John Baymore on fri 10 apr 98
------------------
With all of the current discussion about pyrometers, I thought I'd forward
a copy of a message from a while ago (see date attached) that resulted from
this type of discussion in the past. This message being forwarded was
itself, actually a selected compilation of other prior messages I had sent
to CLAYART on the subject.
A lot of discussion on pyrometers went on in that time period, and for
those with access to the CLAYART Archives, you will find a lot of info on
pyrometers.
---------- Forwarded Message ----------
From: John Baymore, 76506,3102
TO: Clayart, Internet:Clayart=40lsv.uky.edu
DATE: 8/28/96 8:20 PM
RE: Copy of: Pyrometers again.
Subject: digital pyromoters
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=3Ccut=3E...... Why does a digital pyrometer register inaccurately at higher
temps.? =3Cslice=3E............. =3Czap=3E At bisque temps my pyrometer =
reads
pretty close to accurate (1830)-but at =5E10 it reads 2260. Any ideas? =
=3Csnip=3E
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Tom,
The accuracy of a pyrometer is affected by a LOT of things. The main
corollary is basically the more you pay, the more accuracy you get. About
a month ago there was a long, long discussion about this subject. Here is
what I could quickly find and =22cut and paste=22 together from my own past
comments...... you could look at the old =22Digest=22 to see those of =
others:
----------------------------old messages complied--------------------------
One factor is the =22quality=22 of the meter itself...... the accuracy of =
the
electronics. An inexpensive meter is pretty inaccurate. A meter rated at
+/- 2=25 of full scale at 2400F (a pretty good analog meter) is still +/-
almost 50 degrees. So if it reads 2300 you only know it is somewhere
between 2250 and 2350.
-----------next post--------------------
This thermocouple, pyrometer, and controller thing is sort of like the
discussion that's been going on about barium and glaze chemistry.......
there is a lot more to it than first meets the eye. Now we're leaning into
the realm of kilns......... as you know, that' s an area in which I have
some expertise =3Cg=3E.
Many of the things we =22assume=22 to be true from =22experiential logic=22 =
are
simply not. A pyrometer seems like such a simple thing on the surface.
Stick a probe in the kiln and attach a guage to read the result. Yet
there are complexities untold in these devices. The pottery suppliers
haven't helped the situation much either. They have us expecting a cheap
=22magic bullet=22 in these things.
The controller on your kiln uses type K thermocouples, most likely, like
99.9999999999=25 of all equipment supplied to the craft potters. The =
Seebeck
coefficients (temperature change related to voltage change created) for
Chromel / Alumel type thermocouples (K) goes somewhat non-linear at the top
end of the use range. Seriously oversimplified, this means that one unit
temp change no longer results in one unit voltage change. That makes the
software for the controller a bit difficult for the programmer, and the
conversion from microvolts to degrees gets less accurate as the temp goes
up.
For example, the Seebeck for the 50 degree interval from 600C to 650C
changes .3 microvolts per degree Centegrade. As you get up to 850C to
900C, it changes more, from 40.5 uV/dC to 40 uV/dC....... a change of .5
microvolts. From 1150C to 1200C it goes from 37.2 uV/dC to 36.5 uV/dC, a
change of .7 microvolts. From 1200C to 1250C, it changes .8 microvolts.
The software has to take this increasing =22curve=22 into account.
Worse, the ASTM recommends type K for use only up to a max of 1260C. So a
firing to cone 10 (150C/ hr....... 9 hours+/-) is pushing the absolute
limit of their usefulness by a great factor...... 105 degrees C over the
max. use rating of the sensor. So they simply should not be used for cone
9-10 kilns. Yet they are all the time.
Continuing this complex picture, that rating is for PROTECTED (more on that
later) thermocouples of AWG (wire thickness) of 8 ......... yes =
EIGHT=21=21=21=21
(pretty darn thick) The maximum use rating goes DOWN to 1090C (1970ish F)
for AWG 14 (look at household 14-2 wire for a size comparison), 980C for
AWG 20, and 870C for AWG 24.
