AZ-Stew
01-07-2009, 12:50 AM
About a year ago I picked up a cheap ($30 retail) toaster oven at a yard sale for $10. Figured on using it to heat treat boolits. Just got around to trying it to see if it would be suitable. I've read a lot of posts here complaining about wild temperature swings and decided to investigate for myself. A couple of years ago I picked up one of Harbor Freight's digital thermometers for the princely sum of $26 (+tax) to use for a casting thermometer. It's been a great tool for that concern. As it turns out, it's also a great tool for investigating the inner workings of a cheap toaster oven because the thermocouple can be placed almost anywhere within the volume of the oven cavity.
The first test consisted of weaving the thermocouple wire through the oven shelf grating so that it would hold a constant position. The tip (measuring point) of the thermocouple (henceforth called the TC) wire rested about an inch and a half inside the front glass door. I turned on the oven, set the dial for 400 degrees and watched the digital thermometer. The readings quickly climbed until they reached nearly 500 degrees before the heater shut off and the temperature began to decrease. I later learned that the thermostat is on the outside of the oven cavity inner wall, so the heat has to sink through that wall before the thermostat realizes that the oven is over temperature. No big deal. This just means that the oven must be turned on and the internal temperature allowed to normalize before putting the boolits inside. More on this later.
It took about a half hour for the oven to reach a stable temperature. A disappointing 340 degrees. I turned the temperature dial on the oven up to maximum (450+) and again waited for the oven to normalize. This time it settled out at 360 degrees, still not hot enough to heat treat. Well, says I, it's time to do some tweaking. I pulled the plug, opened the oven door and found something else to do for about a half hour while the oven cooled back to room temp. After removing some screws (too many) and figuring out how the sheet metal cabinet was assembled, I had finally uncovered the heart of the beast. The bad news was that the thermostat had been factory set (wrongly) and an anti-tampering adhesive had been applied to the adjustment screw. Fortunately, it succumbed to some persuasion in the form of a lot of grunting and groaning on my part as I applied excessive torque with a screwdriver while holding the thermostat against rotation with a pair of pliers. Fortunately, I didn't break anything. I had to guess at the proper direction to turn the screw. The thermostat has markings, but they aren't very clear. I took a small increment of change and then reassembled the oven to repeat the test. A stroke of good luck! I turned it the right direction, but not enough. After a half hour the oven settled at 390 degrees. Still not high enough, but at least I knew what I was doing when I took the oven apart again.
About ten minutes later I was ready to repeat the test. This time I hit 410. I was getting close, but there were some things about the test that bothered me. At each temperature increment I've mentioned so far, I've listed the average temperature. The minimum and maximum averages were about 13-15 degrees either side of the nominal average. I wondered if food would cook properly with such "wide" swings, though these are smaller than some others I've read about here. I began to think about the thermocouple placement and the fact that I wasn't measuring the temperature of the boolits (which is the object of the whole exercise), but rather the air temperature inside the oven. In addition, the TC was relatively close to the glass door of the oven and I could easily feel a lot of heat radiating out the front of the oven. Surely this didn't contribute to temperature stability inside the oven, or to my measurements. The other day, Jeff in NZ, I believe, mentioned that he had insulated a small oven to reduce temp swings, but I wanted to find out if this is necessary, or whether the oven can stand on its own for heat treating.
I came to the conclusion today that if I want to know how stable the temperature of the boolits will be during heat treatment, I should probably measure the temperature of one. And not near the oven door, but on the pan where the boolits will rest during treatment, farther back in the oven. When I got home from work I chucked up a reject boolit (RCBS .30-150FN) in my lathe and drilled a hole in the base about half the length of the boolit. I threaded the TC wire up into the hole and used a bit of casting scrap to wedge the TC wire in place. I placed the boolit on the pan and plugged in the oven. This time the initial temperature swing exceeded 600 degrees! I watched through the oven door as my boolit oozed into a puddle in the middle of the pan. Well, says I, this isn't going to give me exactly the results I want, because the boolit is no longer round, but more of a non-descript lump and it's unlikely that the TC is still in the middle of the mass. While I waited for the temperature to normalize, I grabbed another reject boolit and drilled out the base for the TC wire. The oven stabilized at about 440 degrees, this time with the TC near the middle of the oven, rather than near the glass door. Also, due to the mass of metal surrounding the TC, the temperature swings were much smaller, more like +/- 7 degrees. I'm onto something!
I pulled the melted boolit out of the oven and used a propane torch to melt it off the TC wire. I inserted the TC wire into the fresh boolit, placed it on the pan and shut the oven door. The oven quickly came back up to temp and settled at 447 +/- 4 degrees! I also discovered that air currents in the vicinity of the oven wreak havoc with temperature stability. Air currents passing through the louvers in the side of the oven where the thermostat resides fool it into premature heating cycles. When I sat still near the oven, rather than standing over it and breathing on it, the temperature was more stable. When I get time I'm going to watch this process, collect the numbers and run them through an Excel spreadsheet to find the true average and SD, which I suspect will be +/-2 or less. Now, this I can work with! When the pan in the oven is full of boolits, the mass of material will stabilize the temperature further.
