Basic Thermometry Concepts: Thermometer Technologies
As we have discussed previously, there are thermometers that measure many different types of physical characteristics, but the five most common include: liquid expansion devices, bi-metallic devices, resistance temperature devices (RTD's 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 coil is made of two different types of metal that expand in different (yet predictable) ways when exposed to heat. The expansion of the coil with heat pushes the needle on the dial. Bi-metal thermometers are very inexpensive, and typically take several 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 that don't need any electricity to function. Bi-metal thermometers also go out of calibration very easily and need to be re-calibrated weekly or even daily using an ice bath. Adjustments can be made using a simple screw that rewinds the metal coil.
Electronic thermometers (RTD's, thermistors and thermocouples) measure the affects of heat on electronic currents. Resistance devices (RTD's and thermistors) take advantage of the fact that electrical resistance reacts to changes in temperature along predictable curves.
Both the thermistor and its high-precision "standards thermometer" cousin - the RTD - measure resistance in a resistor attached to an electronic circuit to calculate temperature. Thermistors typically use ceramic beads as resistors, while RTD's often use either platinum (a highly stable metal) or other metal films.
With thermistors, resistance decreases with temperature and with RTD's, resistance increases. Both thermistors and RTD's 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 (called 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 when 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 (32°F/0°C) or an electronic "cold junction" compensation that aids in the calculation. Thermocouples can detect temperatures across wide ranges and are typically very fast.