Micromod Micro-PWC: 53PW6000 MicroPWC Operators Manual - Manual (Page 77)

Micro-PWC OPERATOR’S MANUAL
Trend Displays 63
In the above list, the Compression Value is shown to the right of the colon. To enter a new compression
value directly, click the Compression button. A pop-up window (
Figure 6-6 ) appears. This window con-
tains an entry box, and OK and Cancel buttons.
Click the entry box to call up the text entry cursor, then enter the compression value to be used. Click OK
to complete the operation. If the value entered is outside the range (0.05 to 50), the following error mes-
sage will be displayed:
Value not in range
(0.05 - 50.00)
Click OK to close the pop-up. Also, if the combination of the segment size configured for the trend and the
Compression Value entered will result in too many points being plotted in a segment, the following error
message will be displayed:
Segment size too large
Click OK to close this error window, and enter a lower compression value. This condition can occur when
the configured segment size is larger than the default segment size (60 seconds).
Caution must be exercised when interpreting data displayed on a compressed display. By experimenting
with viewing the same trend display using a number of different compression ratios, it is easy to see that
although all the displays are correct representations of the compressed trend data, the traces displayed
may be deceiving if not viewed with care.
When the display options MIN, MAX, or AVG are in effect, remember that the values of the points between
the displayed points could have varied widely. Even when INSTantaneous is the display option in effect, a
compressed display can be subject to misinterpretation. (The examples which follow in this section illus-
trate how this is true for instantaneous values.)
When viewing a compressed trend display of process variables, the value of which repeat over time, be
especially careful. “Aliasing” can cause the trace for the signals to appear as a straight line, as a magnified
duplicate of the original trace, even as a mirror image of the original trace.
To see the reason for the generation of these possibly misleading traces, the way in which compressed
displays are “drawn” must be considered. Every
nth sample is plotted on the graph as a point, and the
points are connected by straight lines. Therefore, the position of the extracted points along the original
trace will affect the shape of the trace on the new compressed display. For example, if an Historical trend is
TC00061A
Figure 6-6. Compression Value Entry Window