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Effects of temperature

The equilibrium between gravity and the spring force in a vertical seismometer is disturbed when the temperature changes. Although thermally compensated alloys are available for springs, a self-compensated spring does not make a compensated seismometer. The geometry of the whole suspension changes with temperature; the seismometer must therefore be compensated as a whole. However, the different time constants involved prevent an efficient compensation at seismic frequencies. Short-term changes of temperature must therefore be suppressed by the combination of thermal insulation and thermal inertia. Special caution is required with seismometers where electronic components are enclosed with the mechanical sensor: they heat themselves up when insulated and are then very sensitive to air drafts, so the insulation must at the same time suppress any possible air convection (see section 5.3). Long-term (seasonal) changes of temperature do not interfere with the seismic signal (except when they cause convection) but may drive the seismic mass out of its operating range. Eq. (34) can be used to calculate the thermal drift of a passive vertical seismometer when the temperature coefficient of the spring force is formally assigned to the gravitational acceleration.


next up previous contents
Next: The homogeneous triaxial arrangement Up: Design of seismic sensors Previous: Direct effects of barometric
Erhard Wielandt
2002-11-08