small furnace?

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bert weiss

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May 30, 1999, 3:00:00 AM5/30/99
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I want to build a small furnace for the purpose of making and coloring
frit. I work with clear and colored (cobalt or copper) frits. Mostly I
am fritting up float glass. I may get interested in coloring or
fritting lead crystal but that is not immediate. Propane is my fuel of
choice. This is not a 24/7 operation.

I have gotten conflicting opinions on what configuration I should go
with. One opinion is that I should build a free standing pot furnace.
This would allow me to use different pots for different purposes. It
would also offer the possibility of removing the hot pot and pouring the
glass out. I'm told this setup will give me cleaner glass.

Another opinion is that I should build an invested pot furnace. I'm
told this would be easier to build, and be easier to run.

I am thinking about scaling the furnace to a 55 gal drum or there
abouts. I imagine castable refractory is a good route to make a
relatively inexpensive durable rig.

If I run this rig outdoors, intermittently do I need a safety system?
Will a venturi burner do the trick?

I'll be interested to hear some opinions on what is the most practical
approach from more experienced furnace users and designers.

Bert Weiss

Bert Weiss Glass Studio
http://www.customartglass.com
Painted Art Glass
Custom Productions
Architectural and Sculptural Cast Glass
Collaborative Art Glass
Lighting design

Terry Harper

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May 30, 1999, 3:00:00 AM5/30/99
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bert weiss wrote in message <37515B11...@bigfoot.com>...

>I want to build a small furnace for the purpose of making and coloring
>frit. I work with clear and colored (cobalt or copper) frits. Mostly I
>am fritting up float glass. I may get interested in coloring or
>fritting lead crystal but that is not immediate. Propane is my fuel of
>choice. This is not a 24/7 operation.
>
>I have gotten conflicting opinions on what configuration I should go
>with. One opinion is that I should build a free standing pot furnace.
>This would allow me to use different pots for different purposes. It
>would also offer the possibility of removing the hot pot and pouring the
>glass out. I'm told this setup will give me cleaner glass.
>
>Another opinion is that I should build an invested pot furnace. I'm
>told this would be easier to build, and be easier to run.
>
>I am thinking about scaling the furnace to a 55 gal drum or there
>abouts. I imagine castable refractory is a good route to make a
>relatively inexpensive durable rig.
>
>If I run this rig outdoors, intermittently do I need a safety system?
>Will a venturi burner do the trick?
>
>I'll be interested to hear some opinions on what is the most practical
>approach from more experienced furnace users and designers.
>
Bert

Too many years ago to reveal we were doing lots of experimental melts.
Almost universally they were done in pots holding about 1 or 2 pounds, which
we slip cast and fired in the lab. This was using a gas-fired furnace with
two tangential nozzle-mix burners and high/low gas flow temperature control,
with a thermocouple in the wall of the furnace.

At some stage as you scale up, it becomes impossible to lift the pot out
with ease, because of the weight and the need to grip with tongs. At that
stage you have to ladle the glass out, or have a means of tilting the pot to
pour it direct. Alternatively use a small day tank, and mount on a pivot. I
think it is AFT in England, at Retford, who have designed something on these
lines.

You are talking about oil drum size. Is this the glass containment? If so it
may need to be a clay pot, which would have to be fired in situ before use
to be any good, and may not last more than one firing. You will not get away
with lining an oil drum with castable, in my opinion, although Didier
Taylor's 150 Zircon Patch is a surprisingly good material. A zircon-mullite
castable could also work well, but not inside an oil drum. As for firing,
the castable has to be prefired, and it needs to follow a careful heat-up
curve. You need a mould from which you can release it. A simple programmable
controller would be a good investment. The firing chamber can be a pile of
bricks held in a simple steel frame, with some insulation. In fact you could
probably use 2800 IFB for short term use, maybe coated internally with a
thin veneer of castable with a suitable rating.

