Guralp Systems CMG-5T Compact - Manual (Page 16)

CMG-5T Compact
Note that if you have used the Scale... option described above,
you will need to take the scale factor into account to produce the
correct number of counts. For example, if the C2 (calibration
input) signal has been scaled by a factor of 40, the signal
strength as measured by the Ampl Cursors must be divided by
40 to yield the correct value.
9. Convert to volts using the µV/Bit values given on the digitizer's
calibration sheet for the relevant input ports and compare the
returned signal with the input calibration signal (C2).
10. Suppose we measure the following values:
Input calibration signal strength (C2) 697,221 counts
Returning signal strength (Z3) 701,512 counts
The calibration sheets provide us with the remaining values
needed to calibrate the sensor. For example, we might have:
Sensor acceleration response 0.254 V/ms -2
Equivalent accel. from 1V calibration 1.968 ms -2
Digitizer input port sensitivity 3.507212 µV/Bit
Calibration channel sensitivity 3.491621 µV/Bit
From these, we calculate that the calibration signal is producing
697,221 × 3.491621 = 2,434,431 µV (2.434 V). This corresponds
to an equivalent input acceleration of 2.434 × 1.968 =
4.791 ms -2 .
The sensor's acceleration response is given as 0.254 V/ms -2, so
that an acceleration of 4.791 ms -2 will produce an output of
0.254 × 4.791 = 1.217 V (1,216,904 µV), which corresponds to a
count number at the digitizer's input port of
1,216,904 / 3. 507212 = 346,972 counts.
Because this calibration is being carried out with a differential-
output sensor, the count number observed at the digitizer
should be double this: 693,944 counts. All Güralp Systems
sensors use balanced differential outputs.
The actual signal at the digitizer of 701,512 counts is within
1.5% of this value, indicating that the sensor is adequately
calibrated.
11. If you know the local value of g, you can also perform absolute
16 Issue E