CZECH METROLOGY 
INSTITUTE  Okružní  31, 638 00 Brno  
legmet@cmi.cz  
+420 545 555 414 
 
1. ------IND-  2019 0492 CZ-  EN- ------ 20191021 --- --- PROJET 
 
Executive summary for the EC (not part of this legislation)  
'Gas flow sensors with a multi -hole orifice plate' may be placed on the market and put into use in the 
Czech Republic as legally controlled measuring instruments pursuant to Act  No 505/1990 on 
metrology, as amended.Pursuant to the Act, legally controlled m easuring instruments are instruments 
that are included in the list of the types of legally controlled measuring instruments (Implementing 
Decree No  345/2002) and, at the same time, intended (by the manufacturer/importer) for 
measurements of relevance to th e protection of public interests in areas of consumer protection, 
contractual relations, imposition of sanctions, fees, tariffs and taxes, health protection, 
environmental protection, occupational safety, or the protection of other public interests protect ed 
by special legislation. This is therefore a similar purpose as that which is used to identify stipulated 
products - non-automatic measuring and weighing instruments pursuant to Directives 2014/31/EU 
and 2014/32/EU.The requirements of this legislation do not apply to measuring instruments not 
placed on the market in the Czech Republic for the above purposes, identified by Act No 505/1990 
on metrology. 
The specified metrological requirements are comparable to the relevant requirements of the European 
standards and apply to the basic design of gas flow sensors with a multi -hole orifice plate.  
The purpose of this notified legislation is to lay down metrological and technical requirements for 
these legally controlled measuring instruments.This legislation also stipulates tests for type approval 
and verification of legally controlled measuring instruments of this type.  
(End of executive summary)  
 
 
PUBLIC DECREE  
 As the authority with substantive and territorial jurisdiction for stipulating metrological and technical 
requirements for legally controlled measuring instruments and stipulating test methods for type approval 
and verification of legally specified instruments pursuant to § 14(1) of Act No  505/1990, on metrology, 
as amended (hereinafter the ‘Metrolog y Act’), and in accordance with the provisions of § 172 et seq. of 
Act No  500/2004, the Code of Administrative Procedure (hereinafter the ‘CAP’), the Czech Metrology 
Institute (hereinafter the ‘CMI’) commenced ex officio proceedings on 154.2016 pursuant to  § 46 of the 
AC, and based on the supporting documents, hereby issues the following:  
 
 Measure of a General Nature No  0111- OOP -C094- 18 
2 I. 
MEASURE OF A GENERAL  NATURE  
number:0111- OOP -C094 -18 
 
laying down metrological and technical requirements for legally controlled measuring 
instruments, including the t esting methods for type approval and verification of legally 
controlled measuring instruments:  
'gas flow sensors with a multi -hole orifice plate'  
 
1 Basic definitions  
For the purposes of this general measure, terms and definitions pursuant to VIM and VIML1 as well as 
the terms and definitions stated below shall apply.  
1.1 
measurement system for measuring the flow quantity of gas  
a set of one or more measuring instruments (components of measuring instruments) and, often, other 
equipment arranged and adapted to provide information about the qualitative and quantitative 
characteristics of gas 
1.2 
gas flow sensor with a multi -hole orifice plate  
a gas flow sensor based on the principle of measuring differential pressure upstream and downstream of 
a multi- hole ori fice plate, inserted in a defined manner into pipes with specified geometric parameters  
1.3 
multi -hole orifice plate  
a thin plate with circular openings -  an orifice plate  
1.3.1 front side A  
the side of the multi -hole orifice plate that is installed facing  the influx of gas, with higher quality 
requirements  
1.3.2 back side B  
the side of the multi- hole orifice plate that is installed facing the outflow of gas, with lower requirements 
for quality, and with a space for markings.  
