The Mini-Lehman Seismometer
By Charlie Thompson
The full-sized Lehman...complete with insulating enclosure was taking up too much room in our garage. I decided to reduce the size of the design and fit it inside a PetSmart 5 gallon glass aquarium tank. This allows easy checking of the seismometer for 6-legged pests and spiders. Period is about 6.2 seconds which is 1/2 the period of my larger Lehman. The unit uses oil damping for now...may change back to eddy current damping if the oil system causes problems. The unit experimentally went on line October 17th, 1999 providing round the clock seismic data.
In the photos below you can see that three levelling screws (now with their ends ground to a sharp point) are used to "tune" and level the device. It was difficult to get a pendulum period in excess of 6.2 seconds so I settled for that. Not shown in the photos below are the pickup coil and oil damping system. These two photos are before I put the sensor into service. See the third photo below for "in-service" view.
Below is the Mini-Lehman in service in my garage...the light bulb heats the air and kills the convection currents that would otherwise boil up from the warm concrete slab against the cold air (in the wintertime).
(You can see the oil damper on the end of the boom; the leveling screws now are wrapped in teflon tape for smooth operation)