Guidance

MCERTS for water flowmeters Part 1: performance standards and test procedures for open channel instruments

Updated 24 August 2026

Applies to England

Introduction

The Environment Agency set up the Monitoring Certification Scheme (MCERTS) to provide guidance on the standards you must meet to monitor emissions that affect the environment. The scheme is based on international standards. It provides for the product certification of instruments, the competency certification of personnel and the accreditation of laboratories.

This guidance is for:

• manufacturers of water flowmeters for use on open channels and closed conduits operating under open-channel flow conditions who need to know what standards to meet to be able to use their equipment for regulatory purposes in England and Wales
• test organisations who carry out testing of the equipment on behalf of manufacturers to see if it meets the required standard

We designed MCERTS to support the measurement requirements specified in environmental permits by making sure that equipment used to derive monitoring data is fit for purpose.

Open channel continuous water flowmeters

A continuous water flowmeter is an instrument that measures the flow-rate or totalised volume of water passing along an open channel or conduit. It computes such quantities from measurements of one or more properties which have a defined relationship with the flow-rate.

Open channel continuous water flowmeters are used to measure the flow of water in open channels and closed conduits operating under open channel flow conditions. They are referred to as open channel instruments (OCIs). They are used to measure the flow of treated discharges from wastewater treatment works, industrial processes, untreated wastewater and receiving waters.

For pipes to operate under open-channel flow conditions, they should always have free surface flow. This means they should not become surcharged or behave as a pressurised pipe.

Certification body

We have appointed CSA Group Testing UK Ltd (CSA Group) as the certification body to operate MCERTS on our behalf.

CSA Group Testing UK Ltd complies with EN ISO/IEC 17065 Conformity assessment – Requirements for bodies certifying products, processes, and services.

If you have any questions about the certification process or would like more information on how to make an application, please contact CSA Group Testing UK.

More information about MCERTS is available in the Monitoring emissions to air, land and water collection on GOV.UK.

If you have any general questions about MCERTS, please contact the Environment Agency.

Certification committee

The certification body appoints a certification committee to make certification decisions. The committee is made up of appropriately independent, competent person or group.

MCERTS certification

Product certification comprises of 2 phases. These are:

• laboratory and field testing – used to determine performance characteristics, where such testing requires a highly controlled environment (field testing is carried out on processes representative of the intended industrial sectors and applications)
• surveillance - initial and continuing which comprises an audit of the manufacturing process (to confirm that the manufacturer has provisions to ensure manufacturing reproducibility and to control any design changes to ensure that they do not degrade performance below the MCERTS requirements)

MCERTS continuous flowmeter scheme

MCERTS for continuous flowmeters provides the product certification of OCIs and closed conduit instruments. Information about closed conduit instruments is available at MCERTS performance standard for water flowmeters part 2: closed conduit instruments. 

Water flowmeters may be certified against either one or both parts of this standard.

1. Summary

In this section we introduce the scheme.

1.1 Background

1.1.1 The European standard which forms the technical basis of the MCERTS requirements for OCIs is ‘EN 17694-1:2023 – Hydrometry – Minimum performance requirements and test procedures for water monitoring equipment – Devices for the determination of flow – Part 1: Open channel instrumentation’

1.1.2 The requirements in this document and EN 17694-1 are intended to be technology transparent and allow the product certification of any technical solution that meets the requirements. The principles upon which the general requirements, minimum performance requirements, test procedures and statistical procedures for evaluation of test data are based are set out in section 5 of EN 17694-1.

1.1.3 Definitions of terms used in this document are provided in section 3 of EN 17694-1. Additional terms are provided in this document.

1.1.4 The Environment Agency requires operators of regulated processes to use MCERTS certified continuous water flowmeters.

1.1.5 It is the user’s responsibility to make sure that the selection, installation and operation of an OCI is appropriate to the application.

1.1.6 EN 17694-1 specifies general requirements, minimum performance requirements and test procedures applicable to OCIs used to determine either volumetric flow-rate, total volume or both.

1.1.7 It applies to level sensors with associated electronics designed to be used with a conventional gauging structure. The requirements and test procedures for gauging structures, such as weirs and flumes, are excluded. The stage discharge characteristics for many of these structures are established and published in national and international standards.

