Stacy E. Apelt - Bladesmith
ilmarinen - MODERATOR
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Knifemaker / Craftsman / Service Provider
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- Aug 20, 2004
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I recently had a question about how to check a HT oven to see if the pyrometer/TC is reading right.
While you can borow r buy a second pyrometer, you still won't know the exact temp iunless the one you borrow has been recently calibrated.
The simplest and best way is using Tempil-Sticks to calibrate the oven.
Tempil is a company that makes a crayon like test stick that will melt at a very exact temperature. The mark made with the stick will melt and turn glassy looking and usually a bit darker when the corresponding temperature is reached. They are available at a huge range of temperatures. A stick will do hundreds of tests. They are very low price, too. I recommend getting 1440, 1450, 1460. You will need a test coupon of steel about 2X2X.25" ( mild steel is fine). Draw a line on the coupon with each stick in order, and write the temperature rating at the end of each line. The coupon now wil test for 1440/1450/1460F.
Start the calibration by pre-heating your oven to 1430F. You can do the tests manually, or program the controller to make 5 degree steps with 5 minute soaks.
Place the coupon in the pre-heated oven and let soak for 5 minutes. Remove and look at the lines. None should have melted. Put back and raise the temp 5 degrees Soak for 5 minutes and check the lines. Repeat the 5 degree raises and 5 minute soaks until the 1440F line melts. Note the temp the oven pyrometer shows. Raise in 5 degree steps and make notes until the other two melt. At this time you will have a graph showing the oven reading at 1440, 1450, 1460. Take the average variance and use that as a compensation for making HT regimes.
Lets say the three readings were 1435, 1450, and 1455. That would be a deviation of -5, 0, and -5. Total variance over the three readings is -10, so the average is -3.33. This shows that on an average the oven reads just a bit low, but sometimes is dead on. Any temperature you want for a HT regime should be raised 3 degrees to get a very exact result. If you want the steel to soak at 1475, set the controller to 1478.
The process works best in an electric HT oven, but a PID controlled forge could be checked using a similar process. The only difference is that you would use a piece of pipe and make the marks inside the pipe. This will keep the marks out of the flames. For testing a forge, a good choice would be 1425, 1450, 1475.
Other uses for Tempil-sticks are checking things you pre-heat. In welding you may want to heat an anvil to 300F before doing a weld repair. Just draw a line on the hot anvil to check when you are at 300F.
You can test your kitchen oven to see how far off it is at 400F, or any other device you want an accuracy check on.
While you can borow r buy a second pyrometer, you still won't know the exact temp iunless the one you borrow has been recently calibrated.
The simplest and best way is using Tempil-Sticks to calibrate the oven.
Tempil is a company that makes a crayon like test stick that will melt at a very exact temperature. The mark made with the stick will melt and turn glassy looking and usually a bit darker when the corresponding temperature is reached. They are available at a huge range of temperatures. A stick will do hundreds of tests. They are very low price, too. I recommend getting 1440, 1450, 1460. You will need a test coupon of steel about 2X2X.25" ( mild steel is fine). Draw a line on the coupon with each stick in order, and write the temperature rating at the end of each line. The coupon now wil test for 1440/1450/1460F.
Start the calibration by pre-heating your oven to 1430F. You can do the tests manually, or program the controller to make 5 degree steps with 5 minute soaks.
Place the coupon in the pre-heated oven and let soak for 5 minutes. Remove and look at the lines. None should have melted. Put back and raise the temp 5 degrees Soak for 5 minutes and check the lines. Repeat the 5 degree raises and 5 minute soaks until the 1440F line melts. Note the temp the oven pyrometer shows. Raise in 5 degree steps and make notes until the other two melt. At this time you will have a graph showing the oven reading at 1440, 1450, 1460. Take the average variance and use that as a compensation for making HT regimes.
Lets say the three readings were 1435, 1450, and 1455. That would be a deviation of -5, 0, and -5. Total variance over the three readings is -10, so the average is -3.33. This shows that on an average the oven reads just a bit low, but sometimes is dead on. Any temperature you want for a HT regime should be raised 3 degrees to get a very exact result. If you want the steel to soak at 1475, set the controller to 1478.
The process works best in an electric HT oven, but a PID controlled forge could be checked using a similar process. The only difference is that you would use a piece of pipe and make the marks inside the pipe. This will keep the marks out of the flames. For testing a forge, a good choice would be 1425, 1450, 1475.
Other uses for Tempil-sticks are checking things you pre-heat. In welding you may want to heat an anvil to 300F before doing a weld repair. Just draw a line on the hot anvil to check when you are at 300F.
You can test your kitchen oven to see how far off it is at 400F, or any other device you want an accuracy check on.
