[time-nuts] HP 10811 Response I Replies

[Perry Sandeen]

Esteemed Colleagues,

I will attempt to make a coherent technical reply to the remarks made about my 10811 posting. 
But first a bit of qualification.  My remarks refer to the 10811 and that vintage of OCXO’s.

Wrote: Doing what you describe will result in a very sensitive humidity sensor, having eliminated the thermometer effect.

I do not understand.  I believed that since the OCXO temperature will be substationally higher than the surrounding temps, any residual moisture would migrate to a lower temperature.  The fiberglass insulation inside the vacuum flask prevents air currents, reduces air volume, and provides mechanical isolation.

If I err, what would be a better solution?

There are similar, well lauded, designs using large blocks of styrofoam instead (Shera).  Why would foam be better that a vacuum bottle?

Wrote: In any event, none of this affects crystal aging or frequency jumps.

So true, but I never said it would.  All I proposed was to as much as possible eliminate any exterior variables so all one would see is only the crystal aging characteristics.

Wrote:  This is what limits the E1938A which is hermetic and has a thermal gain 1000 times better than the 10811.

I know it is a splendid OCXO, a much later design and costs your new-born child IF a used one can be found.

Rick, you have probably forgotten more about OCXO’s than I’ll ever know.  My questions are not argumentative but are for seeking knowledge.

Wrote: Just how good is the thermos bottle in this case? (as in degrees / watt). You can get some very good vacuum ones and some pretty poor Styrofoam ones.

I don’t know anything about its degrees/watt.  It is a true wide mouth glass vacuum thermos with a protective plastic inner lining that I bought a long, long time ago.  It was sold for taking hot soup to work.

Speaking of Styrofoam.  If you want to use one try to get one from Omaha Steaks.

Wrote: Imagine that the set point is variable, and can be set below the desired temperature.  Then imagine that the set point can approach the desired temperature more closely as it gets closer to the desired temperature.

... And you will have discovered (100 years late) the PID controller.

It still works on the concept of successive approximation.  It may have less over-shoot on start-up and the amount of over-shoot may not have a negligible effect.  But it still will be there.  

Perhaps if one was designing a new product that might be the best solution.  However my remarks were about the HP 10811.  It isn’t applicable in this situation.    

Wrote: 30 years ago I was designing PID controllers, with a little microprocessor magic, that could quickly arrive at the set point temperature and never, I repeat, never, exceed that temperature.  Someone's internal organs would have become toast if it did.

From 1976 to 2001 I repaired life support equipment including hemodialysis machines (factory trained) and blood warmers.  They only used thermistor bridge or mechanical thermostats.  And yes we had to make sure the blood temperature never, ever went over 102F.

Probably at that time reliable µprocessor control was not generally cost effective.

But I don’t think that PIC technology you did would apply to OCXO’s.  Here’s why: heat is added to either large amount of cold fluids entering the body or to substitute for the body’s inability to maintain proper body heat (radiant infant warmers). You have a large ongoing thermal sink.

In an extremely well insulated OCXO as I understand it, there is almost no thermal sink once the set point is reached. And then there is the cost issue.

Again, not to argue with your type of controller.  I’m just considering the current surplus OCXO’s that are available.

Regards,

Perrier