1.1.8 It also applies to integrated velocity area instruments comprising level and velocity sensors that may be separate or combined in a single assembly.

1.1.9 It also applies to sensors that determine the mean water velocity through a channel.

1.1.10 Performance tests for product certification of an OCI against these MCERTS requirements shall be carried out in accordance with the procedures specified in this document and EN 17694-1.

2. Scope of the MCERTS scheme

MCERTS certified OCIs can be required for use on a range of sites including:

• processes under the Environmental Permitting Regulations
• consented discharges for processes regulated through provisions of the Water Resources Act, 1991
• sites under the Urban Wastewater Treatment Regulations (UWWTR)
• sites under the Radioactive Substance Regulations
• sites under other relevant directives, for example, the Water Framework Directive
• other monitoring of the aquatic environment

Information on flow and event duration monitoring installations is available in Minimum requirements for self-monitoring of flow: MCERTS performance standard and MCERTS: performance standards and test procedures for event duration monitors.

3. General requirements

This section describes general requirements for the identification, design and operation of OCIs and the information required in manufacturer’s documentation.

The general requirements shall be assessed by inspection or the manufacturer’s statement.

3.1 Requirements for OCIs

3.1.1 OCIs shall meet the general requirements specified in EN 17694-1 (Section 6.1).

3.1.2 MCERTS certified OCIs shall have a unique designation that clearly identifies the OCI as a MCERTS certified model. To enable identification of an MCERTS certified OCI, reference may also need to be made to versions of software and firmware installed in the OCI.

3.1.3 Any additional equipment required for the OCI to operate continuously on site within the MCERTS requirements shall be available from the manufacturer. This shall include but not be limited to, upstream strainers or filters, sunshades, batteries and secondary enclosures.

3.1.4 Attaching an ancillary device to the OCI shall not reduce the rated operating conditions for any environmental parameter.

An ancillary device is any additional equipment that may be required for operation of the OCI on site but which does not normally form part of the OCI package. Examples include external data loggers or telemetry equipment, power conditioning devices, lightning protection, external pumps or compressors required for automatic cleaning systems.

3.1.5 An electronic OCI shall have a means of automatically communicating faults to a remote system.

3.1.6 An OCI intended for regulatory reporting shall have a means for preventing unauthorised resetting of the total recorded volume.

3.1.7 Measurement resolution of level sensors shall meet or exceed the requirements shown in Table 1. This requirement is equivalent to the performance class requirements of EN ISO 4373:2022 Hydrometry – Water level measuring devices.

Table 1: MCERTS level measurement performance requirements for level sensor

Certification range	MCERTS Class 1	MCERTS Class 2	MCERTS Class 3
≤1m	≤1mm	≤2mm	≤10mm
1.0 to 5.0m	≤2mm	≤5mm	≤50mm
5.0 to 20.0m	≤10mm	≤20mm	≤200mm

3.2 Requirements for associated documentation

3.2.1 Manufacturers’ associated documentation shall meet the requirements specified in EN 17694-1 (section 6.2).

3.2.2 The manufacturer’s documentation shall include a statement on the OCI’s maximum and minimum range for all measurement parameters.

3.2.3 For OCIs with non-contact velocity sensors, the manufacturer shall provide guidance on the water surface conditions required to maintain performance within the MCERTS requirements.

4. Performance requirements

This section describes the performance requirements for OCIs.

4.1 Expression of performance requirements

Performance requirements of OCIs shall be expressed as specified in EN 17694-1 (section 7.1).

4.2 Performance requirements for OCIs

The performance requirements for OCIs shall be as specified in EN 17694-1 (section 7.2). However, Tables 2 to 4 shall be used in place of Table 2 of EN 17694-1 to reflect the performance requirements and classes of the MCERTS scheme.

MCERTS level instruments class 2, integrated velocity-area instruments class 1 and velocity sensors class 1 are equivalent to the performance requirements specified in EN 17694-1. MCERTS level class 1 exceeds the performance requirements defined in EN 17694-1.

