Common Temperature Sensors
Temperature is a measure of the average kinetic energy of particles in a system. Adding heat to a system causes its temperature to rise. In the first part of this series, George details the characteristics of common temperature sensors.
Temperature is one of the most often measured quantities. Accurate temperature measurement is crucial for many automatically controlled processes. As electronic engineers we need temperature expressed as an electrical quantity, be it voltage, current, frequency, or a digital number. Consequently, I shall not discuss mechanical thermometers in this series. I’ll only touch on thermodynamic principles. For the purpose of this article we only need to understand that temperature is a measure of the average kinetic energy of the particles in a system. Adding heat to a system causes its temperature to rise. We’ll discuss some of the physics of heat later with respect to contactless thermometers.
Temperature value is commonly expressed using different scales, so let’s begin with a review of the common ones and show how they relate to each other. The most common and popular around the world today is the Celsius scale, which is expressed by a degree of Celsius (°C) and sometimes called a centigrade. Swedish scientist Anders Celsius (1701–1744) used two defining points for it. He designated the low point where the water was freezing as 0°C and the high point where water was boiling at 1 atmosphere (Atm) pressure as 100°C. About 60 years ago, those two points were redefined such that the absolute zero—where all molecular motion stops, known as 0K (Kelvin) equal to –273.15°C—became the new low point. The temperature where water exists in gas, liquid, and a solid state became the new high point. That triple point equals to 0°C, or +273.15K. This changed nothing on the Celsius scale, but calibrating temperature became much more accurate as opposed to the experimental measurements.
The Kelvin temperature scale (K) was introduced by William Thomson (Lord Kelvin). It is just the Celsius scale offset by –273.15 and used primarily in science.
In the United States, the Fahrenheit scale (°F) is prevalent in public use, although most scientific and engineering work is performed using °C or K. The Fahrenheit scale was introduced by German scientist Daniel Gabriel Fahrenheit (1686–1736). Its two defining points are 0°F, which is the coldest temperature of brine (essentially sea water), and 100°F, which is the typical highest core body temperature of humans (37.8°C).
Nowadays the defining points for the Fahrenheit scale have also been modified in reference to the Celsius scale. The triple point water temperature of 0°C now equals 32°F and the boiling point of water 100°C is 212°F when measured at sea level. The Celsius and the Fahrenheit scales intersect at –40°. That is –40°C = –40°F. To convert from Fahrenheit to Celsius (and vice versa) is just basic mathematics:
There are several other scales one might come across (e.g., Römer, Newton, or Delisle), but it is unlikely you ever will. Two other scales are sometimes mentioned in old novels and science fiction, such as those written by Jules Verne. The Réaumur scale has two defining points: 0°R is identical to 0°C and 80°R is equal to 100°C. The Rankin scale, on the other hand, is based on the Fahrenheit scale, except that its zero coincides with 0K.
Having reviewed the temperature scales, let’s turn our attention to the sensors that convert temperature into an electrical signal capable of being processed by electronics. Table 1 lists the commonly used temperature sensors and their main characteristics. We’ll take a separate look at contactless temperature sensing in an upcoming article.
Let’s begin with the thermocouple. In 1822 Thomas Seebeck discovered that when two wires of dissimilar metals were connected, as shown in Figure 1a, and each of the two junctions was exposed to a different temperature, a small current would flow through the loop. This phenomenon, known as Seebeck Effect, became the basis for measuring temperature with the use of a thermocouple. Thermocouples do not measure an absolute temperature; instead, they measure the temperature differential between the two junctions. The “hot” and the “cold” junction names are merely arbitrary, with the “cold” junction being considered a reference. The “hot” junction can be colder than the “cold” junction, in which case the current is reversed.
For the “cold” junction to act as a reference, it needs to be exposed either to a steady, accurate temperature or its immediate temperature needs to be accurately measured. In the past the cold junction used to be located in an environment with known, constant temperature, such as a vessel with melting ice maintaining 0°C, or a temperature stabilized oven. Nowadays, the cold junction is usually placed on a printed circuit board with the rest of the electronics or inside an integrated circuit (IC). The accuracy of the cold junction temperature measurement determines the achievable precision of the entire thermocouple setup. The cold junction temperature is usually determined by absolute temperature sensors located in its close proximity. Diodes, RTDs, and thermistors are popular. When the cold junction and its temperature measurement are an integral part of an IC, a diode is usually the most convenient temperature sensor to be used.
To measure Seebeck voltage, the loop needs to be broken and a circuitry to measure the voltage added. Connecting to the “voltmeter” (i.e., some electronic interface circuit) via two terminals (or solder joints), as shown in Figure 1b, creates additional parasitic junctions due to the inclusion of, most often, copper wire connections. These two or often more additional parasitic junctions do not cause a problem as long as they are isotherm—meaning, they are maintained at the same temperature. This should be relatively easy to achieve by carefully laying the circuit, connectors, and packaging.
There are eight standard thermocouples: J, K, N, E, R, S, T, and B. They are optimized for different operating temperatures. Their output voltage is a function of the temperature difference between the junctions and their type, such as J, K, and so on. For example, a type K thermocouple composed of Chromel (Nickel-Chromium) and Almel (Nickel-Aluminum) has a useful range from –200°C to 1,250°C (–328°F to 2,282°F). The ability of thermocouples to measure extremely high and low temperatures while maintaining mechanical ruggedness and low weight is one reason for their wide popularity in the aerospace, military, and manufacturing industries.
The thermocouples’ output voltage versus temperature is described—for some types by up to the fourteenth-order polynomials. If the thermocouple is used to measure temperature within a fairly narrow range, the temperature-versus-output voltage function can be sometimes reasonably linear. Otherwise, a common solution for conversion can be look-up tables provided by the thermocouple manufacturers or mathematical functions built into the software.
The output voltage of a thermocouple is quite small—typically, on the order of 40 to 60 µV/°C. Even though the circuit operates at a very low impedance, it must be well protected against common mode electromagnetic interference (EMI), voltage transients, and so forth. Due to the microvolt levels, the signal processing circuits must be designed for low noise, low drift, and high stability. For operation in safety-critical systems, it is necessary to be able to detect a fault in the thermocouple. The most common failure mode is an easy-to-detect open circuit.
The thermocouple interface is made simple by specialized ICs, such as the now relatively old Analog Devices AD596 or the newer Linear Technology LTC2983. The latter IC has the polynomials built in, can interface with all kinds of sensors, has built-in filters, and has the transient protection. When the sensed temperature is lower than the reference the output voltage goes negative. LTC2983 can handle it with just a single 2.85-to-5.25-V power supply. The measured temperature is output on the SPI bus in both Celsius and Fahrenheit.
RESISTOR TEMPERATURE DETECTORS
You should now have a decent understanding of temperature sensing with thermocouples. In the next part of this series, we will investigate temperature sensing by diodes, resistance temperature detectors (RTDs), thermistors, and touch on contactless detectors.
Analog Devices, “Thermocouple Conditioner and Setpoint Controller,” AD596, 1998, www.analog.com/media/en/technical-documentation/data-sheets/AD596_597.pdf.
Linear Technology, “LTC2983—Multi-Sensor High Accuracy Digital Temperature Measurement System,” www.linear.com/product/LTC2983.
M. Mayes, “Temperature-to-Bits: One IC for All Sensor Types, 0.1°C Conformity,” Vol. 24, Number 4, LT Journal of Analog Innovation, 2015.
PUBLISHED IN CIRCUIT CELLAR MAGAZINE • JUNE 2016 #311 – Get a PDF of the issue