Charles,
I have two modified Lehman derivative sensors, both made by John C. Cole,
one is a Mini-Lehman with a stable pendulum period of 14 seconds, it has the
ball bearing/flat surface mountings, the other is a Tmax with a stable
period of 32 seconds, it has the roller bearings supports. I check them once
a week and the last time I have to "center" the Mini, was about 4 months
ago. I am currently tuning/playing with the Tmax but it remains centered if
I do nothing with it. You can check their performance by the results posted
at the PSN web page. I am fortunate enough to have local quakes all the
time, because living in an active area as Central America, there is no
single day without an event. Besides I have 1 of the 10 most active
volcanoes 16 miles away, so my sensors detect high freq events as close as
they can be and as far as the recent 14k Km away, in Asia. The Mini starts
"flopping" at 20 seconds, and the Tmax at around 36 seconds. You can take a
look at pictures and details at John Cole's web page at
http://pages.prodigy.net/fxc/JC.html and at Frank Cooper's at
http://pages.prodigy.net/fxc/.
Hope this helps.
Best regards from Fraijanes, Guatemala.
Rolando


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