ABB 266CRH Instructions Manual page 65

266CRH / 266CRT, 266CSH / 266CST MULTIVARIABLE PRESSURE TRANSMITTER | OI/266CXX/MODBUS-EN REV. A
Square root to the third power
The square root transmission function of x3 can be used for flow
measurements in open chutes (see Figure 43 and Figure 44) 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.
Figure 43: Rectangular measuring weir
h
4 ÷ 5 h
max.
Figure 44: Trapezoidal weir
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 Figure 45); 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.
h
Figure 45: V measuring weir
Customer-specific linearizaton characteristic curve
The transmission function with a customer-specific linearization
curve is normally used for volume measurement in tanks with
unusual shapes. There is an assignment to a freely-definable
transmission characteristic curve with a maximum of 22
reference points. The first reference point is always the zero
point, the last is the upper range value. These two reference
points cannot be changed. In between a maximum of 20 points
can be freely entered.
These max. 22 points are defined by extrapolating the tank fill
data.
After they are determined, the 22 reference points are loaded
into the device, via a HART handheld terminal or via an
appropriate configuration program such as 'Asset Vision Basic'.
M10813
4 ÷ 5 h
max.
65
M10814