I have often thought you might use
some kind of PID loop to maintain a temp
which is the local maxima or minima.
For here that might be constant 140degreesF
or 20DegreesF

A PID loop is like the Automobile Cruise control.

Proportional Integral Derivative

It produces an error signal which decides how much power
to deliver to the heater or cooler.

It has to run 24 hours a day
and
might be expensive to power.


-----Original Message----- 
From: Christopher Chapman
Sent: Friday, May 20, 2011 1:15 PM
To: dyouden@............. ; psnlist@..............
Subject: Re: Photos







Dave Youden dyouden@.............
Sent: Mon, 16 May 2011 20:45
Subject: Photos

Here is the link to the website where my pictures are posted. http://marvelousmachines.blogspot.com/  David H. Youden Hi David, You 
have been busy! A few comments. Can you check http://www.aspe.net/publications/Annual_2005/PAPERS/5FLEX/1859.PDF please? I can't get 
it to display. Good insulating screening boxes are essential. Trying to control the seismometer temperatures may not be such a good 
idea. The heaters, thermometers and control circuit often cause small drifts / transients. It is very difficult to control the 
temperature of a large enclosure with millidegree accuracy. It is more effective to allow the seismometer temperature to drift very 
slowly.  You can buy Al-Clad wirewound resistors which you can bolt onto the Aluminum top-plates to heat them. If you put these 
plates INSIDE the TOP of the clear plastic enclosures, they will stabilise the internal air column. You only need a few watts of AC 
power. The internal temperature will drift very slowly, but there will always be a small positive internal air temperature gradient 
very effectively inhibiting any convection. Crossed foil suspensions are relatively difficult to construct and they are not the 
quietest. Ordinary steel foil is liable to corrosion. Stainless steel foil may be difficult to obtain. Collect and cut up old 
electric shaver foils? Rolling foil and wire suspensions are easy to make and have lower internal friction losses eg a figure of 8 
loop of fine wire glued around two SS cylinders? You can buy fine Nickel plated music wire from D'Addario. Other suspensions which 
work well are circular section crossed Tungsten Carbide rods and a flat, very hard Stainless Steel plate rolling on a Stainless 
Steel ball bearing (Smallparts). I use a large scalpel blade for the plate, stuck to the end of the arm with two component Acrylic 
Glue. This is tough, unlike Epoxy which tends to crack off smooth surfaces.  What noise level were you able to obtain with your 
electro-optical detector, please? I managed to get down to about 14 nano metres using large area Si photo cells and a temperature 
stabilised tungsten filament bulb.  Regards, Chris Chapman






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