Do you or have you ever calibrated your Thermometer ?

[gray wolf]

I know as long as the reading is the same all the time we can set up a base line.
So if our metal goes to slush at 200* and goes liquid at 275* as long as it is repeatable we are OK.
But I like to know as close as possible what is happening. I use the Tel-true that many of us use. The only thing I have that I can trust is boiling water 212*
I know that differences in altitude can effect the boiling point of water,
My GPS say's I am at 1,565 Feet above sea level.
The Tel-True hash marks are in 10* increments so I have mine just over 210*
in the boiling water.
What I don't know is this: If it is off by lets say 5* at 212* How much will it be off at 6 or 700* Is the error ex-potential or logarithmic ?
I know many will say who cares, some will say --hey I never use one,
close enough is good enough, I cast when it's shiny, and so on.
But what you all think of my question.
Julie say's I definitely need supervision.

[garbear]

I am not a math wiz but if you know 5* at 212* as an example then it comes down to a ratio equation. now i'll open mouth and insert my foot.
Garbear

[Southern Son]

I think the only way that you will be able to confirm if the error is ex-potential or logarithmic would be to find a second temperature that you are certain of (perhaps melting point of pure tin) and see if it too is out by the same amount or if the difference has changed. My missus is saying that I should not be allowed near anything that children should not be doing unsupervised, so you might have to test my hypothasis (sp?).

[chboats]

GW - I under stand your problem. I have the same situation. If it is off 5 at 212 is it off 5 at 636 or is it off 15 or even more. I looked into getting mine calibrated but it would cost more than the thermometer.
What to do, what to do.

Carl

[btroj]

What thermometer?

[geargnasher]

GW, don't calculate error, calibrate your thermometer at the higher temps using KNOWN alloy, like certified pure lead. This will get you a rock-solid 621F, right near the bottom of the ww casting range, plenty good for anything else.

As a matter of fact I just re-calibrated mine tonight before checking some solder for slush range, I used ten pounds of pure lead I have just for the purpose, it's just enough to give the minimum depth for an accurate thermometer measurement in my pot. I recalibrated my thermocouple for my PID controller while I was at it, using the melting point of the pure lead as my guide.

FWIW, my Tel-Tru purchased from Swede Nelson shows lead fully liquid at just under midway between the 620 and 630 hash, so two or three degrees is pretty darn close.

Gear

[gray wolf]

The only lead I have is unused roofing lead -- sheet lead

[lylejb]

Is the error ex-potential or logarithmic

Could be either, or simply linear. All depends on the quality of the thermometer.

If, for example, your thermometer uses a bi-metal coil ( as most do) that was poorly manufactured ( the coil) it could be off 5 deg @200, then 10@ 400, then 20@ 800, for example. the coil is gaining error as temp rises in this example.

If, for another example, the coil was made correctly but the thermometer was not calibrated, then 5deg @ 200, could be 5@ 400, and 5 @800. the coil is working correctly, but is simply off by 5 deg.

It would also be possible to mismanufacture a coil that does just the opposite of the first example. Instead of gaining, it might lose temperature. 5@200 might be 0 @ 400, and -5 @ 800.

It could also be possible to combine these, for example, a coil that gains temp in a thermometer calibrated too low.

short answer is.....there is no hard rules.

this is why bimetal thermometers are rarely used for anything that has to be precise.

For our use, we really don't need fraction of a degree accuracy, or for that matter even 10 degree accuracy.

Bimetal thermometers are inexpensive to manufacture, even if overpriced once you add a makers name. for most things, there close enough.

[geargnasher]

probably close to pure, but I just about guarantee not REAL close, since it doesn't have to be for it's purpose. Same with zinc wheel weights, you could use those if you have enough to make five pounds, but they are an alloy of who knows what, so it would also be a guess.

Gear

[Bradley]

The temperature of boiling water is known. The temperature at which both lead and linotype melt is known. So there are three standards you could use.

[theperfessor]

My home built PID registered 213F in boiling water and 623F for the melting point of some lead roof sheathing that I believe is close to 100% pure lead. I gotta believe that's plenty close enough for casting bullets.

[btroj]

I hear you Professor.is closer than 1 or 2 degrees really needed for carting bullets?
Heck, I don't even own a thermometer so I obviously don't care about temp that much.

I have to wonder if we, as casters, get hung up on little things because we can measure them? Pot and mold temp amongst them. This along with hardness. Seems to me we got along very well for a long time without measuring these things so precisely. I have to wonder, do these measurements really help us in a meaningful way? Do they just add to the confusion for a beginner who generally tries to worry everything?

Not trying to rain on any parades, just asking the question. Is something relevant just because we can measure it?

[gnoahhh]

I don't use a thermometer. I just dip my finger in.

Seriously, I've used the same thermometer for 30 years now. It's readings provide the base line data that I have developed for my own style of casting and alloys used. What is 700deg for me may not be 700deg in the real world but I know that 700deg by my thermometer is where I want to be for X alloy.

[casterofboolits]

I have used a Lyman thermometer for about 30 years. I think I paid about $14.00 for it. New in the box, so that gives a rough time frame!

I cast about 700 to 725 degrees and this works well for my alloy which I make about 250 to 300 pounds at a time, normally a 50/50 mix of WW and range scrap. This is what I use for all my 45 casting. For 9mm/38 super/38/357 and 40/10mm I add one pound of sweetner to my RCBS 10 kilo pots. My sweetner is 90/10 of Lino/tin. I use a Lyman four cavity 358477 38-150-SWCPB mould to check the consistancy of my alloy and my target is 150 grains plus/minus 1 grain.

I check my pot temps at the start of a casting session and as long as my boolits are filling out and weighing what I want, I'm good to go. I don't think I have changed the temp setting on my pots for years.

I developed this method when I had my casting business and have cast and sold a couple million boolits over 25 years. Consistancy is my goal.

[theperfessor]

I cast for years w/o any type of thermometer with no problems. Like a previous poster pointed out, I knew where to set my Lyman 20lber to get good bullets from various molds and alloys by using fillout and the 5 second freeze rule for the sprues. And I could go back to that today and do the same thing. The only reason I built a dual PID controller (to control lead pot and hot plate) was because I wanted to have better data and temperature control for an ongoing mold material/alloy material shrinkage test I'm working on.

But for most casting I could easily do without it.

[MT Gianni]

I don't worry about calibrating my thermometer as I used it as a reference only. If one mold likes an alloy @ 750F by that thermometer that is what I take the melt to. If another is best cast @ 625 mine will reference that also.
I calibrate electronic correctors for gas meters 3 months of the year and a 2 point calibration is minimum. This is for a thermometer with a range of -40 to +160, or 200 Degrees F. It is common to recalibrate an instrument and retest to find it accurate to +or - .025%. These relatively cheap [for the industry] correctors only run $1500 or so. I have no idea what the standard would be for one under $25 with a 1000 degree range as my lead thermometer has.