Thermocoule precision?

[Lauri Levanto]

Sean wrote in another thread:
". I'm positive the
glass is being fired at the temp. the controller is reading.

How accurate are the controllers really?
I trust it to give pretty much the same reading at the same temperature. If I remember right the thermocoule
came with 10% accuracy. Over a 800 C range
it may be 80 C off at worst.
Not to mention the difference between glass and thermocouple.

Just now I have a 6 kg (12 lb) casting in the kiln.
Cooling down AFAP, the small thermocouple head
cools faster than thhis bulk of glass.

The veterans keep saying that kiln are different
and the temps given here are but indicative.

Is it kilns or thermocoules that are different?

-lauri

[Tony Smith]

A standard K-type thermocouple has a precision on 2.2°C or 0.75% which ever is greater. So, at 800°C, that would be 6°C. Your controller electronics could add additional error, but it should be minimal.

Other thermocouple types are listed here
http://www.omega.com/temperature/pdf/tc_color_codes.pdf

Unfortunately, when casting, there is no good way to know what the core temperature is unless you were to embed a thermocouple in the mold material (which you should be able to do if you are curious) then you could correlate it to the air temperature in the kiln and use that to slow down or speed up your ramp rate.

Tony

[Tim Swann]

Try calibrating the thermocouple in boiling water. Your thermocouple should have come with a calibration procedure.

Tim

[Bert Weiss]

Lauri

The thermocouple is simply reading the air temp in the kiln. The actual glass temp is a completely different story. Our challlenge is to control the air temp so that the glass ramps up and down in a way that works for us. Therein lies the challenge. The amount of mass inside the kiln including the kiln walls anf floor determines the speed of heating and cooling. Each different kiln design behaves a bit differently which is why there are different firing schedules to get the same results.

The glass reacts to what I call quality of heat. The factors that make up that quality are a mixture of time and temperature. I have noticed that the edges on glass fired in my kiln with lots of blanket and board covering up the floor are considerably different than the edges on a brick floor covered with 1/4" paper, fired to the same schedule. I actually like the brick edges better as they are better fused.

[Tony Smith]

Most thermocouples attached to kiln controllers don't come with calibration procedures. Beyond that, their reading at 212° has little bearing at what they are doing at 1000° or 1500°. You could just as easily compare the temperature to a mercury thermometer at room temperature. There is often some nonlinearity from the controller in the middle of their range.

Tony

[Wallace Venable]

A thermocouple in a kiln is NOT simply reading air temperature. In addition, it is being heated by radiation. If it can "see" elements, the total it indicates might be significantly different. Think of a piece of black metal in the sun - it might be too hot to touch, although the air temperature seems moderate. I would hazzard a wild guess that these differences are on the order or 25 to 50°F.

A thermocouple embedded in a plaster or ceramic based mold will NOT indicate the temperature of the glass because the mold acts as an insulator. On the way up, it will read high, on the way down it will read low. The difference will depend on the thicknesses involved. In addition, the glass is not all at the same temperature. That is why we "soak" and "ramp," to make sure the temperature gradients are small.

If we are proceeding with sufficient caution, the "air" temperature is a good indicator. The problem is guessing whether we are using sufficiently conservative programs, and that must be determined by experiment. Remember even Corning cracked the first Palomar telescope mirror - they got the second one right.

Attempts to create simple explanations have a tendancy to create simplifications.

[Brock]

Right on, Wallace! I helped a well known glass artist do a large casting, somewhere, sometime. He/she had 4 thermocouples in the kiln, 3 in various spots through the kiln walls and one buried in the mold material.

The large piece, approx. 140 lbs. of glass, did not survive. No matter what was tried and how long the glass soaked, the mold itself comtributed to a 40 degreeF difference in the between the top and bottom of the casting.

Brock

[Tony Smith]

A thermocouple embedded in a plaster or ceramic based mold will NOT indicate the temperature of the glass because the mold acts as an insulator. On the way up, it will read high, on the way down it will read low. The difference will depend on the thicknesses involved. In addition, the glass is not all at the same temperature. That is why we "soak" and "ramp," to make sure the temperature gradients are small.

