Chapter 5, An Inexpensive Vertical Seismometer for Classroom Demonstrations: Example Experiments

Developed for the Mid America Earthquake Center Workshop

at Georgia Tech, June 22-25, 1998

Observing movement.

For adjusting period and studies of the motion of the seismometer, the motion may be directly observed. However, for small movements, those that are caused by slight ground movements, electronic output is helpful.

1) A resistivity meter may be connected directly to the sensor. The sensors we used vary in resistance from a few hundred ohms (in light) to 20,000 ohms (in dark). The resistivity meter will follow the movement as the shutter moves up and down.

) Alternatively, an oscilloscope can be connected to the light sensor. To get a reading, construct a voltage divider using a 6 v dc supply. With a 10k ohm resister in series with the light sensitive resistor, the voltage across the light sensitive resistor will vary from 5 volts to less than 0.5 volts as the shutter moves across the sensor. The variation in voltage can be observed on the oscilloscope. Measurements such as relative amplitude of various vibrational sources can be instructive.

3) If a digital computer interface is available, such as Vernier Software=s Serial Box Interface, then the output can be connected directly to a computer and saved for analysis. The Serial Box Interface is available for IBM or Mac computers and is capable of digitizing 50 samples a second at a 12 bit resolution. Supporting software is available from Vernier Software. Analysis may include measures of damping and free period. The circuit and setup for this type of measurement is shown in figure 8.

 

Applicable Experiments

(these are ideas, you may need to look in texts for standard experiments and detailed explanations for students)

1. Hooks law for springs, in finding the zero length (see description of instrument).

2. Finding center point for balance system. The vertical component long period seismometer can be adjusted beyond a point where the period goes to infinity. At this spring and mass adjustment position the lever arm will either move up or down depending on whether it is started above or below, respectively, the center point. At the center point, the lever arm could go either way, depending on which way it is pushed first. Finding this point is important in determining where the zero position should be placed for reference.

3. Determining free period. Free period can be determined by removing all damping and counting the seconds it takes for a number of cycles of vibration.

4. Determining damping ratio. The damping is determined by how much smaller each successive swing is when the system is free to vibrate and decay slowly with damping. If the system does not vibrate it is critically damped. Seismometers are usually set at 0.7 critical damping.

5. Determination of gain can be an exercise. One measures the displacement of the lever arm at the sensor and the voltage out on the meter. The gain is the volts per millimeter of displacement. .