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Thermometers are made to measure different types of physical characteristics, but the five most common are: liquid expansion devices, bi-metallic devices, resistance temperature devices - RTDs and thermistors, thermocouples and infrared radiation devices.
Bi-metals have dial displays. The dial is connected to a spring coil at the center of the probe. The spring is made of two different types of metal that, when exposed to heat, expand in different but predictable ways. Heat expands the spring, pushing the needle on the dial. Bi-metal thermometers are cheap and typically take minutes to come to temperature. Not to mention, their entire metal coil has to be immersed in the material being measured to get an accurate reading (usually more than an inch or two).
Liquid thermometers and bi-metals are mechanical thermometers needing no electricity to function. Bi-metal thermometers lose calibration very easily and need to be re-calibrated weekly or even daily using a simple screw that rewinds the metal coil.
Electronic thermometers - RTDs, thermistors and thermocouples - measure the effects of heat on electronic current. Resistance devices, RTDs and thermistors, take advantage of the fact that electrical resistance reacts to changes in temperature along predictable curves.
Both the relatively inexpensive thermistor and the high-precision RTD - measure resistance in a resistor attached to an electronic circuit to measure temperature. Thermistors typically use ceramic beads as resistors, while RTD's often use either platinum or metal films.
With thermistors, resistance decreases with temperature and with RTDs, resistance increases. Both thermistors and RTDs may have a higher degree of accuracy than thermocouples, but their range is limited by comparison and they are generally not as fast.
Thermocouples work on the principle, that when connected to two different metals across a span with a temperature difference, an electronic circuit is generated. The voltage of the generated circuit changes with variations in temperature in predictable ways. Common thermocouples weld together nickel and chromium - Type K, copper and constantan - Type T or iron and constantan - Type J and place the weld at the very tip of the thermometer probe. Since thermocouples only generate voltage if there is a difference in temperature along the circuit (and the difference in temperatures needs to be known to calculate a temperature reading), thermocouples either have a cold junction where part of the circuit is brought to the ice point (0°C/32°F) or an electronic cold junction compensation that aids in the calculation. Thermocouples can detect temperatures across wide ranges and are typically quite fast.