Okružní  31, 638 00 Brno  
legmet@cmi.cz  
 +420 545 555 414 
 
1. ------IND-  2019 0287 CZ-  EN- ------ 20190628 --- --- PROJET 
Executive summary for the EC (not part of this legislation)  
‘Venturi tube gas flow sensors’ may be placed on the market and put into use in the Czech Republic as 
legally controlled measuring instruments  in accordance with Act  No 505/1990 on metrology, as 
amended. Pursuant to the Act, legally controlled measuring in struments are instruments, which 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 in the protect ion of public interests in the fields 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 protected by 
special legislation . This means that their purpose is similar to that used for defining specified products 
– measuring instruments and non- automatic weighing instruments – under Directives 2014/31/EU 
and 2014/32/EU. The requirements of this legislation do not apply to measuring instruments placed 
on the market in the Czech Republic for other than the above purposes defined by Act No 505/1990 
on metrology. 
The purpose of this notified legislation is to lay down metrological and technical requirements for 
legally controlled measuring instruments of the type specified above. This legislation also lays down 
the tests for the purposes of type -approval and verification of legally controlled measuring 
instruments of the type specified above.  
 (End of executive summ ary.)  
 
 
 
P U B L I C  N O T I C E 
 
As the authority with substantive and territorial jurisdiction in the matter of laying down metrological 
and technical requirements for legally controlled measuring instruments and stipulating the testing 
methods for type-approval and verification of legally controlled measuring instruments pursuant to 
§ 14(1) of Act No  505/1990, on metrology, as amended (hereinafter referred to as the ‘Metrology Act’), 
and in accordance with the provisions of § 172 et seq. of Act No 500/2004, the Administrative Procedure 
Code (hereinafter referred to as the ‘APC’), on 15  April  2016, the Czech Metrology Institute (hereinafter 
referred to as the ‘CMI’) commenced ex officio proceedings pursuant to §  46 APC, and, on the basis of 
supporting  documents, issues the following:  
 
 General Measure No 0111- OOP -C091- 18 
2 I. 
D R A F T   G E N E R A L   M E A S U R E 
number: 0111- OOP -C091 -18 
 
laying down metrological and technical requirements for legally controlled measuring 
instruments, including the testing methods for type -approval and  verification of legally 
controlled measuring instruments:  
 ‘Venturi tube gas flow sensors’  
The specified technical and metrological requirements are at a level comparable to that of the relevant 
requirements of the European standards and apply to the basi c design of the Venturi tube gas flow sensor.  
 
1   Basic definitions  
For the purposes of this general measure, the terms and definitions pursuant to VIM and VIML1 and the 
terms and definitions below shall apply:  
1.1    
gas flow quantity measuring system  
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    
Venturi tube gas flow sensor  
a gas flow sensor based on the principle of measuring the difference between the pressure upstream of 
the Venturi tube and in its throat inserted in the defined manner into a pipe with specified geometric 
parameters  
1.3    
Venturi tube  
a primary element consist ing of a conical convergent inlet connected to a cylindrical section (the 
‘throat’) and a conical expanding section (the ‘divergent section’)  
1.4   pressure tapping (in the wall)  
annular or circular cavity drilled into the pipe wall in such a way that the edge of the cavity is flush with 
the pipe’s inner surface  
                                                      
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  General Measure No 0111- OOP -C091- 18 
3 1.5    
static pressure of gas flowing through a pipe  
pressure which can be measured by connecting a pressure- measuring device to the pressure tapping in 
the wall  
1.6    differential pressure  
difference between the (static) pressures measured at the pressure tappings in the wall, one of which is 
upstream of the Venturi tube and the other of which is in the throat of the Venturi tube, inserted into a straight pipe through which gas flow occurs, considering all the differences in the amounts of tappings 
upstream of the Venturi tube and in its throat.  
1.7    
diameter ratio β 
the ratio of the diameter of the nozzle throat orifice to the internal diameter of the pipe upstream of the 
nozzle  
1.8   
pipe Reynolds number ReD 
dimensionless parameter expressing the ratio between the inertia and viscous forces in the pipe upstream 
of the nozzle  
1.9    
flow coefficient C  
coefficient, defined for an incompressible fluid flow, which relates the actual flowrate t hrough the nozzle 
to the theoretical flowrate  
1.10    
expansion coefficient  
coefficient characterising the degree of compressibility of the fluid  
2   Metrological requirements  
During verification of Venturi tube gas flow sensors, the metrological requirements applicable when 
they were placed on the market shall apply.  
2.1   Operating conditions  
The operating conditions are specified by the manufacturer taking into account the range of the 
anticipated ambient temperatures during operation and the ra nge of temperatures of the gas to be 
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.  
The following condit ions shall apply to Venturi tube gas flow sensors for the use of the individual types 
of Venturi tubes:  
 Venturi tube with a cast convergent section:  2·10
5 ≤ Re D ≤ 2·106 
 Venturi tube with a machined convergent section:  2·105 ≤ Re D ≤ 1·106 
 Venturi tube  with a rough -welded sheet -iron convergent section:  2·105 ≤ Re D ≤ 2·106 General Measure No 0111- OOP -C091- 18 
4 2.2   Limits of use  
Venturi tube gas flow sensors with a cast convergent section may only be used for:  
 internal pipe diameter D: 100 mm to 800 mm 
 diameter ratio β  (d/D): 0.30 to 0.75  
 the inner surface roughness of the tube and the pipe shall comply with the specified requirements  
Venturi tube gas flow sensors with a machined convergent section may only be used for:  
 internal pipe diameter D: 50 mm to 250 mm 
 diameter ratio β  (d/D): 0.40 to 0.75  
 the inner surface roughness of the tube and the pipe shall comply with the specified requirements  
Venturi tube gas flow sensors with a rough- welded convergent section may only be used for:  
 internal pipe diameter D: 200 mm to 1  200 mm 
 diameter ratio β  (d/D): 0.40 to 0.70  
 the inner surface roughness of the tube and the pipe shall comply with the specified requirements  
2.3   Relative uncertainty of the Venturi tube flow coefficient  
Subject to compliance with all technical requirements and installation requirements, the following 
relative uncertainties of the flow coefficient C  apply to the individual types of Venturi tubes within 
limits not requiring the relative uncertainty of the f low coefficient to be increased by additional 
uncertainties:  
 Venturi tube with a cast convergent section:  0.7 %  (C = 0.984)  
 Venturi tube with a machined convergent section:  1 % (C = 0.995)  
 Venturi tube with a rough -welded sheet -iron convergent secti on: 1.5 %  (C = 0.985)  
If straight lengths of pipe with additional uncertainty are used, this uncertainty shall be added to the 
uncertainty of the flow coefficient. Straight lengths of pipe carrying additional uncertainty may only be 
used for the section upstream of the Venturi tube.  
