Reflow Toaster Oven - Part 2

Having an oven is one thing, knowing what it's capable of is just as important. In order to accurately control the temperature it's not enough to simply turn the heating element on when it's too cold and off when it's warm enough. The heating element has quite a bit of thermal mass, it will stay warm for quite some time even when you turn it off, which means that it will keep on heating its environment, so the temperature in the oven may still rise even though the heating element has been turned off.

If I want my reflow oven to be accurate, I need more data on it. How fast does it heat up? How fast does it loose heat? What is the maximum temperature? How fast can it reach that temperature? The only way to get answers is by simply testing it.

So just stick a probe in the oven, connect it to an Arduino, turn the oven on and start logging!

The probe is a K-type thermocouple I got from eBay for a few bucks.

It's connected to a MAX6675 Cold-Junction-Compensated Thermocouple-to-Digital converter which converts the minuscule voltage generated by the thermocouple to a digital value. The MAX is connected to an Arduino which simply polls it about every second and prints the value on the console.

The first value is seconds since the last reset, the second one the temperature in degrees Celsius. I simply copied the output from the console to a file, named it .csv and opened it in Excel. The result is a nice little graph.

Just a few notes here. You can see a little bulge around 460 seconds. The thermostat of the oven is connected directly to the heating element. Around 440 seconds the heating element aparently reached 280 degrees C (which is what the thermostat was set to). It took a bit of time for the heating element to cool down, but it did so by warming up the rest of the oven. Around 530 seconds the element turns on again and the temperature starts to rise "rapidly" again.

When the temperature reached 245 degrees C, the wire of the probe got so hot that it was starting to melt the handle on the oven door it was touching. So that's when I turned the oven off and set the door ajar. I have no clue what the little hickup around 706 seconds is.

The time spent above liquidus (217 deg C) is 220 seconds, which is well above what's recommended (60-120 seconds). The time until peak temperature is 641 seconds, which is also well above the recommended maximum of 8 minutes.

Long story short: it's too slow, needs thermal insulation.