[volt-nuts] DIY Air bath

[Bob Smither]

Will wrote:
> Varying room temperature is a remarkable source of error when making
> measurements in home environment. It would be nice to have a simple
> air bath for the items under test. And even better if it was big
> enough for the multimeter too. There are commercial units available
> but very expensive.
> 
> A DIY solution seems to be possible: a simple PI(D) controller built
> around an NTC resistor, op-amp, pass-transistor and heater plus a fan
> to keep the air moving inside the box. That approach allows
> temperature higher than room temperature only but in most cases
> stability is important and not the absolute value. A Peltier element
> is almost as easy to drive as a heater resistor, but dissipated heat
> probably makes the thermal design much more challenging.
> 
> I am not an expert and there many questionmarks related air
> circulation, thermistor location and probably other things I can't
> even imagine.

Hi Will,

For years I have used a modified Rival brand consumer grade convection oven as a
test chamber.  Although I designed it for use up to 200C, it can be used at any
temperature above ambient.

The oven has a usefully large "cooking" chamber - 16" x 14" x 8.5".  The heater
and fan are in a section above the chamber separated by a metal partition.  The
heated air exits that section and enters the chamber through several slots in
the metal partition.

I redid the wiring so that the fan runs all the time and I designed a simple on
/ off controller to control the heater element.  The controller is in series
with the oven bimetallic thermostat which serves as an over temperature safety
cutoff - I just set it well above the temperature I set the controller to.

The controller uses a diode that is placed in the chamber as the sensing
element.  A comparator compares the diode voltage to a fixed level derived from
a reference and a 10 turn pot.

The heater is switched by a solid state relay that is controlled by the
comparator output.  The SSR has the nice feature of zero voltage turn on that
reduces any electrical interference.

I have tried different placements of the diode sensor.  If it is located in the
heater / fan section above the chamber the system is very over damped and it
takes a long time to reach equilibrium (the diode "overreacts" to the power
applied to the heater element and shuts down).  If it is located in the chamber,
the oven will overshoot the target temperature as the applied power is not
sensed quickly enough.  The best location seems to be in the chamber but close
(~1") to one of the slots where the heated air enters the chamber.

Although the SSR is an on / off device, when close to the set point it behaves
in almost an analog fashion as there is some 60 cycle pickup on the sense lead
(that goes to the diode sensor) so in fact it operates at various duty cycles as
it applies power to the heater.

I just ran an experiment with three thermocouples in the chamber at different
positions (close to wall, in middle of chamber, ...).  The lowest setting on my
controller is above room temperature at 34C.  Here are the minimums and maximums
through several cycles of the controller at the three different locations as
recorded on a Doric Trendicator 412A:

Position 1: 33.8C - 34.2C
Position 2: 33.7C - 34.2C
Position 3: 33.7C - 34.3C

I have used this oven for over 25 years.  Recently the fan motor finally gave up
but it was easy to replace.

Hope this gives you some ideas.

Bob Smither

-------------- next part --------------
A non-text attachment was scrubbed...
Name: smither.vcf
Type: text/x-vcard
Size: 140 bytes
Desc: not available
URL: <http://www.febo.com/pipermail/volt-nuts/attachments/20120113/dec353d2/attachment-0001.vcf>