Thermal control schematic

[ilClod]

I'm looking for some info about building a hi-precision thermally controlled
water tank. Could anyone give me some hints, links, or suggestions of any
kind?

Thanks in advance.

 

[CWatters]

I'm looking for some info about building a hi-precision thermally controlled
water tank. Could anyone give me some hints, links, or suggestions of any
kind?

Use one of these (or similar) to control a relay that switches a heating
element..

http://www.siliconchip.com.au/cms/A_30523/printArticle.html

Use a motorised paddle to stir the water to ensure no hot spots exist and
reduce any temperature under/overshoot.

Take extreme care if mixing water and mains electricity!

 

[ilClod]

In message [email protected],

Use one of these (or similar) to control a relay that switches a
heating element..

http://www.siliconchip.com.au/cms/A_30523/printArticle.html

Well, I don't know if it's enough. The basic idea was a temp sensor, such as
an LM35 or similar, and a temperature control circuit (don't know if analog
or digital yet) with a precision of a tenth of degree.
The precision is important, because it is needed for photographic film
development.
If it's better to make it digitally, i would have some problems, because i
don't have experience with PICs and similars.

Use a motorised paddle to stir the water to ensure no hot spots exist
and reduce any temperature under/overshoot.

I thought it could be a good idea using PWM driven peltier cells as
actuators and a PI regulator. But I don't know how to build a PI
regulator... :-/

Take extreme care if mixing water and mains electricity!

I'll surely do!

 

[ilClod]

In message [email protected],

Sure. Put a load of ice cubes in it and stir. Keep adding ice cubes
as necessary.

Oh? You wanted to be able to _change_ the temperature?

That'd be quite good. ))
Would you suggest me to have a pot of warm water ready to pour in the tank?


Seriously, need to build this thermally controlled tank for my dark room,
and I need an extremely precise control (1/1O °C), vith water temp
adjustable between 15°C (59°F) and 42°C (108°F) in steps of 1°C.

Is that so difficult?

--

il Clod! on VFR RC36 I "il Daily"
ICQ UIN 97056271

Son giunto alla conclusione che, almeno una volta nella vita, bisogna
usare Windows a fondo, perche' e' un'esperienza che rende uomini.
Almeno, io, dopo, ho sempre due palle cosi'.
-- Leonardo Serni

 

[Tweetldee]

In message [email protected],


That'd be quite good. ))
Would you suggest me to have a pot of warm water ready to pour in the tank?


Seriously, need to build this thermally controlled tank for my dark room,
and I need an extremely precise control (1/1O °C), vith water temp
adjustable between 15°C (59°F) and 42°C (108°F) in steps of 1°C.

Is that so difficult?

What is the volume of water that you need to control?
What kind of container will the water be in?
Is the container insulated? How well is it insulated?
Will the water be circulating?

To get temperature control of 0.1 degrees, those things become very
important. Trying to control temperature on any useful volume of liquid in
an uninsulated container will be nearly impossible.
And because you want to cool it below room temperature, that means that a
large peltier array is probably your best first choice as the active
element, or if the volume is too great for a peltier array, a
refrigerator/heater will be needed. Then, you're in a whole new arean,
because of the combination of two disparate systems trying to control the
same environment.

Are you sure you need 0.1 degree precision?

--
Dave M
MasonDG44 at comcast dot net (Just subsitute the appropriate characters in
the address)

Never take a laxative and a sleeping pill at the same time!!

 

[CWatters]

Seriously, need to build this thermally controlled tank for my dark room,
and I need an extremely precise control (1/1O °C), vith water temp
adjustable between 15°C (59°F) and 42°C (108°F) in steps of 1°C.

Is that so difficult?

Problems with accurate temperature and heat flow control (poor
"calorimetry") led to the downfall of several scientists who thought they
had (and may have actually) discovered Cold Fusion

My understanding is that professional photographic systems use a
thermostatically controlled valve to control the mixing of cold and hot
water that flows continiously. The resulting mixed water at a controlled
temperature goes into the insulated tank and the overflow goes down a drain.
The temperature of the incoming cold water obviously needs to be below that
of the min temperature you want to achieve (15C) with this type of system.

I think this web log appears to describe how one person thinks it could be
done at home..

http://carbonphoto.cicada.com/darkroom/2002/09/14.html

Not being a photographer I looked up what other people use.... This site
seems to cover the popular options but I doubt any of those shown achieve
temperature control to +/- 0.1C - althought they may measure the temperature
with a display that has a 1/10 C digit.

http://wilson.dynu.net/colordkrm.asp

The Jobo CPA processor..

http://wilson.dynu.net/images/jobotube.jpg

...doesn't even appear to have an insulated cover over the top of the bath.

Sorry if this isn't what you are talking about - like I say I'm no
photographer.

Colin

 

[ilClod]

In message [email protected],

Sorry if this isn't what you are talking about - like I say I'm no
photographer.

