[time-nuts] enclosure temperature controllers

[Don Latham]

I know, I know, I should use God's units as revealed by the French, but it
was easier in this case to follow the instructions as printed by the
Chinese...
All is true. A bang-bang is probably OK if the thermal first order filter
is long enough?
Don

Dave Baxter
> Hmmm.  Another two digests missing.....
>
>
> Hi...
>
> I feel that a simple "Bang Bang" (?) controller is not going to give you
> what you seek.
>
> To get truly accurate and stable temperature control, you will almost
> certainly need a full "PID" controller, with some sort of variable
> control of the heating element.  "Phase control" (EMI is fun)  or "Burst
> Fire" (can make lights flicker with big heaters) if powered from the
> utility AC feed.  DC of course is another option.  But you will also
> need to "Measure" the temperature in one form or another, not just
> having a comparator (or two with dead band) and simple switching.
>
> With heating (and cooling) systems, there are significant time lag's
> between applying the heating/cooling stimulus, and the sensor seeing
> that change (let alone whatever you are trying to stabilise.)   Those
> time constants can be a real pain in the but, resulting in long settling
> times if you don't want considerable over/undershoot, but with a "Good"
> controller, you should easily get within a couple degree of what you ask
> for.  With a big thermal mass, probably 1'F is doable, but it'll take
> time to settle.
>
> Any sudden changes due to external (or internal) conditions though,
> could again "kick" it off kilter.  (Clasical PID servo system theory
> etc)
>
> I used to work on GC-MS systems (Finnigan MAT) where there are multiple
> controlled heating systems, the GC oven being the most capable
> controller.  CPU controlled full PID type, controlling what was
> effectively a fan assisted oven.  I learnt a lot, as to what you can
> (and cant!) do with temperature controls.
>
> I have also seen small systems (metal block heating) using power
> transistors as the heating elements, with a "Type K" thermocouple, or
> Platinum probe used (depending on the application and temperature range)
> as the sensor, all bonded to the work piece.
>
> Anyway, what's all this degree F nonsense?  SI units, 'C (or 'k for the
> purists, or is that 'K?) is surely the way to go, like the rest of the
> world.
>
> Have Fun.
>
> Dave B.
> G0WBX.
> Tin Hat and fire extinguisher ready... :))
>
>
> --------------Original Message----------------
>
> Message: 4
> Date: Mon, 15 Mar 2010 18:42:19 -0700
> From: jimlux <jimlux at earthlink.net>
> Subject: Re: [time-nuts] enclosure temperature controllers
> To: Discussion of precise time and frequency measurement
> 	<time-nuts at febo.com>
> Message-ID: <4B9EE1FB.4080007 at earthlink.net>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> Don Latham wrote:
>> hello the group:
>> I've just installed a new "mercury-free" thermostat in my shop. I
>> looked the thing over and realized it has everything needed to control
>
>> a "constant temperature" enclosure. There's a diode used as temp
>> sensor,easy to remote with two wires, a place to put two AA batteries,
>
>> a pot to adjust the set temperature, all needed circuitry for a
>> bang-bang controller with 1deg F hysteresis, uses a latching relay so
>> the batteries last a long time, a low battery indicator, and an easily
>
>> backengineered circuit board, and a low price. There is provision for
>> external power as well. The one I bought is a rite-temp model 6005;
>> distributed by home despot. And it's relatively cheap, about 20
>> devalued rasbuckniks. All that's required is whatever needed for a
>> heater and its power supply. Unfortunately, not for 110 volts but I
>> didn't look at the relay to get the capability; that may be a lawyer
> problem rather than a capacity problem.
>> Don
>>
>
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-- 
Dr. Don Latham AJ7LL
Six Mile Systems LLP
17850 Six Mile Road
POB 134
Huson, MT, 59846
VOX 406-626-4304
www.lightningforensics.com
www.sixmilesystems.com