ABB 268D/V Operating Instruction page 36

ADDENDUM FOR DIFFERENTIAL PRESSURE TRANSMITTERS:
SELECTABLE OUTPUT FUNCTIONS
1.0 LINEAR
Using this function, the relationship between the input (measured value), expressed in % of the calibrated span and the output
is linear, e.g. at 0% input, corresponds 0% output (4mA), at 50% input corresponds 50% output (12mA) and at 100% input
corresponds 100% output (20mA).
2.0 SQUARE ROOT (X)
Using this function, the output (in % of the span) is proportional to the square root of the input signal in % of the calibrated span:
the instrument, e.g., gives an analog output proportional to the rate of flow.
To avoid the extremely high gain with the input approaching zero, the transmitter output is linear with the input up to 4%,
programmable in order to ensure a more stable output near zero. This also allows an easier zero adjustment and performs a
reduced zero error for ambient temperature variations.
An explanation is given in fig. 2 For an input variation from 0 to 0.5% the output varies linearly. For input variation greater then
0.5% up to a value programmable from 1 to 4%, the output still varies linearly. Then at input values greater then 4% the output
follows the applied transfer function. In the figure it is also indicated the default selection.
To convert from a pressure value within the calibrated span to a percent of flow, first express the pressure as a percent of calibrated
span, then take the square root of this pressure percentage and multiply by 10.
Example : Transmitter calibrated 0÷400 mbar - with 196 mbar, pressure input, the percentage of flow is determined as follows:
To convert from a percentage of the calibrated flow to the equivalent output current (see figure), first divide the percentage of flow
by 100, then multiply this figure by the 16 mA adding also the live zero 4 mA.
Output
÷
4 20 mA
7.2 mA
(20%)
5.6 mA
4.08 mA
4 mA
(0%)
- 36 -
196
x 100 = 49% of calibrated pressure
400
√ 49 x 10 = 70% of calibrated flow
70% calibrated flow
x 16 mA + 4 mA d.c. = 15,2 mA d.c.
100
slope 19
slope 1
0 0.5% 1%
Default
selection
slope 5.57
4%
programmable
Fig. 2
Square
root
mathematical
Square Root Curve
instrument output
with Square Root
Selection
Input