ABB 2600T Series Operating Instruction page 60

Square root to the third power
The square root transmission function of x3 can be used for
flow measurements in open chutes (see Fig. 42 and Fig. 43)
with rectangular measuring weirs or trapezoidal measuring
weirs, as well as Venturi flumes in accordance with ISO 1438.
For these devices the relationship between flow and dammed
up height h (differential pressure measured by the transmitter)
is proportional to h3/2 or to the square root of h3. For other
Venturi flumes or Parshall flumes this relationship is not right.
For this function the output (in % of the measuring span) is
proportional to the square root to the third power of the input
signal in % of the set measuring span. With the aid of the
formulas cited, the device provides an output signal that is
proportional to the flow rate.
Fig. 42: Rectangular measuring weir
Fig. 43: Trapezoidal weir
60 OI/266CXX/266JXX/HART-EN Rev. B | 266CRx, 266JRx, 266CSx, 266JSx
Square root to the fifth power
The square root transmission function of x5 can be used for
flow measurements in open flumes of V measuring weirs
(triangular weirs) in accordance with ISO 14398 (see Fig. 44);
the relationship between flow and dammed up height h
(differential pressure measured by the transmitter) is
proportional to h5/2 or to the square root of h5.
With this function the output in % of measuring span is
proportional to the square root to the fifth power of the input
signal in % of the set measuring span. The device delivers an
output signal that behaves proportionally to the calculated flow
rate.
Fig. 44 V measuring weir