Last week we had a new oven installed. Our old el-cheapo one that came with the house was never in Karen’s good books. Small, dirty, incapable of getting to a high temperature and generally giving the impression that it was about to die at any moment. Indeed, it did a couple of Christmases ago – it refused to do anything. It turned out that the clock/timer chip had given up and was sending a permanent signal to the oven that it should turn off. The repair man took about a minute to diagnose it and another minute to bypass the clock chip – which meant we couldn’t use automatic on/off settings or anything fancy but at least we could turn the oven on and off manually.
But the days of that oven are gone, and it’s been replaced by something much more sensible. (I don’t do product placement so I won’t tell you make it is here) The thing that most strikes me about the new one, other than it does what you tell it, is how well thermally insulated it is. It’s a cabinet-mounted oven, rather than part of a freestanding cooker. When we turned up the old one high, the oven door got very hot and the cupboards next door became rather warm too. With the new one, the cupboards next door hardly change in temperature, and the oven door itself, is only slightly warm. This has the unintended benefit of meaning the baby is rather safer in the kitchen – if he puts his hands against the oven door he will barely notice a temperature change. (Not that we encourage him to try – his latest kitchen trick is trying to climb into the dishwasher – something that even the cat though better of)
The thermal resistance of a material is defined in terms of the temperature differential between its front and back surfaces needed to drive a given amount of power (heat per unit time) through the material. Thermal resistance obviously depends on both how thick the material is (double the thickness, double the resistance) and also its area (double the area and you’ll get twice the heat flow for the same temperature differential). We can take the area out of the equation by moving to a U-value – this describes the power loss for a degree kelvin (or celsius) difference between a unit area of the two surfaces. A low U-value means the material is a good insulator. Still, U-value depends on how thick the material is. Taking this out of the picture and we get thermal conductivity, which is an intensive measure of how good a material is. Whatever is surrounding our new oven and is in our oven door surely has a pretty low thermal conductivity. The thought has occured that there is a vacuum somewhere – that has low thermal conductivity indeed!
P.S. While putting in links for some of these terms I notice that there is some ambiguity in what people refer to as thermal resistance. I was taught it as being the temperature differential required per unit power (so SI units kelvin/watt – analogous to electrical resistance), but it also seems to be used as the reciprocal of the U-value. Not the same thing.