Chris
  I'm not sure what the periods are. The STM8 style ~30" long appears to have a period (with feedback) of 82 sec. I get occasional spikes I think from the concrete slab it rests on. When this happens I get a single cycle which has a period of 82 secs. I have plans in the future to put a secondary coil near the feedback coil to provide a induced force to measure the period etc. The 12" STM8 style vertical has a background oscillation of 167 sec but this could be from anything including electronics. The Lehman has a damped period around 20 sec. I could make it longer but it's located near an exterior wall in my garage and I would be spending every day adjusting its position in the spring and fall. I have plans for a basement. Hopefully this will assist in stability. The STM sytle verticals need slight motor adjustment every few days In the spring and fall. 
  Regards
  Barry
  

ChrisAtUpw@....... wrote:
  In a message dated 2008/08/23, barry_lotz@............. writes:

  I made two modest versions of Sean Thomas Morrissey's design, a 30" and a smaller 12" version. 

Hi Barry,

       What periods do you get from your seismometers, please?

       Do you experience any serious tilt driifts with the Lehmans, please?

       Regards,

       Chris Chapman 
Chris
  
I'm not sure what the periods are. The STM8 style ~30" long appears to have a period (with feedback) of 82 sec. I get occasional spikes I think from the concrete slab it rests on. When this happens I get a single cycle which has a period of 82 secs. I have plans in the future to put a secondary coil near the feedback coil to provide a induced force to measure the period etc. The 12" STM8 style vertical has a background oscillation of 167 sec but this could be from anything including electronics. The Lehman has a damped period around 20 sec. I could make it longer but it's located near an exterior wall in my garage and I would be spending every day adjusting its position in the spring and fall. I have plans for a basement. Hopefully this will assist in stability. The STM sytle verticals need slight motor adjustment every few days In the spring and fall. 
  
Regards
  
Barry