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Hello everyone. We had a pretty successful raku firing today. We did many test and it seems that our pyrometer could be off by 400 degrees. Has anyone else experienced this? When I called the company I purchased it from, they said they had never had one that didn't register correctly. It seems that at 1800 the glazes are not hot at all, by 2000 they start to bubble, and by 2250 degrees they smooth out. When we opened the kiln all the glazes looked burnt, not glowing (daytime), but once out of the combustibles they were beautiful.

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I have four pyrometers and they all read very differently but not 400 degrees off.

I have 2 digitals company, one has 2 probes. Even the two probes on the same pyrometer read differently when next to each other. It is frustrating.

Marcia

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Any piece of equipment can have manufacturing flaws. could you have a bad, yes. one way to test it is to use witness cones during your firing and see if the cone temperature is about same as the pyrometer temperature. Some pyrometers are adjustable. This could at least show about how far off the temperature is.

Good luck

Tom

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Guest Herb Norris

Did the manufacturer offer any suggestions as to what MIGHT be the problem?

If this were happening to me, I would replace/adjust each link of the "chain" in the temperature monitor process.

It starts at the thermocouple, so I would replace that first, keeping all other parts of my firing routine the same. A dropped or defective thermocouple could be the problem.

If the replacing the T/C yields the same results, I would replace the wire next, using the same thermocouple.

If you had another pyrometer that you know is accurate you could run it in parallel with yours, and see what happens.

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The manufacturer did not suggest anything. I will try using the cones again. The last time they did not bend. The manufacturer said that cones don't always work well when a kiln is being fired this quickly. I will call him again today. Thanks.

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Guest JBaymore

A few questions........

 

Is the pyrometer analog (a dial face with a needle) or digital?

 

What type of thermocouple does the pyrometer require? (Some digitals are autosensing in that they can adjust automatically to different termocouples, some require a manual setting change.)

 

What type of thermocouple are you using? (Type K, type S, type J etc.)

 

What type of insulkation in on the thermocouple? (bare wire, ceramic, ceramic double hole, ceramic double insulators, etc.)

 

Are you using a thermocouple protection tube around the thermocouple? If so, made of what material and what diameter?

 

If you are using an extension wire between the meter and the thermocouple, what exactly is it? (Supplied with the thermocouple, a piece of copper wire, and so on)

 

How far into the chamber away from the refractory wall is the tip of the termocouple projecting?

 

At the point where the thermocouple connects to the extension wire, how hot is that point getting?

 

At the point that the thermocouple (or extension wire) is connecting to the meter, how hot is that point getting?

 

 

All this stuff can help us maybe determine "what's up".

 

best,

 

....................john

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Guest JBaymore

I have four pyrometers and they all read very differently but not 400 degrees off.

I have 2 digitals company, one has 2 probes. Even the two probes on the same pyrometer read differently when next to each other. It is frustrating.

Marcia

 

 

 

Marcia,

 

The pyrometers that most potters use are relatively inexpensive units and don't typically get calibratet regularly like industry does. So this is not all that surprising. And unless the unit is self-calibrating, as a thermocouple is used, its resistance changes as the metals deteriorate..... thereby changing the temperature readings.

 

Analog pyrometers are a bear in thie reguard.

 

For Orton cone 9-10 range firing, Type K thermocouples are not even recommended for use since they go quite non-linear near the end point range. It is over their top end use range. Digital pyros software usually compensates for this, but the thermocouple is deteriorating pretty quickly at those temps. And reduction is not good for the thermocouples.... they should be in protedtion tubes to help prevent rapid deterioration.....by many potters don't do this. (The protection tubes add in a bit of thermal lag on the termperature reading too.)

 

I'm using a rather expensive Omega digital unit on my noborigama (with multiple probles connected). It is quite accurate when I calibration test the thermocouples. Unfortunately some of this is the ole' "you get what you pay for".

 

Luckily, most of what craft potters do is not all that demanding as far as truly accurate temperature reading is concerned.

 

best,

 

....................john

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My pyrometer is an analog and says type K on the digital face. The company I purchased it from told me not to adjust it.I guess that depends on the elevation you are at? I realize that I sound ignorant, because I'm not even sure what part is called the thermocouple. There is a ceramic piece around what I call the" probe" that goes into the spyhole. I place it as close to the ware as possible about 4-5 inches into the kiln. There is a square part on the bottom of this probe with 2 holes and wires going into the probe. This gets hot. Next time I fire I will check to see if the wire near the meter is hot.

 

 

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Guest Herb Norris

My pyrometer is an analog and says type K on the digital face. The company I purchased it from told me not to adjust it.I guess that depends on the elevation you are at? I realize that I sound ignorant, because I'm not even sure what part is called the thermocouple. There is a ceramic piece around what I call the" probe" that goes into the spyhole. I place it as close to the ware as possible about 4-5 inches into the kiln. There is a square part on the bottom of this probe with 2 holes and wires going into the probe. This gets hot. Next time I fire I will check to see if the wire near the meter is hot.

 

 

 

 

 

Centered, I guess we're all "ignorant" until we get schooled!

The elevation does not enter into the temp reading or accuracy of the pyrometer.

The thermocouple is actually inside the ceramic tube that makes up your probe; i guess technically the whole assembly is more properly called "the probe" but we all tend to call the whole thing "the thermocouple."

Anyway, the thermocouple is really pretty delicate, and should not be banged around, dropped, stepped on, or abused in any way. It should be babied to continue to be reliable. If it IS dropped or hit with shelves, ware, etc. it would be suspect, and might be the source of temp errors. That's why I suggested replacing it with a new or used one that you know is accurate, if you can do it.