An inspirator nozzle mix burner on propane or natural gas ought to be
suitable, but time is going to be the problem, and a controller will
definitely help, with a control valve on the gas line. You also should have
safety shut-off valves even working outside.

I suspect that you can buy a simple pot furnace for maybe USD20-30k. That
would be suitable for industrial use and would comply with most applicable
regulations. A DIY effort is obviously going to cost a lot less, but you may
have a learning curve. Have you ruled out electric heating totally? It may
have advantages for intermittent use. Molybdenum disilicide (Kanthal Super)
heating elements are the best way. You could then use insulating fibre
modules for the walls of the heating chamber (like Clinotherm).
--
Terry Harper
Acting Webmaster, The Omnibus Society http://www.omnibussoc.org
E-mail: terry....@btinternet.com
URL: http://www.btinternet.com/~terry.harper/


Brad Shute

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Jun 2, 1999, 3:00:00 AM6/2/99
to
Bert,

Either of the design opinions you got - freestanding or invested pot - will do
what you want. They do, however, each have their strong and weak points. You
will have to decide which is more important to you, durability or the ability
to change/remove pots easily. An invested pot will take more abuse, but is more
of a pain to rebuild when the time comes and you can't swap the crucible to
change colors. As you mentioned, with a freestanding pot you can change the
crucible or remove it for pouring the glass (depending on the design). Either
one is reasonable easy to build. Something you might keep in mind while
deciding is that cobalt is an extremely strong glass colorant and will
contaminate anything melted in the same pot for a long time to come. This may
or may not affect you adversely, as I assume that you will only be melting
various shades of blue (cobalt and copper), but if you melt the copper after
the cobalt it WILL affect the color (it might look nice, too).

Either setup should give you clean glass, the cobalt contamination issue
notwithstanding. Depending on how large a pot you use, an insulating castable
(APGreen Kast-O-Lite 30) and or ceramic fiber insulation should work. You might
check Henry Halem's "Glass Notes" and Dudley Gibberson's "A Glassblower's
Companion" for design ideas.

If you want a cheap (comparatively speaking) and easy way to build this, just
build what is essentially a glory hole with a castable floor and stick a
crucible (or two or three) inside it. A propane venturi burner for heat. It
isn't high tech, but it's easy and will work.

A safety system is always a good idea, especially with propane, which is
heavier than air and therefore has a tendency to "puddle" when there is a leak.

E-mail me if you want any more info.

Brad

P.S. Disregarding Terry's "advice" might be a good idea, since a great deal of
what he wrote either doesn't apply to what you asked (I'm sure you figured that
out already) or is simply wrong. My guess is that you were planning on spending
a little less that $20,000 or $30,000 for this...

****************

> bert weiss wrote in message <37515B11...@bigfoot.com>...

> >I want to build a small furnace for the purpose of making and coloring
> >frit. I work with clear and colored (cobalt or copper) frits. Mostly I
> >am fritting up float glass. I may get interested in coloring or
> >fritting lead crystal but that is not immediate. Propane is my fuel of
> >choice. This is not a 24/7 operation.
> >
> >I have gotten conflicting opinions on what configuration I should go
> >with. One opinion is that I should build a free standing pot furnace.
> >This would allow me to use different pots for different purposes. It
> >would also offer the possibility of removing the hot pot and pouring the
> >glass out. I'm told this setup will give me cleaner glass.
> >
> >Another opinion is that I should build an invested pot furnace. I'm
> >told this would be easier to build, and be easier to run.
> >
> >I am thinking about scaling the furnace to a 55 gal drum or there
> >abouts. I imagine castable refractory is a good route to make a
> >relatively inexpensive durable rig.
> >
> >If I run this rig outdoors, intermittently do I need a safety system?
> >Will a venturi burner do the trick?
> >
> >I'll be interested to hear some opinions on what is the most practical
> >approach from more experienced furnace users and designers.
> >
> Bert
>

Terry Harper

unread,
Jun 3, 1999, 3:00:00 AM6/3/99
to
Brad Shute wrote in message <3755A3B2...@aol.com>...