Table 2: MCERTS OCI performance requirements for level sensors % error and water level by class

Determinand	Symbol	MCERTS clause	Level sensor MCERTS – class 1 %error – Water level	Level sensor MCERTS – class 2 %error – Water level	Level sensor MCERTS – class 3 %error – Water level
Bias	b	6.3.2, 6.3.11	±0.1	±0.3	±1
Lower limit value for bias (see note 1)	N/A	6.3.2	±0.2	±0.5	±1
Repeatability	SR	6.3.2	0.05	0.15	0.5
Supply voltage	XV	6.3.3	0.025	0.075	0.25
Output impedance	XO	6.3.4	0.025	0.075	0.25
Water temperature	XFT	6.3.5	0.025	0.075	0.25
Ambient air temperature	XT	6.3.6	0.025	0.075	0.25
Relative humidity	XRH	6.3.6	0.025	0.075	0.25
Incident light	XLX	6.3.9	0.025	0.075	0.25
Sensor location	XSL	6.3.8	0.025	0.075	0.25
Direct solar radiation	XSR	6.3.7	0.05	0.15	0.5
Accuracy of computation	XAC	6.3.10	0.025	0.075	0.25
Combined performance requirement	UC	6.3.14	0.2	0.5	1.5
Warm up	N/A	6.2	To be reported	To be reported	To be reported
Response time (see note 2)	N/A	6.3.12	30 seconds	30 seconds	30 seconds

Note 1: The lower limit value for bias is only applicable from the minimum value to the minimum value plus 10% of the measurement range.

Note 2: The response time of an OCI will be assessed over a variation in level of at least 20% of the measurement range.

Table 3: MCERTS OCI performance requirements for integrated velocity area instrument % error and volumetric flow-rate or totalised volume passed, by class

Determinand	Symbol	MCERTS clause	Integrated velocity area instrument MCERTS – class 1 % error – Volumetric flow-rate or total volume passed	Integrated velocity area instrument MCERTS – class 2 % error – Volumetric flow-rate or total volume passed	Integrated velocity area instrument MCERTS - class 3 % error – Volumetric flow-rate or total volume passed
Bias	b	6.3.2, 6.3.11	±1.5 (see note 5)	±4	±6.5
Lower limit value for bias (see note 3)	N/A	6.3.2	±5	±10	±10
Repeatability	SR	6.3.2	1	2	4
Supply voltage	XV	6.3.3	0.5	1	2
Output impedance	XO	6.3.4	0.5	1	2
Water temperature	XFT	6.3.5	0.5	1	2
Ambient air temperature	XT	6.3.6	0.5	1	2
Relative humidity	XRH	6.3.6	0.5	1	2
Incident light	XLX	6.3.9	0.5	1	2
Sensor location	XSL	6.3.8	0.5	1	2
Direct solar radiation	XSR	6.3.7	1	2	4
Accuracy of computation	XAC	6.3.10	0.25	0.5	1
Combined performance requirement	UC	N/A	2 (see note 5)	5	8
Warm up	N/A	6.2	To be reported	To be reported	To be reported
Response time (see note 4)	N/A	6.3.12	30 seconds	30 seconds	30 seconds

Note 3: The lower limit value for bias is only applicable from the minimum value to the minimum value plus 10% of the measurement range.

Note 4: The response time of an OCI will be assessed over a variation in flow-rate of at least 20% of the measurement range.

Note 5: These individual performance requirements exceed the performance requirement of EN 17694-1, where the overall MCERTS class is equivalent, due to previous higher MCERTS standards in these classes, the no backsliding principle has been applied.

