PSN-L Email List Message

Subject: Thanks for comments
From: Roger Sparks rsparks@..........
Date: Sat, 12 Feb 2005 12:55:53 -0800

Thanks to all for comments in response to my request regarding plungers 
in liquid damping systems.

Some day (probably soon) I will undertake to change the liquid damping 
to magnetic eddy current damping.  I think it will be cleaner, less 
frequency sensitive, and less temperature sensitive.


psn-l-digest-request@.............. wrote:

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>| Message 1                                                           |
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>Subject: Shape of vane or plunger in liquid damping systems
>From:    Roger Sparks 
>Date:    Tue, 08 Feb 2005 16:34:39 -0800
>Hello All,
>I am looking for some feedback on my thoughts on liquid damping systems 
>for amateur seismometers.
>All of the liquid damping systems that I have seen use vanes or 
>plungers.   As a beginning amateur seismologist, I made a vertical 
>seismometer and used a washer (about 1 1/2 inch in diameter) in liquid 
>for a damping system.  I adjusted the viscosity of the oil to get an  
>return overshoot (past center) about as described in several 
>build-it-yourself sources.
>As I gained experience with my instrument, I noticed that I was not 
>detecting local quakes that I thought I should detect. (I live in 
>Washington state and we have a lot of local quakes).  From the 
>literature, I learned that local quakes are higher frequency, so I 
>guessed that my instrument and amplifier were not detecting or passing  
>the higher frequency signals.  I easily increased the pass band of the 
>amplifier, but still very little signal from local quakes. 
>Then I considered how the plunger of my damping system must be acting as 
>a low pass filter due to the characteristics of fluid flow at higher 
>velocities.  I reasoned that the plunger must move a column of fluid at 
>some velocity.  A fluid moving at a velocity would contain some energy  
>E = mass * velocity squared and divided by 2.  I further reasoned that 
>if the frequency doubled, then the distance traveled in a given time 
>period would also double, and the velocity would also have to double.  
>If that was correct, then the energy required to set the system into 
>motion to move a unit distance, would increase by a factor of 4 when the 
>frequency doubled.  That is a characteristic of a low pass filter system.
>I further considered that I was using a large diameter plunger and 
>expecting  fluid to move from the bottom center of the disk to the top 
>center each half cycle.  That certainly could not happen at higher 
>frequencies.  I reasoned that the path length from bottom center to top 
>center doubled if the  plunger diameter doubled.  A longer path would 
>require that the fluid velocities would have to be greater if the 
>displacement was equal for both large and small plungers.  Again, stored 
>energy in the fluid due to velocity would be energy E = mass * velocity 
>squared and divided by 2.
>I reasoned that the two factors would compound if the frequency doubled. 
>Thus comparing two dampers, one twice the diameter of the other, the 
>larger diameter plunger would require 16 times the energy to move at a 
>doubled frequency through a unit distance compared to the smaller 
>plunger which only requires 4 times the energy to move through the same 
>unit distance at the same doubled frequency.
>To test my ideas, I drilled several holes in my plunger, thinking that 
>the center to center distance would drop dramatically.  This occurred, 
>and I began seeing a much improved response to local quakes.  There was 
>little change in response to more distance quakes.
>If a few holes helped, the ultimate would be to go to a vertical vane 
>which would consist of thin plates parallel to the direction of intended 
>motion.  The cross section of the structure at right angles to the 
>motion would be as small as possible.  The damping then would have to 
>come from drag or friction as the liquid moved along the smooth sides of 
>the plates.  From my text books, I noticed that if the flow was laminar, 
>then the friction would be related only to velocity, not to velocity 
>squared.  While the flow was laminar, the friction would increase with 
>frequency in a linear relationship.  At larger movements and very high 
>frequencies, the flow would be turbulent and the friction again would 
>become related to the velocity squared.
>My parallel vane damper worked very well and now I observe local quakes 
>frequently.  There is still room for improvement with further reduction 
>of the structure cross section and more care in making the plates flat 
>and parallel. 
>In writing this, I hope that others with a better theoretical knowledge 
>about fluid flow will critique  my logic for accuracy.  Does a larger 
>plunger really require 16 times the energy at doubled frequency compared 
>to 4 times the energy for doubled frequency needed by the smaller (1/2 
>diameter) plunger? (to move the same displacement)
>Wishing all the best,


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