Rockwell Automation 441-PEN25 Enpac Ex Data Collector - Manual (Page 51)

Publication GMSI00-UM001A-EN-E - February 2005
Setting Up Measurements 45
Next, find the total collection time for the time waveform measurement using
this formula.
Number and Type of Averages
The number and type of averages are part of the collection specification
(Setup > Collection). See Figure 3.4 on page 40. You select the collection
specification when you set up the measurement definition.
Emonitor and the Enpac Ex support the following number and types of
averages for data collection.
• Linear - Up to 4096 averages.
• Time Synchronous - Up to 255 averages, and requires a trigger.
Averaging is useful for reducing random errors. Random errors include
background vibration due to some source other than the machine being
measured. If you collect more averages, you have fewer random errors.
However, collecting more averages requires more time.
Overlap processing (Percent overlap) speeds up the averaging process. The
overlap amount determines how much of the time samples are overlapped for
successive averages. The greater the overlap, the faster the sample can be
collected. The disadvantage is that the greater the overlap, the less new data
there is, and the greater the influence of random errors.
EXAMPLE
If you had a Maximum frequency value of 7200 CPM and
a Number of lines value of 400, you would:
1. Convert CPM to Hz.
2. Find the total collection time.
collection time
spectral lines
Fmax (Hz)
------------------------------- =
7200 CPM
60
-------------------------- 120 Hz =
400 lines
120 Hz
--------------------- 3.3 seconds =