[time-nuts] enclosure temperature controllers

[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
>