Water meter accuracy class is a crucial parameter that defines the precision of a water meter’s measurements. It indicates the maximum permissible error (MPE) in the meter’s readings at different flow rates. Simply, it tells us how close the meter’s measurement is to the actual water volume passing through it.
Modern water meter accuracy standards, such as ISO 4064:2014, classify water meters into two main accuracy classes: Class 1 and Class 2. Class 1 meters are more precise than Class 2 meters. This classification is essential for residential water meter accuracy and various other applications.
Standards Defining Water Meter Accuracy Classes
Several international standards define water meter accuracy classes. Let’s look at the requirements specified in each:
ISO 4064-1:2014 & OIML R 49:2013
Accuracy class 1 water meters:
The MPE for the upper flow rate zone (Q2≤Q≤Q4) is ±1 %, for temperatures from 0.1 °C to 30 °C, and ±2 % for temperatures greater than 30 °C.
The MPE for the lower flow rate zone (Q1≤Q< Q2) is ±3 % regardless of the temperature range.
The MPE for the upper flow rate zone (Q2≤Q≤Q4) is ±1 %, for temperatures from 0.1 °C to 30 °C, and ±2 % for temperatures greater than 30 °C.
The MPE for the lower flow rate zone (Q1≤Q< Q2) is ±3 % regardless of the temperature range.
Accuracy class 2 water meters:
The MPE for the upper flow rate zone (Q2≤Q≤Q4) is ±2 %, for temperatures from 0.1 °C to 30 °C, and ±3 % for temperatures greater than 30 °C.
The MPE for the lower flow rate zone (Q1≤Q< Q2) is ±5 % regardless of the temperature range.
MPE stands for Maximum Permissible Error
AWWA C700 Series
The American Water Works Association (AWWA) standards provide detailed accuracy requirements for different types of water meters. While they don’t use the Class 1 and Class 2 terminology, they specify accuracy requirements for various flow rates:
AWWA C700 (Displacement-Type meters):
Normal test flow rates: ±1.5%
Intermediate test flow rates: ±2.0%
Low test flow rates: ±3.0%
AWWA C701 (Turbine-Type meters):
Normal test flow rates: ±1.5%
Intermediate test flow rates: ±2.0%
Low test flow rates: ±5.0%
Old Water Meter Accuracy Standards: Class A, B, C, and D
Before the introduction of the new Class 1 and Class 2 system, water meters were classified as A, B, C, and D. This old system was defined in the ISO 4064:1993 standard. Here are the details of the old classification:
Class A is the lowest and Class D is the highest.
Lower zone (Qmin to Qt): ±5%
Upper zone (Qt to Qmax): ±2%
| Nominal Diameter | Qn (m³/hr) |
Qmax (m³/hr) | Class A | Class B | Class C | Class D | ||||
|---|---|---|---|---|---|---|---|---|---|---|
| Qmin (litres/hr) |
Qt (litres/hr) |
Qmin (litres/hr) |
Qt (litres/hr) |
Qmin (litres/hr) |
Qt (litres/hr) |
Qmin (litres/hr) |
Qt (litres/hr) |
|||
| 15(½) | 1.0 | 2.0 | – | – | – | – | – | – | 7.5 | 11.5 |
| 15(½) | 1.5 | 3.0 | 60 | 150 | 30 | 120 | 15 | 22.5 | 11.25 | 17.25 |
| 20(¾) | 2.5 | 5.0 | 100 | 250 | 50 | 200 | 25 | 37.5 | 18.75 | 28.75 |
| 25(1) | 3.5 | 7 | 140 | 350 | 70 | 280 | 35 | 52.5 | 26.25 | 40.25 |
| 25(1) | 6.0 | 12 | 240 | 600 | 120 | 480 | 60 | 90 | – | – |
| 32(1½) | 6.0 | 12 | 240 | 600 | 120 | 480 | 60 | 90 | – | – |
| 40(1½) | 10 | 20 | 400 | 1000 | 200 | 800 | 100 | 150 | – | – |
Regardless of the water meter level, the common flow and overload flow rates for meters of the same caliber remain consistent. The maximum permissible error is approximately 5% from the minimum flow rate up to the low zone (excluding the boundary flow rate), and about 2% from the boundary flow rate to the high zone. However, the measurement characteristics vary among water meters of different accuracy levels.
The main difference between these classes was in the values of Qmin (minimum flow rate) and Qt (transitional flow rate), which determined the meter’s measuring range. Class A had the narrowest measuring range, while Class D had the widest.
Typical Applications for Different Accuracy Classes
Class 1 meters (or Class C and D in the old system) are typically used in applications requiring high precision, such as:
Industrial processes with strict water management needs
Large commercial establishments
Utility revenue metering in areas with high water costs
Class 2 meters (or Class A and B in the old system) are more commonly found in:
Residential applications
Small to medium-sized commercial establishments
General purpose metering where slightly lower precision is acceptable
BMAG Metre can offer a wide range of water meters with accuracy grades class 1 and class 2 /class B and class C.
References:
ISO 4064:2014 “Water meters for cold potable water and hot water”
OIML R 49:2013 “Water meters for cold potable water and hot water”
AWWA C700 Series Standards “Cold-Water Meters”
ISO 4064:1993 “Measurement of water flow in closed conduits — Meters for cold potable water”
European Measuring Instruments Directive 2014/32/EU