Compliance with technical requirements shall be ascertained when verifying the individual components 
of the gas flow sensor and compliance with installation requirements shall be ascertained when verifying 
the calculator unit fo r the measurement of gas flow quantity.  
2.4   Relative uncertainty of the expansion coefficient  
Subject to compliance with all of the specified technical requirements, the following relative uncertainty 
of the expansion coefficient ε applies to the types o f Venturi tubes referred to above:  
(4 + 100β8)
1pp∆ % 
where Δ p is the differential pressure and p 1 is the absolute static gas pressure upstream of the Venturi 
tube.  
Compliance with technical requirements shall be ascertained when verifying  the individual components 
of the gas flow sensor.  
3   Technical requirements  
During verification of Venturi tube gas flow sensors, the technical requirements applicable when they 
were placed on the market shall apply.  General Measure No 0111- OOP -C091- 18 
5 3.1   Design  
The basic design of the Venturi tube gas flow sensor consists of the following parts:  
 the Venturi tube (with a cast, machined or rough- welded sheet -iron convergent section);  
 pressure tappings;  
 a stabilising straight length of pipe upstream of the Venturi tube with a minimum length of 2D  
(measured from the front end of the Venturi tube’s inlet section).  
The Venturi tube consists of the inlet cylinder (A) connected to the conical convergent section (B), the 
cylindrical throat (C) and the conical divergent section (E). The inn er surface of the Venturi tube shall 
be rotationally symmetrical and concentric with the pipe centreline.  
The design and installation of the Venturi tube shall ensure that it is not subject to plastic or elastic deformation during operation under the press ure of the flowing gas or, as the case may be, that the 
deformation is only within the specified limits.  
The Venturi tube and straight pipe sections may be made of any kind of material with a known 
longitudinal expansion coefficient provided that all the r elevant requirements of this regulation are 
continuously complied with under operating conditions. 
3.2   Venturi tube with a cast convergent section  
It is manufactured by casting in a sand mould or using other methods that leave the surface of the 
convergent section similar to that created by casting in sand. The throat is machined and the junctions 
between the cylindrical and conical sections are rounded.  
3.2.1   Inlet section (A)  
3.2.1.1   Length of the inlet section  
The length of the cylindrical inlet section (A) shall not be less than D  or 0.25 D + 250 mm.  
3.2.1.2   Internal diameter of the inlet section  
The internal diameter D of the cylindrical inlet section shall comply with the requirements specified in 
Article  2.2. 
The diameter D  shall be measured at the level of the upstream pressure tappings and the number of 
measurements shall be at least equal to the number of pressure tappings (a minimum of four). The diameters shall be measured both near each pair of pressur e tappings and between these pairs. Diameters 
shall also be measured along the length of the cylindrical inlet section, at levels other than at the pressure 
tappings. The mean value of the internal diameter D  of the inlet section  (A) shall be determined as the 
simple arithmetic mean of all measurements performed.  
3.2.1.3   
Cylindricity of the inlet section  
No diameter along the cylindrical inlet section may differ by more than 0.4 % from the value of the 
mean diameter.   
3.2.1.4   Roughness of the inner sur face of the inlet section  
The inner surface may be left unmachined provided that the condition of the surface is the same as that 
of the convergent section (B).  
3.2.2   Convergent section  (B) 
The convergent section is limited upstream by the plane containi ng the intersection of the cone 
frustum  (B) with the inlet section  (A) or their prolongations and downstream by the plane containing 
the intersection of the cone frustum  (B) with the throat  (C) or their prolongations. General Measure No 0111- OOP -C091- 18 
6 3.2.2.1   Apex angle of the convergent  section  
The convergent section shall be conical and shall have an apex angle of 21°  ± 1°. 
3.2.2.2   Overall length of the convergent section  
The overall length of the convergent section measured parallel to the centreline of the Venturi tube shall 
be approximately equal to 2.7 ( D – d). 
3.2.2.3   Profile of the convergent section  
The convergent section is joined to the inlet section (A) by a curvature of a radius R 1 = 1.375D  ± 0.275D .  
The deviation of the conical section of the convergent section from its ideal shape (represented e.g. by 
a shape template) may not exceed 0.004D . 
The inner surface of the conical section of the convergent section is taken as being a rotating surface if 
two diameters situated at the level perpendicular to the axis of revolu tion do not differ from the value of 
the mean diameter by more than 0.4 %.  
It shall be checked in the same way that the joining curvature of radius R 1 is a rotating surface.  
3.2.2.4   Inner surface roughness of the convergent section  
The inner surface of t he convergent section may be left unmachined, however, it may not have any 
surface defects or impurities. The roughness parameter Ra  of the inner surface of the convergent section 
shall be  ≤ 10-4 D. 
3.2.3   Throat (C)  
The throat shall be cylindrical with a diameter d, limited upstream by the plane containing the 
intersection of the cone frustum  (B) with the throat (C) or their prolongations, and downstream by the 
containing the intersection of the throat (C) with the co ne frustum  (E) or their prolongations. The 
distance between those two planes shall be equal to d ± 0.03d. The throat is connected to the convergent 
section by a curvature of radius R 2 and to the divergent section by a curvature of radius R 3. 
For Venturi tubes with a cast convergent section, the length of the cylindrical part of the throat shall be less than d/3. In addition, the length of the cylindrical section between the end of the joining curvature 
R
2 and the plane of the pressure tappings , as well as the length of the cylindrical section between the 
plane of the pressure tappings and the beginning of the joining curvature R 3 shall not be less than d /6. 
