MCERTS Flowmeter Standard Rewritten: What the August 2026 Update Means for Permit Holders
TL;DR: On 24 August 2026 the Environment Agency substantially revised the MCERTS flowmeter scheme. The product standard now adopts EN 17694:2023 and splits into Part 1 for open channel instruments and Part 2 for closed conduit instruments, with three performance classes and Class 4 removed. If your permit requires flow monitoring, the MCERTS flowmeter on your discharge must still measure total daily volume to within ±8%, and the update changes what you check when you buy the next one.
Last updated: 11 September 2026
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Key takeaways
- The Environment Agency published a substantially revised MCERTS flowmeter scheme on 24 August 2026, with EN 17694:2023 as its technical foundation.
- The single flowmeter standard becomes two parts: Part 1 for open channel instruments and Part 2 for closed conduit instruments. A product can be certified against either part or both.
- Closed conduit instruments are graded in three classes, with combined performance limits of 2%, 5% and 8%. Class 4 has been removed.
- The site rule has not moved. A permitted discharge measured as total daily volume needs a target uncertainty of ±8% or better at 95% confidence, and new or replacement meters need MCERTS product certification.
- The same regime asks new installations to leave enough exposed pipe to clamp on a second meter for verification: at least 450 mm or 1.5 pipe diameters, whichever is greater.
What changed in the MCERTS flowmeter scheme?
If you first need to know whether your site needs MCERTS at all, read what MCERTS is and when a site needs it.
The MCERTS flowmeter scheme is the Environment Agency’s product certification for water flowmeters. On 24 August 2026 it was substantially revised. It now adopts EN 17694:2023, it is split into Part 1 for open channel instruments and Part 2 for closed conduit instruments, it introduces certification classes for velocity sensors, and it removes Class 4.
That is the first rewrite of substance in six years. The MCERTS performance standard for water flowmeters was first published on 7 April 2014 and was last updated on 4 March 2020, when a section on combined performance and loop powered instruments was added. The 2026 version keeps that combined performance idea and rebuilds the rest around the European hydrometry standard.
The standard is written for two audiences: manufacturers of water flowmeters and the test organisations that assess them. Certification is operated by CSA Group Testing UK Ltd, which works to EN ISO/IEC 17065, and an MCERTS flowmeter certificate is valid for 5 years. The Environment Agency describes both parts as technology transparent, which means any technical solution that meets the requirements can be certified, whatever its measuring principle.
What is the difference between Part 1 and Part 2?
Part 1 covers instruments on open channels, and on closed conduits running under open channel conditions, such as a part full sewer. Part 2 covers closed conduit instruments in pressurised pipes and in partially filled pipes that may become surcharged. The split mirrors EN 17694, which is itself published in two parts.
Each part grades an MCERTS flowmeter by class. For closed conduit instruments, Part 2 sets out the limits below, and every MCERTS flowmeter in that part is tested against them for bias, repeatability and a set of influence quantities such as water temperature, ambient temperature, humidity and stray currents.
| Closed conduit class | Bias limit | Repeatability limit | Combined performance |
|---|---|---|---|
| Class 1 | ±1.5% | 1% | 2% |
| Class 2 | ±4% | 2% | 5% |
| Class 3 | ±6.5% | 4% | 8% |
Part 1 uses the same three bias figures for integrated velocity area instruments, and adds much tighter classes for level sensors, from ±0.1% bias in Class 1 to ±1% in Class 3, because a small level error becomes a large flow error at a weir or flume. Field trials under Part 1 log the output at intervals of no more than 15 minutes, and Part 2 sets a maximum response time of 30 seconds for all three classes.
Who needs an MCERTS flowmeter?
You need MCERTS flow monitoring if you hold a permit under the Environmental Permitting Regulations that says you must monitor liquid flow, such as sewage or trade effluent, and collect and record the results. The Environment Agency’s minimum requirements for self-monitoring of flow names industrial sites and water utility sewage treatment works among those it covers.
Two separate certifications sit behind every MCERTS flowmeter, and they are easy to confuse. MCERTS product certification says a model of meter met the performance standard under test. MCERTS inspection certification says your installation, on your site, measures the flow it is meant to measure. The August 2026 rewrite is to the first. The second sits in the minimum requirements standard, last revised on 28 August 2024.
For new or replacement flowmeters that measure total daily volume, an MCERTS flowmeter is not optional. The minimum requirements standard says any meter fitted after April 2010 shall hold suitable MCERTS product certification, and an exception can only be made where no suitable certified product exists for the installation and the regulator agrees in writing. Natural Resources Wales permits may also call for certification against the same standard.
What does ±8% on the daily volume actually mean?
It means the total daily volume your permit specifies must be measured by your MCERTS flowmeter installation with a target uncertainty of ±8% or better, at a 95% confidence level. On a discharge of 1,000 cubic metres a day that is a band of 80 cubic metres either side. The figure applies to the whole measurement, so where a discharge runs through more than one flowmeter their uncertainties are combined.
This is where a laboratory class meets a real chamber. A Class 1 MCERTS flowmeter installed with a bend just upstream, a partly blocked approach or a sensor in the wrong place can still miss ±8% on site. That is why the inspection side exists, and why the standard asks inspectors to assess hydraulic influences both upstream and downstream across the full range of flow conditions.
