CZECH 
METROLOGY 
INSTITUTE  Okružní 31, 638 00 Brno  
legmet@cmi.cz  
 +420 545 555 414 
 
1. ------IND-  2019 0295 CZ-  EN- ------ 20190702 --- --- PROJET 
Executive summary for the EC (not part of this legislation)  
‘Venturi tube transmission medium 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 control led measuring instruments 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 to the protection 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 intere sts 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 thi s legislation do not apply to measuring instruments not 
placed on the market in the Czech Republic for the above purposes, defined by Act  No 505/1990 on 
metrology.  
The purpose of this notified legislation is to lay down metrological and technical requireme nts 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 exec utive summary.)  
 
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 the ‘Metrology Act’), and in 
accordance with the provisions of §  172 et seq. of Act  No 500/ 2004, the Code of Administrative 
Procedure (hereinafter the ‘CAP’), on 15 April  2018, the Czech Metrology Institute (hereinafter the 
‘CMI’) commenced ex officio proceedings pursuant to § 46 CAP, and, based on supporting documents, issues the following:  
 
 
I. 
D R A F T   G E N E R A L   M E A S U R E 
number: 0111- OOP -C093 -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 transmission medium flow sensors’  General Measure No  0111- OOP -C093- 18 
2 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 basic design of the Venturi tube transmission 
medium 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    
transmission medium  
the fluid used for the transfer of heat; for the purposes of this General Measure, this means the water 
vapour, i.e. the gaseous form of water and/or condensate  
1.2    
transmission medium 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 the transmission medium  
1.3    
Venturi tube transmission medium flow sensor  
a transmission medium 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  
1.4    
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’)  
1.5   pressure tapping (in the wall)  
an 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 internal surface  
1.6    static pressure of the transmission medium flowing through the pipeline  
pressure which can be measured by connecting a pressure- measuring device to the pressure tapping in 
the wall  
1.7    differential pressure  
the 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 in a 
                                                      
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 -C093- 18 
3 straight pipe through which the flow of the transmission medium occurs, when any difference in height 
between the tappings upstream of the Venturi tube and in its throat has been taken into account  
1.8    
diameter ratio β 
the ratio of the diameter of the nozzle throat o rifice to the internal diameter of the pipe upstream of the 
nozzle  
1.9   pipe Reynolds number Re
D 
a dimensionless parameter expressing the ratio between the inertia and viscous forces in the pipe 
upstream of the nozzle  
1.10    
flow coefficient C  
a coefficient, defined for an incompressible fluid flow, which relates the actual flowrate through the 
nozzle to the theoretical flowrate  
1.11    
expansion coefficient  
a coefficient characterising the degree of compressibility of the fluid  
2   Metrologic al requirements  
During verification of Venturi tube transmission medium 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 range of temperatures of the transmission 
medium to be measured. If there are significant temperature differences between the ambient 
temperature and the temperature of the flowing transmission medium, the transmission medium flow 
sensor shall be thermally insulated by suitable means.  
The following conditions shall apply to Venturi tube transmission medium flow sensors for the use of 
the individual types of Venturi tubes:  
 Venturi tube with a cast convergent section:  2·105 ≤ 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 
2.2   Lim its of use  
Venturi tube transmission medium flow sensors with a cast convergent section shall only be used for:  
 internal pipe diameter D: 100 mm to 800 mm 
 diameter ratio β  (d/D): 0.30 to 0.75  
 the internal surface roughness of the tube and the pipe shall comply with the specified 
requirements  General Measure No  0111- OOP -C093- 18 
4 Venturi tube transmission medium 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 internal surface roughness of the tube and the pipe shall comply with the specified 
requirements  
Venturi tube transmission medium 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 internal 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 flow coefficient to be increased by additional 
uncertainties:  
 Ventur i 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 section:  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 shall only be used for the section upstream of the Venturi tube.  
Compliance with technical requiremen ts shall be ascertained when verifying the individual components 
of the transmission medium flow sensor.  
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 of Venturi tubes referred to above:  
(4 + 100β
8)
1pp∆ % 
where Δ p is the different ial pressure and p1 is the absolute static transmission medium pressure upstream 
of the Venturi tube.  