Table 4: MCERTS OCI performance requirements for velocity sensor % error and mean channel velocity, by class

Determinand	Symbol	MCERTS clause	Velocity sensor MCERTS – class 1 %error – Mean channel velocity	Velocity sensor MCERTS – class 2 %error – Mean channel velocity	Velocity sensor MCERTS – class 3 %error – Mean channel velocity
Bias	b	6.3.2, 6.3.11	±1.9	±3.8	±6.25
Lower limit value for bias (see note 6)	N/A	6.3.2	±4	±8	±8
Repeatability	SR	6.3.2	0.96	1.92	3.84
Supply voltage	XV	6.3.3	0.49	0.98	1.96
Output impedance	XO	6.3.4	0.49	0.98	1.96
Water temperature	XFT	6.3.5	0.49	0.98	1.96
Ambient air temperature	XT	6.3.6	0.49	0.98	1.96
Relative humidity	XRH	6.3.6	0.49	0.98	1.96
Incident light	XLX	6.3.9	0.49	0.98	1.96
Sensor location	XSL	6.3.8	0.49	0.98	1.96
Direct solar radiation	XSR	6.3.7	0.99	1.98	3.96
Accuracy of computation	XAC	6.3.10	0.24	0.48	0.96
Combined performance requirement	UC	N/A	3.95	4.9	7.8
Warm up	N/A	6.2	To be reported	To be reported	To be reported
Response time (see note 7)	N/A	6.3.12	30 seconds	30 seconds	30 seconds

Note 6: The lower limit value for bias is only applicable from the minimum value to the minimum value plus 10% of the measurement range.

Note 7: The response time of an OCI will be assessed over a variation in flow-rate of at least 20% of the measurement range.

4.3 Computational accuracy

OCIs shall meet the computation accuracy requirements specified in EN 17694-1 (section 7.3).

4.4 Data retention

OCIs shall meet the data retention requirements specified in EN 17694-1 (section 7.4).

4.5 Environmental requirements

OCIs shall meet the environmental requirements specified in accordance with EN 17694-1 (section 7.5).

4.6 Water temperature requirements

OCIs shall meet the water temperature requirements specified in accordance with EN 17694-1 (section 7.6).

4.7 Installation influences

OCIs shall meet the installation influences requirements specified in accordance with EN 17694-1 (section 7.7).

4.8 Field test requirements

OCIs shall meet the field test requirements specified in accordance with EN 17694-1 (section 7.8).

5. Provisions for test organisations

This section describes the provisions for organisations testing OCIs.

5.1 General requirements for testing

The testing of OCIs for MCERTS shall be conducted in line with the requirements specified in EN 17694-1 (section 8.1).

The results of other tests may be acceptable to the certification body if they are shown to be equivalent to MCERTS and carried out independently. Manufacturers’ own test data may also be considered if it also meets the MCERTS requirements for testing and data quality.

5.2 Laboratory test conditions

OCIs shall be tested at the reference conditions and test point values specified in EN 17694-1 (section 8.2).

5.3 Reporting

Test results shall be reported as specified in EN 17694-1 (section 8.3).

6. Laboratory test procedures

This section describes the laboratory test procedures.

6.1 Guidance for establishing a test plan

Before testing is started, a test plan which meets the requirements of EN 17694-1 (section 9.1) shall be agreed with the MCERTS certification committee.

6.2 Verification by inspection

6.2.1 The OCI shall be inspected to verify conformity with the requirements in EN 17694-1 (section 9.2).

6.2.2 The OCI shall be inspected to verify conformity with the additional general requirements listed in this document.

6.2.3 The OCI’s associated documentation shall be inspected to verify that the general requirements listed in this document are fulfilled.

6.3 Performance tests

Performance tests should be carried out following the agreed test plan.

6.3.1 Loss of power for electronic OCIs

Determine the influence of loss of power on the performance of the OCI in accordance with EN 17694-1 (section 9.3.1).

6.3.2 Bias (mean error) and repeatability

Determine the bias and repeatability of the OCI in accordance with EN 17694-1 (section 9.3.2).

6.3.3 Supply voltage

Determine the influence of changes in supply voltage on the OCI in accordance with EN 17694-1 (section 9.3.3).

6.3.4 Output impedance

For OCIs fitted with analogue outputs only, determine the influence of changes in output impedance on the OCI in accordance with EN 17694-1 (section 9.3.4).

The output is a displayed reading, or a digital or analogue electrical signal, generated by an instrument in response to a measurand.

6.3.5 Water temperature

Determine the influence of changes in water temperature on the OCI in accordance with EN 17694-1 (section 9.3.5).

