Any bakers out there? How accurate (or precise, I forget which) should my plain Jane Sears oven be? I have two separate oven thermometers, but they disagree with each other by about 15 degrees. At an oven setting of 350, one read 315 and the other 325 or so. Can a repairman test and adjust the oven for proper temperature setting?
Check your thermometers by putting them in a pot of boiling water. Whichever one reads closest to 212 F you keep and trash the other. Unless you live at a particularly high elevation, water should boil within a degree or two of 212 F.
I have nothing to offer but sympathy. My plain ol’ Sears unit is on the way out. I can cook most things in it, but baking is pretty much out. It usually takes me at least double the time to get a toothpick to come out clean on a pan of brownies. I’ve given up.
To answer the second question, yes, the oven can typically be adjusted such that the desired temp setting equals oven temp. If it can’t, then replacement of the t-stat pickup bulb assembly is indicated.
Or you can just make a habit of turning the knob up 25-50° past the desired temperature.
Yeah, but when you buy a new oven years later, you wonder why you keep burning shit. 
Are they bimetallic thermometers? If so, throw them away.
If you want good accuracy, buy a thermocouple & thermocouple readout. (Nowadays, a lot of cheap multimeters can read thermocouples.) Install the thermocouple in the oven, and run the thermocouple wires out of the oven and into the readout. When you’re using the oven, ignore the oven’s readout and adjust the temperature control knob to get the temperature you desire as read from the thermocouple readout.
Of course, there is the question of how accurate the thermocouple & readout is. I run a calibration lab at work, and have calibrated hundreds of thermocouples & readouts. Most of the time, the thermocouple + readout has a maximum error of around 3 °C to 4 °C over the entire operating range. Even better accuracy can be obtained by creating a look-up table. If you want uber-accuracy, use a PRT.
They can usually be adjusted by pulling the temp knob and adjusting the screw behind it.
Electronic temp readouts usually have instructions in the installation guide. Check the mfgr site for support by model number.
They will also have a way to contact a tech.
Whats a PRT?
Sea level think at work again. [Boiling point vs. altitude.](StackPath Point.htm) In Denver or Albuquerque (the only cities I’ve lived in) water boils at ~203F.
Also, many oven thermometers are not suitable for immersion.
What’s a good way to calibrate a basic oven? Don’t they constantly cycle on and off leaving the temperature at (for example) 350 +/- 25 degrees?
Of course, I didn’t meant the whole thermometer–just the probe.
We calibrate a lot of thermal and environmental (T & RH) chambers where I work. The simplest method is to stick a standard thermocouple in the center of the oven, run the thermocouple’s wires out of the oven, and plug the wires into a standard thermocouple readout. The key word here is standard. Using even better in-house standards, I calibrate our standard thermocouple & readout to a tolerance of ±1 °C. Hence the lowest tolerance we can assign to a chamber under test is ±4 °C. This is good enough for 99% of chambers.
Some of our customers want us to measure the temperature gradients inside the chamber. To do this, I built a 9-channel thermocouple standard. We place a thermocouple at each corner in the oven and one in the center.
Yes - the heaters are controlled using PWM in a PID loop. In English, this means the heaters are rapidly turned on and off. Now I can’t speak for home ranges, but it’s a non-issue for industrial chambers. The thermal time constant of the system is much longer than the PWM frequency. Assuming the thermal load remains constant, temperature fluctuations due to the PWM are virtually non-existant.
Denver and Albuquerque are high altitude cities. Basically for baking over about 3000 feet is high altitude.
Your basic oven thermometer. You will note the lack of a probe. 
I have a small digital one with a probe that hooks over the door. I’d thought most of them were similar, but apparently not. How are you supposed to read that if the door has no window in it, like mine? Obviously, you can open the door, but doesn’t that cool of the inside some?
Most people have ovens that were built at least last century 
But seriously, who makes an oven w/o a window in it?
Kenmore (Sears), apparently. This one isn’t all that old; it has a digital clock built in with a vacuum fluorescent display, so it can’t be much more than 20 to 25 years old, or so. Hey, it came with the house.
checks kitchen I dunno, the brand name is pretty well eroded… It looks like it might once have said “Hotspot”; that’s GE, right? Mind you, from the color scheme and other clues, I suspect that my kitchen was installed some time in the 1950s.