[volt-nuts] DIY Air bath

[Will]

Thanks for the practical advice. Exactly what I was looking for.

There are a few heater related things I would like to ask before
testing in real life

What is the optimal location of the heater, close proximity with the
blower or some distance?

High power heater and low duty cycle or vise versa?

Is there any specific reason to use a light bulb, except low price and
easy availability? I have some resistors which have bare wire on
ceramic body. I think they would be even faster than a light bulb. And
smaller size too.

Will


2012/1/22, Neville Michie <namichie at gmail.com>:
> 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