3.2.3.1   Internal diameter of the throat  
The diameter of the throat orifice d shall meet the requirements specified in Article 2.2.  
The diameter d shall be measured in the plane of the throat pressure tappings. The number of 
measurements shall be at least equal to the number of pressure tappings (a minimum of four). The 
diameters shall be measured both near each pair of pressure tappings and between these pairs. Diameters 
shall also be measured along the length of the throat, at levels other than at the pressure tappings. The 
mean value of the internal throat diameter d shall be determined as  the simple arithmetic mean of all 
measurements performed.  
3.2.3.2   Throat cylindricity  
The Venturi tube throat shall be cylindrical, no diameter in any cross -section measured in the cylindrical 
section of the throat may differ by more than 0.1 % of the v alue of the mean diameter.  
3.2.3.3   Throat joints curvature profiles  
The throat joint curvatures of radius R 2 and R3 shall form the rotation surfaces. This requirement is 
satisfied when two diameters situated at the level perpendicular to the axis of revolution do not differ 
from the value of the mean diameter by more than 0.1 %.  General Measure No 0111- OOP -C091- 18 
7 The radius of curvature R 2 shall be equal to 3.625d ± 0.125d. The radius of curvature R 3 shall be between 
5d and 15 d.  The values of radii R 2 and R 3 shall be checked using a template.  
The deviation between the template and the Venturi tube shall evolve in a regular way for each curvature 
so tha t the single maximum deviation that is measured occurs approximately midway along the template 
profile. The value of this maximum deviation shall not exceed 0.02d . 
3.2.3.4   Throat surface roughness 
The roughness parameter Ra of the inner surface of the throat of the Venturi tube and the adjacent 
curvature shall be ≤ 10-4 d. 
3.2.4   Divergent section  (E) 
The divergent section shall be conical and may have an apex angle between 7° and 15°.  
The smallest diameter of the divergent section shall not be less than  the diameter of the throat of the 
Venturi tube.  
If the outlet diameter of the divergent section is less than the diameter D , the Venturi tube is called 
‘truncated’. If the outlet diameter of the divergent section is equal to D , the Venturi tube is called ‘not 
truncated’.  
3.3   Venturi tube with a machined convergent section  
It is manufactured in the same way as a Venturi tube with a cast convergent section, however, in this 
case the convergent section is machined, as is the throat and the conical inlet section. The junctions may, 
but do not have to be, rounded.  
3.3.1   Inlet section  (A) 
3.3.1.1   Length of the inlet section  
The length of the cylindrical inlet section  (A) shall not be less than D . 
3.3.1.2   Internal diameter of the inlet section  
The inter nal diameter D  of the cylindrical inlet section shall comply with the requirements specified in 
Article  2.2. 
The diameter D  shall be measured at the level of the upstream pressure tappings and the number of 
measurements shall be at least equal to the number of pressure tappings (a minimum of four). The 
diameters shall be measured both near each pair of pressure tappings and between these pairs. Diameters 
shall also be measured along the length of the cylindrical inlet section, at levels other than at the pr essure 
tappings. The mean value of the internal diameter D  of the inlet section  (A) shall be determined as the 
simple arithmetic mean of all measurements performed.  
3.3.1.3   Cylindricity of the inlet section  
No diameter along the cylindrical inlet section may differ by more than 0.4 % from the value of the 
mean diameter.   
3.3.1.4   Inner surface roughness of the inlet section  
The inner surface of the inlet section shall have the same surface finish as the inn er surface of the 
throat  (C). General Measure No 0111- OOP -C091- 18 
8 3.3.2   Convergent section  (B) 
The convergent section is limited upstream by the plane containing the intersection of the cone 
frustum  (B) with the inlet section  (A) or their prolongations and downstream by the plane containing 
the intersection of the cone frustum  (B) with the throat  (C) or their prolongations. 
3.3.2.1   Apex angle of the convergent section  
The convergent section shall be conical and shall have an apex angle of 21°  ± 1°. 
3.3.2.2   Overall length of the convergent section  
The overall length of the convergent section measured parallel to the centreline of the Venturi tube shall 
be approximately equal to 2.7 ( D – d). 
3.3.2.3   Profile of the convergent section  
The convergent section is joined to the inlet section  (A) by a curvature of radius R 1 < 0.25D  (ideally 
R1 = 0).  
The deviation of the conical section of the convergent section from its ideal shape (represented e.g. by a shape template) may not exceed 0.004D . 
The inner surface of the conical section of the convergent section is taken as being a rotating surface if 
two diameters situated at the level perpendicular to the axis of revolution do not differ from the value of the mean diameter by more than 0.4 %.  
It shall be checked in the same way that the joining curvature of radius R
1 is a rotating surface.  
3.3.2.4   Inner surface roughness of the convergent section  
The inner surface of the convergent section shall have the same surface finish as the inner surface of the throat (C).  
3.3.3   Throat (C)  
The throat  shall be cylindrical with a diameter d, limited upstream by the plane containing the 
intersection of the cone frustum  (B) with the throat (C) or their prolongations, and downstream by the 
containing the intersection of the throat (C) with the cone frustum  (E) or their prolongations. The 
distance between those two planes shall be equal to d ± 0.03d. The throat is connected to the convergent 
section by a curvature of radius R
2 and to the divergent section by a curvature of radius R 3. 
For Venturi tubes with a  machined convergent section, the length of the cylindrical section of the throat 
between the end of the curvature R 2 and at the level of pressure tappings in the throat shall not be less 
than 0.25d  and the length of the cylindrical section between the lev el of the pressure tappings in the 
throat and the beginning of the joining curvature R 3 shall not be less than 0.3d.  
3.3.3.1   Internal diameter of the throat  
The diameter of the throat orifice d shall meet the requirements specified in Article 2.2.  