>
>P.S. Disregarding Terry's "advice" might be a good idea, since a great
deal of
>what he wrote either doesn't apply to what you asked (I'm sure you figured
that
>out already) or is simply wrong. My guess is that you were planning on
spending
>a little less that $20,000 or $30,000 for this...
>
That figure is what you would have to pay if you go to a furnace builder and
ask him for a recuperative single pot furnace, to hold a covered or open
pot, installed and ready to go. A very small gas-fired furnace, top loaded,
is going to cost you some refractory, some steelwork, some combustion
equipment and piping, and some minimal controls (an optical pyrometer is the
absolute minimum). Plus time and effort. If you are going to run it alone, a
programmable controller would be a very wise investment.

Most of what I said is relevant, but without knowing what is "small" it is
difficult to be precise. Your idea of modifying a glory hole is fine, but
you will need to be able to close the front with an insulating door to get
melting temperatures.

The small pots which we made and used were only ever used once. Cost was
minimal anyway. Thermal shock and corrosion are the two enemies to re-use.
Some pots will always leak or break, so you need some grog on the furnace
siege, such as molochite, to prevent the bottoms of good pots from getting
stuck to the floor with molten glass.

Just trying to be helpful, and certainly not trying to mislead.

Bert Weiss

unread,
Jun 3, 1999, 3:00:00 AM6/3/99
to
Terry

I understand your corporate approach to glass melting. My father, who
recently retired, sold optical pyrometers and programmable controllers
of the very finest quality to the glass and steel industry. I still have
access to his engineering pals for advise and equipment maintenance.
The temperature control equipment that I use on my 130 amp electric kiln
is way more sophisticated than I have seen in anybody else's art glass
studio.

I have never seen an optical pyrometer in an art glass blower's shop.
I've never even seen a programmable controller on a gas glass blowing
furnace. I have been assured that if one wants to consistently make a
particular color or expansion value for a glass that this equipment is
necessary. I've still never seen it being used in a small studio. Most
people have a gas valve and an air valve and they open or close them by
feel and look. I've never even seen a thermocouple in a furnace. The
seat of the pants is sufficient to melt and blow glass, particularly
melting cullett.

I plan to run my simple small furnace out doors with a venturi burner so
that I needn't worry about electrical power outages. All I really need
is a gas line from the 500 lb tank that is in my yard to supply the heat
for my house.

I don't plan to spend much money to build this furnace. I need a 55 gal
drum, some blanket, a bag or 2 of castable, some steel (to make and
hinge a lid), and a burner. I will probably end up with a 5 lb pot,
although I would prefer a 10 pounder.

I have been frustrated by how difficult it is for me to get things like
cobalt colored lead crystal. Lenox once offered to make me 500 lb for
$12 a pound. This isn't really practical for a small studio without a
contract for such product. With a small furnace of my own I could spend
a couple of days and make some for the cost of clear cullett, a bit of
cobalt or copper, and some propane.

Lately I have been fusing crushed blue frit between 2 layers on 1/2" or
1/4" glass. One catch is that the 1/4" and the 1/2" aren't compatible,
so they each need their own bucket of frit made from leftover scraps of
the sheet glasses. If I don't use this scrap it has to go to the dump.

I lean toward the freestanding pot design.

