Classification of Temperature Sensors

Classification of Temperature Sensors

The temperature sensor is the most common in industrial IoT, agriculture, medical field etc. According to the mode of action, it can be divided into contact and non-contact. Contact temperature sensors can be divided into thermocouples, RTDs and integrated temperature sensors. Non-contact temperature sensors mainly include infrared temperature sensors and fiber optic temperature sensors. We will mainly introduce the contact temperature sensor below.

1: Thermocouple
 measurement principle
Thermocouples are made of two different conductors or semiconductor materials welded or twisted together and are divided into hot and cold ends. The hot end is inserted into the object to be measured, and the cold end is placed outside the thing to be measured. If the temperature of the two ends is different, a thermal potential will be generated in the thermocouple circuit. Since the thermoelectric potential is a function of the measured temperature, the value of the electric potential can be converted to a temperature value once it is measured.

 Temperature range: Commonly used thermocouples from -50 ~ +1600 ℃ can be measured continuously. Some special thermocouples can be calculated as low as -269 ℃ (such as gold, iron, nickel and chromium), up to +2800 ℃ (such as tungsten – rhenium).
 Structure of thermocouple: The shape of the thermocouple bonding end consists of 3 types, as shown in the figure below

 Common thermocouple types:

Type Thermal electrode materials
Positive Pole Negative Pole
R Platinum-rhodium 13% Platinum
S Platinum-rhodium 10% Platinum
B Platinum-rhodium 30% Platinum-rhodium 6%
J Iron Copper-Nickel
T Copper Copper-Nickel
E Nickel Chromium Copper-Nickel
K Nickel Chromium Nickel Aluminum
N NiCrSi Nickel-silicon

2: RTD
 measurement principle
RTD is based on the thermal effect of resistance for temperature measurement. The resistance of a metallic conductor varies with temperature. Therefore, the temperature can be measured by simply measuring the change in resistance of the metallic conductor. The following approximate relationship can generally express the resistance value and temperature of the metal. Rt=Rt0[1+α(t-t0)] Rt is the resistance value at temperature t. α is the temperature coefficient.
 Characteristics:
Temperature range: -200~500℃
Accurate measurement
Good stability
Reliable performance
 Common RTD:
RTDs are mostly made of pure metal materials. Platinum and copper are mostly used today. Platinum resistors with good stability, high precision, a certain non-linearity and the smaller resistance change at high temperature are suitable for neutral and oxidizing media. Copper resistors have a linear relationship between resistance and temperature. With a large temperature coefficient, it is ideal for non-corrosive media, easily oxidized above 150°C.
3: Thermistors

The resistance value of thermistors varies with temperature. The resistance that increases as the temperature increases is called a positive temperature coefficient thermistor (PTC). The resistance that decreases as the temperature increases is called a negative temperature coefficient thermistor (NTC). Unlike RTDs, which use pure metal, thermistors usually use ceramic or polymer.
4: Integrated temperature sensor
(1) Analog temperature sensors
 Measurement principle
It integrates the driver circuit, the signal processing circuit and the necessary logic control circuit on a single IC. It has the advantages of small size, easy to use, high sensitivity, good linearity and fast response.
 Common analog temperature sensors
Voltage output type: M3911、LM335、LM45、AD22103
Current output type: AD590
LM135\235\335 series is a high-precision easy-to-calibrate integrated temperature sensor produced by National Semiconductor (NS). It is a voltage output temperature sensor. Its operating characteristic is similar to the Zener regulator. Its characteristics are as below.

sensitivity: 10mV/K
less than one dynamic impedance
Working current range: 400uA ~ 5mA
Precision: 1 ℃
Temperature range:
LM135 -55 ℃ ~ +150 ℃
LM235 -40 ℃ ~ +125 ℃
LM335 -40 ℃ ~ +100 ℃
Package: TO-46, TO-92, SO-8
These components are widely used in temperature measurement, temperature difference measurement and temperature compensation systems.
The AD590 is a current output temperature sensor from American Analog Devices. Its characteristics are as below.
sensitivity: 1uA/℃
Supply voltage: 3 ~ 30V, It can withstand 44V forward voltage and 20V reverse voltage.
Temperature measurement range: -55 ℃ ~ +150 ℃
The output current range: 223uA ~ 423uA
Maximum non-linear error: 0.3℃,
Power consumption: 2mW
1uA change in output current corresponds to a 1°C change in temperature. The output current signal can be transmitted over a distance of more than 1km.
It is suitable for multi-point temperature measurement and long distances temperature measurement and control.

(2) Digital Temperature Sensors
 measurement principle
It integrates the sensitive components, A/D converter unit, memory on a single chip.
Integration of sensitive components, A/D converter unit, memory on a single chip. It outputs digital signals directly. It is easy to use but slow to respond.
 Common digital temperature sensors
The DS18B20 is the world’s first digital temperature sensor that supports a one-wire bus interface, manufactured by Dallas Semiconductor, Inc.
 Characteristics
Supply voltage range: 3 ~ 5.5V
Temperature measurement range: -55℃ to +125℃
±0.5°C accuracy from -10°C to +85°C
Programmable 9 to 12 bits resolution
Applications include thermostatic controls,
industrial systems, consumer products,
thermometers, or any thermally sensitive

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