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Re: Calorimeter



Original poster: "Dr. Duncan Cadd by way of Terry Fritz <twftesla-at-uswest-dot-net>" <dunckx-at-freeuk-dot-com>

Hi Gary!

This is fascinating stuff!

Date: 16 January 2001 04:57
Subject: Calorimeter


>Original poster: "Gary Johnson by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <gjohnson-at-ksu.edu>
>

<snip>

>Impedances rise when the spark occurs.
>


Glad you've been able to confirm this.  I observed this via
a thermocouple ammeter in the ground lead of my micro coil
(15VA).  Base current plummets when a spark is drawn - in
the case of my 15VA coil from around 250mA to less than the
meter registers.  Any clues as to ballpark figures under
spark loading? I guessed k-ohms.

>One interesting quirk is that below breakout, the coil gets
hot first, and
>then heats the air, so the peak temperature in the
calorimeter will be
>observed a few minutes after power is removed. Above
breakout, however, the
>spark heats the air first, which then heats the coil.

That sounds spot on - the circulating power heats the coil
if it can't break out.

<snip>

>input power). The 16 gauge coil and one of the 22 gauge
coils were made as
>identical as possible. They use the same coil form, same
winding length, and
>the same number of turns. Both are resonant at 123 kHz when
using a
>particular toroid. Both are tight wound but the 22 gauge
coil has thicker
>insulation. The 22 gauge coil has a lower input impedance
and puts more
>power into the spark (faster temperature rise) for a given
input power. John
>Freau is right when he suggests using smaller gauge wire!
>


Sounds like a confirmation of age-old wireless practice -
space winding the turns reduces proximity effect losses.  I
worked out the details and according to my old B&S wire
tables the 22awg hookup wire spacing in your coil is around
1 wire diameter if the winding pitch is the same as for
close wound 16awg.  The old radio books I have suggest
anything from 0,6 - 0,9 wire diameters as the ideal spacing.
This is interesting. Or are you saying that the increased
efficiency is more than the reduction of proximity effect
losses would suggest, as you mention compensating for them
earlier?

>A third observation, and the reason I have not been more
precise with my
>numbers, is that humidity appears to be the biggest factor
in the input
>impedance.  My lab is a large metal building in eastern
Kansas. Temperatures
>at the coils range from zero to 100 degrees F, and the
humidity probably
>ranges from 20 to 90 percent. On December 29 I measured the
input impedance
>of the 16 gauge coil as 87 ohms. This morning I measured
122 ohms using what
>appear to be identical procedures. I put a desiccant inside
the calorimeter
>and watched the impedance drop to 100 ohms. (The desiccant
saturated at that
>point). Then I put in a humidifier and watched the
impedance climb to 345
>ohms. A similar test with the 22 gauge coil of identical
dimensions saw the
>impedance climb from 90 to 150 ohms.
>


I assume that the presence of humidity is causing dielectric
heating of the water molecules by the E field and hence an
additional loss of non-negligible proportions.  That
permanent dipole moment has a lot to answer for.  Moving the
water molecule around is a very lossy business, hence the
popularity of industrial rf drying of wood, curing of glue
etc.  I'm wondering if the presence of high humidity
increases the rate at which the air heats under "no
breakout" conditions, which it may well do if the water
vapour is being directly stimulated by the E field.
However, the test with the 22awg hookup wire coil suggests
that a significant part of this loss is due to water in very
close proximity to the wire, where the E field gradients
will be highest - whatever is going on appears to be
primarily a short range effect.  Perhaps a good, thick
coating of PolyGloop (TM) on top of the windings is a Good
Thing after all.  Maybe when you've run all the other
experiments you want to try, you could give one of the
magnet wire coils an extra-thick coat of goo and see if the
humidity losses are affected.  The old radio tomes suggest a
good soaking in wax as an excellent treatment for coils
subject to "tropical" conditions, so I assume this is what
they were dealing with.  If the losses are not reduced by a
thick coat of goo, it looks like surface tracking may be
involved.  Boy, have you opened up a can of worms!

Congratulations on rigging up a really useful test system!

Dunckx