So...... what guage (AWG) are your thermocouples? I think your kiln's
manufacturer uses 8 AWG...... they do on the single pyros they sell. Not
sure on the controllers though. Look at the household wire....look at your
thermocouples. If it is AWG 14, it is only recommended to be accurate by
ASTM (within all the other constraints) for use up to about cone =
4=21=21=21=21=21=21
Enlightening, huh?
Next, you have to look at the design accuracy of the manufacturing process
to start with. ASTM specs that the initial calibration of type K is +/-
2.2C or .75=25 (whichever is GREATER) between 0 and 1200C. That is the BEST
it will ever be....when it is brand new. At 1200C, that allowable .75=25 is
+/- 9 degrees. At cone nine, plus or minus 9 degrees C is about 1/3 a cone
(ignoring the heat-work concept for simplicity)=21
Plus, you have to deal with the accuracy of the meter (or controller). Good
analog meters are only accurate within 2=25 of full scale to start with. If
full scale is 1300C then it is only accurate +/- 26 degrees C, and THAT is
of the thermocouple output which is also only +/- 9 degrees C=21
Additionally, type K is subject to oxidation when used IN AIR above 750C.
This oxidation results in the electrical output changing, and the need for
constant recalibration. This recalibration requires some electronic
equipment that costs a lot of =24, and some skill to use. (Industry
recalibrates on a regular schedule to maintain precise production control.)
Type K should not be used in reduction atmospheres or sulpherous ones, so
ASTM also says they are best used enclosed in a sealed tube with an inert
gas........ (a thing you'll never see in a potters kiln=21) BTW..... A
vacume on the protection tube is also a no-no.
In general, for cone 9-10 use, type K is useless for REALLY accurate work
at the top of the temperature range. They are used on =22potters kilns=22
simply because they are CHEAP, and most potters won't pay for the platnium/
10=25 platnium-rhodium (type S) thermocouples and control systems that =
really
should be used (and the more expensive plated switches and contacts that
entails). Type K simply are more resistant to oxidation than the really
cheap types E, J,and T at temps over 500C, so they got pressed into use.
So you DO just get used to the fact that the temperature shown is not
really the temperature in the kiln (pretty bad since that is what you are
lead to beleive you are paying for=21 :( You also get used to the
fact that the accuracy will change with time. You =22work around=22 the
inherent inaccuracy. So all the fancy =22scientific=22 sounding precision =
of
these controllers supposedly offer is subject to a lot of variables.
What these controllers SEEM to promise (incredible accuracy), CAN be done,
but not at the price point that they have to occupy to meet the budget of a
studio craftsperson. Industry has much better stuff.
YET.................................., it is still far better than the old
=22seat of the pants=22, look in the kiln, sniff the wind, discuss it for a
half-hour, throw salt over your shoulder, stuff we used to do (I fire a
wood kiln....I still do all that stuff=21=3CBg=3E).
Luckily, most potter's applications are not all that critical at the top
end of the firing for cone 9-10 work. So the manufacturers get away with
type K in most cases. If you are getting into trouble and need more
accuracy, you will have to change controllers to one that uses the platnium
thermocouples. And it'll cost you some =24=24=24. But from the sound of it=
in
the thread, you just need to understand the limitations of the tool, and
work with it.
PS: If your readings are that far off and you DO have type S (or type R or
B) thermocouples, call the kiln supplier. Something is really off there
with the controller.
---------next post -------------
You got good info from whoever you talked to. Yes, you will need to
=22match=22 the circuit again to make the readings accurate (or at least as
accurate as they can be =3Cg=3E....... see other post). If you are not into
this =22tech weenie=22 type stuff, you probably will do best to have your
supplier do the math and the conversion for you. They gave the right
answer, so they probably know how to do it correctly.
