The European MID (measuring instruments directive) which came into force on 30 October 2006 specifies the performance of a water meter in a slightly different way to the old regulations. The performance of a water meter is now defined by four flow rates and the R number – the ratio between the nominal flow rate and the minimum flow rate:

Minimum Flow Q1

This is the lowest flow rate where the meter can measure to an accuracy of +/- 5%

Transitional flow rate Q2

At this flow rate accuracy of the meter increases from +/-5 % to +/-2%. MID requires that this equates to 1.6 x Q1.

Nominal flow rate Q3

This flow rate is defined as 80% of the maximum flow rate (Q4), Q3 will be a value from the R5 series of preferred numbers (1, 1.6, 2.5,4 etc.). prior to this Qn was 50% of Qmax.

Maximum Flow rate Q4

This is the highest flow rate the meter can experience for short periods without damage. This figure represents the flow through the meter when the head loss across the meter is 0.64 bar, previously this was the flow rate when head-loss was 1 bar.

R number

Found by dividing the maximum flow rate by the minimum flow rate - Q4/Q1. This gives an expression of the flow range over which the water meter is capable of providing useful information, the higher the number the wider the working range of flow rates.

Currently on their data sheets Arad show both the old method of defining performance and the new.