Bert

Bert Weiss Glass Studio
http://www.customartglass.com
Painted Art Glass
Custom Productions
Architectural and Sculptural Cast Glass
Collaborative Art Glass
Lighting design


Terry Harper wrote:
>
> Brad Shute wrote in message <3755A3B2...@aol.com>...
> >

> >P.S. Disregarding Terry's "advice" might be a good idea, since a great
> deal of
> >what he wrote either doesn't apply to what you asked (I'm sure you figured
> that
> >out already) or is simply wrong. My guess is that you were planning on
> spending
> >a little less that $20,000 or $30,000 for this...
> >

> That figure is what you would have to pay if you go to a furnace builder and
> ask him for a recuperative single pot furnace, to hold a covered or open
> pot, installed and ready to go. A very small gas-fired furnace, top loaded,
> is going to cost you some refractory, some steelwork, some combustion
> equipment and piping, and some minimal controls (an optical pyrometer is the
> absolute minimum). Plus time and effort. If you are going to run it alone, a
> programmable controller would be a very wise investment.
>
> Most of what I said is relevant, but without knowing what is "small" it is
> difficult to be precise. Your idea of modifying a glory hole is fine, but
> you will need to be able to close the front with an insulating door to get
> melting temperatures.
>
> The small pots which we made and used were only ever used once. Cost was
> minimal anyway. Thermal shock and corrosion are the two enemies to re-use.
> Some pots will always leak or break, so you need some grog on the furnace
> siege, such as molochite, to prevent the bottoms of good pots from getting
> stuck to the floor with molten glass.
>
> Just trying to be helpful, and certainly not trying to mislead.

> --
> Terry Harper
> Acting Webmaster, The Omnibus Society http://www.omnibussoc.org
> E-mail: terry....@btinternet.com
> URL: http://www.btinternet.com/~terry.harper/

--

Terry Harper

unread,
Jun 4, 1999, 3:00:00 AM6/4/99
to
Bert Weiss wrote in message <37572171...@customartglass.com>...

>
>I have never seen an optical pyrometer in an art glass blower's shop.
>I've never even seen a programmable controller on a gas glass blowing
>furnace. I have been assured that if one wants to consistently make a
>particular color or expansion value for a glass that this equipment is
>necessary. I've still never seen it being used in a small studio. Most
>people have a gas valve and an air valve and they open or close them by
>feel and look. I've never even seen a thermocouple in a furnace. The
>seat of the pants is sufficient to melt and blow glass, particularly
>melting cullett.
>
>I plan to run my simple small furnace out doors with a venturi burner so
>that I needn't worry about electrical power outages. All I really need
>is a gas line from the 500 lb tank that is in my yard to supply the heat
>for my house.
>
>I don't plan to spend much money to build this furnace. I need a 55 gal
>drum, some blanket, a bag or 2 of castable, some steel (to make and
>hinge a lid), and a burner. I will probably end up with a 5 lb pot,
>although I would prefer a 10 pounder.
>
Bert

If there is one thing which is vital to making glass it is some means of
measuring temperature. The squinting into the fire method of temperature
assessment is a recipe for failure. Thermocouples help, but never tell you
exactly what you want to know, and are inherently unreliable long term. An
optical pyrometer is the only reliable way of measuring temperature inside a
hot enclosure. It is a good investment.

Inspirator burners work fine, but you should link your air and gas valves if
you want to have some degree of control over your firing. Make the burner
tangential and low down, just above the floor.

I would not bother with the steel top for your oil-drum furnace. Line it
with two types of castable, an insulating outer layer at the sides, and a
2800 grade for the inner face and base. Get a couple of mullite or
sillimanite tiles for the top cover, say 18x9x2, as you will want to move
them apart to regulate the internal pressure of the combustion chamber, and
to get at your pot. Also to allow you to measure the temperature with the
optical pyrometer! The tiles stop radiation which is your main source of
heat loss, apart from combustion gases.

Don't try sticking a bare thermocouple into the combustion chamber, because
it will not give you reliable readings. A sheathed one will help, on the
basis that some information is better than none, but you still need the
optical pyro to calibrate that in terms of furnace temperature. Once you
have tried a bit of control, you will not go back. Borrow an optical pyro
and try. Ask your Dad. In melting, the margin between success and failure
can be very narrow.

MikeFirth

unread,
Jun 6, 1999, 3:00:00 AM6/6/99
to
In article <37572171...@customartglass.com>, Bert Weiss
<be...@customartglass.com> writes:

>I have never seen an optical pyrometer in an art glass blower's shop.

