[volt-nuts] DIY Air bath

Neville Michie namichie at gmail.com
Sun Jan 22 00:51:29 UTC 2012


Hi,

I have made many air baths over the years.
One very effective technique is to instal a fan or fans to cause the  
air to swirl around
the inside of the box in a cyclonic pattern. One fan, near the side,  
mounted tangentially,
is usually enough. Random air stirring is not nearly so effective.
To sample the temperature, a very small thermistor is mounted off the  
opposite side
of the box. (half way rround the air circulation)
The air mass, about 1 kilogram per cubic metre, is now mixed with a  
time constant of
1-5 seconds. The thermistor, with a good air velocity across it has a  
time constant
of a second or two. Using light bulbs as heaters (shielded to contain  
the radiation)
the unit has a bang bang controller without any hysteresis. Unlike  
most electric ovens,
the device behaves as a fast response thermal mass, heating at say  
1/100 degrees per second.
When the set point is crossed it cools at a similar rate. Control  
continues cycling on and off
every few seconds with a triangle wave of better than 1/10 degree.
For it to work you need:
1 well stirred air mass
2 fast response temperature sensor
3 fast response (low mass) heater.
4 no time delay or hysteresis in the power control.
In cases where switching is too noisy, I insert a mono to keep the  
heater off for 2 secs each time it turns off.
The unit then develops a 2-10 second switching cycle but with no  
delay to the heater
being turned on when the set point is crossed.
This is only good for projects were the ambient 1-2m/s air flow does  
not hurt the project.
Often it helps to keep the temperature gradients down.

cheers, Neville Michie

On 22/01/2012, at 1:20 AM, Will wrote:

> Thanks for the comments. Yesterday I found an old vine cooler / fridge
> probably meant for hotel use. Aluminium profile frame and double glass
> door, stainless steel inside and outside. The old compressor has been
> removed so there is plenty of space for the electronics.
>
>
> 2012/1/17, Marvin E. Gozum <marvin.gozum at jefferson.edu>:
>> Some DIY ideas for an environmental chamber.  Note, one thing most
>> don't do is alter relative humidity.  To do that you can inject  
>> steam.
>>
>> A key item is getting a chamber big enough for your need.  The heat
>> control is fairly easy to design, many approaches.  Cooling to the
>> 40F ish area using a Peltier device is simple solution for a chamber
>> but its cost efficiency and regulation could be a problem as the cuft
>> of the chamber grows.  I think the heating part is generally easy and
>> reliable, its getting the right size to save the labor of building
>> one from scratch, that has both thermal glass, and insulation to  
>> build on.
>>
>> In the eevblog.com post earlier, Dave uses a reptile incubator, there
>> is a variant sold as a portable cooler/refrigerator under the Coleman
>> name but the Peltier is set only to cool.  The ReptiPro 5000 or its
>> clones, has reportedly unreliable electronics, either the Peltier or
>> its thermostat are prone to fail early, but its worth a mod to get a
>> good sized chamber that has the infrastructure to build both a
>> heating and cooling chamber in one.
>>
>> You can find incubators new or eBay that will save you the labor of
>> building more tightly regulated heat only chambers.  Infant animal
>> incubators, egg incubators and bio-lab incubators are typical  
>> search terms.
>>
>> Toaster or convection ovens work for just heating, and the only value
>> of changing or supplementing the thermostat is for tighter regulation
>> and adding a fan to insure the heat is evenly distributed in larger
>> chambers.
>>
>> If you are lucky to find a used human baby incubator, and have the
>> room for it, it has both the size, fan, and regulation for good even
>> heating with portholes for working inside the chamber.
>
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