Guralp Systems CMG-3T - Manual (Page 28)

CMG-3T
Sensor Sensor type code
Units
(V/A)
CMG-3T, 360s – 50 Hz response CMG-3_360S_50HZ V
CMG-3TB or 3V / 3ESP borehole,
30s – 50 Hz response
CMG-3B_30S_50HZ V
CMG-3TB or 3V / 3ESP borehole,
100s – 50 Hz response
CMG-3B_100S_50HZ V
CMG-3TB or 3V / 3ESP borehole,
120s – 50 Hz response
CMG-3B_120S_50HZ V
3.4 Calibration with a hand-held control unit
If you prefer, you can inject your own calibration signals into the
system through a hand-held control unit. The unit includes a switch
which activates the calibration relay in the seismometer and 4 mm
banana sockets for an external signal source. As above, the equivalent
input velocity for a sinusoidal calibration signal is given by
v = V / 2 π f R K
where V is the peak-to-peak voltage of the calibration signal, f is the
signal frequency, R is the value of the calibration resistor and K is the
feedback coil constant. R and K are both given on the calibration sheet
supplied with the 3T.
The calibration resistor is placed in series with the transducer.
Depending on the calibration signal source and the sensitivity of your
recording equipment, you may need to increase R by adding further
resistors to the circuit.
3.5 The coil constant
The feedback coil constant K is measured at the time of manufacture
and printed on the calibration sheet. Using this value will give good
results at the time of installation. However, it may change over time.
The coil constant can be determined by tilting the instrument and
measuring its response to gravity. To do this, you will need apparatus
for measuring tilt angles accurately.
1. Measure the acceleration due to gravity, g, at your location.
2. Tilt the instrument slightly, and measure its attitude and the
gain of the mass position output for the component you wish to
calibrate.
28 Issue N