Accelerometers can be statically calibrated on a tilt table. Starting from a horizontal position, the fraction of gravity coupled into the sensitive axis equals the sine of the tilt angle. (A tilt table is not required for accelerometers with an operating range exceeding ; these are simply turned over.) Force-balance seismometers normally have a mass-position output which is a slowly responding acceleration output. This output can, with some patience, likewise be calibrated on a tilt table; the small static tilt range of sensitive horizontal seismometers may however be inconvenient. The transducer constant of the calibration coil is then obtained by sending a direct current through it and comparing its effect with the tilt calibration. Finally, by exciting the coil with a sinewave whose acceleration equivalent is now known, the absolute calibration of the broadband output is obtained. The method is not explained in more detail here because we propose a simpler method. Anyway, seismometers of the homogeneous-triaxial type cannot be calibrated in this way because they do not have X,Y,Z mass-position signals.
The method which we propose (for horizontal components only; program TILTCAL) is similar to what was described under 8.2 but this time we excite the seismometer with a known step of tilt, and evaluate the recorded output signal for acceleration rather than displacement. This is simple: the difference between the slopes of the deconvolved velocity trace before and after the step equals the tilt-induced acceleration; no baseline interpolation is involved. In order to produce repeatable steps of tilt, it is useful to prepare a small lever by which the tilt table or the seismometer can quickly be tilted forth and back by a known amount. The tilt may be larger than the static operating range of the seismometer; one then has to watch the output signal and reverse the tilt before the seismometer goes to a stop.