wit h scale marks which 
stand out in contrast to the scale  (i.e. the silver lines).  
 
3.3 The upper  limit of the maximum flow rate and the corresponding minimum flow 
rate for a gas meter are given in Table 1.  
 
Table 1 
Qmax 
m3/h Upper limit of Q min 
m3/h 
1 
1.6 
2.5 
4 
6 
10 
16 
25 
40 
65 
100 
160 
250 
400 
650 
1000  0.016  
0.016  
0.016  
0.025  
0.040  
0.060  
0.100  
0.160  
0.250  
0.400  
0.650  
1.000  
1.600  
2.500  
4.000  
6.500  
 
4. Verification, inspection and maximum permissible errors 
 CNMV 3 1 
6 
  
4.1 The traceability  of verification and inspection equipment s is required.  
 
4.2 Prior to  verification or inspection , gas meter s shall be placed at least 12 hours in 
the place in which the verification is implemented . 
 
4.3 External airtight test of gas meter s 
4.3.1 For gas maters with maximum working pressure not greater than  10 kPa : 
block the outlet of gas meter s; conduct the test with air pressure of 10  kPa, 
and hold for  3 minutes ; the leakage  rate shall be less than 67 Pa  / min. 
4.3.2 For gas maters with ma ximum working pressure greater than 10 kPa: b lock 
the outlet of gas meter s; conduct the test with maximum working pressure , 
and hold for  3 minutes ; the leakage  rate shall be less than 2%. 
4.3.3 When applying for verification to the dedicated weights and me asures 
authority or conducting self -verification, the applicant s or the measuring 
instrument enterprises entitled to conduct self -verification  shall submit  or 
prepare  quality report/ certificate , complied with the requirements of 
Section 4.3.1 or 4.3.2 and issued by testing lab s accredited by accreditation 
bodies of ILAC MRA, original manufacturers of said gas meters or 
enterprises holding business license of measuring instrument repairer, and 
relevant  supporting documents  in accordance with the quantity of gas 
meters to be tested . If necessary, t he dedicated weights and measures 
authority should check the external airtight of these gas meters.  
 
4.4 Pressure absorption test  
The total pressure absorption of gas meters, averaged over a measuring cycle, 
with a f low with ambient condition, at flow rate as Q max, shall not exceed the 
values given in Table 2.  
Table 2  
Maximum Flow  rate 
 
m3/h Maximum permissible average 
value for total pressure 
absorption  (including safety 
detection control unit)  
Verification  
Pa Maxi mum permissible average 
value for total pressure 
absorption  (including safety 
detection control unit)  
Inspection  
Pa 
1 to 10 (inclusive)  
16 to 65 (inclusive)  
100 to 1000 
(inclusive ） 200 
(242)  
 
300 
(330)  
 
400 
(440)  220 
(242)  
 
330 
(363)  
 
440 
(484)  CNMV 3 1 
7 
  
 
4.5 When implement ing error  test of gas meters  for verification and inspection , the 
procedure shall be conducted at specified flow rates and minimum volume s for 
verification and inspection  listed in Table 3. The difference  between the real 
flow rate and the flow rate listed in Table 3 shall not be greater  than 5%.  
For gas meters subject to type approval , the error test shall be conducted at flow 
rate Q max and 0.2 Q max one by one. Besides , 5% of the gas meters applied for 
verification shall be sampled and tested for error at flow rate 3 Q min. If the 
number of meters applied for verification is less than 100, it is counted as 100. 
In case any of the samples failed the test, the original sample s should be 
re-tested. If any of them failed the test again, the whole batch of gas meters 
applied for verification shall be rejected . 
For gas meters with Qmax greater  than 16 m3/h that are not subject to type 
approval , all meters shall be verified one by one for error test at flow rates Qmax 
and 0.2 Q max.  
For gas meters with Qmax not greater  than 16 m3/h that passed initial verification 
before the implement of type approval, all meters  shall be conducted error test 
one by one at  flow rate Qmax and 0.2 Q max when apply for re -verification. 
Besides, 5% of the gas meters applied for verification shall be sampled and 
tested for error at flow rate 3 Q min. If the number of meters applied for 
verification is less than 100, it is counted as 100. In case any of the samples 
failed the test, the original samples could  be re -tested. If any of them failed the 
test again, the whole batch of gas meters applied for verification shall be 
rejected . For gas meters with Qmax greater  than 16 m3/h that passed initial 
verification  before the implement of type approval, all meters  shall be 
conducted error test one by one at  flow rate Qmax and 0.2 Q max when apply for 
re-verification.  
 
