Rev. ... 2002-03-11; 2003-02-26, 2004-01-08, 2005-03-10, -04-20
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The three most common methods of measuring temperatures involved in furnace glass work are infrared radiation (IR), thermocouple (TC) and resistance temperature device (RTD). IR is very nice but, especially in the temperature ranges of molten glass, very expensive $1100-1200 (Omega). RTD's are limited to annealing temperatures (1200F)
THERMOCOUPLES & THERMOMETERS Rev. 7/4/94 2001-07-31
[Moved in from annealer.htm and
modified]
A thermocouple is two different metals, usually wire, welded together to produce a junction that works in reaction to the other end of the wires. When the junction is heated (or cooled), a voltage is produced along the wires because of the change in temperature.* Any pair of metals will do this (the reaction is part of what causes corrosion), but some do it much better (higher voltage, better linearity.) Because the effect will occur with any metals, care must be taken in wiring and switching thermocouples to avoid introducing additional voltages by creating other junctions. [This site, Thermocouple Calc, will calculate voltage vs. temp and vice versa for a number of different kinds. This site, Thermocouple Technical Reference, has tables of information including millivolt outputs for a wide range of temps. 2005-04-20]
Thermocouples are given letter identifications: J, K, etc. For
the highest temperatures, platinum (Type R, S or B) is needed ($125-400
each.) For annealers, kilns, and glass furnaces, K type are used,
although they need protection at furnace temps or get eaten up
over a couple of months and lose accuracy, although the amount of
loss seems small to me. (At $16-24 vs $125-400, a lot can get
eaten.) Higher temperature ratings go to thermocouples with more
material (thicker wire) because the failure mode is oxidation of
the iron in the type K. Type K is made of Chromel and Alumel. A
type S is Platinum and Platinum with 10% Rhodium. I have not used
type R.
K type is connected with special yellow (+) and red (-) insulated wire with the metals in the wire matched to metals in the thermocouple and I use a standard mini-connector (right) on my devices that lets me also use commercial thermocouple probes and measuring devices. Type K thermocouples are typically $16-25, connectors $5.50 a pair. (Each connector has two flat blades, one wider than the other. In the image a blue coated small wire thermocouple is attached to the plug at the top and the yellow cased red and yellow lead wires are attached to the socket at the bottom.)
VOLTAGE
OUTPUT - Thermocouples put out varying voltages with temperature and the
change over each ten degrees also changes. For this reason, some kind of
conversion must be done or the temperature reading will only be approximate.
Also, wiring must be carefully done because the voltages are small.
K & S type thermocouples are defined above.
The upper graph shows the line of millivolts vs. temperature in Celsius. The red line is a K type. The green line is S type. S type output is about 1/5th K type. The blue line is S multiplied by 5 to show the different shape of the curves.
The
lower graph shows the difference in mv at each 10 degrees with the S type
multiplied by 5. The K type is much closer to level, but has a funny
little dip at roughly 190C. The S type has constantly increasing millivolt
output for each 10 degrees. 2005-04-20
METERS - In order to measure temperatures, I looked at analog and digital pyrometers. Analog uses a dial with a moving needle, while digital displays digits (numbers.) Less expensive analog meters with little adjustment can be used with K-type at the temps of annealing, sagging and fusing because of the levelness of the curve above. I chose digital for convenience and accuracy and durability, since reading most analog displays to closer than ten or twenty degrees is guessing after squinting at the needle if it hasn't broken when dropped. I looked at dedicated temperature measurement units as well as adapters. I decided to buy an adapter that uses a digital voltmeter for the display. [This is early. I have also bought volt meters with K type temp reading, which have come down considerably in price, to $39.99, as time has passed. And a K-type C only for $29.99 here 2004-01-08]
Costing less than the cheapest digital thermometers, but more
than analog, the adapter includes amplification and compensation
so the reading in millivolts is equal to temperature and the
voltage is read with a cheap voltmeter. I use the adapter with
the lowest cost Radio Shack Digital Voltmeter ($17 on sale.) The
TA-1A adapter is by A.W.Sperry and costs $66.67 from Grainger
with shipping, [Grainger has since dropped Sperry. Call 800-645-5398
for source.] A single purpose temperature meter is about $110. [2001-08-16
Omega www.omega.com has come
out with an $88 analog to voltage adaptor SMCJ-K
which looks to be a better layout than the TA-1A. which connects
to some meters by directly plugging into the meter's banana
jacks, but needs an adaptor for cheaper ones. Omega also has a
neat, but $249, talking DMM
with 2 K-type inputs.] [Thermocouple
Converter TMT-80T-K
$70.50 another
product] [TA-1A
SITE] [2005-03-10 More recently, meters costing as little as $39.99
have come on the market - on sale - with a K-type jack. 2005-03-10]
Note that it is also possible to build your own converter using a chip sold by Analog Devices which is discussed here.
The TA-1A adapter comes with a thin bead K-type probe rated to 1400F, except that the Teflon insulation is only rated to 500F. I started using it for testing (and eventually ruined the insulation) while waiting for more serious thermocouples. If only the tip is touched to 1400F presumably it would work. For higher temperature work, a TC made of 16 or 14 gauge wire has a higher rating. Since the thermocouple voltage is created along the temperature gradient, the connection at the tip only has to be okay, not perfect and broken thermocouples can be welded.