The di ameter d shall be measured at the level of the throat pressure tappings. The number of 
measurements shall be at least equal to the number of pressure tappings (a minimum of four). The diameters shall be measured both near each pair of pressure tappings and  between these pairs. Diameters 
shall also be measured along the length of the throat, at levels other than at the pressure tappings. The 
mean value of the internal throat diameter d shall be determined as the simple arithmetic mean of all 
measurements per formed.  
3.3.3.2   
Throat cylindricity  
The Venturi tube throat shall be cylindrical, no diameter in any cross -section measured in the cylindrical 
section of the throat may differ by more than 0.1 % of the value of the mean diameter.  General Measure No 0111- OOP -C091- 18 
9 3.3.3.3   Throat joints curvature profiles 
The throat joint curvatures of radius R 2 and R3 shall form the rotating surfaces. This requirement is 
satisfied when two diameters situated at the level perpendicular to the axis of revolution do not differ 
from the value of the mean diameter by more than 0.1 %. 
The radius of curvature R2 shall be less than 0.25d (ideally equal to zero). The radius of curvature R3 
shall be less than 0.25 d (ideally equal to zero). The values of radii R 2 and R3 shall be checked using a 
template.  
The deviation between the template and the Venturi tube shall evolve in a reg ular way for each curvature 
so that the single maximum deviation that is measured occurs approximately midway along the template 
profile. The value of this maximum deviation shall not exceed 0.02d .  
3.3.3.4   Throat surface roughness 
The roughness  parameter Ra of the inner surface of the throat of the Venturi tube and the adjacent 
curvature shall be ≤ 10-4 d. 
3.3.4   Divergent section (E)  
The divergent section shall be conical and may have an apex angle between 7° and 15°.  
The smallest diameter of the diver gent section shall not be less than the diameter of the throat of the 
Venturi tube.  
If the outlet diameter of the divergent section is less than the diameter D , the Venturi tube is called 
‘truncated’. If the outlet diameter of the divergent section is equa l to D, the Venturi tube is called ‘not 
truncated’.  
3.4   Venturi tube with a rough- welded sheet -iron convergent section  
It is manufactured by means of welding. Tubes of large dimensions are typically not machined, in the 
case of smaller dimensions, the t hroat is machined.  
3.4.1   Inlet section  (A) 
3.4.1.1   Length of the inlet section  
The length of the cylindrical inlet section  (A) shall not be less than D . 
3.4.1.2   Internal diameter of the inlet section  
The internal diameter D of the cylindrical inlet section shall comply with the requirements specified in 
Article  2.2. 
The diameter D  shall be measured at the level of the upstream pressure tappings and the number of 
measurements shall be at least equal to the number of pressure tappings (a minimum of four). The 
diameters shall be measured both near each pair of pressure tappings and between these pairs. Diameters 
shall also be measured along the length of the cylindrical inlet section, at levels other than at the pressure 
tapping s. The mean value of the internal diameter D  of the inlet section  (A) shall be determined as the 
simple arithmetic mean of all measurements performed.  
3.4.1.3   Cylindricity of the inlet section  
No diameter along the cylindrical inlet section may differ by  more than 0.4 % from the value of the 
mean diameter.   General Measure No 0111- OOP -C091- 18 
10 3.4.1.4   Inner surface roughness of the inlet section  
The inner surface shall be clean, without dross and slag left after welding.  The roughness  parameter Ra  
of the inner surface of the inlet section  shall be ≤  5·10-4 D. 
3.4.2   Convergent section (B)  
The convergent section is limited upstream by the plane containing the intersection of the cone 
frustum  (B) with the inlet section  (A) or their prolongations and downstream by the plane containing 
the in tersection of the cone frustum  (B) with the throat  (C) or their prolongations. 
3.4.2.1   Apex angle of the convergent section  
The convergent section shall be conical and shall have an apex angle of 21°  ± 1°. 
3.4.2.2   Overall length of the convergent section  
The overall length of the convergent section measured parallel to the centreline of the Venturi tube shall 
be approximately equal to 2.7 ( D – d). 
3.4.2.3   Profile of the convergent section  
There shall be no joining curvature between the inlet sect ion (A) and the convergent section (B) other 
than that resulting from welding. There shall be no joining curvature between the convergent section (B) 
and the throat  (C) other than that resulting from welding. The internal welded seams shall be flush with 
surrounding surfaces. They shall not be located in the vicinity of the pressure tappings.  
The deviation of the conical section of the convergent section from its ideal shape (represented e.g. by 
a shape template) may not exceed 0.004D . 
The inner surface of  the conical section of the convergent section is taken as being a rotating surface if 
two diameters situated at the level perpendicular to the axis of revolution do not differ from the value of the mean diameter by more than 0.4 %.  
3.4.2.4   
Inner surface roughness of the convergent section  
The inner surface shall be clean, without dross and slag left after welding.  The roughness  parameter Ra  
of the inner surface of the convergent section shall be ≤ 5·10-4 D. 
3.4.3   Throat (C)  
The throat shall be cylindri cal with a diameter d, limited upstream by the plane containing the 
intersection of the cone frustum  (B) with the throat (C) or their prolongations, and downstream by the 
containing the intersection of the throat (C) with the cone frustum  (E) or their prol ongations. The 
distance between those two planes shall be equal to d ± 0.03d for any type of Venturi tube.  
3.4.3.1   Internal diameter of the throat  
The diameter of the throat orifice d shall meet the requirements specified in Article 2.2.  
The diameter D  shall be measured at the level of the pressure tappings in the throat and the number of 
measurements shall be at least equal to the number of pressure tappings (a minimum of four). The diameters shall be measured both near each pair of pressure tappings and  between these pairs. Diameters 
shall also be measured along the length of the throat, at levels other than at the pressure tappings. The 
mean value of the internal throat diameter d shall be determined as the simple arithmetic mean of all 
measurements per formed.  
3.4.3.2   
Throat cylindricity  
The Venturi tube throat shall be cylindrical, no diameter in any cross -section measured in the cylindrical 
section of the throat may differ by more than 0.1 % of the value of the mean diameter.  General Measure No 0111- OOP -C091- 18 
11 3.4.3.3   Venturi tube throat profile  
There shall be no joining curvature between the convergent section (B) and the throat  (C) other than that 
resulting from welding. There shall be no joining curvature between the throat  (C) and the divergent 
section  (E). The internal welded seams shall be flush with surrounding surfaces. They shall not be 
located in the vicinity of the pressure tappings.  
