Using Chart Scales - Omega RD8800 User Manual

Chapter 3 Getting Started
The outputs of the conditioning block are the values represented in the Digital Windows and are real-world engineering
values. The outputs of the conditioning block are also fed through to a scale block where a chart scale is applied
to each point. The chart scales determine what part of the full scale range will be used by the display/record block.
There are two sets of 8 scales available to the user. The second set is an alternate set which can be applied to the
base points in place of the normal set when triggered by an external event. Each chart scale may be applied to any
one or more of the base points to provide an output scaled point. These scale points are then applied to the
display/record block which consist of the visual information or recorded information that the user requires. Up to
twelve scaled points are applied to the bar graph display. Up to twelve may be displayed on the chart and up to
fifteen may be recorded to disk/card. Note that points recorded to disk/card need not be the same as points
displayed on the bar graph or the chart, however, in the case of the chart, any point which is not recorded to disk/
card will have limited browse capability. The user should also be aware that applying scaling other than full scale
output to recorded data limits the data to the range between scale end points. It is possible to store the base point
with different scaling than is used to display it on the chart using one of the computational channels.

3.3 Using Chart Scales

To better understand the use of input and output scaling, and how scaling base points affects the data, consider the
following example, highlighted by Figure 3-11.
Figure 3-11 Scaling Example
Assume the user wants to record the output from a pressure transducer which gives a 0 to 5 Vdc output signal.
This coincides to a pressure of 0 to 3000 pounds per square inch (PSI). The process being monitored typically runs
at 2200 PSI ±10%; this is the area of interest. The output of the transducer which peaks at 5 V is connected to live
input channel 1 and the 12.5 Vdc full-scale range is selected to cover this range. Since the maximum input voltage
will be only 5 V on a 12.5 Vdc range, use the input scale option to set the low end at 0.0 Vdc, and the high end at
5.0 Vdc. To convert this directly to PSI, the output scale is set at low point 0.0 and high point 3000.0. The base
point now becomes 0 to 3000 for an input of 0 to 5 Vdc, scaled linearly across the range. The engineering units can
be set to PSI and the point tag can be set to any label that identifies the process. For more details refer to Section
5.6. Apply scale A to the base point; this too is explained in Section 5.6.
It is then necessary to define chart scale A to suit the requirement (see Section 5.5.2). In order to maximize the
display resolution, the user is interested in pressure ranges from 2000 to 2500 only. Scale A is thus set for a low
end of 2000, a mid-range of 2250 and a high end of 2500. This scale point is now assigned to the bar graph and the
chart graph. The chart will thus display from 2000 to 2500 as will the bars, maximizing the display resolution for the
value of interest. The user can choose to record this particular value, in other words, what you see on the chart is
what is recorded to disk/card, values from 2000 to 2500. Or, if so desired, the user can choose to record a second
point which will show pressures from 0 to 3000. To do this, use the channel 1 as a base point for a computational
channel and multiply this value by 1 which is then recorded to disk/card.
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