Mechanical errors can be generalized as physical characteristics of the flow meter which attribute error to the measured flow rate. A common mechanical error associated with rotary flow meters is frictional losses. Frictional losses can in some cases become the dominant source of error depending on the flow rate and meter design. For example, in the case of low flow rates, it is common for the frictional losses in the bearing to influence the ability for the impeller to rotate proportionally to the fluid flow rate. This relatively large frictional force will eventually equal the force of water flowing through the meter, dictating the minimum flow rate.

Additional mechanical errors can originate from inaccuracies in mechanical movement. In the case of a flow meter, it is generally assumed for magnets to be equally spaced and mounted planar to the top of the impeller. Assembly errors; however, can influence the positioning of components, therefore altering the magnetic field sensed by the Hall sensor. An important characteristic of a mechanical flow meter is the trueness in how the impeller rotates with reference to the locating shaft. Misalignment in this movement can be descried as a wobble in the rotating assembly, altering the magnet angle and distance to the Hall sensor.

Figure 5-1 represents the horizontal impeller simulated with 0 degree, ±1-degree, ±2.5-degree, and ±5-degree vertical misalignment with the center shaft. The simulation emulates the mechanical positioning of the horizontal impeller used with 8 individual ¼-in diameter N42 disc magnets positioned equally along the impeller with alternating magnetic poles. Position 0° represents the maximum positive misalignment, and 180° represents the maximum negative misalignment. The simulation ranges from 0° to 180°, as it is assumed the maximum negative misalignment is 180 degrees out of phase of the positive misalignment.