The deviation between the template and the Venturi tube shall evolve in a regular way for each curvature 
so that the single maximum deviation that is measured occurs approximately midway along the template 
profile. The value of this maximum deviation shall not exceed 0.02d . 
3.4.3.4   Throat surface roughness 
The roughness  parameter Ra  of the inner surface of the throat of the Venturi tube shall be ≤ 10-4 d. 
3.4.4   Divergent section (E)  
The divergent section shall be conical and may have an apex angle between 7° and 15°.  
The smallest diameter of the divergent section shall not be less than the diameter of the throat of the 
Venturi tube.  
If the outlet diameter of the divergent section is less than the diameter D , the Venturi tube is called 
‘truncated’. If the outlet diameter of the divergent section is equal to D , the Venturi tube is called ‘not 
truncated’.  
3.5   Differential pressure tapp ings for Venturi tubes  
The pressure tappings upstream of the Venturi tube and in the throat shall be made in the form of separate 
pipe wall pressure tappings interconnected by annular chambers or piezometric rings, or a ‘triple T’ 
arrangement.  
If d ≥ 33.3 mm, the internal diameter of the pressure tappings shall be between 4  mm and 10 mm. 
Moreover, this diameter shall never be greater than 0.1 D for the upstream pressure tappings and 0.13d  
for the throat pressure tappings. 
If d <  33.3 mm, the internal diameter of the throat pressure tappings shall be between 0.1d to 0.13d  and 
the diameter of the upstream pressure tappings shall be between 0.1 d and 0.1D . 
At least four pressure tappings shall be provided for a pressure measurement upstream of the Venturi 
tube and in the throat. The centrelines of the pressure tappings shall meet the centreline of the Venturi 
tube, shall form equal angles with each other and shall be contained in planes perpendicular to the 
centreline of the Venturi tube.  
At the outlet, the  opening of the pressure tapping shall be circular. The edges shall be flush with the pipe 
wall and free from burrs. If joining curvatures are required, the radius shall not exceed one -tenth of the 
diameter of the pressure tapping. The pressure tappings sh all be cylindrical over a length at least 2.5 
times the internal diameter of the tapping (measured from the inner wall of the pipeline).  
For Venturi tubes with a cast convergent section, the spacing between the upstream pressure tappings 
situated on the in let section and the plane of intersection of the inlet section  (A) and the prolongation of 
the convergent section (B) shall be equal to  
0.5D  ± 0.25D  for 100 mm  < D < 150 mm;  
0
25,05,0D D−  for 150 mm  < D < 800 mm.  
For Venturi tubes with a machined convergent section and with a rough- welded sheet -iron convergent 
section, the spacing between the upstream pressure tappings and the plane of intersection of the inlet 
section  (A) and the convergent section (B) or their prolongations shall be equal to 0.5D  ±0.05D . General Measure No 0111- OOP -C091- 18 
12 The spacing between the plane containing the axes of the throat pressure tappings and the intersection 
plane between the convergent section and the throat (or their prolongations) shall be equal to 
0.5d ±  0.02d for all Venturi tubes described abov e. 
The area of the free cross -section of the annular chamber of the pressure tappings shall be greater than 
or equal to half the total surface area of the tapping openings connecting the chamber to the pipe.  
The other technical requirements for the design of the pressure tappings are specified under the relevant 
requirements of the notified standards attached to this general measure.  
3.6   Straight lengths of pipe  
3.6.1   Design  
The minimum straight lengths of pipe upstream of the Venturi tube to be complie d with in specific 
applications shall be determined according to the diameter ratio β and the type and mutual arrangement 
of at least two adapting pieces placed upstream of the Venturi tube.  
The straight length of pipe between the first adapting piece upst ream of the Venturi tube and the actual 
Venturi tube may be made of one or more pieces, however, the part of the pipeline up to the length of 
2D upstream of the Venturi tube shall be made of one piece.  
3.6.2   Pipeline circularity and cylindricity  
The pipe line shall be cylindrical over a length of at least 2 D upstream of the inlet section of the Venturi 
tube. The pipeline meets the circularity and cylindricity requirements if none of the measured internal 
pipe diameters in any plane differs by more than 2  % from the mean value of the measured pipe 
diameters.  
At the point of connection to the Venturi tube, the mean pipeline diameter shall be within 1 % of the 
diameter D  of the inlet section of the Venturi tube.  
The internal pipe diameter downstream of the Venturi tube shall not be less than 90 % of the diameter 
of the end of the divergent section of the Venturi tube.  
3.6.3   Quality of the inner surface of the pipeline upstream of the Venturi tube 
The inner surface of the pipeline shall meet the specified surface roughness requirements for at least the 
minimum st raight length upstream of the Venturi tube determined by the basic design of the gas flow 
meter (see Article 3.1). The inner surface of the pipeline over the length of 2D  upstream of the Venturi 
tube (measured from the front end of the inlet section of the  Venturi tube) shall have a relative roughness 
of Ra /D ≤ 3.2·10-4.  
3.6.4   Coaxiality of the Venturi tube  
The spacing between the centreline of the Venturi tube and the axis of the upstream pipeline (measured 
at the level of the pipe’s connection to the i nlet section of the Venturi tube) shall be less than 0.005D . 
The maximum deviation from angular coaxiality between the Venturi tube and the upstream pipeline shall be 1°. 
The sum of the deviation due to mounting and half of the permitted deviation of the internal diameter D 
(see Article  3.6.2 – the equality of diameters at the point of connection of the pipe to the inlet section of 
the Venturi tube) shall be less than 0.0075D . 
3.7   Installation requirements  
The installation position of the Venturi tube shall be clearly defined by its design or schematically 
indicated directly on the Venturi tube.  General Measure No 0111- OOP -C091- 18 
13 4   Measuring instrument markings  
4.1   General  
All inscriptions and markings shall be readily visible, legible and indelible under normal conditions and 
shall be the source of information necessary for the faultless implementation of the Venturi tube gas 
flow sensor into the gas flow quantity measuring system in relation to the oth er components of the 
measurement system.  
