Station Calibration

The following calibration procedures have been taken from the manual for each of the respective seismometer.

Long period Vertical

Spring Adjustment procedure:

  1. Adjust the spring tension, by adjusting the upper spring tension nut (part 12), until pendulum will oscillated in the range of limit stops. Fine adjustments for periods of 30 seconds or longer are made by adjustment of the sliding weight (part 54).  This is best done by gently tapping the weight. It is best to keep the weight near the center of its range.
     
  2. When the pendulum is oscillating freely, tighten set screw (part H9) in the clamp ring (75B) making sure that the front edge of the clamp ring is flush with the adjusting screw (part 18) using a straight edge. If the pendulum deflects upward when the clamp set screw (part H9) is tightened loosen the set screw and lower the tension nut (part 12) slightly and tighten the set screw (part h9) keeping them flush.  Repeat this step until the pendulum is oscillation in the range of its stops.
     
  3. Center the pendulum in its range by moving the sliding weight (part 54).  If the weight must be moved to far from the center of its range or if the pendulum unstable and will not oscillate in its range, check and repeat step 2, then tilt the base down 1/16th inch using the front leveling screw (about 2 full turns).
     
  4. Continue step 3 adjustments until the pendulum is oscillating at a 30 second period.  The Period can be lengthened by tilting the base upward or shortened by tilting downward by turning the single leveling screw at the font. Use an accurate stopwatch to time three 2mm peak-to-peak oscillations to determine the period. It is best to use the mid-point as the observation point for the timing.  The pendulum should be able to freely oscillate four to five times at a 30 second period when the coils are open circuited. If the system seems over damped check the connections for shorts or for debris in the magnets. 

To Check for period stability:

  1. Make sure that the instruments are away from drafts or air motion. This test can be done with the cover off as long as the instrument is shield from air drafts.
     
  2. Move the sliding weight (part 54) away from the hinge until the pendulum oscillates about a point nine or ten divisions below the center for several oscillations and measure the period.
     
  3. Move the sliding weight toward the hinge until the pendulum oscillates about the center point for several oscillations and measure the period.
     
  4. Move the sliding weight towards the hinge until the pendulum oscillates about a point nine or ten divisions above the center position and measure the period.
     
  5. If the period is not uniform check to make sure step 3 in the Spring Adjustment procedure has been completed correctly. Period linearity should be adjusted to plus or minus three percent.
     
  6. If the period linearity is not within three percent the following adjustments can be made.
    1. To make the period longer at the upper position: loosen the clamp screw (part 74) and raise the adjusting screw (part 18) with the hex adjusting nut (part 19) moving about ½ -turn clockwise. Tighten the clamp screw (part 74) and repeat steps three and four in the spring adjustment procedure.
       
    2. To make the period shorter at the upper position: loosen the clamp screw (part 74) and lower the adjusting screw (part 18) with the hex adjusting nut (part 19) moving about ½ -turn counterclockwise. Tighten the clamp screw (part 74) and repeat steps three and four in the spring adjustment procedure.

Long Period Horizontal

Period Adjustment procedure:

Give a slight deflection to the pendulum and time ten free oscillations to obtain the period. Repeat several times to obtain a mean period.

To shorten the period, turn leveling screw (part 2A) clockwise. To lengthen the period, turn the leveling screw counter clockwise.  If any drift from the zero position occurs, readjust leveling screws at opposite end of base plate.  Smaller adjustments should be made as the desired period is approached.

 

 

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