are filtered with the same weighting or band filter used for the displayed measure-
ments. Channel A is not recorded.
Please note that the logged data is calculated only for the last display interval (ap -
prox. 1 s). For human vibration, however, interval RMS values are measured which
are integrated over the entire measuring time. Thus there will be differences be-
tween the displayed and the recorded values.
To start the data logger press F3 to open the main menu, select “Data memory” /
“Data logger” / “Start logging” (Figure 30). You will be prompted to select a file
number. This number will be used later on to identify the log file. The memory can
hold 100 log files. File numbers can be 00 to 99. If the se-
lected file should already contain data you will be asked to
delete it. Now you select whether RMS or peak values are
to be logged. Press OK to leave the menu and to start log-
ging. After a settling time the unit will start to record mea-
surements. The active log mode is indicated in the upper
right display corner by the text “LOG!” (Figure 31).
Logging can be stopped by pressing F3.
Please note that the data logger will be halted while data is transferred via the USB
interface. This may result in missing records.
The automatic shut-off function (see chapter 10.3) is not in use while logging.
The maximum number of records per log file is 32,000 which is sufficient for about
10 hours of continuous recording.
7. Frequency Analysis
The VM31 provides a simple FFT function for the detection of main frequencies. It
displays a 125 line peak spectrum of acceleration.
Press F3 and select “Frequency analysis” to open
the FFT screen (Figure 32). You see the frequency
components of channels X/Y/Z
2
in their respective
channel colors.
The line above the diagram shows frequency and
magnitude of the longest spectral line.
The keys ◄► move the cursor. Below the dia-
gram you can see the cursor readout.
Press F1 or F2 to change the frequency range. Four
ranges are selectable:
•
3 to 244 Hz
•
7 to 488 Hz
•
15 to 977 Hz
•
30 to 1954 Hz
By the OK key you can switch between automatic and fixed amplitude scaling. In
the help line you see the function of the OK key when you press it next time. In
most cases automatic scaling will be the best choice.
Pressing the key ▼saves the spectrum. The VM31 memory can hold up to 1000
FFTs (see also chapter 8.3).
2
There is no FFT function for channel A.
22
Figure 32: Frequency analysis
Figure 31: Log indi-
cation