UK Flow Meter Standards Explained: MID, WRAS and Custody Transfer
Ask a plant manager whether their new flow meter needs to be "MID approved" and you will often get a confident yes, whether or not the application requires it at all. The acronyms UK flow meter buyers meet, MID, WRAS, UKCA, ATEX, get used almost interchangeably in conversation, and the confusion is understandable: each covers something different, and none of them measures the same thing as a calibration certificate.
Nixon Flowmeters has specified and supplied flow meters into UK water utilities, chemical processing, food and beverage, and oil and gas since 1969, and we field this question weekly. This article sets out plainly what each scheme covers, organised around the question that decides which, if any, applies to your application: what is the reading used for? If money changes hands on it, one set of rules applies; if it touches drinking water, a different set applies. For most industrial process measurement, none of them do, and paying for the wrong one is money misspent.
Approvals Follow the Use, Not the Meter
The mistake we see most often is treating "approval" as a general badge of quality a good meter simply has. It is not. Every scheme here answers a specific regulatory question about a specific use; a meter properly approved for one application may need no approval at all for another.
Three use-cases cover almost every UK flow meter application. Billing or trade: the reading determines what one party charges another, triggering MID. Contact with drinking water: the meter's wetted materials sit in a potable system, triggering WRAS materials approval, regardless of billing. Hazardous atmospheres: the meter sits somewhere an explosive gas or dust atmosphere could form, triggering ATEX or its UK equivalent, regardless of what the reading is used for.
Most process measurement, internal to a plant rather than an invoice, falls into none of these categories. Which applies has nothing to do with which technology you choose; turbine, magnetic or ultrasonic is a separate decision, covered in our technology comparison guide.
MID: The Framework for Meters Used in Trade
The Measuring Instruments Directive (MID) is the legal framework for measuring instruments used in trade, the category regulators call legal metrology. It applies to instruments whose reading determines an amount one party pays another: a utility's billing meter, a fuel dispenser, a gas meter feeding an invoice.
MI-001, MI-002 and MI-005: The Annexes Buyers Actually Meet
MID splits into instrument-specific annexes. The three a flow meter buyer meets most are MI-001 (water meters), MI-002 (gas meters and volume conversion devices) and MI-005 (liquid meters other than water, covering fuel and similar duties). Each annex sets the accuracy classes and testing an instrument's design, or "pattern", must meet, demonstrated through a sealed configuration and ongoing in-service verification.
UKCA, CE and the Honest Answer on Marking
Since the UK left the EU, exactly which mark, UKCA, CE, or both, applies to a given instrument is precisely the sort of detail that shifts with government transition arrangements. In broad terms, UK law retains the same underlying MID requirements alongside a UK marking scheme. Rather than quote a date that may have moved, check the current position with your approved body or GOV.UK before you specify, or ask us; we track it because we have to.
WRAS: A Materials Approval, Not an Accuracy Approval
This is the confusion this article exists to untangle: WRAS approval has nothing to do with measurement accuracy. The Water Regulations Advisory Scheme administers a materials approval on behalf of UK water suppliers, confirming wetted materials will not contaminate drinking water, under the Water Supply (Water Fittings) Regulations. It assesses material safety and backflow protection, not accuracy.
A meter can be WRAS-approved with no MID approval (fine, if never used for billing), or MID-approved yet lack WRAS approval (a problem, in a potable system). Asking for "MID and WRAS" on a billing water meter is asking the right question twice.
Custody Transfer: The Top Tier
Custody transfer sits above ordinary billing: measurement where ownership of a bulk product, oil, gas, refined fuel, changes hands, and the value at stake justifies a far more rigorous regime. Beyond an approved instrument, it demands proving against a reference standard, physical sealing against tampering, and an audit trail both parties can rely on.
Nixon's turbine flow meters are specified into custody transfer and allocation metering for exactly this reason: a mechanical, pulse-output design with traceable calibration history suits the top tier's discipline. We cover proving, sealing and audit trail practice in full in a dedicated custody transfer article.
ATEX in One Paragraph
ATEX certifies that a meter is safe to install in a potentially explosive atmosphere: flammable gas, vapour or combustible dust, nothing more. It says nothing about measurement accuracy: a meter can be ATEX-certified and metrologically ordinary, just as an accurate custody transfer meter might carry no hazardous area rating if never installed anywhere hazardous. Since Brexit, UK hazardous area certification runs alongside the EU ATEX regime under separate UK arrangements; check the current requirement for your site and zone rather than assuming the two are interchangeable.
Calibration Is Not Approval
This is the section that saves customers real money. A calibration certificate is a traceable record of one meter's performance on a specific day, against a reference standard. An approval, MID or otherwise, is a statement that a design meets a legal scheme, checked once and policed through sealing. The K-factor on a turbine meter's certificate, explained in our guide to turbine flow meter accuracy, is exactly this kind of record.
Most industrial buyers need calibration, not approval. If your meter informs a process control loop and nobody's invoice depends on the number, a UKAS-traceable certificate tells you everything you need. Specifying MID approval for internal measurement, because it sounds rigorous, adds cost and nothing useful.
At a Glance: Which Scheme Applies to Which Use
| Use case | Scheme | What it covers | Who checks it |
|---|---|---|---|
| Billing or trade | MID (MI-001/002/005) + UK marking | Accuracy class, sealed design | Approved body, in-service checks |
| Drinking water contact | WRAS | Material safety, no contamination | WRAS, UK water suppliers |
| Custody transfer | MID + custody practice | Approved instrument, proving, sealing | Both parties, independent prover |
| Hazardous atmosphere | ATEX / UK equivalent | Ignition safety, not accuracy | Approved body, zone rating |
| Internal process measurement | Usually none; calibration instead | Traceable performance on the day | UKAS calibration lab |
Frequently Asked Questions
What is a MID approved flow meter?
A meter whose design is assessed against the Measuring Instruments Directive for its category (water, gas or liquid), then sealed and verified for lawful trade use. It excludes internal process readings.
What does WRAS approval mean?
It means the product's materials are assessed safe for contact with drinking water, under the Water Supply (Water Fittings) Regulations. It is about material safety, not measurement accuracy.
Do I need an approved meter for internal process measurement?
Usually not. If the reading informs an internal process and no invoice depends on it, a traceable calibration certificate is what you need, not a MID or WRAS approval.
What is the difference between calibration and approval?
Calibration records how one meter performed on a given day against a reference. Approval is a legal statement that a design meets a defined scheme, checked once and policed through sealing.
What approvals does a billing water meter need in the UK?
It needs MID approval, because the reading sets a charge, and WRAS approval, because it sits in contact with drinking water. A compliant billing meter needs both.
Getting the Scheme Right, or Knowing You Need None
Approvals exist to protect the public and the customer, not to decorate a datasheet. If you are unsure which scheme, if any, your application needs, ask us before paying for the wrong one; the Nixon team deals with this every week.
