Guralp Systems CMG-3TB - Manual (Page 63)

Operator's guide
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.
4.5 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 a mounting
harness for the sonde and 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.
3. Repeat this measurement for several tilt angles.
4. For the vertical sensor, the input acceleration is given by
a = g sin φ, whilst for the horizontal sensor, it is
a = g ( 1 – cos φ ).
Calculate the input acceleration for each of the tilt angles used,
and plot a graph of mass position output against input
acceleration.
5. The gradient of the line obtained gives the sensitivity of the coil
(in V/m/s
2
, if g was measured in m/s 2
and the mass position in
V.)
6. The coil constant K is equal to this sensitivity divided by the
value of the displacement feedback resistor, given on the
calibration sheet.
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