I would agree with this. In the couple of dozen shops I have been in, an
optical pyrometer ($2000+) would be a significant portion of the equipment
budget

>I've never even seen a programmable controller on a gas glass blowing
>furnace.

This, on the other hand, is not true. I have seen several shops with these,
to
control the overnight cooking timing of the batch. On the other hand, usually

the controller is the same one that is controlling the anneallers, so it is
just one
channel of a 5 channel unit. More common is a temperature control with
thermocouple in the furnace and gas control with air follower (or vice versa).

Several of the school systems, which are fully code compliant, have temp
controls,
with or without the programmable.


Mike Firth, Hot Glass Bits furnace glassblowing newsletter
Mike...@compuserve.com Fir...@concentric.net Mike...@AOL.com
Home Page: http://ourworld.compuserve.com/homepages/MikeFirth


MikeFirth

unread,
Jun 9, 1999, 3:00:00 AM6/9/99
to
In article <7j9842$he3$1...@plutonium.btinternet.com>, "Terry Harper"
<Terry....@btinternet.com> writes:

>If there is one thing which is vital to making glass it is some means of
>measuring temperature. The squinting into the fire method of temperature
>assessment is a recipe for failure. Thermocouples help, but never tell you
>exactly what you want to know, and are inherently unreliable long term. An
>optical pyrometer is the only reliable way of measuring temperature inside a
>hot enclosure. It is a good investment.

Let me outline the problem I have with this paragraph.
I will assume when we say "optical pyrometer" we are not talking about the old

calebrated electric filament, voltage supply, etc. where the brightness is
matched,
but we are talking about infrared pyrometers that measure at a distance.
I turn to my favorite source of information, the Omega Temperature Handbook
(free from 1-800-TC-OMEGA, www.omega.com) where I find that a high temperature
infrared thermometer (display on the back of a gun shaped unit) can be had for
$1995 or more and bare units requiring housings and connections can be had for
about $1,000. All of the units listed claim +/-1% of reading plus 1 or 2
degrees F.
Now, 1% of reading at glass temperatures is +/-20 degrees or more. That
means
a temperature measurement of 2040F might be between 2020F and 2060F which
makes a difference in the handling of the glass even in my limited experience.
On the other hand, for $79 I can get a single input Type K digital
thermometer that
is stated to have +/-0.3%+2F of reading and for $140 I can step to 0.2%+2F and
for
$295, I can step to 0.1%+1.8F. That last one translates to about +/-4F at
2050F.
Of course there is that much mentioned drift of thermocouples that can affect

temperature measurements. But what is the drift we are talking about? In the
back of the Omega book is an article on the advantages of the Type N
thermocouple
which has about the same range as the Type K and is proclaimed to have better
durability and less drift. But when we look carefully at the data presented we
find
that while the wonderful N is much better than a K, exhibiting, for example
under 2C
(about 3.6F) at 1077C in 1200 hours (16 gauge wire) and well under 3C at 1202C
for
1200 hours (8 gauge wire) the K type (8 gauge) is only showing on the order of
6-8C
drift over the same time.
In other words, the thick K type is showing about 12-15F inaccuracy as it
ages
well under the +/-20F accuracy of the optical pyrometer at best. Since, in an
economy situation, a furnace could be adjusted using the read out of a
controller
for other devices (an autotune controller for example) the incremental cost of
measuring a furnace with a thermocouple might be $25 for an added heavy
thermocouple ($75 if a thermowell is used) vs $1-2,000 for an optical
pyrometer.
Most glass workers have made the choice to buy 10 or 15 pipes rather than
one
infrared pyrometer.