                                                      
1 TNI 01 0115 International Vocabulary of Metrology –  Basic and General Concepts and Associated Terms 
(VIM) and International Vocabulary of Legal Metrology (VIML) are part of the technical harmonisation 
compendium ‘Terminology in the field of metrology’, which is publicly available at www.unmz.cz.  Measure of a General Nature No  0111- OOP -C094- 18 
3 1.4 
pressure tap (in wall)  
an annular or circular cavity drilled in the wall of a pipe using such a method that the edge of the cavity 
fits with the interior surface of the pipe.  
1.5 static pressure of the gas flowing through the pipe  
pressure which can be measured by connecting a pressure -measuring device to the pressure tapping in 
the wall  
1.6 differential pressure 
the difference between (static) pressures measured in pressure taps in the wall, one being upstream of 
and one being downstream of the multi -hole orifice plate inserted  in a straight pipe through which the 
gas flows, taking into account all differences in tap height in front of and past the multi -hole orifice plate  
1.7 
ratio of diameters β  
the ratio of the diameter of the orifices in the multi -hole orifice plate to the i nternal diameter of the pipe 
upstream of the multi -hole orifice plate  
1.8 pipe Reynolds number Re
D 
a dimensionless parameter expressing the ratio between the inertial forces and the frictional forces in 
the pipe upstream of the multi -hole orifice plate  
1.9 
flow coefficient C  
a coefficient defined for a flow of incompressible liquid; it indicates the ratio of the actual flow through 
the multi -hole orifice plate to the theoretical flow  
1.10 
expansion coefficient  
a coefficient characterising the degree of comp ressibility of the liquid in question  
2 Metrological requirements  
The specified metrological requirements are comparable with the relevant requirements of European 
standards and apply to the basic design of the gas flow sensor with a multi -hole orifice pla te. 
During verification, gas flow sensors with a multi -hole orifice plate are subject to the metrological 
requirements that applied to their being placed into circulation.  
2.1 Operating conditions  
The operating conditions are specified by the manufacturer in consideration of the range of anticipated 
ambient temperatures during operation and in consideration of range of temperatures of the gas being 
measured.If there are significant temperature differences between the ambient temperature and the 
temperature of the flowing gas, the gas flow sensor shall be thermally insulated by suitable means.  Measure of a General Nature No  0111- OOP -C094- 18 
4 The following conditions apply to gas flow sensors with a multi -hole orifice plate when using individual 
types of pressure taps:  
 with corner pressure taps:  for 0.40  ≤ β ≤ 0.56:  
for β > 0.56:  ReD ≥ 5,000  
ReD ≥ 16,000 β2 
  with flange pressure taps:  ReD ≥ 5,000 and ReD ≥ 170 β2D, where D is in millimetres  
 with taps at distances of D and D/2: for 0.40  ≤ β ≤ 0.56:  
for β > 0.56:  ReD ≥ 5,000  
ReD ≥ 16,000 β2 
2.2 Limits of use  
Gas flow sensors with a multi -hole orifice plate may only be used for:  
  internal pipe diameter D: 25 mm to 1000  mm 
  minimum orifice plate hole diameter di: 6.25 mm  
  minimum characteristic dimension of the multi -hole orifice 
plate d : 12.5 mm  
  ratio of diameters β (d/D): 0.4 to 0.65 
2.3 Relative uncertainty of the multi -hole orifice plate flow coefficient  
The following relative uncertainties of flow coefficient C  apply to a multi- hole orifice plate under 
compliance with all technical requir ements and installation requirements within the limits of undesired 
increases in the relative uncertainty of the flow coefficient by additional uncertainty: 0.75 % for 
0.40 ≤  β ≤ 0.65.  
If D < 71.12 mm, then the relative uncertainty 0,9(0,75−𝛽𝛽)�2,8−𝐷𝐷
25,4�%, in which D  is in 
millimetres, must be added to the above -mentioned relative uncertainty values of the flow coefficient.  
If β > 0.5 and Re D < 10,000, then the relative uncertainty 0.5 % must be added to the above -mentioned 
relative uncertainty values of t he flow coefficient.  