YMMV, but at least for this $10 yard sale special, it is ready for prime time and will do a proper job of heat treating boolits if set up correctly and left undisturbed to do its work. I believe most of these cheapie ovens will. I think the wild temperature swings that have been reported are more a function of poor test setups than of poor equipment. (BTW, it's a Black & Decker).
Regards,
Stew
The first test consisted of weaving the thermocouple wire through the oven shelf grating so that it would hold a constant position. The tip (measuring point) of the thermocouple (henceforth called the TC) wire rested about an inch and a half inside the front glass door. I turned on the oven, set the dial for 400 degrees and watched the digital thermometer. The readings quickly climbed until they reached nearly 500 degrees before the heater shut off and the temperature began to decrease. I later learned that the thermostat is on the outside of the oven cavity inner wall, so the heat has to sink through that wall before the thermostat realizes that the oven is over temperature. No big deal. This just means that the oven must be turned on and the internal temperature allowed to normalize before putting the boolits inside. More on this later.
It took about a half hour for the oven to reach a stable temperature. A disappointing 340 degrees. I turned the temperature dial on the oven up to maximum (450+) and again waited for the oven to normalize. This time it settled out at 360 degrees, still not hot enough to heat treat. Well, says I, it's time to do some tweaking. I pulled the plug, opened the oven door and found something else to do for about a half hour while the oven cooled back to room temp. After removing some screws (too many) and figuring out how the sheet metal cabinet was assembled, I had finally uncovered the heart of the beast. The bad news was that the thermostat had been factory set (wrongly) and an anti-tampering adhesive had been applied to the adjustment screw. Fortunately, it succumbed to some persuasion in the form of a lot of grunting and groaning on my part as I applied excessive torque with a screwdriver while holding the thermostat against rotation with a pair of pliers. Fortunately, I didn't break anything. I had to guess at the proper direction to turn the screw. The thermostat has markings, but they aren't very clear. I took a small increment of change and then reassembled the oven to repeat the test. A stroke of good luck! I turned it the right direction, but not enough. After a half hour the oven settled at 390 degrees. Still not high enough, but at least I knew what I was doing when I took the oven apart again.
About ten minutes later I was ready to repeat the test. This time I hit 410. I was getting close, but there were some things about the test that bothered me. At each temperature increment I've mentioned so far, I've listed the average temperature. The minimum and maximum averages were about 13-15 degrees either side of the nominal average. I wondered if food would cook properly with such "wide" swings, though these are smaller than some others I've read about here. I began to think about the thermocouple placement and the fact that I wasn't measuring the temperature of the boolits (which is the object of the whole exercise), but rather the air temperature inside the oven. In addition, the TC was relatively close to the glass door of the oven and I could easily feel a lot of heat radiating out the front of the oven. Surely this didn't contribute to temperature stability inside the oven, or to my measurements. The other day, Jeff in NZ, I believe, mentioned that he had insulated a small oven to reduce temp swings, but I wanted to find out if this is necessary, or whether the oven can stand on its own for heat treating.
I came to the conclusion today that if I want to know how stable the temperature of the boolits will be during heat treatment, I should probably measure the temperature of one. And not near the oven door, but on the pan where the boolits will rest during treatment, farther back in the oven. When I got home from work I chucked up a reject boolit (RCBS .30-150FN) in my lathe and drilled a hole in the base about half the length of the boolit. I threaded the TC wire up into the hole and used a bit of casting scrap to wedge the TC wire in place. I placed the boolit on the pan and plugged in the oven. This time the initial temperature swing exceeded 600 degrees! I watched through the oven door as my boolit oozed into a puddle in the middle of the pan. Well, says I, this isn't going to give me exactly the results I want, because the boolit is no longer round, but more of a non-descript lump and it's unlikely that the TC is still in the middle of the mass. While I waited for the temperature to normalize, I grabbed another reject boolit and drilled out the base for the TC wire. The oven stabilized at about 440 degrees, this time with the TC near the middle of the oven, rather than near the glass door. Also, due to the mass of metal surrounding the TC, the temperature swings were much smaller, more like +/- 7 degrees. I'm onto something!
I pulled the melted boolit out of the oven and used a propane torch to melt it off the TC wire. I inserted the TC wire into the fresh boolit, placed it on the pan and shut the oven door. The oven quickly came back up to temp and settled at 447 +/- 4 degrees! I also discovered that air currents in the vicinity of the oven wreak havoc with temperature stability. Air currents passing through the louvers in the side of the oven where the thermostat resides fool it into premature heating cycles. When I sat still near the oven, rather than standing over it and breathing on it, the temperature was more stable. When I get time I'm going to watch this process, collect the numbers and run them through an Excel spreadsheet to find the true average and SD, which I suspect will be +/-2 or less. Now, this I can work with! When the pan in the oven is full of boolits, the mass of material will stabilize the temperature further.
YMMV, but at least for this $10 yard sale special, it is ready for prime time and will do a proper job of heat treating boolits if set up correctly and left undisturbed to do its work. I believe most of these cheapie ovens will. I think the wild temperature swings that have been reported are more a function of poor test setups than of poor equipment. (BTW, it's a Black & Decker).
Regards,
Stew