As far as placement, you might be putting it too far into the kiln, which wolud be a source of errors. The probe only needs to project about 2 - 3 inches into the kiln for an accurate reading, and the place where the wire attaches to the probe should be insulated from the heat as much as possible. Under no circumstances should the wire itself ever go into the kiln, nor should the spot where the wire attaches to the thermocouple.

It just takes some experimenting/fooling around to get at the problem; with JBaymores help, you're bound to fix it! Seasoned Warrior could help too, I wonder where he is hiding?

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Guest JBaymore

Ok.... I was figuring it likely was an analog type. That explains a lot.

 

There are some REALLY good and relatively accurate analog units out there,........ but thay are typically NOT the ones that craft potters use. I have an ancient horizontal scale industrial Honeywell unit that probably cost $500 "back in the day" that is really sweet in its accuracy. It has a number of calibration functions built into it.

 

BUT... most of the cheap analog pyrometers that are "in circulation" in the pottery community (let's say in the sub $100 dollar range) are not very accurate as an inherent part of the product itself. If you look at the meter face, somewhere in VERY tiny print in a corner might be something that says the accuracy as a percentage of full scale deflection. So if it says 2.5% of full scale (typical)... and the scale on the unit goes from 0 to 2500 (typical) that means that when it says 1800 degrees F, it is somewhere between 1737.5 F and 1862.55 F. That is if everything ELSE that can affect the temperature is "perfect". Likely it is not.

 

Such meters are useful only for the most vague "it is getting hotter or colder and approximately how fast in big round numbers" type of uses.

 

Then there is the likely lack of what is called "cold junction compensation". A thermocouple is nothing but two dissimilar metals welded together at one point. The dielectric properties of this junction cause a tiny, weenie, itsy, bitsy electrical current to be created when the junction is heated. Well...... every set of connections of dissimilar metals does this too. So where the thermocouple wire connects to the meter itself creates another set of tiny thermocouples, since the meter connections are likely copper based and the thermocouple is composed of two different metals. These two thermocouples (each side of the two leads) creates its own little currents depending on what the temperature at THAT point is. These tiny currents "conflict" with the current created by the thermocouple that you WANT to read. So if the back of the meter is getting hot or cold, that affects the readings too.

 

If you are using a thermocouple extension wire between the actual thermocouple and the meter, that also can cause secondary "thermocouples" at the junction locations. Most "official" extension wires are made of metals that are almost fully compatible with the thermocouple thay are intended to "extend", so this is not a big problem. But if the extension wire is some OTHER metal, that situation creates significant "extra thermocouples" that are each measuring the temperature at their particular location and are adding or subtracting their current from the real thermocouple's.

 

It sounds like you are using a unit (from your description) that has a thermocouple protection tube. That protection tube (looks like a cut off porcelain broom handle?) causes some thermal lag between the kiln chamber and the metal tip of the actual thermocouple that is located INSIDE that tube. The heat energy from the kiln chamber has to heat the protection tube, the heat energy has to then get conducted thru the porcelain tube wall, and then by radiation it has to heat the air contained inside the tube, which then heats up the thermocouple itself. In really fast firing situations (mayb like a fast raku climb), this thermal lag can cause the readings at any point in time to be "behind" the actual chamber temperature.

 

That protection tube stuff is just "information" for you about how these things work... and is NOT all that germane to your issue....... your situation is apparently reading "hot".

 

 

It is possible that the location that you have chosen to place the thermocouple probe (protection tube containing the thermocouple unit) is such that the flame from the burner is traveling up along the outside kiln refractory wall and is impinging directly onto the tube. This means that the pyrometer will read WAY hot while the actual temperature of the stacking of ware in the central area is also exhibiting a normal "thermal lag". The temperature of the gases circulating in the kiln when it is on a "climb cycle" will always exceed the temperature of the wares and kiln furniture. Otherwise, there will be no driving force to cause heat energy transfer (Laws of Thermodynamics). The larger the differential in temperature between the two, the more readily the heat energy gets transfered to the wares.

 

I am guessing your "issue" is a combination of a meter with poor accuracy to start with, some cold junction issues, and mostly the location of the probe itself.

 

A bit more infomation about the nature of the kiln you are using along with the pyrometer/thermocouple unit would be helpful. Can you post a picture?

 

best,

 

..............john

 

PS: It is easy for me to spend your money wink.gif but check out Omega Engineering in CT for good temperature measurement equipment. http://www.omega.com/ I have been using their products in the consulting and construction work I do for my kiln clients for probably about 27-30+ years.

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John, You are a wealth of info. I am using the zen portable raku kiln. The 4 inch spyhole is directly across from the gas coming into the kiln. I was told to place my pyrometer into that spyhole and NOT plug it with fiber(which I had been doing for the first 3 firings. The fiber held it in place, now it just balances in the hole. I was told that the open hole creates some sort of downdraft. Now I am thinking that this may not be the best place for the pyrometer. the only other hole is on top of the kiln and that gets too hot i am guessing. At this point I am pulling my ware after it has bubbled and smoothed out but I don't like constantly checking it since I was told we should not look through the hole as it could damage the retina and with welding glasses on I cannot see much of anything. When I open the kiln most of the ware looks burnt and not glowing as much as I have seen from other kilns and was wondering if I should leave it in for a bit longer? My concern is that I don't want the clay (cone 05) to melt. Maybe I should use cone 10 while I figure this out. Thank you for responding to my never ending questions.

Debi

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