If a straight length of pipe with additional uncertainty is used, then this uncertainty must be added to the 
uncertainty of the flow coefficient.The use of a straight length of pipe load weighted with additional 
uncertainty is limited to either only the section upstream of the orifice plate, or only the section 
downstream of the orifice plate.  
Compliance with technical requirements is determined during the verification of the individual parts of 
the gas flow sensor, and compliance with installation requirements is determined within the scope of 
verification of the evaluation unit for the measurement of gas flow quantity. 
2.4 Relative uncertainty of the expansion coefficient  
In compliance with all of the established technical requirements, it holds true that the relative uncertainty 
of the expansion coefficient ε is 5Δ𝑝𝑝
𝜅𝜅𝑝𝑝1 %, in which κ is the isentropic exponent, Δ p is the differential 
pressure and p1 is the absolute static pressure of the gas upstream of the orifice plate.  
Compliance with technical requirements is ascertained during the verification of the individual parts of 
the gas flow sensor.  Measure of a General Nature No  0111- OOP -C094- 18 
5 3 Technical requirements  
The specified metrological requirements are on the level of the relevant requirements of the European 
standards and apply to the basic design of the gas flow sensor with a multi -hole orifice plate.  
During verification, gas flow sensors with a multi -hole orifice plate are subject to the technical 
requirements that applied to their being placed into circulation.  
3.1 Desig n 
The basic design of a gas flow sensor with a multi -hole orifice plate includes the following parts:  
  orifice plate;  
  pressure taps;  
  a stabilising straight length of pipe upstream of the orifice plate with total minimum length of 10 D, 
or a shorter length if specified in the type approval certificate;  
  a stabilising straight length of pipe past the orifice plate with total minimum length of 2 D, or a shorter 
length if specified in the type approval certificate;  
The design and installation of the mul ti-hole orifice plate must ensure that it is not subject to plastic or 
elastic deformation during operation under the pressure of the flowing gas, or that said deformation is within specified limits.  
3.2 Orifice plate  
The part of the orifice plate with diameter D  that is located inside the pipe after installation must be 
circular and concentric with the pipe axis.The front and back side of the orifice plate must be parallel.  
Unless specified otherwise, all requirements placed on the design of the orifice plate only apply to the 
parts that are located in the pipe.  
3.2.1 Front side A  
3.2.1.1 Flatness  
The maximum deviation between the linear straight edge of length D , positioned in a discretionary 
direction above diameter D  on the orifice plate, and the plane of the front side of orifice plate A must be 
less than 0.005 D  – d)/2, i.e. the inclination of the surface must be less than 0.5 %.  
3.2.1.2 Surface roughness  
Inside a circle of diameter D, concentric with the axis of th e pipe in which the orifice plate is installed, 
the orifice plate must have a roughness parameter of Ra  < 10-4d. 
3.2.2 Back side B  
The back side of the orifice plate must be flat and parallel with the front side.The roughness of the 
surface on the back sid e of the orifice plate need not achieve the quality required on the front side of the 
orifice plate.  
3.2.3 The thickness e of the holes in the orifice plate and orifice plate thickness E  
The thickness e of the holes in the orifice plate must be between 0.0 05D and 0.02D .The difference 
between the values of e  measured at an arbitrary point of the orifice plate holes may not be greater than 
0.001D . Measure of a General Nature No  0111- OOP -C094- 18 
6 The thickness of the orifice plate E  must be between e and 0.05D .In the case of 50 mm ≤ D ≤ 64 mm, E 
can be up to 3.2 mm.  
For orifice plates with D  < 200 mm, the difference between the values of E  measured at an arbitrary 
point must not be greater than 0.001D .For orifice plates with D  < 200 mm, the difference between the 
values of E  measured at an arbitrary point must not be greater than 0.2 mm.  
3.2.4 Chamfer angle α 
If the thickness E of the orifice plate is greater than the thickness of the orifice plate holes e, then the 
orifice plate holes on the back side of the orifice plate must be chamfered; the chamfer angle α must be 
45° ± 15°.The chamfered surface must be finished smoothly. 
