Hach-Lange ORBISPHERE Model 51x User Manual - Manual (Page 75)

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Inputs/Outputs Menu
8.5 Analog output characteristics
Analog output characteristics
“Linear” analog output The "Linear" output is the default setting for the analog output. It is
illustrated below (4-20 mA output is shown, 0-20 mA or 0-5 V settings are
similar).
The goal of this setting is to use all the points available on the slope from
4 mA to 20 mA to show the range of measurements that are usual in the
measured process. Setting the output this way allows for the highest signal
resolution for the actual conditions.
The downside is that any measures below the set range will have the same
analog signal locked at 4 mA. Similarly, any measure over the set range will
have the same analog signal locked at 20 mA. Settings must be made in
balancing these aspects.
Settings
For the output, set ML and MH in the current measuring unit (e.g. °C for a
temperature output). When a compound unit is selected the smallest unit will
be used (e.g. ppb for a "ppm-ppb" compound unit).
These points should be set keeping in balance the following conditions (see
illustration above):
•
The smaller is Range 1, the better is the analog signal resolution within
the expected range of measure.
• In Range 0 the analog output only shows that measurement is below the
ML value. Similarly in Range 2 the analog output only shows that
measurement is over the MH value.
The formula to compute the measurement knowing the current I (or voltage
U) and the resolution R is given in the following table:
Linear Range Measurement M Resolution R
4-20 mA 20 > I > 4 M = ML + (MH - ML)
*
(I - 4) / 16 R = (MH-ML) / 808
0-20 mA 20 > I > 0 M = ML + (MH - ML)
*
I / 20 R = (MH-ML) / 1010
0 - 5 V 5 > U > 0 M = ML + (MH - ML)
*
U / 5 R = (MH-ML) / 1010