Neon John

unread,
Jun 9, 1999, 3:00:00 AM6/9/99
to

MikeFirth wrote:

> I turn to my favorite source of information, the Omega Temperature Handbook
> (free from 1-800-TC-OMEGA, www.omega.com) where I find that a high temperature
> infrared thermometer (display on the back of a gun shaped unit) can be had for
> $1995 or more and bare units requiring housings and connections can be had for
> about $1,000. All of the units listed claim +/-1% of reading plus 1 or 2
> degrees F.
> Now, 1% of reading at glass temperatures is +/-20 degrees or more. That
> means
> a temperature measurement of 2040F might be between 2020F and 2060F which
> makes a difference in the handling of the glass even in my limited experience.
> On the other hand, for $79 I can get a single input Type K digital
> thermometer that
> is stated to have +/-0.3%+2F of reading and for $140 I can step to 0.2%+2F and
> for
> $295, I can step to 0.1%+1.8F. That last one translates to about +/-4F at
> 2050F.

(takes off his glass hat and puts on his metrologist's one)

This is an apples & oranges comparison.

First, it is important to understand that Omega makes almost nothing
and simply private labels other peoples' products and sells them at
high markups. (hey, someone has to pay for that 100 lbs of paper
they send you when you request a catalog set.) Those instruments
are either Raytek or Wahl units and are available for significantly
less directly from them.

The 1% of reading specification quoted for the pyrometer is the
overall system accuracy and is typically derived by root-sum-squared
combination of:

* sensor absolute error. (the error inherent in the sensor)
* indicator absolute error. (the error inherent in the indicator.)
* Error in precision of indication. (how precise the indication is
for identical input)
* error in resolution of indication.
* error of repeatability (the spread of indication for subsequent
identical inputs.)
* errors of long term drift.

Note also that the error specifies how far the indication will
disagree with a calibrated standard. It does not represent how
precise or repeatable the instrument is. Both these figures are
typically an order of magnitude better than the absolute accuracy.
This matters because while we don't care so much what the absolute
temperature is, we do care that the temperature at which we find the
glass to work properly be repeated consistently. In other words, it
is important that the instrument hit that sweet mark you
grease-penciled on the display dial.

It should also be noted that as reputable brand names who sell to
the military and GSA, Raytek and Wahl derive their error
specifications using standard procedures. This is not the case for
inexpensive instruments.

The cheap T/C indicator's specification is only for certain
laboratory conditions and even then only applies to the A/D
conversion. I've looked at a LOT of these instruments in my former
professional capacity. Here are some problems:

* Cold junction compensation - top of the list. A precision
instrument connects to the T/C through heavy metal blocks with long
thermal time constants and have cold junction temperature sensors
embedded in the metal. Cheap instruments have stamped metal
connections with some sort of low precision temperature sensor
located somewhere inside the case. Typically a cheap forward-biased
diode.

* Linearization - the temperature vs EMF curve is non-linear.
Analog instruments handle this with a non-linear dial. Quality
instruments handle linearization with either calculated (low
accuracy) or table lookup (high accuracy) linearization. Cheap
instruments do either a) nothing, b) analog approximation using the
transfer curve of a diode to approximate the T/C curve or c) table
linearization using only a few segments, sometimes as few as 2.
Such an instrument can be repeatable but not accurate.

* Reference stability. This refers to the internal voltage
reference that the T/C signal is compared to. Cheap instruments use
either simple external references or worse, the reference built into
many A/D chips. These show both poor long term stability and poor
temperature compensation. The latter is important in a glass shop
because of the extremes in ambient temperature encountered.

* Power supply stability - cheap instruments are typically poor in
this area. As the batteries run down, the reading changes. This
affects both the accuracy and the repeatability (ability to hit your
mark.)

Without individual characterization I'd never rely on a cheap meter
to be better than perhaps 5% of the indication.

> Of course there is that much mentioned drift of thermocouples that can affect
>
> temperature measurements. But what is the drift we are talking about? In the
> back of the Omega book is an article on the advantages of the Type N
> thermocouple
> which has about the same range as the Type K and is proclaimed to have better
> durability and less drift. But when we look carefully at the data presented we
> find
> that while the wonderful N is much better than a K, exhibiting, for example
> under 2C
> (about 3.6F) at 1077C in 1200 hours (16 gauge wire) and well under 3C at 1202C
> for
> 1200 hours (8 gauge wire) the K type (8 gauge) is only showing on the order of
> 6-8C
> drift over the same time.

