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Water Metering Approvals – An Overview

Historical Approval Process

Like water meters, worldwide water meter approvals have undergone significant changes in the past 10 years, and in most respects have been carried out with the objective of unifying independent country requirements under one approval certification process.  As a result, the now internationally recognised standard for metering of cold potable water is, OIML R49.

Originally, when a country adopted a standard for Water Metering, they usually registered it under their own approval body, such as BS 5728 within the United Kingdom.  Requirements for gaining these approvals were often devised around the meter technology when they were released, which at the time, were mechanical meters.  As technologies improved, amendments were made, leaving several varying standards, for essentially the same measurement principle.

IS0 4064

However, as well as having a specific “Country Approval”, it was deemed appropriate that an international standard for metering devices was also required and thus, ISO 4064 became the universal standard for many years, setting the ‘precedent’ for future approvals.  

Nevertheless, with the introduction of Electronic Water Metering, the principle of measurement, not the accuracy of the meters, caused problems within these older standards.  Having said that, whilst ISO 4064 was once seen as the correct approval, it simply was unable to provide the necessary testing criterion for the new generation of meters.


Moreover, approval standards emanating from the European Union, such as EN or the often requested ‘EEC’ standards no longer have any credence or are legally acceptable, as they have been harmonized into OIML. In fact, from the very beginning, they were never based upon a metrological testing standard, as they were merely a means to sell into the respective territories.

International Organization of Legal Metrology - OIML

With this widespread confusion, it was accepted that clarity was required and therefore, OIML, the International Organization of Legal Metrology, an intergovernmental treaty organization was established to promote the global harmonization of legal metrology procedures. Since that time, OIML has developed a worldwide technical structure that provides the Countries that have signed up to the Organization with metrological guidelines for the elaboration of national and regional requirements, concerning the manufacture and use of measuring instruments for legal metrology applications.

OIML R49 was legally adopted for metering of cold potable water and represents a blend of BS 5728, ISO 4064, EN 14154 and many other standards from around the world, encompassing regulations for every criterion of water metering.  This standard represents all stipulations and regulations pertinent to cold potable water metering from the Regulatory body and thus, supersedes all other standards.

Currently, there are 57 member states and 58 corresponding members across the World, all adopting the metering requirement stipulated within the OIML R49 test requirements.

Nonetheless, several member countries, such as Brazil initially required authentication of the OIML R49 certification, and these were usually carried out at their approval body i.e. INMETRO. However, being such a considerable user of meters INMETRO has now decreed that OIML R49 shall be the only applicable standard.


In respect to the United States, their measuring principle is very different to most other Countries and subsequently apply the American Water Works Association (AWWA) standards but they are in addition to OIML.


There is much “Nomenclature” within metrology, but since OIML was introduced, a number of issues have been raised in respect to the difference between the Flow Rate symbols for OIML and the generally understood ones, for ISO.

The flow rate error tolerance curve for ISO has 4 key points, Qmin, Qt, QN, and Qmax. OIML similarly, has 4 key points, Q1, Q2, Q3, and Q4, but the two ranges are not directly comparable.

Each Standard has the necessary boundaries to the testing range, minimum and maximum for ISO and 1 and 4 for OIML.  The high flows for a certain meter, in both cases, never change and are defined, but the low flows can be different, depending on the sensitivity of the meter. 

For ISO, the Class of the meter and the Nominal flow of the meter define this low flow value.  For example, a QN 1.5 m³/hr meter of Class C will have a Qmin of 15 l/hr, whilst a Class D type would have a lower Qmin of 11.25 l/hr.  For OIML, the low flow is defined by its Q3 value and a ratio defined as Q3/Q1, which can be either 250, 200, or 160.  A comparable QN 1.5 m³/hr meter in OIML terms would have a Q3 of 2.5 m³/hr, and thus, the low flow, Q1 would be 10 l/hr, 12.5 l/hr, and 15.625 l/hr respectively.

The transition from the ±5% tolerance to the ± 2% flow is marked as Qt in ISO and Q2 in OIML, but these are not the same value and therefore, the same meter would be tested at different flow rates, depending on the standard.  As with Qmin, Qt will differ, depending on the Class of the meter.  However, Q2 does not change relative to the Q3/Q1 ratio.

QN and Q3 are used to define the meter in terms of categorisation, but again, they would not be the same value for the same meter.  For ISO, it is referred to as the nominal flow or average, which is half of the maximum flow.  However, for an OIML meter, its Q3  value is significantly higher than that of QN in ISO, but its value can only be taken from OIML R49-1 3.1.3.

In summary, the following applies:-


Qmin = Minimum Flow

Qt = Transitional Flow

QN = Nominal (Average) Flow

Qmax = Maximum Flow


Q1 = Minimum Flow. Q1= Q3/(Q3/Q1) where (Q3/Q1) has a value of either 250, 200 or 160.

Q2 = Transitional Flow. Q2 = Q1 + 60% or Q2/Q1 = 1.6

Q3 = Selected from a defined specified list as shown in OIML R49-1 3.1.3.

Q4 = Maximum Flow. Q4 = Q3 + 25% or Q4/Q3 = 1.25

Even though these sets of points cannot be compared, the above illustrates a guide to switching from ISO to OIML nomenclature.

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