Compliance with technical requirements shall be ascertained when verifying the individual components of the transmission medium flow sensor.  
3   Technical  requirements  
During verification of Venturi tube transmission medium flow sensors, the technical requirements applicable when they were placed on the market shall apply.  General Measure No  0111- OOP -C093- 18 
5 3.1   Design  
The basic design of the Venturi tube transmission medium 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 internal 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 pressure of the flowing transmi ssion medium 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 relevant re quirements 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 in the plane 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, in planes other than the plane of 
the pressu re 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 shall differ by more than 0.4 % from the value of the 
mean diameter.   
3.2.1.4   Roughness of the internal surface of the inlet section  
The internal 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 containing the intersection of the cone frustum (B) with the inlet section (A) or their prolongations and downstream by the plane containing the 
intersect ion of the cone frustum (B) with the throat (C) or their prolongations.  General Measure No  0111- OOP -C093- 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 blended to the inlet section (A) by a curvat ure of radius 
R1 = 1.375D  ± 0.275D .  
The deviation of the conical section of the convergent section from the ideal shape (represented e.g. by 
a shape template) shall not exceed in any place 0.004D . 
The internal surface of the conical section of the convergent section is taken as being a surface of 
revolution if two diameters situated in the same plane 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 surface of revolution.  
3.2.2.4   Internal surface roughness of the convergent section  
The internal surface of the convergent section may be left unmachined, however, it shall not have any 
surface defects or impurities. The roughness  parameter Ra of the internal 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 
plane 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 cast convergent section, the lengt h 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 
R2 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, in planes other than the plane of the pressure 
tappings. The mean value of the internal throat diameter d shall be determined as the simple arithmetic 
mean of all measurements p erformed. 
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 shall differ by more than 0.1 % of the value of the mean diameter.  General Measure No  0111- OOP -C093- 18 
7 3.2.3.3   Throat joi nts curvature profiles 
The throat joint curvatures of radius R2 and R3 shall form surfaces of revolution. This requirement is 
satisfied when two diameters situated in the same plane perpendicular to the axis of revolution do not 
differ from the value of th e mean diameter by more than 0.1 %.  
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 that the single maximum deviation that is measured occurs at approximately midway along the template profile. The value of this maximum deviation shall not exceed 0.02 d. 
3.2.3.4   
Throat surface roughness 
The roughness parameter Ra  of the internal 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 shall 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 internal diameter D of the cylindrical inlet section shall comply with the requirements specified in 
Article  2.2. 
The diameter D  shall be measured in the plane 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, in planes other than the plane of 
the pressure tappings. The mean value of the internal diameter D  of the inlet section (A) shall be 
determined as the si mple arithmetic mean of all measurements performed.  
3.3.1.3   Cylindricity of the inlet section  
No diameter along the cylindrical inlet section shall differ by more than 0.4  % from the value of the 
mean diameter.   General Measure No  0111- OOP -C093- 18 
8 3.3.1.4   Internal surface roughness of t he inlet section  
The internal surface of the inlet section shall have the same surface finish as the internal surface of the 
throat (C).  
3.3.2   Convergent section (B)  
The convergent section is limited upstream by the plane containing the intersection of t he 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 the ideal shape (represen ted e.g. by 
a shape template) shall not exceed in any place 0.004D . 
The internal surface of the conical section of the convergent section is taken as being a surface of 
revolution if two diameters situated in the same plane perpendicular to the axis of rev olution 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 surface of revolution.  
3.3.2.4   Internal surface roughness of the convergent section  
The internal surface of the convergent section shall have the same surface finish as the internal 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 
plane 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 R2 and the plane of pressure tappings in the throat shall not be less 
than 0.25 d and the length of the cylindrical section between the plane of the pressure tappings in the 
throat and the beginning of the joining curvature R 3 shall not be le ss 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 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, in planes other than the plane of the pressure General Measure No  0111- OOP -C093- 18 
9 tappings. The mean value of the internal throat diameter d shall be determined as the simple arithmetic 
mean of all measurements performed.  
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 shall differ by more than 0.1 % of the value of the mean diameter.  
