John Baymore on tue 16 jul 96
From: "Shelley S."
Subject: Re: Small cones vs. Controller
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John,
Speaking of thermocouples, pyrometers, etc., on a simple analog pyrometer for a
gas kiln, .........if I want to change the length of the wire, I was told I
would need to change the resisters in order to recalibrate it for the new wire
length. Is
this true? Is there something else I would need to do as well?.....
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Shelley,
I am not sure "simple" is quite the right term for any pyrometer after the last
post . Or this one either . I am probably going to give you more than
you need to know. But hopefully it will be of some use.
First of all, my brain is doing some "short circuit" stuff lately. The line
"......... the conversion from microvolts to degrees gets less accurate........"
in my last post should have said "millivolts" not "microvolts". A bit of a
decimal place error there . Also the "uV" should have been "mV".
You got good info from whoever you talked to. Yes, you will need to "match" the
circuit again to make the readings accurate (or at least as accurate as they can
be ....... see other post). If you are not into this "tech weenie" 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 "sees". 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 = R1 + R2
+ R3 + .........Rx. Resistors in parallel are a little different. The
following expresses that relationship for two resistors in parallel: Rt = R1R2
/ R1 + R2.
So here is some more info about pyrometers to help out (or confuse ) 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!!!
These junctions of dis-similar metals will create another millivolt signal that
will combine with the other "real" signal (either plus or minus depending on
polarity....or "direction"..... of electon flow). Worse yet, there will be TWO
new thermocouples (each junction)..... one Copper / Alumel and one Copper /
Chromel each producing different signals! 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!
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 ("extension" grade wire is slightly
different than "thermocouple" grade wire for type K), there are those two
undesirable "thermocouples" formed at the junction. They are pretty "poor"
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! 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 "isothermal block", 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! That reference temperature in industry is
called the "ice point", cause it is precisely 32 degrees F. You could keep the
cold junction in an ice water bath . Industry uses an electronic means to do
this same thing...... but it costs $$$. 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 "think " the clod junction is at 32
degrees F. 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
"off" and compensates for it in calculations. Very little industrial
temperature measurement is done with anything but digital systems now. The "ice
point" 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 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) "thermocouple extension
wire". 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 "K", 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 "in the clear"
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 "potter type" 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! BIG BUCKS!) 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 "false" thermocouple.....
the wire, not the real probe. I usually use AWG 14 stuff called "EXPP-K-14"
from Omega Engineering. It is an alloy that works with type K thermocouples,
but is what is called "extension grade". 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 "system", 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 (*Note-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.
[* 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! 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.]
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
"trade" this type of erroneous, induced signal is called "noise". 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 "see". This may be
printed on the meter as "external resistance". 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. They measure the "pressure" of
electricity in the system, like a gas guage. The scale that is printed on the
front is really a conversion from millivolts to degrees that the manufacturer
worked out for us.
So that is a bit more on pyrometers.
..............................john
John Baymore
River Bend Pottery
Wilton, NH
76506.3102@Compuserve.com
Shelley S. on tue 16 jul 96
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John,
Thanks for your response. Just a few more questions from the pragmatic end of
things. As you say, my supplier had the right idea. However, they gave me
resistors good for ten feet of wire, assuming this would also be ok for 6 feet
of wire. Is this true or do the wire and resistors have to be an exact match?
Can you have resistors with greater capacity than the length of the wire? Re
your comments, the thermocouple/wire I have is Chromel/Alumel. Rather than
extend the wire, I just planned to replace it, since it seems less complicated.
Also, excuse my ignorance, but when you say the resistors are in series or
parallel, is that the physical location of them?
Thanks again.
Shelley
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