3.2.5 Edges of the orifice plate holes  
The front edge of the orifice plate holes must not have any burrs or other damage.The edges must be 
sharp and rig ht-angled.The radius of curvature of the front edge of the orifice plate holes must not be 
greater than 0.0004d.  
The angle between individual orifice plate holes and the front edge of the orifice plate must be 
90° ± 0.3°. 
The back edge(s) of the orifice pl ate holes need not meet the qualitative requirements for the front edges 
of the orifice plate holes.  
3.2.6 Orifice plate hole diameters d i 
The diameters of the orifice plate holes d i must meet the requirements specified in Article 2.2.  
Orifice plate hole diameter d i is the mean value of the measurement of at least four diameters at 
approximately the same angles to each other.  
3.2.7 Cylindricity of the orifice plate holes  
The orifice plate holes must be cylindrical, and no diameter measur ed in the cylindrical part of the orifice 
plate holes may differ by more than 0.05 % of the mean of diameters d i. 
3.2.8 Surface roughness of the orifice plate holes  
The surface roughness of the cylindrical part of the orifice plate holes must not influence  the 
measurement of the sharpness of the edge.  
3.2.9 Orifice plate material  
The orifice plate may be made of any kind of material with a known longitudinal expansion coefficient, on condition that it will continuously meet all of the relevant requirements of this regulation under 
operating conditions.  
3.2.10 Design execution of the orifice plate for flow in both directions  
With orifice plates intended for measuring gas flow in both directions, the technical and metrological 
requirements for the front side A  of the orifice plate must be applied on both sides of the orifice plate 
(edges).The orifice plate holes must not be chamfered.  Measure of a General Nature No  0111- OOP -C094- 18 
7 3.3 Differential pressure taps  
Flange pressure taps, corner pressure taps or pressure taps at distances D  and D/2 may be used fo r 
differential pressure taps.  
In the case of a bi -directional version of a gas flow sensor with centric orifice and pressure taps at 
distances D a D/2, two pairs of said taps must be used upstream and downstream of the orifice, 
depending on the direction of flow.  
3.4 Straight lengths of pipe upstream and downstream of the orifice plate  
3.4.1 Design  
The minimum straight length of pipe upstream of the orifice plate which must be complied with in the 
specific application shall be determined based on the ratio of diameters β  and according to the type and 
mutual arrangement of at least two adapters placed upstream of the orifice plate.  
The straight length of pipe between the first adapter in front of the orifice plate and the orifice plate 
itself can consist of o ne or more pieces, but the part of the pipe up to a length of 2 D in front of the orifice 
plate must always consist of one single piece.  
The design and execution of the straight lengths of pipe must facilitate the performance of the tests 
defined in Article  5.2.4 or Article  6.2.4.  
3.4.2 Pipe straightness  
The straightness of the stabilising minimum straight lengths of pipe is be deemed adequate if the surface 
deviation of the pipe from a straight line running parallel to the axis of the pipe does not exceed 0.4 % 
of its length upstream and downstream of the orifice plate.  
3.4.3 Mean value of the inner diameter of the pipe  
The internal diameter D of the pipe is the arithmetic mean of at least twelve measured diameter values, 
i.e. four diameters lying at approximately the same angles to one another at each of at least three cross 
sections evenly distributed at a length of 0.5 D upstream of the orifi ce plate.Two of these cross- sections 
must lie on the plane of the front pressure tap and at a distance of 0.5D  from the front pressure tap.If a 
welded joint is located at a length of 0.5D  upstream of the front pressure tap, then the third cross section 
must be placed precisely into said joint.  
3.4.4 Circularity and cylindricity of pipe  
The internal shape of the pipe must be circular along the entire minimum straight length of pipe.  
No inner pipe diameter may differ from the mean value of inner diameter D  by more than 0.3 % in any 
plane in the part of the straight length of pipe up to a distance of 2D  upstream of the orifice plate.  
