The size of the water meter
Water meters are sized on their nominal flow rate. This is called the Qn and is given in cubic meters per hour (one cubic meter is 1,000 litres of water). The water meters maximum flow rate is twice the Qn. If the required flow rate is known then a water meter can be selected so that the required flow rate falls between the nominal and maximum flow rates. If the flow rate is not known then it is generally safe to select a meter of the same nominal size (DN) as the pipework it is to be connected to.

What class of meter is required?
The class does not indicate the accuracy of the water meter but at what flow rate the meter meets the common accuracy figures. These are ± 5% at the meters minimum flow rate and ± 2% in the meters normal range (between Qt and Qmax) for cold water meters. The figures for hot water meters are ± 6% and ± 3% respectively. The higher the class of water meter the higher the accuracy at very low flow rates.
When deciding if a low flow reading is required it should be remembered that even a class A Qn 2.5 (a 3/4" meter) will start to read, within its tolerance band, at a flow rate of 1.66 l/m (a basin tap will flow at between 6 and 10 l/m)
If all that is required is an overall indication of the amount of water used then a class A or B meter is sufficient (most mainland European Water Authorities use single and multi jet water meters as they maintain their accuracy for a long time).

If the total of a number of secondary meters has to relate very closely to a master meter then a class C meter should be selected.
If the effects of dripping taps and low flows caused by float operated valves are to be taken into consideration then a class D meter should be selected.

Wet or Dry Dial
Wet dial meters are used for cold water applications where the meter is subject to climatic changes (e.g. a meter mounted outside a building but still protected from frost) which could cause condensation to form on the face of the dry dial meter making it difficult to read. This should be balanced against the possibility of water borne contamination getting into the meter. The type of meter must be selected based on site conditions, but in all cases dry dial meters should be used in applications where the water quality is suspect, ie. contaminated or cloudy.