3.3.3.3   Throat joints curvature profiles  
The throat joint curvatures of radius R2 and R3 shall form surface s of revolution. This requirement is 
satisfied when two diameters situated in the same plane 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 0.25 d (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 regular way for each curvature so that the single maximum deviation that is measured occurs at approximately midway along the 
template profile. The value of this maximum deviation shall not exceed 0.02 d.  
3.3.3.4   
Throat surface roughness 
The roughness parameter Ra  of th e internal 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 shall 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’.  
3.4   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 throat 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 in the plane 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, in planes other than the plane of 
the pressure tappings. The mean value of th e internal diameter D  of the inlet section (A) shall be 
determined as the simple arithmetic mean of all measurements performed.  General Measure No  0111- OOP -C093- 18 
10 3.4.1.3   Cylindricity of the inlet section  
No diameter along the cylindrical inlet section shall differ by more than 0.4  % from  the value of the 
mean diameter.  
3.4.1.4   Internal surface roughness of the inlet section  
The internal surface shall be clean, without dross and slag left after welding. The roughness  parameter 
Ra of the internal 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 
intersection 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 section (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 the ideal shape (represented e.g. by 
a shape template) shall not exceed in any place 0.004D . 
The internal surface of t he conical section of the convergent section is taken as being a surface of 
revolution if two diameters situated in the same plane 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   Inter nal surface roughness of the convergent section  
The internal surface shall be clean, without dross and slag left after welding. The roughness  parameter 
Ra of the internal surface of the convergent section shall be ≤  5·10-4 D. 
3.4.3   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 
plane 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 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 in the plane 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, in planes other than the plane of the pressure 
tappings. The mean value of the internal throat diameter d shall be determined as the si mple arithmetic 
mean of all measurements performed.  General Measure No  0111- OOP -C093- 18 
11 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 shall differ by more than 0.1 % of the value of the mean diameter.  
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 at approximately midway along the 
template profile. The value of this maximum deviation shall not exceed 0.02 d. 
3.4.3.4   
Throat surface roughness 
The roughness parameter Ra  of the inter nal 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 shall 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 tappings 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 in the form of 
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 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 perpendic ular to the centreline 
of the Venturi tube.  
At the point of break- through, the hole 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 0.1 
of the diameter of the pressure tapping. The pressure tappings shall 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 inlet 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.  General Measure No  0111- OOP -C093- 18 
12 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 . 
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 above.  
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 holes 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 complied 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 upstream of the Venturi tube and the actual 
Venturi tube shall be made of one or more pieces, however, the part of the pipeline up to the length of 
2D upstream of the Venturi tube s hall be made of one piece.  
3.6.2   Pipeline circularity and cylindricity  
The pipeline 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 equal within 1 % to 
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 internal surface of the pipelin e upstream of the Venturi tube  
The internal surface of the pipeline shall meet the specified surface roughness requirements for at least 
the minimum straight length upstream of the Venturi tube determined by the basic design of the 
transmission medium flow  meter (see Article 3.1). The internal 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 in the plane of the pipe’s connection to the inlet 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 . General Measure No  0111- OOP -C093- 18 
13 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.  
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 info rmation necessary for the faultless implementation of the Venturi tube 
transmission medium flow sensor into the transmission medium flow quantity measuring system in relation to the other components of the measuring system.  
4.2   Markings  
4.2.1   Markings on the Venturi tube  
The Venturi tube shall carry the following information:  
a) the 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 form 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 transmission 
medium flow, the flow direction shall be indicated on the Venturi in such a manner that it cannot be 
confused.  
4.2.2   Marking of the Venturi tube transmission medium flo w sensor  
Venturi tube transmission medium flow sensors shall carry the following information:  
a) the name of the manufacturer and type (design variant);  
b) the 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 / n ominal 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 transmission medium flow sensor which is permanently 
connected to the straight pipe section 0.5D  upstream of the Venturi tube (installation chamber, General Measure No  0111- OOP -C093- 18 
14 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 the transmission medium flow sensor, a ll of these sections shall be marked with the type 
approval mark. Any important separate sections of the Venturi tube transmission medium flow sensor 
shall generally be marked with the type approval mark.  
