The drive assembly can be either direct or magnetic. The magnetic drive provides for a fixed seal in the housing between the propeller and the indicating head. Direct drives use a packing-type seal. The propeller measures the velocity of the water in the pipeline or meter section. Therefore, when ordering a saddle-type meter, the pipe I.D. must be specified.
Some companies use a correction factor to account for various I.D.'s rather than changing gears.
This should be checked when buying the meter. A recommended practice is to buy the meter already installed in a pipe section complete with straightening vanes. The size of the meter selected will depend on the nominal size of the irrigation pipeline, the range of flows to be measured, and the head loss characteristics of the meter. The range of flow should be the highest consideration. At lower flow rates, the accuracy of the meter will drop off as illustrated in Figure 1 .