PSN-L Email List Message

Subject: Re: Your Seismometer
From: "rem11560@............
Date: Sat, 3 Jul 2004 01:50:21 GMT

Hi Jason,

 Here are my answers to your questions, to the best of my ability to answer them

>>1. Greatly reduce the mass of the pendulum.
>How much weight is preferable--is there a rule of thumb (or better yet, mathematical formulas) based on the boom size and/or other parameters?
  Answer: There is no theoretical lower limit on mass, except a quantum one. You could't reduce the mass enough for that to have any effect. The practical limits are susceptibility to air currents, friction in the upper and lower pivots, and over damping caused by the input impedance of the amplifier. Sensors do not have to drive styluses over smoked paper any more. 

>>2. Stiffen the superstructure laterally with cross bracing. 
>It seems pretty sturdy but I can easily add another cross-brace.
    Answer: Just make sure that you have at least one diagonal member.

>>3. Having a magnet on the pendulum makes the sensor responsive to stray magnetic fields. Put the coil on the pendulum and the magnet on the base. 
>Easy enough, but where should I run the wires?  Along the boom or up the suspension?
    Answer: I run my wires down the boom, and let them dangle in a loose coil on the way to a terminal block.

>>4. Be sure to use a cover which shields the sensor from ambient temperature change and drafts. You may need a heater inside at the top to prevent convection of air upward from the baseplate.
>Will make an enclosure of Celotex.  How about a small appliance lamp instead?
    Answer: Others with experience with large Lehman sensors could answer this with more authority. I would think a small appliance lamp would be OK.

>>5. Put a one microfarad capacitor across the input terminals of your amplifier. 
>In parallel with the damping resistor?
    Answer: Yes, at the amplifier terminals. Larry Cochrane doubts the necessity of this, but I am using one of his amplifiers, and it was certainly necessary for me.



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