4.2   Markings  
4.2.1   Markings on the Venturi tube  
The Venturi tube shall carry the following information:  
a) serial number;  
b) the value of the internal diameter D 20 of the inlet section of the Venturi tube relative to the 
reference temperature of 20  °C; 
c) the type -approval mark;  
d) suitable marking to identify the type and placement of the differential pressure tappings, if 
pressure tappings are a fixed component of  the design of the Venturi tube.  
In cases where the above information will not be visible once the Venturi tubes have been incorporated 
into the pipe, the Venturi tube shall also carry a separate additional label with the relevant information, 
which will f orm part of the system securing the Venturi tube against unauthorised disassembly or 
replacement.  
In cases where the Venturi tube could be incorrectly installed in terms of the direction of gas flow, the 
flow direction shall be indicated on the Venturi in such a manner that it cannot be confused.  
4.2.2   Marking of Venturi tube gas flow sensors  
Venturi tube gas flows sensors shall carry the following information:  
a) the name of the manufacturer and type (design variant);  
b) serial number and year of manufacture;  
c) the value of the internal diameter D 20 of the inlet section of the Venturi tube relative to the 
reference temperature of 20  °C; 
d) the value of the internal diameter d20 of the Venturi tube throat relative to the reference 
temperature of 20  °C; 
e) the type -approval mark;  
f) nominal size DN/nominal pressure PN;  
g) indication of the direction of flow;  
h) suitable marking to identify the type and placement of the differential pressure tappings, if 
pressure tappings are a fixed component of the straight lengths of pipe;  
i) the measured flow range or the value of maximum flow. 
This information shall be placed on a part of the gas flow sensor which is permanently connected to the straight pipe section 0.5D  upstream of the Venturi tube (installation chamber, connection flange 
upstream of the Venturi tube, straight length of pipe over a maximum of 2D  upstream of the Venturi 
tube).  
If the straight length of pipe upstream of the Venturi tube comprises multiple pipeline sections in the 
basic design of th e gas flow sensor, all of these sections shall be marked with the type- approval mark. 
Any important separate sections of the Venturi tube gas flow sensor shall generally be marked with the 
type-approval mark.  General Measure No 0111- OOP -C091- 18 
14 4.3   Marking with official marks  
Suitable spaces shall be provided for placing the type- approval mark and the official mark(s).  
It shall be possible to secure the Venturi tube against unauthorised disassembly or replacement.  
5   Type -approval of measuring instruments  
The process of type -approval of Venturi tube gas flow sensors comprises the following tests and 
activities:  
a) external inspection;  
b) check of the geometric parameters.  
5.1   External inspection  
The external inspection shall include the following:  
 the completeness of the required technical documentation;  
 the conformity of the metrological and technical characteristics specified by the manufacturer in 
the documentation with the technical and metrological requirements of this regulation specified 
in Chapters  2 and 3;  
 the Venturi tube, pressure tappings and straight lengths of pipe in terms of any mechanical damage 
or traces of corrosion which would preclude further testing; and  
 specification of the spaces where the basic geometric parameters of the Venturi tube gas flow  
sensor will be indicated and spaces for placing official marks.  
5.2   Check of the geometric parameters  
5.2.1   Test equipment  
Suitable equipment with valid metrological traceability shall be used to check the geometric parameters.  
5.2.2   Reference ambient temperature for testing  
5.2.2.1   Venturi tube  
The test ambient temperature shall be within the interval of (18 to 22)  °C and changes in ambient 
temperature may not exceed 2  °C during the period of the test.  
5.2.2.2   Straight lengths of pipe  
Pipe ≤ DN 300:  The test ambient temperature shall be within the interval of (15 to 25)  °C and 
changes in ambient temperature may not exceed 2  °C during the period of 
measurement.  
Pipe ≤ DN 300:  The test ambient temperature shall be within the interval of (10 to 30)  °C and 
changes in ambient temperature may not exceed 5  °C during the period of the 
test. 
5.2.3   Check of the geometric parameters of Venturi tubes  
The value of the internal diameter D 20 of the inlet section of the Venturi tube shall be established in 
accordance with the requirement under Articles 2.2 and 3.2.1.2, or, if applicable, 3.3.1.2 or 3.4.1.2, and 
shall be relative to the reference temperature of 20  °C. General Measure No 0111- OOP -C091- 18 
15 The value of the internal diameter d20 of the throat of the Venturi tube shall be established in accordance 
with the requirement under Articles  2.2 and 3.2.3.1, or, if applicable, 3.3.3.1 or 3.4.3.1 and shall be 
relative to the reference temperature of 20  °C. 
5.2.3.1   Check of the  geometric parameters of Venturi tubes with a cast convergent section  
For Venturi tubes with a cast convergent section, the following additional parameters shall be checked:  
 the length of the inlet section of the Venturi tube (see Article 3.2.1.1);  
 the cylindricity of the inlet section of the Venturi tube (see Article  3.2.1.3);  
 the inner surface roughness of the inlet section of the Venturi tube (see Article  3.2.1.4);  
 the apex angle of the convergent section of the Venturi tube (see Article  3.2.2.1) ; 
 the overall length of the convergent section of the Venturi tube (see Article  3.2.2.2);  
 the design and inner surface roughness of the convergent section of the Venturi tube (see 
Articles  3.2.2.3 and 3.2.2.4);  
 the internal diameter of the throat of the Venturi tube (see Article  3.2.3.1);  
 the cylindricity of the throat of the Venturi tube (see Article  3.2.3.2);  
 the design and inner surface roughness of the throat of the Venturi tube (see Articles  3.2.3, 3.2.3.3 
and 3.2.3.4);  
 the design and apex angle of the divergent section of the Venturi tube (see Article 3.2.4).  
