The present invention relates to water meters for the installation in pipe lines of a nominal width-group and a method for operating such water meters, which operate according to the Woltmann principle. The present invention addresses the problem of avoiding the calibration expenditures for each of the nominal widths of the nominal width-group and the disadvantages entailed therein. The solution of this problem, stated generally, lies therein that in the water meters at the relevant site, namely in the annular channel (16) directly in front of the impeller wheel (8), independently of the nominal width of the connected pipe line, a substantially rectangular velocity distribution (19) obtains with stream lines directed substantially parallel to the wall of the annular channel (16), with these flow conditions being substantially identical at identical throughflows. |