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 transmission medium 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:  
 check of completeness of the required technical documentation;  
 check of 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;  
 check of 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 th e Venturi tube transmission 
medium 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 shall 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 shall not exceed 5  °C during the period of the 
test. General Measure No  0111- OOP -C093- 18 
15 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 3.3.1.2, or, if applicable, 3.2.1.2 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 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 internal 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 internal 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 V enturi 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 internal 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 an gle 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 parameters 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 internal 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 internal 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 internal 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 internal 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);  General Measure No  0111- OOP -C093- 18 
16  the design and internal 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 internal 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).  
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 internal 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 differential pressure tappings  
Compliance with the requirements for differential pressure tappings, depending on the type of the 
Venturi tube under Article  3.5 and the manufacturer’s specifications, is asce rtained.  
6   Initial verification  
The initial verification of Venturi tube transmission medium 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 Venturi tube transmission 
medium flow sensor.  
6.1   Visual inspection  
The purpose of the visual inspection is to check that:  
 the design of the Venturi tube transmission medium flow sensor confor ms 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 transmission medium flow sensor fails to meet the external inspection requirements, 
no further tests ar e 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 r equirements under Article  5.2.2 shall apply to testing. General Measure No  0111- OOP -C093- 18 
17 6.2.3   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 3.3.1.2, or, if applicable, 3.2.1.2 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 temperature of 20  °C. 
6.2.3.1   Check of the geometric p arameters 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 internal 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 (s ee 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 internal surface roughness of the convergent section of the Venturi tube (see 
Articles  3.2.2.3 and 3.2.2.4);  
 the internal diam eter 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 internal 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).  
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 tube (see Article  3.3.1.3);  
 the internal sur face 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 internal 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 tu be (see Article 3.3.3.2);  
 the design and internal 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).  
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 internal 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);  General Measure No  0111- OOP -C093- 18 
18  the overall length of the convergent section of the Venturi tube (see Article  3.4.2.2);  
 the design and internal 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 internal 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 the 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 internal surface roughness of the pipe (see Article 3.6.3);  
 the coaxialit y 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 V enturi tube according 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 Venturi tube ( d
20) and the internal diameter 
of the inlet section of the Venturi tube ( D20) specified according to Article 6.2.3 shall be indicated in a 
legible and indelible manner in a suitable space on the Venturi tube transmission medium flow sensor 
in accordance with Ar ticle 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 transmission medium flow sensors 
is not performed. 
Long- term compliance with the operating conditions, including keeping the internal surface of the pipes 
in appropriate condition and clean, shall be ensured by the user of the measuring instrument. For thi s 
purpose, the user of the measuring instrument shall perform or ensure the performance of visual 
inspections of the condition of the internal surface of the pipe at intervals which take into account the 
operating conditions and the type of the transmission 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 -C093- 18 
19 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 shall notify Czech technical standar ds, other technical standards or technical 
documents of international or foreign organisations, or other technical documents 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 (on www.cmi.cz
). 
Compliance with notified standards or parts thereof shall be consider ed, 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 co mpliance with the requirements. 
These requirements may also be met by using another technical solution guaranteeing an equivalent 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 transmission medium 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  3.1.2(b) in the Annex ‘List of the Types of Legally Controlled Measuring Instruments’ to 
Implementing Decree No  345/2002 specifying measuring instr uments for mandatory verification and 
measuring instruments subject to type approval, as amended, this type of measuring instrument is 
classified as an instrument subject to mandatory verification.  
This legislation (General Measure) will be notified in ac cordance 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. 
I N S  T R U C T I O N S  
In accordance with §  172(1) CAP, in conjunction with § 39(1) CAP, the CMI has stipulated a time limit 
for comments 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 provisions of § 172(4) CAP, the comments are to be submitted in writing. 
Pursuant to the provisions of § 174(1) CAP, in conjunction wi th the provisions of § 37(1) CAP, 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 signa ture 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 Gene ral Measure shall be posted for 15 days.  General Measure No  0111- OOP -C093- 18 
20  
 
Czech Metrology Institute  
Director General