Sean-Thomas,

Thanks for the info. I'm not sure if these sensors have a calibration coil.
If they did would there be another set of wires coming out of the sensor?
The sensor we got only have 2 wires, some have three but the third is only
a ground wire. There is no resistance between the ground wire and the other
two wires. Is it possible that you need to open up the sensor to gain
access to the calibration coil?

Can you elaborate on how you can test the period using a bridge and the
main coil? Do you use three resistor with the same resistance as the coil
and place the coil as the forth leg of the bridge? One more thing. Do you
know what loading resistor is used for critical damping if the main coil
resistance is around 400 ohms. GeoSpace's web page does not have this
information.

Again, thanks for all of your help.

Regards,
Larry Cochrane
Redwood City, PSN

At 01:54 PM 10/3/99 -0500, you wrote:
>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


_____________________________________________________________________

Public Seismic Network Mailing List (PSN-L)