There are four main temperature sensors used today in modern-day electronics: Negative temperature coefficient (NTC) thermistors, resistance temperature detectors (RTDs), thermocouples, and semiconductor-based integrated (IC) sensors.
Temperature sensors are essential to everyday life. They measure the amount of heat a system or object gives off, typically setting off an alarm when a temperature change exceeds the application’s guidelines. Therefore, temperature sensors play a critical role in providing time for preventative action.
In this article, we will look at the four main temperature sensors, the considerations for each, and the advantages and disadvantages of using them.
What Is A Temperature Sensor?
A temperature sensor is an electrical instrument that measures the temperature of the air, liquid, and solid matter in a wide range of industries and applications.
A temperature sensor works by providing a readable temperature measurement on a meter from electrical signals produced inside a temperature probe. The working principle of a temperature sensor depends on the voltage across the diode inside the temperature probe. The change in temperature is directly proportional to the diode’s resistance. For example, the warmer the temperature, the more resistance, and vice-versa.
Applications Of Temperature Sensors
Temperature sensors are extremely useful to cater to both commercial and consumer needs, which is why they are used in the following industries and applications:
Industrial applications to control the heat of electrical radiators.
Medical applications to monitor temperature measurements in MRI machines and ultrasound scanners.
Laboratory applications in the pharmaceutical industry and environmental temperature control in labs.
Household appliances such as refrigerators/freezers, microwaves, and ovens.
Computers to prevent overheating.
Food and beverage industries for sanitary purposes and minimizing waste.
Contact And Non-Contact Temperature Sensors
Before we look at the types of temperature sensors, it is essential to understand which physical kind of temperature sensor your application requires. There are two main physical kinds of temperature sensors:
Contact Temperature Sensors
Contact temperature sensors are used when you can make good thermal contact with an object, liquid, or gas. They are used to identify the temperature when it is expected below 3400 °F (1700 °C) or above -40 °F (-40 °C).
Contact temperature sensors include thermistors, thermocouples, and resistance temperature detectors.
Non-Contact Temperature Sensors
As the name suggests, non-contact temperature sensors do not have to be in direct contact with what you are measuring. Instead, they use infrared technology to measure the surface temperature remotely.
They are used when:
The object measuring is moving.
Contact could damage the temperature probe or object, such as if the object is extremely hot or corrosive.
Contact could change the temperature.
A large area needs to be measured.
The object is far away or difficult to access, like in space.
Non-contact temperature sensors include fiber optic sensors, radiation thermometers, optical pyrometers, and thermal imagers.
Common Types Of Temperature Sensors
There are four common temperature sensors used in the market today:
Resistance Temperature Detectors (RTDs)
Thermistors (Negative Temperature Coefficient (NTC))
Resistance Temperature Detectors (RTDs)
Resistance temperature detectors, or RTDs, change the resistance of the RTD element directly with temperature. RTDs are made from a film and a glass or ceramic core with wire wrapped around it for greater accuracy. While platinum RTDs (PRTDs) are more expensive, they are the most accurate temperature sensor. In addition to high accuracy, PRTDs offer stable readings, repeatable responses, and they can be used over a wide temperature range (-200 to 600 °C).
Platinum RTDs are available with a 100 Ω and 1000 Ω resistance at 0 °C, which is why they are referred to as PT100 and PT1000.
RTD sensors that are made from nickel and copper are also used because of their lower cost, but, they are not as stable or repeatable as PRTDs.
Compared to other types of temperature sensors, RTDs typically have higher thermal mass, and therefore they usually respond slower to temperature changes than thermocouple temperature sensors. RTDs also require an excitation current to flow through the meter to calculate the resistance.
RTD configurations include two, three, and four-wire options:
Two-wire: Used when the lead length is short enough that the resistance doesn’t affect the accuracy.
Three-wire: This configuration adds an RTD probe to carry the excitation current, providing a way to cancel the wire resistance.
Four-wire: This wire eliminates wire resistance by incorporating separate force and sense leads. This is the most accurate configuration.
Thermistors (Negative Temperature Coefficient)
Thermistors are similar to RTDs, as the resistance varies with temperature. As thermistors have a non-linear temperature relationship, they require correction to interpret the data accurately. This relationship means that thermistor temperature sensors can supply a large resistance variation over a small temperature working range, which is why they are used for measuring temperature in high-tech and set-point applications.
They are typically made from polymer or ceramic covered in a glass surface, which is why they are cheaper and less accurate than RTDs. Thermistors are, however, still accurate compared to other types of temperature sensors, because of their repeatability and quick response to variations in temperature.
Negative temperature coefficient (NTC) thermistors are the most commonly used thermistor to measure temperature. An NTC thermistor’s resistance decreases when temperature increases. Glass-encapsulated thermistors have an operating range of -72.4 to 482 °F (-50 to 250 °C) and standard thermistors have a range of up to 302 °F (150 °C).
Thermocouple temperature sensors are the most commonly used in industrial, automotive, and everyday applications in your home. As they are self-powered, they require no excitation, have quick response times, and they can operate over the widest temperature range (-328 to 3182 °F/-200 °C to 1750 °C.).
Thermocouples are made by joining two dissimilar metal wires, electrically bonded at two points. The connecting end is called the “hot junction”, and the other end is known as the “cold junction”. The varying voltage between the two metals mirrors proportional changes in temperature.
The working principle is very simple. When the two dissimilar metal wires are fused, they create a thermoelectric result, which provides a constant potential difference. The voltage between the metals is called the “Seebeck effect”. If both parts have the same temperature, the potential difference is zero, and therefore, has no output voltage. But, when the parts have different temperatures, the output voltage is relative to the temperature difference.
Thermocouples are made from a variety of different materials, allowing the temperature sensor to measure different temperature ranges and sensitivities. The most common thermocouple sensor used is the K-type, and all other thermocouples are also designated using letters (J, R, and T).
The biggest disadvantage to using a thermocouple sensor is its small output voltage, making it challenging to measure the temperature of the object or substance. Because of the small voltage and cold junction compensation (CJC), thermocouples require precision references with low noise, and often some amplification solutions are needed.
As thermocouples are also non-linear, they require a conversion table to work out the temperature.
Semiconductor-Based Integrated Sensors (ICs)
Semiconductor-based temperature sensors are typically incorporated into an integrated circuit (IC). The two identical diodes with temperature-sensitive voltages monitor temperature changes. Integrated circuit sensors have a linear response, however, they have the lowest accuracy of temperature sensors. This is because ICs have the slowest response across narrow temperature ranges (- -70 °C to 150 °C).
There are two types of ICs:
Local temperature sensors: Measure the temperature using the physical properties of a transistor. They can use an analog or digital output.
Remote digital temperature sensors: Measure the temperature of an external transistor. The transistor is located away from the sensor chip.
Which Temperature Sensor Is The Most Accurate?
If you require the most accurate temperature sensor, an RTD is the best choice. RTD temperature sensors are the most accurate and stable electronic devices for measuring temperature. Here at Atlas Scientific, we only offer high-quality platinum RTD temperature sensors to always provide you with highly accurate readings and low latency.
Depending on which application or industry you work in, will depend on which temperature sensor to use. Platinum resistance temperature detector (PRTD) sensors are the most accurate, yet, negative temperature coefficient (NTC) thermistors, thermocouple sensors, and semiconductor-based integrated sensors (ICs) are also widely used.
If you have any questions regarding temperature sensors or are unsure which temperature sensor will best suit your needs, please do not hesitate to contact the world-class team at Atlas Scientific.
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