There are more error terms than that article indicates. There was a
comprehensive article a few months back in the Instrument Society
(ISA) Journal about T/C error terms. One term we have to deal with
in the temperature range we're interested in is the long term drift
cause by crystalization of the TC metal. This article reported
errors as large as 20 deg at the high end of K/N range. This error
can be annealed out using the proper procedure but most users don't
know to look for this error.

Other error terms:

* Unless selected for the furnace environment, the ceramic
insulators can go conductive at the temperature of interest to glass
workers. This shunts off part of the signal, introducing an error
term. This is worse with precision instruments that read the T/C
voltage than with traditional analog pyrometers that actually
operate off the generated current.

* degradation caused by the T/C connector. Especially significant
for cheap connectors that are made from brass instead of actual T/C
metal.

* Errors from the T/C lead wire. lead wire has a much lower error
specification than T/C wire. this is not a problem when the
transition from the T/C to lead wire is close to the same
temperature as the reference junction but in the case of glass
T/C's, the transition point (connector) is likely to be much hotter
than the readout reference junction. One can run T/C wire all the
way to the indicator but most people don't because of the expense.

* Contamination of the lead wire. Typical lead wire for furnace use
is either silicone impregnated fiberglass or ceramic fiber. Either
jacket can be easily contaminated which alters its electrical
properties. This is a significant consideration for batch
operations where metal oxide dust can settle on the wires

In addition to all that, there are the other uncontrolled factors
that can't be generalized. While the optical pyrometer is a closed
system where error terms can be carefully controlled, a T/C system
is distributed with different elements exposed to different
influences. This is a major consideration if one expects to rely on
a T/C indicator for much better than about 5% overall accuracy.

The reason we can do so well with a relatively inaccurate system is
that we learn to compensate, usually unconciously. The apparent
sweet spot temperature may slowly drift with time but we just
mentally note that "the best place is now half a division below my
mark" or the equivalent with a digital instrument. The problem with
this is that when you have to replace a component or even just
change something, you're back to square one. You erase your mark
and experiment to find it again.

Because the optical pyrometer is self-contained and because it is
remote from the furnace, it minimized these problems. Even the
cheaper sub-$500 instruments are still self-contained and thus
exhibit better long term stability than T/C systems. And one can
easily convert even a cheap instrument to a glass-specific
instrument by buying a cheap optical bandpass filter from Edmund
Scientific to grab only the infrared from the actual glass.

The other significant benefit from an optical pyrometer is that
since it sees down into the melt, it naturally averages the
temperature over the cone of vision. The T/C, on the other hand,
measures only the spot where it is located.

Please don't interpret this article as some sort of condemnation of
T/Cs. It is not, though it is a condemnation of cheap instruments.
People, myself included, do quite well with T/Cs. But an optical
pyrometer simply does it better at moderate cost. I wouldn't look
at it as pyrometer vs blowpipes. I'd look at it as pyrometer vs
lost time trying to get the melt right and keep the color correct.

John

--
John De Armond
johngd...@bellsouth.net
Neon John's Custom Neon
Cleveland, TN
"Bendin' Glass 'n Passin' Gas"

Terry Harper

unread,
Jun 9, 1999, 3:00:00 AM6/9/99
to
MikeFirth wrote in message <19990608215241...@ngol05.aol.com>...

>
> Let me outline the problem I have with this paragraph.
> I will assume when we say "optical pyrometer" we are not talking about the
old
>calibrated electric filament, voltage supply, etc. where the brightness is

>matched, but we are talking about infrared pyrometers that measure at a
>distance.