The difference between inner pipe diameters of two arbitrary sections of pipe may not exceed 0.3 % of 
the mean value of inner dia meter D  along the straight section of pipe between 2D  and 10D  upstream of 
the orifice plate.At the same time, the overlap of the inner diameters of parts of the pipe connected to 
one another, caused by a deviation and/or change in diameter D , may not excee d 0.3 % of the mean 
value of the internal diameter D at any point along the inner circumference of the pipe.  
No internal diameter of a straight length of pipe behind the orifice plate may differ from the mean 
internal diameter D  of the pipe upstream of the  orifice plate by more than 3 %, at least over a length of 
2D from the front side of the orifice plate.  
3.4.5 Quality of the internal surface of the pipe  
The internal surface of the pipe must be clean and free of any visible defects.  Measure of a General Nature No  0111- OOP -C094- 18 
8 Over the stipulated minimum straight length of the basic version of the sensor, the internal pipe surface 
must meet specified requirements for surface roughness Ra  in front of the orifice plate.The value of the 
mean arithmetic deviation (roughness) of the profile Ra must be suc h that 104Ra/D is less than the 
maximum permissible value given in Table 1 and greater than the minimum permissible value given in 
Table 2.  
Table 1 – Maximum permissible value 104Ra/D 
β ReD 
≤ 104 3 × 104 105 3 × 105 106 3 × 106 107 3 × 107 108 
≤ 0.20 15 15 15 15 15 15 15 15 15 
0.30 15 15 15 15 15 15 15 14 13 
0.40 15 15 10 7.2 5.2 4.1 3.5 3.1 2.7 
0.50 11 7.7 4.9 3.3 2.2 1.6 1.3 1.1 0.9 
0.60 5.6 4.0 2.5 1.6 1.0 0.7 0.6 0.5 0.4 
≥ 0.65 4.2 3.0 1.9 1.2 0.8 0.6 0.4 0.3 0.3 
Table 2 – Minimum permissible value 104Ra/D 
β ReD 
≤ 3 × 106 107 3 × 107 108 
≤ 0.50 0.0 0.0 0.0 0.0 
0.60 0.0 0.0 0.003  0.004  
≥ 0.65 0.0 0.013  0.016  0.012  
3.5 Installation requirements  
The installation position of the orifice plate must be clearly defined by its design, or it must be 
schematically marked directly on the orifice plate.  
4 Measuring instrument markings  
4.1 General  
All inscriptions and markings must be easily visible, legible, and indelible and must be a source of the 
information needed for the trouble -free implementati on of the gas flow sensor with a multi -hole orifice 
plate into the gas flow measurement system in relation to its other components.  
4.2 Markings  
4.2.1 Markings on the orifice plate  
The following information must be on the orifice plate:  
a)  serial number;  
b)  the value of the characteristic dimension of the multi -hole orifice plate d20 in relation to a reference 
temperature of 20  °C; 
c)  type approval mark;  
d)  suitable marking for type identification and placement of the differential pressure taps, if the pressure 
taps are a fixed component of the design execution of the orifice plate.  
In cases in which the above -mentioned information is not visible once the multi- hole orifices have been 
installed into the pipe, the orifice plate must also be equipped with a separate additional nameplate with 
the information in question, which will help safeguard the orifice plate against removal or replacement.  Measure of a General Nature No  0111- OOP -C094- 18 
9 In cases in which the orifice plate could be improperly installed against the direction of the gas flow, the 
direc tion of flow must be indicated on the orifice plate by a means which cannot be mistaken. 
4.2.2 Markings on gas flow sensors with a multi -hole orifice plate  
The following information must be on the gas flow sensor with a multi -hole orifice plate:  
a)  the name of the manufacturer and type (design variant);  
b)  serial number and year of production;  
c)  the value of the internal diameter D 20of the pipe in relation to a reference temperature of 20 °C;  
d)  type approval mark;  
e)  nominal size DN/nominal pressur e PN;  
f)  indication of the direction of flow;  
g)  suitable marking for type identification and placement of differential pressure taps, if pressure taps are a permanent part of the design of straight lengths of pipe;  
h)  the measured flow range or the max imum flow value.  
