Using keys ▲▼ you can adjust the chosen value. Upon reaching the maximum
value, e.g. in the 23
rd
hour, the counter starts again from the beginning. Press ◄►
to skip between hour, minute, month, day and year. The date takes the leap year into
consideration. It is, however, important to ensure that no invalid day-month combi-
nations are entered.
Additionally, clock inaccuracy can be corrected. This can be done using the setting
at “Cal.” in ppm (parts per million). The clock frequency can be increased with pos -
itive values and decreased with negative. The sign changes to minus at +254 ppm.
Example: The clock is 5 seconds slow. There are 24 * 60 * 60 s = 86400 seconds in
a day. The difference amounts to 5 s / 86400 s = 58 * 10
-6
= 58 ppm. The adjustable
value is -58 ppm.
To exit the menu press OK followed by F3 repeatedly.
9. Calibration
The VM24 comes with with a factory calibration, which is traceable to the reference
standard of the Physikalisch-Technischen Bundesanstalt (PTB). The calibration is
only valid with the supplied vibration transducer. The serial number of the instru -
ment and the transducer are stated on the calibration certificate. The month and year
of the factory calibration are displayed when the instrument is switched on (cf. 10
on page 10).
The value set upon calibration is the sensor sensitivity in mV/ms
-2
.
This can be viewed in the calibration menu. Open the main menu by pressing F3.
From the main menu scroll down with ▼ until you reach the menu option “Sensor
calibration” and select OK. Within this sub-menu select 'By entering sensitivity'.
The displayed value is the sensor's calibrated sensitivity (27). It should not be
changed, unless a new calibration is being carried out. Press OK followed by F3 re-
peatedly to exit the menu.
Apart from the factory calibration an accuracy test or re-calibration can be carried
out by the user. For this a vibration calibrator is required. Metra offers the instru-
ments VC20 and VC21 (28). They can generate one or more vibration amplitudes
and frequencies with defined accuracy. For the calibration of the VM24 a sufficient
acceleration would be 10 m/s² at 159.2 Hz (radian frequency 1000 s
-1
).
23
Figure 27: Entering the sensitivity