5.2.3.2   Check of the geometric parameters of Venturi tubes with a machined convergent section  
For Venturi tubes with a machined convergent section, the following additional parameter s shall be 
checked:  
 the length of the inlet section of the Venturi tube (see Article 3.3.1.1);  
 the cylindricity of the inlet section of the Venturi tube (see Article  3.3.1.3);  
 the inner surface roughness of the inlet section of the Venturi tube (see Article  3.3.1.4);  
 the apex angle of the convergent section of the Venturi tube (see Article  3.3.2.1);  
 the overall length of the convergent section of the Venturi tube (see Article  3.3.2.2);  
 the design and inner surface roughness of the convergent section of the Venturi tube (see Articles  3.3.2.3 and 3.3.2.4);  
 the internal diameter of the throat of the Venturi tube (see Article  3.3.3.1);  
 the cylindricity of the throat of the Venturi tube (see Article  3.3.3.2);  
 the design and inner surface roughness of the throat of the Venturi tube (see Articles  3.3.3, 3.3.3.3 
and 3.3.3.4);  
 the design and apex angle of the divergent section of the Venturi tube (see Article 3.3.4).  
5.2.3.3   
Check of the geometric parameters of Venturi tubes with a rough -welded convergent section  
For Venturi tubes with a rough -welded sheet -iron convergent section, the following additional 
parameters shall be checked:  
 the length of the inlet section of the Venturi tube (see Article 3.4.1.1);  
 the cylindricity of the inlet  section of the Venturi tube (see Article 3.4.1.3);  
 the inner surface roughness of the inlet section of the Venturi tube (see Article  3.4.1.4);  
 the apex angle of the convergent section of the Venturi tube (see Article  3.4.2.1);  
 the overall length of the convergent section of the Venturi tube (see Article 3.4.2.2);  
 the design and inner surface roughness of the convergent section of the Venturi tube (see 
Articles  3.4.2.3 and 3.4.2.4);  
 the internal diameter of the throat of the Venturi tube (see Arti cle 3.4.3.1);  General Measure No 0111- OOP -C091- 18 
16  the cylindricity of the throat of the Venturi tube (see Article  3.4.3.2);  
 the design and inner surface roughness of the throat of the Venturi tube (see Articles  3.4.3, 3.4.3.3 
and 3.4.3.4);  
 the design and apex angle of the divergent sec tion of the Venturi tube (see Article  3.2.4).  
5.2.4   Check of the geometric parameters of the straight lengths of pipe upstream of the Venturi 
tube  
For the straight lengths of pipe upstream of the Venturi tube, the following geometric parameters shall be checked:  
 the circularity and cylindricity of the pipe (see Article  3.6.2);  
 the inner surface roughness of the pipe (see Article 3.6.3);  
 the coaxiality of the Venturi tube (see Article  3.6.4).  
5.2.5   Check of the geometric parameters of the differen tial pressure tappings  
Compliance with the requirements for differential pressure tappings, depending on the type of the 
Venturi tube pursuant to Article  3.5 and the manufacturer’s specifications, is ascertained.  
6   Initial verification  
The initial verification of Venturi tube gas flow meters and their components shall comprise the 
following tests and activities:  
 visual inspection;  
 test of metrological characteristics;  
 indication of the basic geometric parameters on the relevant part of the Ven turi tube gas flow 
sensor.  
6.1   Visual inspection  
The purpose of the visual inspection is to check that:  
 the design of the Venturi tube gas flow sensor conforms to the approved type;  
 the Venturi tube, the pressure tappings and the straight lengths of  pipe are not mechanically 
damaged and bear no traces of corrosion which 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 Venturi tube gas flow sensor fails to meet the external inspection requirements, no further tests 
are performed.  
6.2   Check of the geometric parameters  
6.2.1   Test equipment  
Suitable equipment with valid metrological  traceability shall be used to check the geometric parameters.  
6.2.2   Reference ambient temperature for testing  
The ambient temperature requirements pursuant to Article  5.2.2 shall apply to testing.  General Measure No 0111- OOP -C091- 18 
17 6.2.3   Check of the geometric parameters of Venturi tub es 
The value of the internal diameter D 20 of the inlet section of the Venturi tube shall be established in 
accordance with the requirement under Articles 2.2 and 3.2.1.2, or, if applicable, 3.3.1.2 or 3.4.1.2, and 
shall be relative to the reference temperature of 20 °C.  
The value of the internal diameter d20 of the throat of the Venturi tube shall be established in accordance 
with the requirement under Articles  2.2 and 3.2.3.1, or, if applicable, 3.3.3.1 or 3.4.3.1 and shall be 
relative to the reference tem perature of 20 °C.  
6.2.3.1   Check of the geometric parameters of Venturi tubes with a cast convergent section  
For Venturi tubes with a cast convergent section, the following additional parameters shall be checked 
during initial verification:  
 the length of the inlet section of the Venturi tube (see Article 3.2.1.1);  
 the cylindricity of the inlet section of the Venturi tube (see Article  3.2.1.3);  
 the inner surface roughness of the inlet section of the Venturi tube (see Article  3.2.1.4);  
 the apex angle of the convergent section of the Venturi tube (see Article  3.2.2.1);  
 the overall length of the convergent section of the Venturi tube (see Article  3.2.2.2);  
 the design and inner surface roughness of the convergent section of the Venturi tube (see  
Articles  3.2.2.3 and 3.2.2.4);  
 the internal diameter of the throat of the Venturi tube (see Article  3.2.3.1);  
 the cylindricity of the throat of the Venturi tube (see Article  3.2.3.2);  
 the design and inner surface roughness of the throat of the Ventu ri tube (see Articles  3.2.3, 3.2.3.3 
and 3.2.3.4);  
 the design and apex angle of the divergent section of the Venturi tube (see Article 3.2.4).  