That's a good starting point.
Thanks a lot

--

il Clod! on VFR RC36 I "il Daily"
ICQ UIN 97056271

Hawkeye Pierce writing home: "Korea's pretty much the same story:
the fighting goes on, the hatred, the violence, the senseless
brutality, men behaving like animals - and then there's the war..."

 

[CWatters]

In message [email protected],


That's a good starting point.
Thanks a lot

...but I do have experience with electronics, control systems and thermal
management.

 

[ilClod]

In message [email protected],

..but I do have experience with electronics, control systems and
thermal management.

That's good.
My idea is building a (more or less) insulated container (ie. a glass
fishtank covered with expanded polystyrene) with an external water pump that
moves water from the top of the tank to the bottom of it, by two pipes, set
at different depth.
Just after the pump, i thought to apply a heat exchanger built with an alu
heatsink and a pair of peltier cells. This cells are driver by a dual supply
PWM circuit, so that I can achieve heating and cooling easily.
The temp sensor should be placed in the middle of the tank, where
presumabily I would put the film container filled with developers etc. I
thought something like LM35...

The problem is building the control circuit, because I don't know nearly
nothing about MCU. I could be able to do it analog, but I'd prefer to avoid
thermal drift phenomena.

--

il Clod!
ICQ UIN 97056271

"You know you're beautiful when he's angry"
-- Hawkeye Pierce

 

[Andyb]

I'm looking for some info about building a hi-precision thermally
controlled
water tank. Could anyone give me some hints, links, or suggestions of any
kind?

Beware of the difference between 'accuracy' and 'resolution' when you're
calibrating your system.

 

[CWatters]

The problem is building the control circuit, because I don't know nearly
nothing about MCU. I could be able to do it analog, but I'd prefer to avoid
thermal drift phenomena.

What did you have in mind for the sensor?

 

[ilClod]

In message I7jZc.232851$%[email protected],

What did you have in mind for the sensor?

Something simple to build and calibrate, and precise enough for my purpose.
I thought LM35 could be fine.
Maybe it could be better if applied on a aluminium heatsink, to increase the
thermal exchange surface. Or is it a bad idea?...

--

il Clod!
ICQ UIN 97056271

Hawkeye Pierce on the war: "In case you haven't noticed, this is ugly.
This is not exciting."

 

[CWatters]

Something simple to build and calibrate, and precise enough for my purpose.
I thought LM35 could be fine.

If you calibrate it then you should be ok (otherwise the datasheet suggest
the accuracy will be around 0.75 degrees).

Maybe it could be better if applied on a aluminium heatsink, to increase the
thermal exchange surface. Or is it a bad idea?

Personally I doubt you can get +/- 0.1C control overall but circulating the
water as you describe is the right way to go. Adding a heat sink reduces the
time constant but hopefully you aren't looking for rapid temperature changes
with no overshoot - just steady state accuracy.

Try this for a starting point of a design...
http://www.elecdesign.com/Files/29/2476/Figure_01.gif
Description here
http://www.elecdesign.com/Articles/ArticleID/2476/2476.html

As shown it would only give control to around 1C but you could improve this
by changing the gain of the input amp and rescaling the ADC. Currently the
input amp has a gain of 2 so that 0-150C at the sensor maps to 8 bits of the
ADC range. If you change the gain to 6 that would map 0-50C to the 8 bit
range. This would give a resolution of 50/256 = 0.2C. That's not enough to
meet your 0.1C spec but you could further improve this by using more bits of
the ADC (eg 9 bits or all 10 bits). Notes: I've not checked that all 10
bits of the ADC are "usable" on the micro they specify. Nor have I looked to
see what language they wrote the code in - if it were C rather than
assembler you would have to find yourself a compiler. The assembler is free
I believe.

You might also look at using the LM35C or LM35CA part - if I've read the
data sheet correctly these have better linerarity.

The PIC 16F8xx series are easily programmed using a home made in circuit
programmer - essentially this is just a modified serial cable connected to
the serial port on a PC.

There is a huge community of engineers using the Microchip PIC series so
it's not hard to find guides on how to get started programming these
devices. The PIC web site is:

http://www.microchip.com

Colin

 

[CWatters]

Just after the pump, i thought to apply a heat exchanger built with an alu
heatsink and a pair of peltier cells.

You could reduce the size of the devices you need if you are prepared to
prime the system with water close to the target temperature - otherwise you
need quite powerful devices. I guess it depends on how much water is in the
tank and how long you want to wait for it to reach the right temperature.

Say you want to heat the water from 20C to 30C and are prepared to wait 10
mins for it to get there. If I've done my sums right I calculate you need
60W per L of capacity. How big is the tank? 10L = 600W. etc

 

[ilClod]

In message [email protected],

Try this for a starting point of a design...

[cut]

Thank you so much! , really helpful.

In case of problems (hope not I'll ask for help again

Thanks by now.