Temperature is one of the most commonly measured quantities of a device or the conditions surrounding it – and particularly for electronic components. This is because electronic devices and circuits generate heat and require some type of thermal management.
There are several types of temperature sensors that work well for such applications and that offer different features or specifications. For example, a temperature sensor may offer an analog or digital output.
In this tutorial, we'll cover some of the most common types. These sensors can be broadly categorized as:
1. Contact Temperature Sensors – These types require contact with an object to detect its temperature and can be used to measure the temperature of a solid, liquid, or gas.
2. Non-contact temperature sensors – detect the temperature of an object or its surroundings using radiation or convection. These sensors are mainly used to measure liquids or gases. However, sensors that use infrared radiation are also capable of detecting the temperature of solid objects.
Within these two classes of temperature sensors, there are several different types to choose from.
Here are some…
Thermistors
The term “thermistor” is an abbreviation for “thermally sensitive resistor”. It is a special type of resistor with resistance that changes according to temperature.
For example:
- If the resistance of a thermistor increases with increasing temperature, it has a positive temperature coefficient and is called a PTC thermistor.
- If the resistance of a thermistor decreases with increasing temperature, it has a negative temperature coefficient and is called an NTC thermistor. Most thermistors are NTCs.
Thermistors have a fairly low response time to any change in temperature. They are passive electronic devices and do not require current to pass through them to generate a voltage output. The physical resistance of a thermistor can vary from a few ohms, kilo-ohms or tens of mega-ohms.
A thermistor is a contact-type sensor that provides analog output and interface into a circuit using a voltage divider network. The potential divider network can also be interfaced with a differential amplifier or the analog input of a microcontroller for voltage reading.
These types of sensors are reliable, highly accurate, and durable. Thermistors are rarely damaged unless they are subjected to extremely high temperatures beyond their maximum limit. However, they can suffer physical damage because they are typically constructed from ceramic-type semiconductor materials such as oxides of nickel, cobalt and/or manganese. The semiconductor material is pressed into discs or spheres and hermetically sealed.
The most important features to consider when choosing a thermistor are:
- Resistance range
- Resistance-temperature curve
- Time constant (i.e. how fast its resistance changes with temperature)
- Physical resistance to room temperature
- Temperature range and power rating according to current flow
NTC thermistors generally have a non-linear resistance-temperature curve due to their exponential nature. But this can be leveled out for certain ranges. Standard thermistors have a typical temperature range between -50˚ to 150˚ C, while glass-encapsulated thermistors go up to 250˚ C.
Most thermistors can be easily interfaced with Arduino or other microcontroller platforms, as long as the selected board or controller has an analog input.
In a microcontroller-less circuit, these sensors can be interfaced with an operational amplifier using a voltage divider network to obtain binary completion for an application (such as if the temperature is lower or higher than a certain threshold).
Resistive Temperature Detectors (RTD)
RTDs have a positive temperature coefficient and their resistance increases with increasing temperature. These types of sensors have a high-purity conductive metal – such as copper, platinum or nickel – that is wound on a coil or in a thin film deposited on a ceramic substrate.
They are precision sensors that offer an extremely linear and precise resistance-temperature curve. However, they have low thermal sensitivity (typically 1Ω/˚C).
RTDs made from platinum are the most commonly used and are called platinum resistance thermometers or PTCs. PTCs are expensive.
Another disadvantage of RTDs and PTCs is that they heat up automatically. This means that their resistance is affected by heat due to the current flowing through them, which can lead to incorrect readings.
RTDs are contact-type sensors that provide an analog output. To compensate for their self-heating characteristic, RTDs are typically interconnected in a circuit using a Wheatstone Bridge network, which has a constant current source connected to it. This is to compensate for any standard errors or additional wires (used for shunt compensation).
Platinum RTDs have a linear resistance-temperature curve that is above the typical range of -200˚ to 600˚ C.
The PTD100 RTD is currently the most popular RTD available in 2-, 3-, or 4-wire packages. It has a resistance of 100Ω at 0˚C, which rises to 140Ω at 100˚C.
To measure temperature using an RTD, it must be connected to a Wheatstone bridge with a constant current source. The voltage output is measured to determine the resistance. The temperature can then be derived via the linear resistance-temperature relationship for a given RTD.
