I don't think the dial ones are very useful, I find them hard to read through the oven glass. Sometimes I can't tell if it says 325 or 350 and that's a big difference. If you have to open the door and bend down to read them, you're letting out the heat.
The CDN grill surface thermometer looks like it might have some potential, though. It looks like the range might be easier to read with more precision.
I find opening the door as little as possible and using an infrared thermometer on the side walls, back walls and other surfaces (like a pan or baking stone) gives me a good idea what the temperature is like in several places.
In engineering classes, we used to talk about the distinction between accuracy and precision. And with ovens the question of exactly where the temperature is being monitored is a separate issue, there will be areas that are noticeably hotter than others. The use of a convection fan is another factor.
A chef friend once told me about the bread test he runs whenever he starts working in a new kitchen, he toasts bread in the oven at several different temperatures and rack positions and that way he knows where the hot spots are.
Most ovens controls measure the temperature in one place, usually along the side wall. You can't always see where the sensor is located though.
Some possibilities for alternatives to dial thermometers: Search for Taylor 5921n or Comark OT600K. The Comark one say it uses mercury, if so I'm surprised it is still available for sale.
I have a Maverick digital oven thermometer with a probe that is designed to hang from one of the oven racks. It gives an 'average' reading rather than instant readings, which is kind of weird. The cables on digital probes tend to wear out especially if subjected to heat over 400 degrees.
I've used it along with my CDN candy thermometer a couple of times, setting the probes side by side to see how they compare. Both gave similar readings, but the average temperature reading tended to lag the instant reading one by several minutes, which is logical, due to the hysteresis effect.
My son gave me a 'meater plus' thermometer with a cordless probe which you stick inside your meat. It gives you an internal temperature as well as an oven temperature on your phone. (The fact that most of the electronics is below the surface of the meat seems to protect it.) They say not to use it with bread, though, that probably doesn't insulate the electronics enough.
The GrillEye looks interesting, it appears to have an operating range up to 300 degrees C (572F). It comes with two probes but it says you can use up to 6 at a time.
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