6.2.3.2   Check of the geometric parameters of Venturi tubes with a machined convergent section  
For Venturi tubes with a machined convergent section, the following additional parameters shall be 
checked during initial verification:  
 the length of the inlet section of the Venturi tube (see Article 3.3.1.1);  
 the cylindricity of the inlet section of the Venturi tub e (see Article 3.3.1.3);  
 the inner surface roughness of the inlet section of the Venturi tube (see Article  3.3.1.4);  
 the apex angle of the convergent section of the Venturi tube (see Article  3.3.2.1);  
 the overall length of the convergent section of t he Venturi tube (see Article  3.3.2.2);  
 the design and inner surface roughness of the convergent section of the Venturi tube (see 
Articles  3.3.2.3 and 3.3.2.4);  
 the internal diameter of the throat of the Venturi tube (see Article  3.3.3.1);  
 the cylindr icity of the throat of the Venturi tube (see Article  3.3.3.2);  
 the design and inner surface roughness of the throat of the Venturi tube (see Articles  3.3.3, 3.3.3.3 
and 3.3.3.4);  
 the design and apex angle of the divergent section of the Venturi tube (s ee Article 3.3.4).  
6.2.3.3   Check of the geometric parameters of Venturi tubes with a rough -welded convergent section  
For Venturi tubes with a rough -welded sheet -iron convergent section, the following additional 
parameters shall be checked during initial verification:  
 the length of the inlet section of the Venturi tube (see Article 3.4.1.1);  
 the cylindricity of the inlet section of the Venturi tube (see Article  3.4.1.3);  
 the inner surface roughness of the inlet section of the Venturi tube (see Articl e 3.4.1.4);  General Measure No 0111- OOP -C091- 18 
18  the apex angle of the convergent section of the Venturi tube (see Article  3.4.2.1);  
 the overall length of the convergent section of the Venturi tube (see Article  3.4.2.2);  
 the design and inner surface roughness of the convergent section of the Venturi tube (see 
Articles  3.4.2.3 and 3.4.2.4);  
 the internal diameter of the throat of the Venturi tube (see Article  3.4.3.1);  
 the cylindricity of the throat of the Venturi tube (see Article  3.4.3.2);  
 the design and inner surface roughness of  the throat of the Venturi tube (see Articles  3.4.3, 3.4.3.3 
and 3.4.3.4);  
 the design and apex angle of the divergent section of the Venturi tube (see Article 3.2.4).  
6.2.4   Check of the geometric parameters of the straight lengths of pipe upstream of t he Venturi 
tube  
For straight lengths of pipe, the following geometric parameters shall be checked during initial 
verification:  
 the circularity and cylindricity of the pipe (see Article  3.6.2);  
 the inner surface roughness of the pipe (see Article 3.6.3) ; 
 the coaxiality of the Venturi tube (see Article  3.6.4).  
6.2.5   Check of the geometric parameters of the differential pressure tappings  
During initial verification, compliance with the requirements for the differential pressure tappings, 
depending on the type of the Venturi tube pursuant to Article  3.5 and the manufacturer’s specifications, 
is ascertained.  
6.2.6   Indication of the basic geometric parameters on the measuring instrument  
The value of the internal diameter of the throat orifice of the Ven turi tube ( d20) and the internal diameter 
of the inlet section of the Venturi tube ( D20) specified pursuant to Article  6.2.3 shall be indicated in a 
legible and indelible manner in a suitable space on the Venturi tube gas flow sensor in accordance with 
Article 4.2. 
7   Subsequent verification  
The procedure for subsequent verification of the Venturi tube is identical to that for initial verification pursuant to Article  6.2.3.  
Subsequent verification of the straight lengths of pipe of Venturi tube gas flow sensors is not performed.  
Long- term compliance with the operating conditions, including keeping the inner surface of the pipes 
in appropriate condition and clean, shall be ensured by the user of the measuring instrument. For this 
purpose, the user of the m easuring instrument shall perform or ensure the performance of visual 
inspections of the condition of the inner surface of the pipe at intervals which take into account the operating conditions and the type of the gas medium being measured. In addition, this inspection shall 
always be performed prior to installing the Venturi tube.  
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. 
The last sentence of Article  6.1 shall not apply. General Measure No 0111- OOP -C091- 18 
19 9   Notified standards  
For the purposes of specifying the metrological and technical requirements for measuring instruments 
and specifying the t esting methods for their type -approval and verification arising from this general 
measure, the CMI shall notify Czech technical standards, other technical standards or technical 
documents of international or foreign organisations, or other technical docume nts containing more 
detailed technical requirements (hereinafter referred to as ‘notified standards’). The CMI shall publish a list of these notified standards attached to the relevant measures, together with the general measure, in 
a manner accessible to the public (at www.cmi.cz
). 
Compliance with notified standards or parts thereof shall be considered, to the extent and under the conditions laid down in this general measure, as compliance with those requirements laid down in 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 eq uivalent or 
higher level of protection of legitimate interests.  
II. 
G R O U N D S  
The CMI has issued this general measure laying down metrological and technical requirements for legally controlled measuring instruments and tests for the type- approval and verification of legally 
controlled measuring instruments – ‘Venturi tube gas flow sensors’ – 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 
instru ment is classified as an instrument subject to mandatory verification.  
This legislation (general measure) will be notified in accordance with Directive (EU)  2015/1535 of the 
European Parliament and of the Council of 9 September  2015 laying down a procedur e for the provision 
of information in the field of technical regulations and of rules on Information Society services.  
III. 
I N S T R U C T I O N S  
In accordance with §  172(1) APC, in conjunction with § 39(1) APC, the CMI has stipulated a time limit 
for co mments of 30 days from the date of posting the draft on the official notice board. Comments 
submitted after this time limit will not be considered.  
The persons concerned are hereby invited to comment on this draft general measure. With a view to the 
provi sions of § 172(4) APC, the comments are to be submitted in writing. 
Pursuant to the provisions of § 174(1) APC, in conjunction with the provisions of § 37(1) APC, it must 
be clearly stated who is submitting the comments, which general measure the comments concern, how 
the draft contradicts legislation or how the general measure is inaccurate. The comments must also 
contain the signature of the person making the comments.  
The supporting documents for this draft general measure may be consulted at the Czech Metrology 
Institute, Legal Metrology Department, Okružní 31, 638 00 Brno, after making arrangements by 
telephone.  
This draft general measure shall be posted for 15  days.  General Measure No 0111- OOP -C091- 18 
20  
 
Czech Metrology Institute  
Director General