The largest factor for inaccuracy is in reading the tube. The operator should read the ball in the exact middle, and at the same plane. Reading the ball at top or bottom and from an angle to the tube increases inaccuracy of the reading. Benefits to the flowmeter, due to the triangular tube is that the level of accuracy is the same regardless of position of the device.
Round tubes increase the amount of inaccuracy with the greater degree of angle away from vertical.
Regarding gauges, we manufacture to ASME Grade B Standards.