Thermocouple please
A thermocouple is the most commonly used contact-type temperature sensor. They are compact, inexpensive, simple to use and provide quick response time to temperature changes. These sensors offer the widest temperature range, which is between -200˚ and 2,000˚ C. A thermocouple is made of two wires of different metals, electrically connected by two junctions. Metals, for example, can be copper and Constantan.
A junction is kept at a constant temperature for reference and is called a cold junction. The other is used to measure temperature and is called the hot junction. Since the temperatures at both junctions are normally different, the potential between them is used to measure the actual temperature.
The junction between the two metals creates a thermoelectric effect, where a constant potential of a few millivolts is formed. This voltage difference between the junctions is called the “seeback effect”. Essentially, it's like a voltage gradient between the two.
When both junctions have the same temperature, they have zero voltage difference. When both junctions are at different temperatures, a voltage proportional to the temperature difference is generated. The voltage difference increases as the temperature difference between the two junctions increases – and until a peak voltage difference is generated. This voltage peak is determined by the characteristics of each metal.
Measuring the voltage output of a thermocouple requires an amplifier. There is normally only a few millivolts of difference with a 10˚C increase in temperature. Chopper and instrumentation amplifiers are commonly used because they offer superior drift stability with very high gains.
The measured voltage output of a thermocouple can be applied to the analog input of a microcontroller or to a regular amplifier circuit for logical completion.
Thermocouples are made from a variety of metals and have different temperature ranges depending on the combination of metals used in their construction. As a result, these sensors are listed and available based on standard codes and lead colors.
Thermocouples are chosen for an application based on its temperature range. Types R, J, and T are commonly used. Although thermocouples are inexpensive, they have low accuracy (0.5˚ to 5˚C tolerance) and a nonlinear temperature curve. Engineers typically need to match the sensor used with a look-up table to determine temperature conversion, control, and compensation.
Thermostats
A thermostat is an electromechanical temperature sensor constructed by bonding two different metals to form a bimetallic strip. When this strip is exposed to heat, it bends due to the different linear expansions of the two metals. Metals can be nickel, aluminum, tungsten or copper.
Thermostats are often used as electrical switches or to control an electrical switch in thermostatic controls. Thermostatic switches are motion controlled and can be:
1. An instant action type – which offers instant ON/OFF operation which is widely used in furnaces, hot water tanks, electric irons and other home heating appliances.
2. A repeat action type – used as dials or gauges and provides gradual temperature changes. These types are assembled as bimetallic coils or spirals and are more sensitive to temperature changes.
Thermostats are available for a wide range of temperatures, but they have low reliability due to high hysteresis. Typically, these sensors are only used in control applications where a precise temperature set point is used to operate like a switch.
Semiconductor-based sensors
Semiconductor-based temperature sensors are dual integrated circuits. Two identical diodes with temperature-sensitive voltage-current characteristics are integrated to detect temperature changes.
Overall, these sensors offer low accuracy (tolerance between 1˚ to 5˚ C), slow response capability (between 5 and 60 seconds), and a narrow temperature range (between -70˚ to 150˚ C).
Many semiconductor-based temperature sensors now come with internal amplifiers, generating an output of around 10mV/˚C. These sensors have better accuracy and high linearity. Semiconductor types are generally used with thermocouples for cold junction temperature compensation.
Infrared Temperature Sensors
Infrared sensors are a non-contact type of temperature sensor. They are photosensitive devices that detect infrared (IR) radiation from the surrounding area or an object to measure heat.
Thermopiles
Thermopiles are one of the most popular types of non-contact temperature sensors. They are used to measure heat as well as gas concentrations.
Thermopiles are often used in industrial process control, medical temperature readers, heat alarms, microwave ovens, road ice detection, and automobiles.
Conclusion
Thermocouples are the most common contact-type temperature sensors because they are inexpensive, offer the widest temperature range, and have acceptable characteristics for most applications.
For better accuracy, precision, and response, however, RTDs and thermistors are preferred. The most important considerations when choosing a contact-type temperature sensor are size, cost, temperature range, and accuracy. These factors also depend on the application.
Non-contact temperature sensors are typically based on detecting infrared radiation. The most important factors to consider when choosing non-contact temperature sensors are cost, reliability, accuracy, and application. Some popular applications that use non-contact temperature sensors are thermal imaging, infrared thermometers, and infrared scanning.