[volt-nuts] Oven thermal insulation

[Charles Steinmetz]

Randy wrote:

>I am working on a voltage reference deisgn that will go into an oven for
>the highest stability.  I am looking for a good insulation material that
>can stand high temperatures safely (up to 80C).  Looking at some HP
>frequency standard ovens I see a hard, light-weight insulation material of
>some type that looks like it would work really well, but I have no idea
>what it is.  Does anyone have any suggestions?

I don't know what HP used, but polysulfone is the usual go-to plastic 
for insulation in that temperature range.  It is available in sheets 
and blocks and is machineable.

Do take care not to over-insulate -- the control loop depends on heat 
flow across the insulator to provide the "pull-down" to 
counterbalance the "pull-up" of the heater.  Too much insulation 
(thermal resistance) and the controller can raise the temperature 
quickly, but it takes forever to lower it when the controller 
overshoots (and controllers always overshoot some if they are set up 
for a normally-damped response).  This results in long settling 
times, instability, or even thermal runaway.  You want the pull-up 
and the pull-down to be roughly symmetrical (rise in internal oven 
temperature per unit time with heater fully on approximately equal to 
decline in internal oven temperature per unit time with heater off).

Generally, moderate thermal resistance combined with thermal 
capacitance (thermal mass) produce optimum system dynamics.

Best regards,

Charles