In a message dated 10/02/2002 3:17:40 AM !!!First Boot!!!, 
randallpratts@.......... writes:


> Allen,
>  
> Have you used the method of calibration you referenced?  It is very easy to 
> set up but my system does not behave quite as advertised.  With the boom 
> blocked I don't get a step function as in fig 4.5.1a but rather an 
> exponential decay.  I find that a bit confusing since I put a steady 
> battery voltage across the coil but I have attempted to determine the curve 
> and adjust subsequent readings by the correct factor over time.  I'm also 
> not clear about para 9 where a0 is computed.  What does that formula really 
> mean?  How would it be adjusted for swings later in the wave train and what 
> is the ' on the end?  Why would later pairs of values work when there is a 
> log decay in the swings?
>  
> Randy 
> 

Hello Randy,

I had used the calibration procedure, that I had previously referred to, many 
years ago. I remember that it worked out OK at the time. For the "indicator", 
I used an old panel meter with its needle adjusted to the mid range location 
on the graduated face. That way I could see the change in voltage polarity 
without destroying the needle movement. The output of the circuit went 
straight to the meter, not through any other circuits that may have contained 
a filter of any type. The value for a0 is determined by the formula given in 
paragraph (ix). This is a general formula using the amplitudes of successive 
swings as well. These successive swings, I had assumed, were obtained when 
repeating the tests. And also obtained by averaging the amplitudes of several 
test sets of waves. 

It was a complicated setup, I preferred simpler ones. I used to determine the 
damping of my electromagnetic seismometers by several means. The more elegant 
solution was using an additional calibration coil attached to the pendulum. 
But the simplest method was to very lightly blow air from my mouth at the 
pendulum. If it had a large mass, a slow fanning motion with a sheet of paper 
worked. A small strip of paper used as a hammer worked well too. The J. 
Lehman walk up method is good if the instrument is under cover, another 
reason to fit a cal coil, or the B. Barnes calibrator.

For my 2 cents worth. I used to allow the pendulums on my instruments to come 
to a rest with a less than a 10% overshoot, in 1 swing, to ensure something 
near critical damping. That way I was sure they were not overdamped. Over the 
last couple of years I have been building BB and VBB instruments using the 
S-T Morrissey Mathcad program to determine seismometer response for near 
critical damping. But I still use the paper strip hammer test at times to 
verify that damping is somewhat correct.

Regards,

Allan Coleman  
In a message dated 10/02/2002 3:17:40 AM !!!First Boot!!!, randallpratts@.......... writes:





Allen,

 

Have you used the method of calibration you referenced?  It is very easy to set up but my system does not behave quite as advertised.  With the boom blocked I don't get a step function as in fig 4.5.1a but rather an exponential decay.  I find that a bit confusing since I put a steady battery voltage across the coil but I have attempted to determine the curve and adjust subsequent readings by the correct factor over time.  I'm also not clear about para 9 where a0 is computed.  What does that formula really mean?  How would it be adjusted for swings later in the wave train and what is the ' on the end?  Why would later pairs of values work when there is a log decay in the swings?

 

Randy 





Hello Randy,



I had used the calibration procedure, that I had previously referred to, many years ago. I remember that it worked out OK at the time. For the "indicator", I used an old panel meter with its needle adjusted to the mid range location on the graduated face. That way I could see the change in voltage polarity without destroying the needle movement. The output of the circuit went straight to the meter, not through any other circuits that may have contained a filter of any type. The value for a0 is determined by the formula given in paragraph (ix). This is a general formula using the amplitudes of successive swings as well. These successive swings, I had assumed, were obtained when repeating the tests. And also obtained by averaging the amplitudes of several test sets of waves. 



It was a complicated setup, I preferred simpler ones. I used to determine the damping of my electromagnetic seismometers by several means. The more elegant solution was using an additional calibration coil attached to the pendulum. But the simplest method was to very lightly blow air from my mouth at the pendulum. If it had a large mass, a slow fanning motion with a sheet of paper worked. A small strip of paper used as a hammer worked well too. The J. Lehman walk up method is good if the instrument is under cover, another reason to fit a cal coil, or the B. Barnes calibrator.



For my 2 cents worth. I used to allow the pendulums on my instruments to come to a rest with a less than a 10% overshoot, in 1 swing, to ensure something near critical damping. That way I was sure they were not overdamped. Over the last couple of years I have been building BB and VBB instruments using the S-T Morrissey Mathcad program to determine seismometer response for near critical damping. But I still use the paper strip hammer test at times to verify that damping is somewhat correct.



Regards,



Allan Coleman