SM500F
Field mountable paperless recorder Appendix E – Math Equations
172 IM/SM500F Rev. Z
E.4 Logarithmic Scales
When 'Scale Type' is set to 'Log' (see page 95), the Chart view's scale bar and graduations are displayed in
logarithmic format with the input signal plotted linearly on the chart:
The recorded value in the Chart and Indicator views is driven by the engineering range selected and
displayed in exponential format:
Example
An application requires a 4 to 20 mA input signal with an associated engineering range of 0 to 1,000,000 to
be displayed in exponential format and plotted on a logarithmic chart with a scale of 1 to 10
+6
.
1. Referring to page 95:
a. Set 'Scale Type' to 'Log'
b. Set 'Low' to '1'
c. Set 'High' to '1.0E+6'
2. Referring to page 97:
a. Set Engineering range 'Low' to '0'
b. Set Engineering range 'High' to '6'
c. Select Engineering Units as required
In this configuration, the input is plotted linearly on the chart but the digital indicator displays the input in
exponential (logarithmic) format in order to match the logarithmic scale displayed on the chart.
Chart View Indicator View
Note. The logarithmic value of the input is obtained by displaying '10 to the power of the
engineering range value'. Therefore, in the case of 0 to 1,000,000, the engineering range
must be set to 0 to 6 as 10
+6
= 1,000,000.
Note. Regardless of the scale type setting:
All alarms on the recording channel are triggered by the channel's linear value.
Data values saved to the archive files are the linear output from the recording channel.
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