# Calculate stream sensitivity

To determine the magnitude correctly you have to configure the sensitivity of all streams of a station correctly. This information is saved at \$SH_INPUTS/sensitivities.txt.

Seismic Handler expects this value in unit (nm/s)/count (velocity per count). This data yields the features of the complete recording chain:

```   .----.
|    |                       ,---.
|    |        ,----.         |   |
`----'--------.____|---------.---'
seismometer   pre-amplifier   digitizer
pre-attenuator
(optional)
```

Because you are able to combine any suitable devices, the resulting gain factor must be calculated from these three input values, which can be found in the devices manuals:

1. seimometer's generator constant or output sensitivity - V/(m/s) (volts per velocity)
2. factor for pre-amplification for adopting different voltage ranges of sensor and digitizer (unit-less)
3. digitizer's bit-weight - V/count (volts per count)

To get the total gain value needed for SH/SHM, simply divide the bit-weight by generator constant (convert to V/nm/s before!).

Now let's look at two examples using a STS/2 seismometer:

• generator constant: 1500 V/(m/s); sometimes given as 2x 750 V/(m/s) - you should use the actual value from the calibration sheet.
• This value would be altered if a pre-amplifier is used: an attenuation by a factor of two will result in 750 V/(m/s), an amplification by a factor of 2 results in 3000 V/(m/s). Our examples will not use any of them, so the generator constant is not changed.
1. STS/2 seismometer and Reftek 130 digitizer:

The value of bit-weight can be found in the manual; it is 1.589e-06 V/count.

Dividing bit-weight by generator constant (converted to V/nm/s) results in the total gain:
1.589e-06 / 1500e-09 = 1.0593

1. STS/2 seismometer and Quanterra Q330:

There is no bit-weight available from the manual. But it can be calculated from the differential input range (Q330: -20 to +20 volts (peek to peek; p-p) resulting in 40 volts total) and the bit resolution (24 bit):

bit-weight = 40 V / 2^24 = 2.3841e-06 V/count

As before we can divide bit-weight by generator constant:
2.3841e-06 / 1500e-09 = 1.5894

Using this recipe it's possible to configure any seismometer-digitizer combination for usage in Seismic Handler.