Mike, I am talking about the disappearing filament type. Leeds and Northrup
made the one which is still widely used. Another was made by Hartmann Braun.
If you are looking into a furnace chamber, the temperature will vary quite
widely. The gun-type I-R pyrometers have a target area which is often larger
than the object of interest. A practised operator can get very consistent
results with an L&N type instrument. Another operator may get equally
consistent results, but clustered around a different temperature. However
both will be much more accurate than a thermocouple at some other point in
the furnace chamber, and will also be able to detect drift in the t/c.

Thermocouples are great for controlling, but not for accurately measuring
the temperature of the work piece. In a combustion chamber they have to be
inside a sheath, or embedded in the furnace wall. In an electrically-heated
chamber they are subject to radiation errors from the heating elements,
which may well be hotter than the work chamber, and from openings which
absorb radiation, leading to lower than correct readings.

In a big continuous melting furnace it is fundamental that the operator's
optical temperature readings are taken and recorded. There are a number of
reasons for doing this, one of which is it makes sure that he looks inside
the furnace. Another is the inherent unreliability of thermocouples or
radiation pyrometers. I know that this is about small furnaces, but I
contend that if you only have one method of measuring temperature, it ought
to be an optical pyrometer.

MikeFirth

unread,
Jun 10, 1999, 3:00:00 AM6/10/99
to
In article <375E202C...@bellsouth.net>, Neon John
<johngd...@bellsouth.net> writes:

>First, it is important to understand that Omega makes almost nothing
>and simply private labels other peoples' products and sells them at
>high markups. (hey, someone has to pay for that 100 lbs of paper
>they send you when you request a catalog set.) Those instruments
>are either Raytek or Wahl units and are available for significantly
>less directly from them.
>

Well, the controller I bought direct from Love Controls cost the same as the
Omega labeled identical unit, so I guess I am just a sucker or too small to
get a cheap price. Actually, I would pay for the Handbook for the information
that is in it.


>This matters because while we don't care so much what the absolute
>temperature is, we do care that the temperature at which we find the
>glass to work properly be repeated consistently. In other words, it
>is important that the instrument hit that sweet mark you
>grease-penciled on the display dial.

>The reason we can do so well with a relatively inaccurate system is


>that we learn to compensate, usually unconciously. The apparent
>sweet spot temperature may slowly drift with time but we just
>mentally note that "the best place is now half a division below my
>mark" or the equivalent with a digital instrument. The problem with
>this is that when you have to replace a component or even just
>change something, you're back to square one. You erase your mark
>and experiment to find it again.

Excuse me, "erase your mark", "half a division below my mark" are we talking

about swinging needle analog displays? You give us a printed page of errors
than
can creep in with thermouples, and then we are going to discuss accuracy of a
grease pencil on a swinging needle display (what 100F per mark?) even if we
limit it
to repeatability somehow being much higher than original accuracy? Ceramics on

a TC can become conductive, and cold junction compensation isn't good on
"cheap"
machines and we are talking about the width of a grease pencil mark? Come on!

And we must certainly be talking about more than workability and
repeatability,
because one early point made in this discussion was cooking the batch, where in

it was stated that while cullet might not require accuracy, cooking batch did.
What
kind of consistant results is someone going to get when workability is a grease

pencil mark and cooking the glass at 2 am depends on finding 2450F somewhere
above the mark?

AStick7910

unread,
Jun 23, 1999, 3:00:00 AM6/23/99
to
but we are talking about infrared pyrometers that measure at a distance.
I turn to my favorite source of information, the Omega Temperature Handbook
(free from 1-800-TC-OMEGA, www.omega.com) where I find that a high temperature
infrared thermometer (display on the back of a gun shaped unit) can be had for
$1995 <<<

Funny you brought these up. My husband sells tools- he brought one of these
gadgets hom, and I used it to see if my pyrometer was working right. I wasn't
real impressed by it. This little toy, I believe, retailed for $800. It was
interesting, but not $800 worth of interesting-
A.S.

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