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Nelson-Nordgren, Astatic Spring, Broadband Force Balance, Vertical (FBV) Seismometers

Seneca II


Seneca II

This experiment was built on a 1.8" x 1.7" base. For comparison, the base sizes for the other seismometers in this series are:

  • Yuma Base: 5.75" x 4.0"
  • Lenape Base: 3.5" x 3.0"
  • Seneca Base: 2.38" x 2.13"
  • Seneca II Base: 1.8" x 1.7"

    Measured instrument parameters:

    1. M0 = 40 grams (needed 20 grams to balance boom with 2.5 sec natural period)
    2. Gn = 1.5 N/A
    3. Ag(hi+) = 90,000 v/m/s
    4. 750 T, #36 AWG, Rc = 42 Ohms
    5. 0.002", 0.72" wide, 17-7 Stainless Spring
    6. 0.25" diameter x 0.25" high, N52 magnet
    7. 0.002" Kapton Flexure, ~0.125" wide
    8. Clamp to clamp spring spacing 0.6"
    9. 0.11" centered pivot height above clamps
    10. 0.015" sensor plate gap
    11. Ca,Cb delta ~20 pF

    This instrument works, however the gain is much too high. The motor is very weak with a coil constant of only 1.5 N/A. There are many ways to increase Gn; additional turns on the coil or a stronger magnet will help. These items are difficult to change, however, due to size constraints. Also this unit is difficult to setup because of its size. It is presently not online.