6.3.6 Ambient air temperature and relative humidity

Determine the influence of changes in ambient temperature and relative humidity on the OCI in accordance with EN 17694-1 (section 9.3.6).

6.3.7 Direct solar radiation

Determine the influence of direct solar radiation on the OCI in accordance with EN 17694-1 (section 9.3.7).

6.3.8 Sensor location

For non-contact velocity sensors only, determine the influence of changes in the sensor location on the OCI in accordance with EN 17694-1 (section 9.3.8).

6.3.9 Incident light

For non-contact OCIs using optical measurement methods only, determine the influence of incident light on the water flowmeter in accordance with EN 17694-1 (section 9.3.9).

6.3.10 Accuracy of computation

Determine the influence of the accuracy of computation on the OCI in accordance with EN 17694-1 (section 9.3.10).

In addition to the test points required in EN 17694-1 at sections 9.3.10.2 and 9.3.10.3, an additional test point shall be used at test point 2 from Table 3 in section 8.2 of EN 17694-1.

6.3.11 Effect of channel size

Determine the effect of channel size on the OCI in accordance with EN 17694-1 (section 9.3.11).

6.3.12 Response time

Determine the response time of the OCI in accordance with EN 17694-1 (section 9.3.12).

6.3.13 Fill level

For OCIs designed for use in pipes operating under open-channel flow conditions, determine the influence of fill level on the performance of the OCI by carrying out the bias (mean error) and repeatability test under each of the following conditions:

• varying water depth with mean water velocity constant at 15%, 50% and 85%, each plus or minus 5% of the velocity operating range
• varying water velocity with the fill depth constant at 15%, 50% and 85%, each plus or minus 5% of pipe bore

For each fill level, calculate and report the bias and repeatability at each test point.

6.3.14 Combined performance requirement

Upon completion of all laboratory testing calculate the combined performance characteristic using the procedure specified in EN 17694-1 (Annex A, section A.5).

7. Field test procedures

This section describes the field test procedures.

7.1 Objective of field test

The objectives of the field test are specified in EN 17694-1 (section 10.1).

7.2 Field test plan

7.2.1 The required details of the field test plan are specified in EN 17694-1 (section 10.2).

7.2.2 The certification body can provide advice on different practical approaches to implementing the field test plan. A questionnaire is available from the certification body to assist in collating the information required to establish whether data from an existing installation would be acceptable for product certification.

7.2.3 The test plan detailing the proposed field trial shall be submitted to the certification body for consideration by the certification committee. The decision of the certification committee on matters related to the test plan and data is final.

7.3 Field test requirements

7.3.1 The requirements of the field test are specified in EN 17694-1 (section 10.3).

7.3.2 Continuous logging of the output data shall be at intervals of no greater than 15 minutes. It should be an average of the preceding period or a spot reading.

7.3.3 Data obtained when conditions are outside the rated operating conditions can be reported to demonstrate performance in excess of the MCERTS requirements.

7.4 Error under field test conditions

The error of the OCI under test shall be determined as specified in EN 17694-1 (section 10.4).

The error is the difference between the value given by the OCI and the reference quantity value.

7.5 Up-time

The up-time of the OCI under test shall be determined as specified in EN 17694-1 (section 10.5).

7.6 Maintenance

The details of any maintenance shall be recorded as specified in EN 17694-1 (section 10.6).

The certification body shall be contacted for approval if any major components are replaced during the field trial.

7.7 Reporting of the field test

The field test shall be reported as part of the report specified in EN 17694-1 (section 8.3).

8. Evaluation of test data

Test data shall be evaluated according to the procedures specified in EN 17694-1 Annex A.

Resolution (u_RES) for level sensors shall be calculated against the certification range.

9. Reference methods

Guidance on reference methods applicable to this document are available in EN 17694-1 Annex B.

10. Reference uncertainty and error calculation

Uncertainty is the parameter associated with the result of a measurement that characterises the dispersion of the values that could reasonably be attributed to the measurand.

Guidance on reference uncertainty and error calculation is specified in EN 17694-1 Annex C.

11. MCERTS product certification

Manufacturers seeking product certification should contact the certification body, who will advise on specific requirements for the water flowmeter under consideration.