This information must be placed on a part of the gas flow sensor that is inseparably joined to a straight 
pipe section 0.5D  in front of the orifice plate (installation chamber, connector flange in front of the 
orifice plate, a straight len gth of pipe at most 2D  in front of the orifice plate).  
The following information must be specified on the connection flange or other location of the straight 
length of pipe past the orifice plate (at a maximum length of 2D  past the orifice plate).  
i)  the manufacturer's name;  
j)  serial number and year of production;  
k)  type approval mark;  
l)  an indication of the direction of flow (if reverse installation is possible).  
If the straight length of pipe upstream of the orifice plate is made up of multiple parts of pipe in the basic 
configuration of the gas flow sensor, then all of these parts must be marked with the type approval 
marking.All major separate parts of the gas flow sensor with a multi -hole orifice plate must generally 
be marked with a type appr oval marking.  
4.3 Marking with official marks  
Suitable areas for placement of the type approval mark and official mark(s) must be provided.  
It must be possible to safeguard the orifice plate against unauthorised removal or replacement.  
5 Type approval of t he measuring instrument  
The type approval process for a gas flow sensor with a multi -hole orifice plate encompasses the 
following tests and activities:  
a)  an external inspection;  
b)  a check of geometric parameters.  
5.1 External inspection  
The following are checked during the external inspection:  
  completeness of the required technical documentation;  Measure of a General Nature No  0111- OOP -C094- 18 
10   the conformity of the metrological and technical characteristics specified by the manufacturer in the 
documentation with this regulation's technical and metrological requirements specified in Chapters  2 
and 3;  
  the orifice plate, pressure taps and straight lengths of pipe in terms of potential mechanical damage 
or potential traces of corrosion which would preclude further testing; and  
  the spaces for indicating the basic geometric parameters of the gas flow sensor with a multi -hole 
orifice plate and spaces for placing official marks are determined.  
5.2 Check of geometric parameters  
5.2.1 Test equipment  
Suitable equipment with valid metrological traceab ility must be used during the check of geometric 
parameters.  
5.2.2 Reference ambient temperature for tests  
5.2.2.1 Orifice plate  
During the test, the ambient temperature must be between 18 and 22 °C and must not change by more than 2 °C.  
5.2.2.2 Straight lengths of pipe  
Pipe ≤ DN 300:  During the test, the ambient temperature must be between 15 and 25 °C and must 
not change by more than 2 °C. 
Pipe > DN 300:  During the test, the ambient temperature must be between 10 and 30 °C and must not change by more tha n 5 °C. 
5.2.3 Check of the geometric parameters of the orifice plate  
The internal diameters d
i of the orifice plate holes must be established in accordance with the 
requirement of Article 3.2.6 and must relate to a reference temperature 20  °C. 
The followin g geometric parameters of the orifice plate are also checked:  
  flatness of the front side (see Article 3.2.1.1);  
 surface roughness of the front side (see Article 3.2.1.2);  
  thickness of the orifice plate holes and thickness of the orifice plate (see Article 3.2.3);  
  chamfer angle (see Article 3.2.4);  
  the edges of the orifice plate holes (see Article 3.2.5);  
  cylindricity of the orifice plate holes (see Article 3.2.7);  
  surface roughness of the orifice plate holes (see Article 3.2.8).  
In the c ase of an orifice plate intended for measuring the gas flow in both directions, all of the tests 
specified in Article 5.2.3 must be carried out for both sides of the orifice plate.  
5.2.4 Check of the geometric parameters of straight lengths of pipe  
The mean internal diameter value D20 of the pipe must be established in accordance with the requirement 
of Article  3.4.3 and must relate to a reference temperature of 20  °C. 
The following geometric parameters are checked for straight lengths of pipe:  
  the straightness of the pipe (see Article 3.4.2);  
  the circularity and cylindricity of the pipe (see Article 3.4.4);  
  the roughness of the pipe's internal surface (see Article 3.4.5).  Measure of a General Nature No  0111- OOP -C094- 18 
11 In the case of a gas flow sensor with a multi -hole orifice plate inten ded for flow in both directions, all 
of the tests specified in Article 5.2.4 must be carried out for the straight lengths of pipe positioned 
upstream and downstream of the orifice plate.  