Table 3 
Maximum 
Flowrate  
(m3/h) Verification  and inspection  flowrate 
m3/h Minimum volume for verific ation and inspection  
dm3 
Qmax 0.2 Q max 3 Q min  Qmax 0.2 Q max 3 Q min  
1 1 0.20 0.048  50 20 10 
1.6 1.6 0.32 0.048  50 20 10 
2.5 2.5 0.50 0.048  50 30 10 
4 4 0.80 0.075  70 50 20 
6 6 1.20 0.120  120 70 30 
10 10 2.00 0.180  200 100 50 
16 16 3.20 0.30 500 300 100 CNMV 3 1 
8 
  
25 25 5.00 0.48 800 400 200 
40 40 8.00 0.75 1200  600 300 
65 65 13.00  1.20 2000  1000  500 
100 100 20.00  1.95 4000  2000  1000  
160 160 32.00  .300 8000  4000  2000  
250 250 50.00  4.80 12000  6000  3000  
400 400 80.00  7.50 20000  10000  5000  
650 650 130.0 0 12.0 32000  16000  8000  
1000  1000  200.00  19.5 60000  30000  15000  
 
4.6 The error of gas meter shall be expressed by percentage; i.e., the ratio of the 
deviation between the indicated value of air volume flowing through the gas 
meter and the standard value from the standard meter  divided by the standard 
value from the standard meter . If the gas meter does not equip a temperature 
conversion device, the reference conditions of the standard value of standard 
meter s shall be  the absolute pressure at the inlet  and the temperature at  outlet of 
the gas meter. If the gas meter equips  a temperature conversion device, the 
reference conditions of the standard value for a  standard meter shall be  the 
absolute pressure at inlet and b asic temperature of the gas meter.  
Error  (%) = 
  value) (standard)  value standard -   value (Indicated
(%) 100 X  
 
(1) When a wet gas meter is used as calibrator, the standard value 
sV  is defined 
as follows.   
P T WG s CCQCFVV  )( )(
 
VWG：Volume of wet gas meter  
CF(Q)：Correction function of instr ument error of the wet gas meter  
CT：Temperature correction quantity between calibrator and gas meter  
Cp：Pressure correction quantity between calibrator and gas meter  
 
 (2) When  sonic nozzle is used as calibrator, the standard value 
sV  is defined as 
follows.  
 
 
 
 
Cd：Discharge coefficient of sonic nozzle  
A*：Cross -section area at throat  
C*：Critical flow function of sonic nozzle  
P0：Stagnation pressure at the upstream of sonic nozzle  
),()(0* *
mm od
sPT MTRtPCACV
CNMV 3 1 
9 
  
T0：Stagnation temperature at the upstream of sonic nozzle  
t： Collection time for verification   
R
：Universal gas constant  
M：Molecular mass of air   
),(m mPT
：Air density under Tm and Pm, in which Tm and Pm respectively stand 
for the temperature and pressure of the meter under test y  
 
4.7 Maximum permissible error s for v erification  and inspect ion of gas meter s 
With the air under room temperature and normal pressure as the medium,  the 
maximum permissible  error s for verification and inspection shall conform to the 
stipulations given i n Table 4. On ver ification of a meter the absolute value of 
each meter error shall not exceed 1 % at flow rates between 0.1  Qmax and Qmax 
where these  errors are all of the same sign.  
 
Table 4  
Flow rate  
(m3/h) Maximum permissible error s 
Verification  Inspection  
Qmin≦Q<0.1 Qmax 
0.1Qmax≦Q≦Qmax ±3% 
±1.5%  -6%,+3%  
±3% 
 
4.8 The period of validity of verification is 10 years, from the day of a verification 
compliance  mark  affixed to the gas meter to the first day of the following month 
of the next 10 years.  
 
5. Verification compliance marks  
5.1  The verification compliance mark  shall be attached with metal wire and seal at 
the opening of the shell on the body of a gas meter.  The duration of validity of 
verification shall also mark at easy scrutiny spot on the fr ont of the meter .