If we are proceeding with sufficient caution, the "air" temperature is a good indicator. The problem is guessing whether we are using sufficiently conservative programs, and that must be determined by experiment. Remember even Corning cracked the first Palomar telescope mirror - they got the second one right.

Attempts to create simple explanations have a tendancy to create simplifications.

Wallace,

I think even you would have to agree that the temperature of the plaster mold will fall somewhere between the temperature of the air and the temperature of the glass since the plaster is the only thing separating the two. During cooling, the heat from the glass is being conducted through the plaster where it is then conducted, radiated and carried away by convective air currents to the exterior of the kiln. Because of plaster's insulating properties, the glass temperature will always be lower than the air temperature on the way up and higher on the way down. But this is true even without a mold.

My point was that a thermocouple embedded in the mold is the closest you will get to the glass temperature without being IN the glass since the air is removed from the equation. If the glass varies in thickness and/or shape, or the mold varies in thickness, then there will be temperature differences throughout the entire piece of glass.

Tony

[Brock]

The main problem is the DIFFERENCE in temperature between the glass exposed to the heat, and the glass insulated by the mold. It is unusual for a mold to be the same thickness all around the glass, especially, a dammed poured plaster mold. When casting smaller lost wax objects, the (new) convential wisdom is to build up the layers of plaster with cloth or fibreglass so that the mold looks roughly like the object, only bigger. This will give you a relatively uniform shell thickness. Brock

[Tony Smith]

The main problem is the DIFFERENCE in temperature between the glass exposed to the heat, and the glass insulated by the mold. It is unusual for a mold to be the same thickness all around the glass, especially, a dammed poured plaster mold. When casting smaller lost wax objects, the (new) convential wisdom is to build up the layers of plaster with cloth or fibreglass so that the mold looks roughly like the object, only bigger. This will give you a relatively uniform shell thickness. Brock

Sorry, I though we were talking about cast glass where all the glass was inside the plaster... my mistake

[Brock]

The main problem is the DIFFERENCE in temperature between the glass exposed to the heat, and the glass insulated by the mold. It is unusual for a mold to be the same thickness all around the glass, especially, a dammed poured plaster mold. When casting smaller lost wax objects, the (new) convential wisdom is to build up the layers of plaster with cloth or fibreglass so that the mold looks roughly like the object, only bigger. This will give you a relatively uniform shell thickness. Brock

Sorry, I though we were talking about cast glass where all the glass was inside the plaster... my mistake

Tony, I wasn't, I don't know about Wallace, maybe he was. Brock

[Tony Smith]

Well then, I agree with your statement. The difference IS a problem when part of the glass is exposed to the heat.

Tony

[Brock]

Well then, I agree with your statement. Aren't we sensible fellas? Brock

[Tony Smith]

Well then, I agree with your statement. Aren't we sensible fellas? Brock

Time for a group hug... Everybody???

(where's David?)

Tony :biggrin:

[Brock]

Well then, I agree with your statement. Aren't we sensible fellas? Brock

Time for a group hug... Everybody???

(where's David?)

Tony :biggrin:

Where's Lani?

[Tony Smith]

Well then, I agree with your statement. Aren't we sensible fellas? Brock

Time for a group hug... Everybody???

(where's David?)

Tony :biggrin:

Where's Lani?

We could have a hugging party

Tony

[Brock]

Kewl! I've still got some Happy Fizzies around here somewhere.

I think their best by date is somewhere in the 3rd millenium. Brock

[Tony Smith]

Kewl! I've still got some Happy Fizzies around here somewhere, I think there best by date is somewhere in the 3rd millenium. Brock

I know I'll regret this, but Happy Fizzies???

Tony

[Brock]

Kewl! I've still got some Happy Fizzies around here somewhere, I think there best by date is somewhere in the 3rd millenium. Brock

I know I'll regret this, but Happy Fizzies???

Tony

Ah, you young whippersnapper! I want to see who knows what they are,
but evidently, they predate you. Brock

[Ron Coleman]

Fizzies

Just like flavored Alka Seltzer. The grape was yucky

http://www.oldtimecandy.com/fizzies.htm

Remember them well