5.2.5 Check of the geometric parameters of pressure taps  
Compliance w ith the requirements for differential pressure taps according to the manufacturer’s 
specifications shall be ascertained.  
6 Initial verification  
The following tests and activities shall be carried out during the initial validation of gas flow sensors with a  multi -hole orifice plate:  
a)  visual inspection;  
b)  test of metrological characteristics;  
c)  marking of the basic geometric parameters on the relevant parts of a gas flow sensor with a multi -
hole orifice plate.  
6.1 Visual inspection  
The purpose of a visual inspection is to check that:  
  the execution of the gas flow sensor with a multi -hole orifice plate matches the approved type;  
  the orifice plate, pressure taps and straight lengths of pipe are not mechanically damaged and whether 
or not they bea r traces of corrosion that would preclude further testing, 
  the content and implementation of markings and inscriptions correspond to the information and 
requirements specified in the type approval certificate for the measuring instrument.  
If the gas flow sensor with a multi- hole orifice plate does not meet the external inspection requirements, 
it is not tested further.  
6.2 Check of geometric parameters  
6.2.1 Test equipment  
Suitable equipment with valid metrological traceability must be used during the  check of geometric 
parameters.  
6.2.2 Reference ambient temperature for tests  
The ambient temperature requirements pursuant to Article 5.2.2 apply to the testing.  
6.2.3 Check of the geometric parameters of the orifice plate  
The internal diameters di of the  orifice plate holes must be established in accordance with the 
requirement of Article 3.2.6 and must relate to a reference temperature 20  °C. 
The following geometric parameters are also checked during the initial verification of the orifice plate:  
  flatness of the front side (see Article 3.2.1.1);  
 surface roughness of the front side (see Article 3.2.1.2);  
  thickness of the orifice plate holes and thickness of the orifice plate (see Article 3.2.3);  
  chamfer angle (see Article 3.2.4);  
  the edges of  the orifice plate holes (see Article 3.2.5);  
  cylindricity of the orifice plate holes (see Article 3.2.7);  Measure of a General Nature No  0111- OOP -C094- 18 
12   surface roughness of the orifice plate holes (see Article 3.2.8).  
In the case of an orifice plate intended for measuring the gas flow in both d irections, all of the tests 
specified in Article 6.2.3 must be carried out for both sides of the orifice plate.  
6.2.4 Check of the geometric parameters of straight lengths of pipe  
The mean internal diameter value D20 of the pipe must be established in acco rdance with the requirement 
of Article  3.4.3 and must relate to a reference temperature of 20  °C. 
During initial verification, the following geometric parameters of the straight lengths of pipe are also 
checked:  
  the straightness of the pipe (see Article 3.4.2);  
  the circularity and cylindricity of the pipe (see Article 3.4.4);  
  the roughness of the internal surface of the pipe (see Article 3.4.5); only the maximum permissible 
value per Table 1 is checked.  
In the case of a gas flow sensor with a multi -hole orifice plate intended for flow in both directions, all 
of the tests specified in Article 6.2.4 must be carried out for the straight lengths of pipe positioned 
upstream and downstream of the orifice plate.  
6.2.5 Check of the geometric parameters of the differential pressure taps  
Compliance with the requirements for differential pressure taps according to the manufacturer’s 
specifications shall be ascertained during the initial verification.  
6.2.6 Marking of basic geometric parameters on the measu ring instrument  
The resultant value of the characteristic dimension d20 of the multi -hole orifice plate calculated from 
internal diameters of orifice plate holes di determined pursuant to Article 6.2.3 must be marked in a 
legible and indelible manner in a suitable location on the orifice plate in accordance with Article 4.2.1.  
The resultant internal diameter value D20 of the pipe determined pursuant to Article 6.2.4 must be marked in a legible and indelible manner in a suitable location on the gas flow sensor in accordance 
with Article 4.2.2.  