Where a permit limits flow rate rather than volume, the standard treats it as instantaneous flow: either a spot reading or an average over 15 minutes or less. Read your permit condition carefully, because the two carry different requirements and some permits carry both.
Does the update change an installation you already have?
Not directly. The revised standard tells manufacturers and test organisations how a flowmeter earns its MCERTS certificate. Your installation is still inspected under the minimum requirements standard, where an MCERTS inspection certificate is valid for 5 years from the date of the initial site inspection and the operator’s management system is reassessed every 5 years.
Where the update will reach you is at the next purchase. When a meter fails, a discharge point is added or a site is rebuilt, the MCERTS flowmeter you specify will be certified against Part 1 or Part 2 of the revised scheme, in a stated class. Ask for the certificate, check which part and class it covers, and check that the class leaves room inside your ±8% once installation effects are added.
If a specification, framework or tender template still asks for a Class 4 instrument, it needs rewriting. That class no longer exists in the scheme.
Where does a clamp-on ultrasonic flow meter fit?
As the check meter. The minimum requirements standard says new installations must have enough exposed pipe to install a clamp-on flowmeter for verification, with a recommended minimum of 450 mm or 1.5 pipe diameters, whichever is greater. It also lists a temporary clamp-on ultrasonic meter as one way to carry out verification or secondary testing of a permanent installation.
That is the regulator describing, in its own standard, the job a clamp-on meter does best: an independent reading, taken from outside the pipe, without breaking into the MCERTS flowmeter installation. The AQUAIOT Clamp-on Ultrasonic Flow Meter fits live pipework from DN8 to DN100 with no cutting and no shutdown, and it reports continuously rather than only on the day of a test.
On a permitted site it earns its place in three roles:
- Verification. A second, independent trace alongside the permanent MCERTS flowmeter on pipework within its range, so drift shows up as a gap between two lines rather than as a surprise at the next inspection.
- The flows the permit does not cover. Process water, washdown, cooling make up and sub-metered buildings, which together explain what arrives at the discharge. Our guide to water balance monitoring shows how those flows reconcile.
- Early warning. A continuous trace catches a stuck valve or a night time flow that should not be there days before a daily total would.
For the permitted measurement itself, the standard expects an MCERTS flowmeter with product certification for that use. The value of continuous clamp-on data is everything around it: the evidence that the permitted meter is still telling the truth, and the explanation when it is not.
What should permit holders do next?
Treat the update as a prompt to check three documents: the monitoring conditions in your permit, your MCERTS inspection certificate and its renewal date, and the product certificate for every MCERTS flowmeter on a permitted discharge. Then plan the next replacement before it is urgent.
- Read the permit condition and note whether it asks for total daily volume, instantaneous flow, or both.
- Find the MCERTS inspection certificate and its date. It runs for 5 years from the initial site inspection.
- For each MCERTS flowmeter on a permitted discharge, record the make, model and product certificate, and which part and class it covers.
- Check the pipework for the 450 mm or 1.5 diameters of straight, exposed pipe that a verification meter needs.
- Put a replacement specification on file now, naming the part and class, so a failed meter is replaced with a certified product rather than whatever is in stock.
CSA Group publishes the MCERTS self-monitoring of flow guidance and bulletins that inspectors work to, and the list of certified products, which is the place to confirm a model before you order it.
Frequently asked questions
What is an MCERTS flowmeter?
An MCERTS flowmeter is a water flowmeter whose model holds MCERTS product certification against the Environment Agency’s performance standard. Since 24 August 2026 that standard is based on EN 17694:2023 and is split into Part 1 for open channel and Part 2 for closed conduit instruments.
What accuracy does MCERTS flow monitoring require?
For effluent flow expressed as total daily volume, the target uncertainty is ±8% or better at a 95% confidence level. Closed conduit instruments are graded in three classes, with combined performance limits of 2%, 5% and 8%.
How long does an MCERTS inspection certificate last?
Five years from the date of the initial site inspection. The operator’s management system is also subject to periodic surveillance audits, and it is reassessed every 5 years.
Can a clamp-on ultrasonic flow meter be used for MCERTS?
The minimum requirements standard names clamp-on meters as a way to verify a permanent installation, and new installations should leave room to fit one. Whether a particular model can be the permitted meter itself depends on it holding MCERTS product certification for that use.
Does the update apply to meters installed before April 2010?
The product certification rule in the minimum requirements standard applies to new or replacement flowmeters fitted after April 2010. For anything older, ask your MCERTS inspector how it is treated before changing the installation.
Where to start
The rewrite is aimed at manufacturers, but its effect lands on operators the next time a meter is bought. A short audit now, of permits, certificates and pipework, means the next replacement is a specification rather than a scramble.
AQUAIOT’s products hub covers the flow, level and quality layer, and our guides to trade effluent monitoring and abstraction licence monitoring cover the two neighbouring consent regimes.
Speak to an AQUAIOT engineer about continuous verification around your MCERTS flowmeter, and we will map the pipework, the check points and the alarm routing with you.