The meter you have needs to see a specific resistance to electron flow in
the overall circuit to be accurate to it's calibrations printed on the face
of the meter. If you change the length of the wire, the total resistance
changes. So if you didn't do anything else, the readings would change. So
you add a resistor of the correct value (Ohms) into the circuit to adjust
the resistance the meter =22sees=22. You will have to add the correct value
resistor to the circuit either in series or parallel to make the total
resistance match what the meter was designed to see.
Resistors in series with the circuit are simply additive in value. Rt =3D =
R1
+ R2 + R3 + .........Rx. Resistors in parallel are a little different.
The following expresses that relationship for two resistors in parallel:
Rt =3D R1R2 / R1 + R2.
So here is some more info about pyrometers to help out (or confuse =3Cg=3E) =
in
the process:
You will also need to use the correct wire to extend the length. You can't
just use copper wire (or anything else). If you do, the place where you
join the thermocouple lead and the copper wire becomes another pair of
thermocouples=21=21=21 These junctions of dis-similar metals will create =
another
millivolt signal that will combine with the other =22real=22 signal (either
plus or minus depending on polarity....or =22direction=22..... of electon
flow). Worse yet, there will be TWO new thermocouples (each junction).....
one Copper / Alumel and one Copper / Chromel each producing different
signals=21 As the the temperature at this junction (probably the outside =
air
around your kiln) changes, the signal will change.
Also note that you need to use a junction between the extension wire and
the thermocouple that keeps both legs of the comnnection made out of the
same materials. If you don't go directly wire to wire (which is a bit of a
bad idea..... more in a second), but put a terminal block in the circuit,
it's contacts need to be rated for type K (or whatever). However, if you
use some sort of pressure connection that is made of copper (or whatever)
that presses the two wires directly onto each other, you can use other
metals without any appreciable problem. Otherwise you create more little
thermocouples=21
Now, on the the need for a terminal block between the extension wire and
the thermocouple. If you use a thermocouple extension wire that is not the
exact same composition as the thermocouples (=22extension=22 grade wire is
slightly different than =22thermocouple=22 grade wire for type K), there are
those two undesirable =22thermocouples=22 formed at the junction. They are
pretty =22poor=22 thermocouples since the metals are not really all that
dis-similar. But they do add error. If each of the legs of the wire is at
a different temperature, that complicates the inaccuracy=21 So you want a
terminal block designed to conduct heat to keep the two legs of the
connector at about the same temperature. This is called an =22isothermal
block=22, and it's job is not only to provide a mechanical connection, but =
to
keep the junctions at the same temperature.
While we are on the subject of the connections between the thermocouple and
the extension wire............ a pyrometer functions by measuring the
difference between the hot junction (the tip of the probe in the kiln
chamber) and the cold junction (usually thought of as the the connection
point to the meter....or where the system goes to copper wire). As the
temperature of EITHER the cold junction or the hot junction changes (or
both), the reading changes.
In other words, to be as accurate as possible the cold junction needs to be
kept at a specific reference temperature=21 That reference temperature in
industry is called the =22ice point=22, cause it is precisely 32 degrees F.
You could keep the cold junction in an ice water bath =3Cg=3E. Industry =
uses
an electronic means to do this same thing...... but it costs =24=24=24. It =
uses
a resistance temperature probe to measure the temperature of the cold
junction and then inserts a corrective voltage into the signal to make the
system =22think =22 the clod junction is at 32 degrees F. (this is called
hardware compensation) Unless you want to spend the bucks, all us potters
need to know is that if the cold junction gets hot, the reading on the
meter is not accurate.
(Industry now uses software compensation more often for the cold junction
via computers. It doesn't correct the voltage, it just knows how much the
signal is =22off=22 and compensates for it in calculations. Very little
industrial temperature measurement is done with anything but digital
systems now. The =22ice point=22 system is going the way of the dodo bird.
Analog meters are pretty ancient technology in industry.)
This need for like metals in self-powered systems (cheap analog) is true of
any switches placed in the system too, unless you start to use some more
electronics (no longer self-powered). There are special switches made for
switching each different type of thermocouple lead. Since the switches are
made of the same alloy as the wire, the temperature issue is not important.