7 Subsequent verification  
The procedure for the subsequent verification of the orifice plate is identical to the procedure for the 
initial verification pursuant to Article 6.2.3.  
Subsequent verification of straight leng ths of pipe of gas flow sensors with a multi -hole orifice plate is 
not performed. 
The user of the measuring instrument is responsible for long- term compliance with operating conditions, 
including keeping it in proper condition and keeping the inner surface  of the pipe clean.For this purpose, 
the user of the measuring instrument performs (or ensures) a visual inspections of the internal surface 
of the pipe at intervals that take into account operating conditions and the type of gas medium being 
measured.This check must also always be performed prior to the installation of the orifice plate.  
8 Measuring instrument examination  
When examining measuring instruments pursuant to § 11a of the Metrology Act at the request of a person who may be affected by incorrect measurement, the procedure under Chapter  6 shall be followed.  Measure of a General Nature No  0111- OOP -C094- 18 
13 9 Notified standards  
For the purposes of specifying the metrological and technical requirements for measuring instruments 
and specifying the testing methods for their type approval and verification arising from this general 
measure, the CMI will notify Czech technical standards, other technical standards or technical 
documents of international or foreign organisations, or other technical documents containing more 
detailed technical requi rements (hereinafter referred to as ‘notified standards’).The CMI will publish a 
list of these notified standards attached to the relevant measures, together with the general measure, in 
a manner accessible to the public ( on www.cmi.cz ). 
Compliance with notified standards or parts thereof is considered, to the extent and under the conditions stipulated by this measure of a general nature, to be compliance with the requirements stipulated by this 
measure to which these standards or parts thereof apply.  
Compliance with notified standards is one way of demonstrating compliance with the 
requirements.These requirements may also be met by using another technical solution guaranteeing an 
equivalent or higher level of protectio n of legitimate interests.  
II. 
GROUNDS  
The CMI has issued this measure of a general nature laying down metrological and technical 
requirements for legally controlled measuring instruments and tests for the type approval and 
verification of legally controll ed measuring instruments – ‘gas flow sensors with a multi -hole orifice 
plate’ – in accordance with § 14(1)(j) of the Metrology Act to implement § 6(2), § 9(1) and (9), and 
§ 11a(3) of the Metrology Act.  
Under item  1.3.11(c) in the Annex ‘List of the Types of Legally Controlled Measuring Instruments’ to 
Implementing Decree No  345/2002 specifying the measuring instruments whose verification is 
mandatory and measuring instruments subject to type approval, as amended, this type of measuring 
instrument is classi fied as an instrument subject to mandatory verification.  
This legislation (Measure of a General Nature) was notified in accordance with Directive (EU) 2015/1535 of the European Parliament and of the Council of 9 September 2015 laying down a procedure 
for the provision of information in the field of technical regulations and of rules on Information Society 
services.  
III. 
INSTRUCTIONS  
In accordance with § 173(2) CAP, no appeals may be made regarding general measures.  
In accordance with the provisions of §  172(5) CAP, decisions on objections are final, and appeals may 
not be made against them.  
Compliance of the measure of a general nature with legislation may be subject to a review process 
pursuant to§ 94 to § 96 of the Administrative Code.A party to the procee dings may initiate review 
proceedings to be conducted by the administrative authority that issued the measure of a general nature.If the administrative authority finds no reason to commence the review proceedings, it shall communicate 
and provide grounds f or this within 30 days.Pursuant to § 174(2) of the Administrative Code,a ruling on 
the commencement of review proceedings can be issued within three years of the effective date of the 
measure of a general nature.  Measure of a General Nature No  0111- OOP -C094- 18 
14 IV. 
ENTRY INTO FORCE  
This measure of a general nature comes into effect on the fifteenth day after its date of publication (§  24d 
of the Metrology Act). 
  
 
Czech Metrology Institute  
Director -General  
 
Posted on:  
Removed on:  
Effective date: