Temperature sensors come in a wide variety and are extensively used in everyday products like automobiles, consumer electronics, and household appliances, each containing one or several temperature sensors.
Among all sensor types, temperature sensors were the earliest to emerge, leading to the development of various kinds like thermocouple sensors, RTD platinum resistance, and integrated semiconductor temperature sensors. With technological advancements, new types of temperature sensors continue to emerge.
Comparison of Common Temperature Measurement Solutions
Temperature sensors are widely used, from industrial process control temperature transmitters to essential household electronic thermometers, all requiring temperature sensors for detection. However, the temperature measurement solutions vary across these applications.
The main types of temperature measurement solutions based on their principles are:
- Platinum Resistance RTD
- Thermistors NTC
- CMOS Temperature Sensors
Thermocouples have the widest temperature range, from -200°C to 2000°C, but require an external reference point, making them relatively complex to use.
Platinum Resistance RTD sensors offer high precision and a broad temperature range but are costly and have complex peripheral circuits.
NTC thermistors are cost-effective but limited in accuracy, characterized by a large temperature coefficient and non-linear output.
CMOS temperature sensors, also known as IC temperature sensors, include both analog and digital output types. Compared to the aforementioned sensors, CMOS sensors offer high linearity, low system cost, high functional integration, simple peripherals, and digital output support. Their main drawback is the limited temperature range, typically between -40°C and 125°C.
A comparative chart can provide a more intuitive understanding:
|Digital Temperature Sensor
|Dependent on calibration
These differences determine their varied application scenarios. Thermocouples and RTD solutions, with their wide temperature ranges and complex usage, are mostly limited to industrial applications.
NTC thermistors, due to their low cost and ease of use, find widespread application in automotive water and oil temperatures, engine intake temperatures, cylinder and exhaust temperatures, and in household appliances like air conditioners, refrigerators, and rice cookers. They are also used in IoT applications for environmental temperature measurements, water temperature probes, and electronic thermometers.
CMOS digital temperature sensors, traditionally in IC form with standard SOP8 pin packaging, are used in board-level temperature measurements in electronic products, such as hard drives and motherboards, primarily with I2C interface output, though some use analog voltage output. As Moore’s Law progresses, the performance of CMOS-based digital temperature sensors continues to improve, with decreasing costs.
Typical Products of Various Temperature Sensors
Thermocouple sensors consist of two different metals joined together at one end by a thermocouple junction. When this junction and the other end of the wire are at different temperatures, a millivolt signal is generated, indicating the temperature at the junction.
These metals generate a small voltage that can be measured and interpreted by a control system. Each metal is insulated, with an outer coating maintaining a closely bonded dual-wire structure.
Platinum Resistance Temperature Sensor: Heraeus PT100/1000, etc.
RTD sensors are specified by their nominal resistance at 0°C, resistance temperature coefficient, and tolerance class. The nominal resistance is the resistance of the sensing element at 0°C. For example, in a PT100 RTD, the nominal resistance at 0°C is 100 ohms, and the material is platinum. The resistance temperature coefficient, which should be as high as possible, indicates the change in resistance per unit temperature change.
For platinum, this coefficient is 0.00392, meaning the resistance changes by 0.00392 ohms for every 1°C temperature change. The tolerance class indicates the accuracy of the RTD at the nominal temperature, 0°C.
IEC 751 defines platinum RTD with an accuracy of ±0.3°C at 0°C. RTD elements are made of platinum, copper, or nickel. Nickel-iron alloys, with thermal conductivity similar to nickel but double the resistivity, are also used as sensing elements.
NTC thermistors, or negative temperature coefficient thermistors, are high-performance ceramics made from a single, high-purity material with a near-theoretical density structure.
They are characterized by small size, minimal resistance and temperature characteristic fluctuations, and rapid response to various temperature changes, allowing for high sensitivity and precision in measurements.
CMOS Temperature Sensors
CMOS temperature sensors use the temperature characteristics of CMOS transistors for temperature measurement. In these transistors, the threshold voltage is inversely proportional to temperature.
Utilizing this characteristic, the temperature can be determined by measuring the threshold voltage. Specifically, a CMOS temperature sensor comprises a series of identically sized and type-matched CMOS transistors.
Applying different currents to these transistors establishes a linear relationship between each transistor’s threshold voltage and temperature. By measuring these voltages, an approximate temperature can be obtained.
ST introduced the latest TMOS sensor STHS34PF80. TMOS sensors, based on CMOS technology, operate on the principle that any object above absolute zero (-273.15°C) emits infrared radiation due to molecular and atomic movement. The magnitude and wavelength of this radiation correlate closely with the object’s surface temperature.
TMOS sensors measure the radiation energy and wavelength within the FOV range to determine the object’s presence and motion status. They can measure absolute temperatures, detecting 5-20um infrared radiation, and are widely used in smart homes, buildings, energy-efficient lighting control, industrial temperature monitoring, IoT, and more.
MEMS Temperature Sensor IC
This sensor features a newly designed ASIC chip, an improved MEMS capacitive humidity sensing element, and a standard on-chip temperature sensing element, significantly enhancing performance and reliability beyond the previous generation.
The next-generation temperature and humidity sensors respond quickly, are more interference-resistant, and maintain stable performance in harsh environments. These sensors are widely used in HVAC, dehumidifiers, testing and measurement equipment, consumer products, automotive, automatic control, data loggers, weather stations, household appliances, humidity control, medical, and other related humidity detection and control areas.
How to Select Temperature Sensors?
Temperature is a vital parameter that must be controlled in industrial production, having direct impacts on the quality of products, equipment safety, and personal safety.
Choosing a temperature detection instrument should not be a blind pursuit of high accuracy, large range, and high automation. Instead, it should be a comprehensive consideration, taking into account the specific process in industrial production, the reality of the measured medium, and economic factors.
The principle to follow is that the upper and lower limits of the measuring instrument’s temperature measurement should exceed the fluctuation range of the medium temperature. The measuring accuracy should meet the technical requirements of the production process.
The method of use should meet the observation needs of the measurement personnel, facilitate routine maintenance and repairs, and, on this basis, select as economical an instrument as possible.
1. Selection by Usage
For local display only, options can include liquid glass, bimetal, and pressure thermometers. If the device needs to measure temperature and alert when the measured temperature approaches the limit, opt for a liquid glass, bimetal, or pressure thermometer with an additional alarm device. If remote display is required, consider a thermistor, thermocouple, or temperature transmitter.
2. Selection by Measurement Range
The temperature of the measured medium is a critical basis for choosing an appropriate detection instrument. For normal temperature measurements, options can include thermocouple thermometers, thermistor thermometers, pressure gauges, and bimetal thermometers.
Organic liquid glass thermometers are characterized by their red indicating liquid, which is conducive to reading, but they can’t carry electrical contacts. So, when measuring mediums at temperatures below 100℃ without needing to send signals, organic liquid glass thermometers can be prioritized.
The primary advantage of bimetal thermometers is their clear scales, shock resistance, and mercury-free feature. Therefore, when the temperature of the measured medium is below 300℃, it is best to choose a bimetal thermometer.
When the medium’s stable temperature is below 150℃, a copper thermistor can be chosen. When the medium’s temperature ranges from 300℃ to 600℃, a nickel-chromium-copper thermocouple can be selected.
However, because the copper alloy wire is easily oxidized, it is best to use a nickel-chromium-nickel silicon thermocouple when measuring steam temperatures exceeding 500℃.
When the medium’s temperature ranges from 600℃ to 1000℃, a nickel-chromium-nickel silicon thermocouple can be selected. When the medium’s temperature ranges from 1000℃ to 1300℃, a platinum-rhodium-platinum thermocouple should be used.
For extremely high temperatures, a radiation high-temperature meter or an infrared high-temperature meter can be selected.
3. Selection by Required Measurement Accuracy
If very high measurement accuracy is required, platinum thermistors, platinum-rhodium-platinum thermocouples, or platinum-rhodium-platinum-rhodium thermocouples can be selected. If the required measurement accuracy is not very high, copper thermistors and nickel-chromium-nickel silicon thermocouples can be chosen.
4. Selection by Chemical Characteristics of the Measured Medium
Most thermocouples are very stable in oxidative or neutral mediums, but they are not suitable for long-term work in reducing mediums. Likewise, platinum thermistors should not be used for long periods in reducing mediums.
Copper thermistors are prone to oxidation when the temperature reaches 100℃. Thermistors are also highly susceptible to oxidative degradation.
Therefore, to prevent such occurrences, the installation of protective sleeves is necessary. The suitable protective sleeve material should be chosen according to the chemical properties of the medium.
For example, for thermocouples: For temperatures below 600℃, medium carbon steel, copper, or lead can be used for the sleeve. For temperatures below 1000℃, austenitic stainless steel (heat and corrosion-resistant) is often chosen.
Additionally, the coordinated use of secondary instruments, thermocouple compensation wires, and free end temperature compensators must be given importance. Installation should ensure the accuracy of detection and be conducive to instrument repair and calibration, avoiding temperature measurement lag.
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