So you need to buy what is called (appropriatly enough) =22thermocouple
extension wire=22. It is made of the same stuff (basically) as the
thermocouples themselves...... at least for type K. So first of all, you
need to know the type of thermocouples you have. Probably, they are type
=22K=22, which is a combination of Chromel and Alumel (synthetic alloys). =
The
meter face may say that right on it. Then you need to get extension wire
for that type of thermocouple. Your supplier probably can sell it to you
in short lengths.
For the type of insulation you need on the wire, you should look at the
environment it will be in (is it mostly in the open, air cooled, or is it
mostly enclosed). If it is very hot, you'll need a wire that has
insulation that will stand that level of heat. I have used a polyvinyl
chloride insulated wire in most kiln applications where I can route the
extension wire well =22in the clear=22 and it has worked fine. It's good =
for
220F temps, has good flame resistance, and is pretty flexible. The next
grade goes to teflon insulation, which should be good for most any =22potter
type=22 applications. It is good to 500F. ( You can even get extension =
wire
with a ceramic fiber braid for insulation that is rated for continious duty
at 2000 degrees=21 BIG BUCKS=21) You supplier probably has the right
insulation on what they get, unless your installation is very atypical.
The guage of the wire is sort of important. Use the thickest guage you can
afford (the thicker the more expensive). The signals in these simple
unamplified systems are extremely low....... you don't want to complicate
that with small diameter, high resistance wire. Plus it is possible for
the resistance in the wire to be great enough when compared to the
resistance in the thermocouple itself, that the meter starts to read a
=22false=22 thermocouple..... the wire, not the real probe. I usually use =
AWG
14 stuff called =22EXPP-K-14=22 from Omega Engineering. It is an alloy that
works with type K thermocouples, but is what is called =22extension =
grade=22.
The code number stands for EXtension grade wire, PPolyvinyl Chloride
insulation, type K wire, 14 AWG thickness.
Next the issue becomes the resistance of the wire to electron flow.
Generally, the thicker the wire, the less resistance. To calibrate your
=22system=22, you'll need to know the resistance per foot of the wire.
Electrical resistance is measured in units called Ohms. The EXPP-K-14
mentioned above has a resistance of 0.146 Ohms per double foot (one foot
running length, but two sections of wire) at 68F (=2ANote-see below). So =
ten
feet would have a total Resistance of 14.6 Ohms. So you will need to know
how long a piece of wire you are using also.
=5B=2A NOTE: As I mentioned in the other post, if you are dealing in fine
accuracy this stuff gets complicated beyond belief. The actual resistance
of the extension wire changes with the temperature of the wire=21 So the
accuracy is affected slightly as the temp of the wire changes from 68F. By
itself, not a significant change for potters, but just another variable
that affects the accuracy picture discussed in the other post.=5D
Another interesting fact to know is that if you route a long length of
extension wire parallel to and close to a live AC line, you can create a
small signal in the extension line by what is called capacitive coupling.
Since we are dealing with desirable signal levels of only millivolts here,
this is a concern. In the =22trade=22 this type of erroneous, induced =
signal
is called =22noise=22. So route your extension away from AC lines, and
preferably at right angles to them if they have to come close.
Next you need to know the resistance the meter likes to =22see=22. This may=
be
printed on the meter as =22external resistance=22. Might not be. If not,
you'll need the help of the meter supplier. You need to know this to get
the new system (meter, extension wire, and thermocouple) to be the same
resistance (Ohms) as the original meter was designed for.
Pyrometers are really just millivolt meters or milliamp meters. They
measure the =22pressure=22 of electricity in the system, like a gas guage or
the volume of flow. The scale that is printed on the front is really a
conversion from electrical values to degrees that the manufacturer worked
out for us.
So that is a bit more on pyrometers.
--------------------end of old messages -------------------
So hopefully that should help you out Tom.
.........................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086
JBaymore=40Compuserve.com
http://www.CraftWEB.com/org/jbaymore/rivrbend/shtml
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