Originally Posted by Kiril
Turbulence could interfere with the accuracy. Those types of meters are best used where you have fully developed flow (i.e. straight pipe of the same size). The typical minimum recommendation for this is 10 diameters upstream and 5 diameters downstream of any fittings or direction changes.
this is one of the reasons that you'll see cavatation in the hair strainer of an inproperly plumbed swimming pool, limited space for equipment forces some installers to resort to rapid directional changes breaking the 10 and 5 rule.