Larry,

Unfortunately I am unfamiliar with the various configurations of
the HS-10-1. I would assume that the period adjusting spring is
part of the design of all the sensors, even without a calibration
coil. I have a file at the U. that might have more info.

For calibration with a bridge in the signal coil/amplifier circuit,
a simple resistive bridge is sufficient. I designed one into the 
remote calibrator for the remote telemetry stations using the L4-C;
most did not have cal coils because the USGS tried to save $$; or
the coils were burned out (a 1.5 V battery will do it).

The bridge is in the conventional shape of a diamond;
the upper right resistor being the seismometer AND its' parallel
damping resistor; for starters this could be about twice the
coil resistance (I can repeat the info on determining the damping
R if you need it). So in your case, the seis is 400 ohms, damped
by 800 ohms, for a net value of (400*800)/(400+800)=267 ohms.

The resistor at the upper left has to match this to balance the bridge.
So I would install a 500 ohm 10-turn potentiometer. The lower two
bridge Rs should be about 100 times the seis R, so I would use
27k (1%) ohms. This high value helps limit the calibration current and 
isolates the amplifier. Match the 27ks with a meter if necessary.

Now consider the bridge as a baseball park, with home plate at the
bottom, 1st base is at the right, 2nd at top, etc.
The seismometer is connected from 1st to 2nd, with 1st being the
common or ground connection. The amplifier is connected between
3rd and 1st, again with 1st being ground. The signal generator must
have a floating output (use large capacitors from the ground side
if it does not), and is connected thru a large (1 meg ohm) pot to
home plate and 2nd base. It does not take much current to drive
the seismometer with the signal coil.

To balance the bridge, clamp the seis by laying it on its side (some
seises, like the L4-C, totally short out in some horizontal positions).
Look at the amplifier output (scope, meter, digitizer), and with a
handy sine wave like 1 to 10 hz, adjust the balance pot (between 2nd
and 3rd) for a minimum output. Keep the voltage low so the amplifier
isn't saturated. What this does is to make the currents in each side 
of the bridge equal: half the current flows from home to 2nd via 1st,
and the other half via 3rd. If the 27k resistors are equal, the voltage
drop across them will be equal, and none of the calibration DRIVE
signal will be seen by the amplifier. However, it IS flowing through
the seis signal coil, so when the seis is set upright (unclamped),
the voltage produced by the motion of the mass will be present at the
amplifier via the balance pot from 2nd to 3rd, which is relatively
small (270 ohms) compared to the amplifier input R of 10k to 100k ohms.

For some seismometers with large inductances, a capacitor needs
to be placed across the balance pot; it needs the same reactance as
the seis inductance at the average calibration frequency.

Regards,
Sean-Thomas

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