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i-metallic thermometer (such as a round outdoor temperature thermometer) is one of the most common, inexpensive thermometers available. The principle of this thermometer is simple. Two different thin metal foils attach in a spiral structure. When temperatures change, the expansion/contraction rate of the two metal foils is different, which changes the tightness of the spiral structure. Aside from use in round outdoor temperature thermometers, bi-metallic sensors are commonly used for oven thermometers and indoor room thermostats. The bi-metallic thermometer is simple, strong, and inexpensive to make, but it’s not as accurate as other thermometers. Digital thermometers have developed dramatically over the last two decades because of growth in the silicon industry; they are inexpensive to make and accurate. Digital devices can connect to analog sensors such as infrared sensors, thermistors, and thermocouples and/or digital sensors. The infrared digital thermometer is becoming popular for measuring body temperature. This sensor is even capable of determining body temperature by just measuring heat emitted from the eardrum.
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hermal cameras (thermography) show temperature distributions (images), not just one temperature point like other sensors. An example of one use of thermography is that of airports, in their effort to prevent the spread of flu. They use thermal cameras to detect fever in passengers. And let’s look at eggs, as another example. Some people like hard-boiled eggs, some like soft-boiled, and some like eggs somewhere in between. This thermal image of sunny-side up eggs shows that egg white cooks faster than the yolk. When the first heat bubble rises in the yolk, the yolk temperature is about 43°C.
hen you look at a thermal image of a campfire, the fire appears hot. The thermal scale shows that people facing the fire are warmer. Fires produce radiant heat. Heat transfers in many ways: there is radiant heat, conductive heat, and convective heat, among others. When frying an egg or meat, the heat going through them from the pan is conductive. When cooking soup in a pot, the liquid is warmed by convective heat. When baking in an oven, the heat is radiant. Modern ovens also have a convection heat option.
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ook at the thermal image of the toaster. We know there’s quite a bit of heat inside the toaster, but the heat shield outside the toaster prevents overheating, so the image doesn’t show much warmth. Note that the electrical outlet shows warmth; this is because it heats up with use.
Calibration
All sensors need calibration because no sensor is entirely accurate. A known temperature is needed to calibrate a sensor. We know that pure water freezes at 0°C (32°F), and we know that typical ocean water (3.5% salinity) freezes at −1.8°C (29°F). To calibrate a sensor, we combine pure water and crushed ice from pure water to make an ice bath; then we put the sensor into this wellmixed ice bath. The sensor should show a temperature of 0.00°C; if it doesn’t, we can see how much the temperature reading shifts from 0°C. www.uaf.edu/permafrost
Temperature Book
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