Flat Spring Seismometer Version 2

The Flat Spring Seismometer is able to measure motion at right angles to the flat surface of the spring but is mechanically restrained in the plane containing the spring.

The Flat Spring Seismometer Version 2

The weakness of the "C" Spring Seismometer is sensitivity in all dimensions and fluid damping. Magnetic damping would solve the second issue but what to do about dimensional stability? I decided that a flat spring would solve the issue but where in the world would I find such a thing?

I decided to build my own (DYI). I would:

• Begin by cutting flat spring material to loaded shape.
• Bend the flat spring into unloaded shape
• Anneal the bent spring to remove the introduced stresses
• Place the annealed spring under load to recover the flat configuration.

More on spring construction later in this article

History and concepts

The tilted first version

The first version of the Flat Spring Seismometer was tilted to lengthen the natural pendulum period. The directivity introduced by tilting eventually motivated me to rebuild the device into a truly vertical sensing machine. Many of the parts from the tilted version were reused and are the reason that the profile of version two is so tall.

Removing the tilt definitely reduced the sensitivity to very low frequency waves (like Love waves). Nonetheless, the instrument remains very able to capture both local and distant quakes (broad band) worldwide.

General Construction Excepting the Spring

All parts are either aluminum or other nonferrous material excepting the magnetic components and steel spring. The base uses 4" x 24" x 1/4" plate aluminum. The towers and safety stop are bolted to the base with L members. Many of the parts are made with adjustment in mind, making final adjustment easier.

The damper and sensor magnets were built using 1" x 1/2" x 1/4" neodymium magnets obtained from K&J Magnets. The damper used 2-4 magnets, the sensor 2 magnets. The coil is from a 120–240-volt relay. Damper blade material can be copper or aluminum with damping increasing with thickness.

Spring Construction

Two annealed springs

The spring in use was cut from a 12" x 12.375" x 0.015" AISI 1075 spring steel sheet. It is 12" long, with the wide end 2" wide and then tapering smoothly to 1" wide. A Dremel cutter was used for cutting the part. [Be careful to follow safety precautions when using cutters like this.]

Blue steel blades from drywall tools have also been successfully used in this application but the AISI number is unknown.

Grind any needed mounting slots before beginning the annealing process. The unloaded spring is now ready to be deformed into a new unloaded configuration.

A short length of 4" iron water pipe can be used to retain the deformed spring. Force the spring inside, taking care to center the narrow end on the wide end. Once this is done, the spring is ready for annealing in a hot environment.

The "hot environment" readily available was our kitchen oven with a self-clean cycle. My computer AI tells me that self-cleaning temperatures are 800-1000 degrees Fahrenheit lasting for 2-4 hours. The technique certainly worked here. Once cooled, the annealed springs slipped out of the retainer with only a little spring back. The flat spring is now ready for use.

Conclusion

By now, my readers can see that this is not a project for the beginning DIYer. It is going to take considerable time and thought to layout precise locations and sizes for parts. Having built two of the machines, I think it worth the effort.

M3.2 2025 3 18 06:23 Near Tacoma, Washington. Sourced about 90 miles west of this seismometer

(c) Roger Sparks 2025