Desk Top Furnace

Have you found any deals on RTD's on eBay? I see that some RTD's go up to

900 Deg F and they seem more accurate and stable than thermocouples. At work we have machines that use a dual RTD, if one of the sensors goes bad, it can detect it. Maintenance changes out the RTD's and throws the old one away, the old one usually has 1 working and 1 bad RTD in it. I need to have some curing techs stash me some back and get a scrap pass to take them home.

RogerN

I think the glass flowing myth is just that but it's still perpetuated. See

. I expect the patterns are not due to sagging but just due to the way the glass is hand made. IIRC the glass can be spun out into a large disc or blown into a large cylinder which is then slit open and flattened, either way produces glass which is not truely flat and you get the distortions you see. Some of the process is shown here starting about 6:50 and continues here

A Variac, some type of suitable thermometer and exquisite patience should work. The

Excellent stuff. I stand corrected!

Next time I want the sagging/distorted effect, I'll just take a torch to new glass!

Excellent video. Def'ly start at 6:50, tho. Fascinating from there on.

I'll sell you mine, but I don't know what temp it is good for. I'll take another look at it tonight. It is brand-new, with manufacturer's tag still on it.

Jon

Well, I kind of have about 15 temperature controls right now. I have 3 Cal

9900's, and 11 Fuji controls with ramp and soak, plus another non-display thermocouple in, triac out control. But someone here might want to give it a try. You can use a 100+ ohm resistor instead of an RTD to test them out. I made some test plugs at work using 110 ohm resistors to test between bad RTD's or bad cables.

RogerN

Not if it's Tempered Glass, you won't... (It'll just shatter.)

The very old glass was hand blown and then hand formed into small panes, and even when they were trying for flat glass it was all distorted from the hand work involved. "Picture Windows" were way off on the horizon, the best you got were multiple mostly clear panes in a larger window unit.

Or they didn't even try for clear and molded it with obscuring ridges and patterns, and a variety of colorants added. Then some bright person got the idea for Stained Glass.

They didn't get truly flat glass or very big sheets till they came up with the idea of using a bed of molten Tin as a casting surface, still cool enough that the glass woiuld solidify on the Tin. And the results were... Float Glass. As big as you want it, they just need to build a wider floating tank and a wider finishing line.

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This is the voltage vs temperature chart for a type K thermocouple. The voltage at heat treating temperatures is easily high enough to read with a digital meter. It's more accurate if you have a cold junction in a glass of ice water. You could calibrate your reading with aluminum, which melts at 660C / 1220F.

Wilkins

Years ago I calibrated a K-type thermocouple using a small crucible filled with pure tin, and a similar one filled with pure copper (electrical wire). From memory the two temperature points were 450 deg. and 1950 deg. Fahrenheit.

It was fascinating to watch the temperature remaining steady while the metal melted during the temperature rise, and while freezing during the temperature drop.

I happend to have tin handy, but as was pointed out other, pure, metals may be used for calibration purposes, eg. lead, zinc, etc.

Flat glass was produced by grinding and polishing both sides long before float glass was thought of. I have a book on industrial processes published in the early 1930's that describes the process. This is the method that was used for making automotive windshield glass at that time... It was tempered glass that would crumble upon breakage, as opposed to producing sharp and dangerous shards.

Wolfgang

If the cold junction is not at 0°C = 32°F (ice-water slurry, close enough) then you can correct by looking at the temperature at the copper-thermcouple junctions (say we're in a 70°F room and the cold end is all at room temperature-- the number is 0.843mV) and add that number to the measured mV (say we measure 21.842mV) before looking it up in the table (we'd get 21.412 + 0.843= 22.255mV => 1000°F)

Otherwise you'd have an error of about 964.5 - 1000 = 35.5°F. (It's not exactly 70 - 32 = 38°F because the thermocouple is somewhat nonlinear).

Thermcouples are close enough for most applications that you don't need to check the calibration (they just work) but it is worth checking against a reference at least once to make sure you have the right type and are using the right table.

Best regards, Spehro Pefhany