Process Unit 73 LF - Knick

Process Unit 73 LF
Output Curve ”Function”
output current [%]
100
80
60
40
20
0
0 20 40 60 80 100
conductivity [mS/cm]
Especially when measuring low conductivities, it is
useful to measure over several decades while at
the same time maintaining a high resolution.
The output curve ”function” allows for a nonlinear
output current characteristic. By defining a 50 %
point, you can spread the beginning and compress
the end of the range as required.
This allows you to create a good approximation of
a logarithmic output curve.
To determine the span corresponding to current
range 0 (4) to 20 mA, set an initial and an end
value for the process variable. In addition, you can
define a 50 % point (at 10 or 12 mA, resp.).
Between initial and end value the output current is
calculated from the following equations:
output current (0 to 20 mA) =
(1 + C) * x * 20 mA
1 + C * x
output current (4 to 20 mA) =
(1 + C) * x * 16 mA + 4 mA
1 + C * x
C = E + I – 2 * X50%
X50% – I
x = M – I
E – I
I: initial value at 0 (4) mA
X50%: 50% value at 10 (12) mA
E: end value at 20 mA
M: measured value
Example:
logarithmic output curve over one decade
Example:
logarithmic output curve over two decades
Approximation of a logarithmic output curve in the
range 10 to 100 S/cm (one decade):
beginning: 10.0 S/cm
50 % point: 31.6 S/cm
end:
100.0 S/cm
Approximation of a logarithmic output curve in the
range 1 to 100 S/cm (two decades):
beginning: 1.00 S/cm
50 % point: 10.0 S/cm
end:
100.0 S/cm
9–20 Capabilities