11.1 Certification process

An OCI shall be a fully functioning water flow monitoring system. OCIs can be supplied with several options, for example different sizes, with remote or integrated transmitters of sensor signals, or where more than one sensor or sensor configuration can be used to cover different ranges. The certification committee shall review how different options may affect performance and decide on the tests that are required for each option.

When selecting the options to be tested, consideration should be given to the specification of the OCI likely to be used for each of the identified applications. For additional sensors, sensor configurations or other options, it may be possible to extend certification by carrying out a subset of the full test programme. Where options are likely to have a significant effect on the performance of the OCI selected, additional tests may be required. Similarly, where different transmitters having different facilities are used with a single sensor, one complete OCI shall undergo the full conformity tests. For additional transmitters, it may be possible to extend certification by carrying out a subset of the full test programme.

11.2 Certification body and committee

The role of the certification body is to assess and certify compliance with the MCERTS requirements for defined applications and or conditions.

In performing this role, the certification body shall consider the relevance of the procedures defined in the MCERTS requirements for the product to be certified.

Following approval from the certification committee. An evaluation is undertaken by an independent reviewer associated with the certification body, who makes a final certification decision.

On request, the certification body shall provide the Environment Agency with the rationale for any certification decision.

11.3 Certification range

The OCI will be certified over the measurement range for which it is tested. If a manufacturer wishes to demonstrate performance over one or more supplementary ranges, some additional testing will be required over those ranges. This additional testing shall at least include evaluations of bias and repeatability.

11.4 Testing 

Manufacturers may commission testing from any organisation, provided that the requirements for testing organisations can be met. Manufacturers’ own test data may also be considered but should be assessed to demonstrate that it conforms to the requirements for test laboratories. This applies to both laboratory and field tests.

The results of previous performance tests may be acceptable to the certification body if they are equivalent to MCERTS requirements and were carried out independently. Any subsequent changes to the OCI shall be demonstrably not significant.

Certain tests may present practical difficulties for certain types of OCIs. In such cases, manufacturers should inform the certification body, so that the most appropriate course of action can be determined.

11.5 Auditing and surveillance

An audit of the manufacturing process shall be conducted by the certification body. This is to confirm that the manufacturer has provisions to make sure of manufacturing reproducibility and to control any design changes that may affect product performance.

Subsequent surveillance audits are normally conducted annually until sufficient evidence of a well-proven, robust system has been collected. Once this has been established, the certification body may extend the interval between audits or require submission of specific audit data for review off site.

11.6 Certificate validity

MCERTS certificates are valid for 5 years. After this time, the product certification is reviewed and any necessary retesting will be identified to maintain the product certification. Modifications to the OCI shall be considered as part of this process.

Assessment for recertification shall be carried out against the MCERTS performance standard current at the time of recertification.

11.7 Modifications to certified OCIs

Modifications and design changes include changes to all components of the flow or level measurement system, including software and firmware.

Any spares or replacement parts for certified OCI must meet the same performance standards as the original parts. Manufacturers may be required to provide evidence that the replacement parts meet the required performance standards of the original equipment as specified by the manufacturer of the OCI.

Modifications to certified OCI are allowable so long as manufacturers can demonstrate that these design changes do not degrade the performance of the OCI below the MCERTS requirements.

Manufacturers shall keep detailed records and drawings of all design changes to OCI, and have provisions for design verification, inspection and testing to make sure that the OCI still meets the required performance standards.

To meet the requirements of product certification, the certification body shall conduct audits of the design changes to OCI. Manufacturers shall notify the certification body of any modifications to equipment that may have a significant effect on OCI performance.

Design modifications or extensions to the range of application of an OCI may require renewed testing. The extent of this renewed testing will depend upon the nature of the modifications to the OCI.

If there is evidence that a modification has only limited effects on the performance of the OCI, then it would not be necessary to retest an OCI completely. In such cases, only a supplementary test would be required to the applicable MCERTS performance standards.

In the case of modifications to software, particularly in measuring instruments, detailed documentation shall be presented to the certification body indicating the nature of the modification as well as resultant effects on operation and functionality. The certification body will then decide if further testing is required.