On 2008-08-16, Joseph Gwinn wrote:
In article ,
"DoN. Nichols" wrote:
[ ... ]
There is a shelf in front of it -- slate or perhaps asbestos
board. Too discolored with age to tell for sure. About 1/4" thick,
FWIW.
Can't be slate at that temperature, so asbestos board is more likely.
Especially if I pull something out at full temperature and leave
it resting on the shelf.
[ ... ]
Hmm ... no vent here, unless the 1" hole in the back was
supposed to be one. But it is not fully at the top of the back.
My Vigor has a 1" diameter vent hole in the top, with a little metal
cover that one swings to the side. I wondered what it was for.
In the top makes more sense than in the back where mine is. So
I just use mine for poking in the thermocouple.
[ ... ]
Chromel and alumel are basically a form of stainless steel.
O.K. I've got the alloy definitions in my old CRC _Handbook of
Chemistry and Physics_ (otherwise known as the "rubber tables". :-)
The Omega.com website has lots of information as well. As does NIST.
And somewhere I have a ton of literature from Omega -- but I
haven't seen it for a while. :-)
[ ... ]
I also added a larger LED to watch the cycling of the heating
element.
The Vigor had a neon pilot connected across the heating element, and so
I left it as it was.
O.K. But neon pilots -- especially after they age quite a bit,
tend to flicker on their own. Of course, if you *really* need to know,
you use the tiny LED in the controller. :-)
[ ... ]
The pyrometer does seem to work, and I'll calibrate it and use it as a
backup for the fancy electronics. It may benefit from a new
thermocouple.
O.K.
I think one can buy pyrometers on eBay and at ham swap meets. There are
now lots of Pyrometers on the dole, having been made redundant by
digital controllers.
:-)
"On the dole" -- are you in the UK by any chance? Or from
there?
[ ... ]
That would certainly work, but would stick out very far to the right.
O.K. Can you punch a couple of holes in the side and mount a
small computer fan in there to circulate air whenever it is running?
Perhaps add a thermostat to keep the fan running for as long as there is
significant heat coming through the wall?
I thought of this, but couldn't find an 115 volt AC fans that were small
enough. I probably can run a 24 volt fan off the controller signal to
the SSR, but the fan would run only when the element was on. Perhaps
that's enough.
Hmm ... a tiny switching power supply from a hamfest to power
it?
I took my mechanical inspiration from pictures of current furnaces with
digital controllers.
O.K. I just went with available Bud Minibox chassis (I didn't
at that time have a shear large enough to accompany my 24" brake to make
my own.)
Doesn't Bud (or Hammond) still make steel chassis? Just like in the
1950s?
I don't know. I have seen only aluminum ones recently.
[ ... ]
There was someone on this newsgroup who had several left over
unused after a project closed down, and who was offering them here. I'm
glad that I got one. I wonder whether we even have the same model? It
would take me a bit of work to verify the model now. Mine has the
plug-in strip along the top which says that it is displaying in degrees
C, but I've switched mine over to display in degrees F, since that
matches the information in _Machinery's Handbook_ more closely.
What I got is a Omega CN7523 controller driving a Omega SSR330DC SSR
which is mounted on an Omega FHS-7 heatsink.
Mine is too hard to get to at night. Easier when I can open the
shop door -- as I intend it to be when I'm using the oven. And a good
fire extinguisher handy, too. :-)
O.K. It is still daylight now, and I just checked. It is the
Omega "CN9000A". (IIRC, there is more fine detail to the number on a
label inside the box.
Yes about the fire issue. I don't run this furnace if I'm not present,
at least not until I have some experience with it. The first thing I
did with it was to measure temperature versus time under power, to see
how long it took and how hot it got. This caused some tightening-up
exercises, which allowed the peak temperature to climb a bit. I've had
it up to 1,000 C or so.
O.K. The highest that I've had mine is 1850 F (1010 C).
[ ... ]
In my Vigor, the hole fits the ceramic sleeve insulating the
thermocouple reasonably tightly.
One can also make bushings from K23 foam firebrick and glue it into
place with furnace cement.
O.K. But I *had* the machinable lava. It machines quite easily
in the lathe (very dusty), and when it is fired it turns pink and *very*
hard.
What make and model is this material? It may be useful.
Well ... I just did a Google search for "machinable lava", and
the first hit is:
http://www.professionalplastics.com/LAVA
(they insist on JavaScript being enabled) which says (in part):
================================================== ====================
Machinable Alumina Silicate L911A (Lava) Machinable Alumina Silicate
L911A (Lava) Aluminum silicate is machinable in an unfired state and
used in applications up to 1000 F After firing, parts are as hard as
carbide & usable up to 2100 F Aluminum silicate has low thermal
expansion Aluminum silicate is low cost Aluminum silicate is available
in wide range of stock up to 12" in diameter.
Some applications include prototype hardware, insulators, bushings,
soldering fixtures, nozzles, welding tips or whatever you need.
================================================== ====================
This site http://www.technicalproductsinc.com/lava.htm seems
to have more details about how it should be fired, and I obviously did
not do it quite right. :-)
[ ... ]
The traditional moving-coil pyrometers are self-powered, and are
optimized to work with a thermocouple. They are accurate enough for
heat treating. Some pyrometers were designed for mounting in a steel
panel, and are inaccurate if not in the panel, so if the panel is
aluminum it may be necessary to provide a piece of sheet steel as well.
Right -- the steel panel shunts off some of the magnetic field
which of course changes the sensitivity somewhat. IIRC, this was
originally mounted in a non-magnetic alloy of stainless, so there is no
problem there.
So, you must avoid the steel.
Omega sells the #14 Type K thermocouples for something like $20 each, so
I wasn't tempted to scrounge for one.
Well ... I already had them, so I did not need to purchase any, yet.
Enjoy,
DoN.
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