Guralp Systems CMG-40T - Manual (Page 11)

Operator's guide
6. Repeat steps 2 – 5 for the north/south and east/west components.
Adjusting the mass position offsets with a Handheld Control Unit
Some 40T units are equipped with a remote mass centring option, which allows
you to adjust the internal potentiometers by applying voltages across control
lines to the sensor:
1. Bring the instrument into 1 second response mode by selecting 1 SEC
VEL from the Velocity Select switch.
2. Measure the vertical mass position output by selecting MASS POS, V
from the Handheld Control Unit's Display Select knob.
3. Set the Centring Select knob to V.
4. Press the spring-loaded switch towards + or – to bring the mass position
offset from negative or positive values towards zero.
5. Repeat steps 3 and 4 for the N/S and E/W components.
6. Return the instrument to broadband mode by selecting BB VEL from the
Velocity Select switch.
Zeroing a 40TD digital instrument
The offset potentiometers in a 40TD are in the same place as on the 40T. To
access them, you will need to remove the digitizer module, which lies on top of
the sensor itself. You can monitor the mass position outputs of the sensor using
a Handheld Control Unit and an adapter cable, available from Güralp Systems.
To change the offsets of a 40TD without digital centring:
1. Check the bubble level on the lid of the instrument, to ensure it is not
tilted. If necessary, re-level the instrument by adjusting its feet.
2. Unscrew the vent cap on the lid to allow the air pressure to equalise.
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