Following a suggestion from someone at Spruce Pine Batch, I ordered $8.50 [now $16] K-type probes from A.R.T. Studio Clay Co. [which is also a source for the large wire porcelain terminal blocks that mount on the thermocouple wires, that Omega does not sell with their heavy TC. 2004--01-08] Neither Grainger nor A.R.T. carry K thermocouple wire separately, so I ordered that for $8.50 for 6' from Seattle Pottery Supply. The adapter uses (as do as most digital thermometers) a standard mini-connector, $7 for 2 from Grainger. For a thermocouple to work most accurately, all the wiring and all the connections should match the metals in the thermocouple. That is why special wire and connectors are needed. Wire and connectors are also available from Omega. [a much better source, 1-800-826-6342]
WIRING NOTE - Many places mention the need for care in wiring and switching but it takes some digging to understand how much care is needed and why. The why is that the thermocouple effect produces small voltages (21.493 mv is specified for 540C, 44.439 mv for 1080C - that's 0.044439 volts) so that it is easy to mess up the reading if other voltages are created. Connecting thermocouple wire to copper produces a new junction. How to avoid the problem? Keep all the pairs of new junctions at the same temp. At ultra precise labs, this is done with thermal junction blocks. In more practical situations, it is likely that the left-hand terminals of a switch or relay are not much different in temp than those on the right. So, if thermocouple wire is connected to a switch (copper or alloy) and a junction is created, the reverse junction is created at the output a fraction of an inch away and the effect is canceled. This clearly does not apply if the iron in the thermocouple is connected to copper wire which is then run several feet (or more) and connected to iron wire again. Thus, special connectors and wires are available, but special switches are in short supply.
Now that I have used the adapter, and the least expensive digital voltmeter from Radio Shack, for some time, I have found it stable and reliable. I have compared its readings with cones and other thermometers and checked the reproducibility of the readings.
The error specified for type K from 559F to 2282 F (293C-1250C) is 0.75% (while R is .25% and is more stable.) 0.75% is a small number, but taken times a large number produces a moderate number, in this case 15F degrees of error at 2000F.
"Iron rusts at low temperatures where condensation can form. Type K (nickel-chromium (Chromel) vs nickel- aluminum (Alumel)) can be used up to 2,300°F in an oxidizing or inert atmosphere. Type K thermocouples exhibit a number of instabilities and inaccuracies, particularly at higher temperatures, changing their emf/temperature characteristics. In reducing atmospheres (lack of oxygen) at temperatures of 1,500 to 1,750°F the positive thermo-element forms a greenish chromic oxide, commonly known as "green rot". This causes a decrease in the electromotive force of the thermocouple. Type K is also subject to aging when exposed to 800 to 1,200°F for a few hours. Temperature cycling above 1,400°F and then below 700°F causes a random error due to changes in the composition (inter-granular structure) of the conductors. " http://www.isa.org/~sarni/temper.pdf.
FLAME DETECTOR - When plain wire is heated
at one point, the thermocouple effects in each direction cancel.
If one could imagine somehow making a metal free connection at
the hot point of the wire, one could get the thermocouple effect
of just one kind of wire between the connection and the end of
the wire. It turns out exactly that can and is done in one kind of flame
detector.
In a furnace, it is important to detect if the flame has
actually ignited, so the furnace does not fill with an explosive
collection of gas and air. One way this is done is to put a the end of a thermocouple wire into the place where the flame is supposed
to be. A hot flame is ionized, so it will conduct. So a voltage
can be created between the burner head (if part of it in the
heated area is metal) and the wire. A sensitive circuit can
detect the voltage, or failing to find it, shut down the gas
supply. 2001-08-27
Thermocouple question
Posted By: Brody brodyshawglass@charter.net
Date: 2/25/2001 - 1:58 p.m.
Hello one and all! Just a quick question...What's the life
expectancy of a K type thermo in a furnace? Now before everyone
shoots off at me, hear (or read) me out. First I know it's gonna
fry. But in how long? 10 sec, 30 sec, maybe a minute or two? I'm
trying to get a temp. reading on my furnace, and looked over at
an old kiln with a pyrometer on it. And thought I might be able
to use it for a "spot" type reading of the temp. I was
thinking of using a glove and holding it just inside of the door,
MOMENTARILY. Like maybe 20-30 secs max. Will this work, or am I
just kinda foolin' myself? Thanks for the answers (go nice!) -Brody
The problem with K type thermocouples is not how long they
will last. I have a couple that I have had in my gloryhole for
hours at a time and they survive quite well thank you. They are
big wire - 8-11 gauge. Small wire will oxidize rather quickly and
be damaged to the breaking point.
The problem everyone talks about is that they drift and become
more inaccurate. My bone of contention is "How much do they
drift and how inaccurate?" The answer turns out to be about
10-20 degrees. For people who are trying to do precision work,
this is too much. For furnace workers, it is less than the
variation from putting the thing in various locations.
The real problem is that for big wire thermocouples, the time it
takes to make a measurement is looooonnnnng. Your hand (or glove)
will fry if you try to hold it. Long is 20-30 seconds before
settling. Also, if the thermocouple has porcelain insulators as
mine do, you can crack them off by heating too fast. You would need to
ease it into place.
--
Mike Firth