Arie,

The period and the mass centering of the HS-10-1 are both accomplished
by adjusting the same part. The HS-10s that I have were originally
encased in deep borehole pressure cases, so they have no dials or
indicators. Some versions have a screw adjustment for the position
(and so the strength) of the calibrator magnet.

For adjustments, there is a rod protruding from the case inside
the calibrator assembly (which may not be complete). The rod, about
1/8" dia, has an approx. 3/8" dia threaded bushing attached to it
with a setscrew (or two). The threads of the bushing are engaged by
the astitizing spring that controls the period. The upper (in Z) end
of the rod may have the calibration coil installed, with fine coilly
leads running to the terminal strip.

There is a three-mark index or scale marked in the inside of the 
cut-aways in the cylinder of the calibrator housing. The lower end 
of the white nylon cal coil assembly should be aligned with the middle 
mark to center the mass. To adjust the centering, loosen the setscrews 
on the threaded bushing and move it appropriately. The mass position 
may change as the screws are tightened.

The period is adjusted by changing the length of the small spring, which
is done by rotating the threaded bushing to engage or release PART of 
A TURN of the spring. I make a mark on the 1/8" shaft to indicate the
position for centering before I loosen it to rotate it. Naturally the 
adjustments interact.  Sometimes the mass position will have to be
raised slightly to get 1 second; slightly tilt the seis to test this.
I have been able to get periods from 0.8 to 1.1 seconds.

I use a function generator, a frequency/period counter, and an oscilloscope
to set the period. The generator drives the calibration coil (or a bridge
with the main coil), and one axis (time) of the oscilloscope, which is operated
in X_Y mode. THe seis signal coil connects to the other axis. Usually
the normal input is used for the signal because it has more gain or
sensitivity. The function generator is adjusted for a flat-line lissajous (sp?)
display, and a period (frequency) counter on the function generator
indicates the seismometer period. THis method can be accurate to
a millisecond, especially if you can finely adjust the function generator
to see the elipse pattern collapse to a line and then reverse.

In liew of the test equipment, look at the amplified signal and measure
the length of time for ten oscillations with a stopwatch.

The adjustment of the period is really not that critical, and +,- 5%
is probably adequate for a critically damped sensor.

Regards,
Sean-Thomas

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