Sensors are one of the key drivers of Industry 4.0 and the Internet of Things (IoT) in factories and workplaces. They help optimize product design, production, quality control, asset management, and maintenance in a factory setup, leading to greater efficiency and productivity gains.
Among the top sensors used in smart manufacturing is a temperature sensor that measures temperature, one of the vital parameters to be measured and controlled in a wide variety of needs and applications that can directly affect material properties and product quality.
One of the commonly used sensors in industrial IoT systems, the temperature sensor, is a device that has the ability to collect temperature data from a resource and then changes it into information that can be understood by another device. It detects temperature changes, produces either an analog or digital output, and can measure the thermal characteristics of gases, liquids, and solids. Accuracy of the temperature reading is essential, as it enables manufacturers to push performance closer to safety limits.
There are a wide variety of temperature measurement systems in use today, depending on what you want to measure and how accurately you want to measure it. However, thermometers are the most common and oldest temperature sensors encountered in simple, everyday temperature measurements. Typically, they are used in heater systems, air conditioners, climatizing units, washing machines, overheating protection, and other appliances.
Several other temperature sensors have been developed in recent years, which can be used in electrically and chemically hostile environments. Temperature sensors are divided into two groups: (a) low-temperature sensors, with a range of −100 to +400 ◦C, using sensing materials such as phosphors, semiconductors, and liquid crystals; and (b) high-temperature sensors with a range of 500 to 2000 ◦C, based on blackbody radiations.
They are also categorized into contact temperature sensors, which require direct contact with the system being sensed, and non-contact temperature sensors, which use convection and radiation to detect the temperature changes. These two sensors could further be classified into electro-mechanical, resistive, and electronic types.
Below is the list of different types of commonly-used temperature sensors, along with their key features.
1. Thermocouple
Thermocouples, a type of electronic sensor, are one of the most routinely used types of sensors due to their simple design, small size, ease of usage, and quick response to temperature changes.
Thermocouples are made of two dissimilar electrical conductors, forming electrical junctions at different temperatures. As a result of the thermoelectric Seebeck effect, temperature changes cause a temperature-dependent voltage, which is, in turn, converted into a temperature reading. Thermocouples can detect temperatures as high as 3000 ◦C and as low as −250 ◦C.
Most widely used in industrial measurement due to its inexpensive, rugged, and reliable nature, it has a wide temperature range, high-temperature measurement; high resistance to shock and vibration; fast thermal response.
Pros:
- Simple
- Inexpensive
- Large variety
- Large temperature range
- Self-powered
- No self-heat
- Rugged
Cons:
- Cold-junction compensation
- Accuracy
- Stability
- TC extension leads
- Non-linear
- Low voltage
- Require reference
- Less stable
2. Thermistor
Also called thermally sensitive resistors, thermistors change their physical appearance with temperature changes. They are passive components, with a resistance much dependent upon temperature. Their effective operating range is −50 ◦C to 250 ◦C. Widely used in the automobile industry to detect the intake and coolant temperature, they have fast thermal response, and lead wire resistance results in a small error. The limitations of these temperature sensors are their limited temperature range and low resistance to shock.
Pros:
- Sensitivity
- Accuracy
- Cost
- Rugged
- Flexible Packages
- Hermetic Seal
- Surface Mount
Cons:
- Non-linearity
- Self-heating
- Moisture failures (non-glass only)
3. Resistance thermometer
Resistance thermometers have a fixed relationship with temperature and their resistance changes as temperature changes. They are made of pure material like platinum, nickel, or copper, with a highly predictable resistance/temperature relationship.
Known for accuracy and stability, these sensors detect temperature changes ranging from −50 ◦C to 500 ◦C for thin film and −200 ◦C to 850 ◦C for a wide film. They are widely used as HVAC, room, duct, refrigerant temperature, motors for overload protection, and automotive for air and oil temperature detection. Resistance thermometers have high precision and stability, strong output signal, high sensitivity, good stability (can maintain temperatures below 0.1 ◦C for a long time). But the limitations are that they are expensive; easily influenced by lead wire resistance, slow thermal response, low resistance to shock and vibration.
Pros:
- Accuracy
- Stability
- Linearity
- More Stable
- More accurate
- More linear
Cons:
- Lead resistance error
- Response time
- Vibration resistance
- Size
- Package limitations
- Expensive
- Small
- Self-heating
- Small base resistance
All sensors have specific advantages and disadvantages. To choose the best temperature sensors that suit your need, you should consider these criteria: cost, temperature range, interchangeability, long-term stability, accuracy, sensibility (output), response time, linearity, self-heating, extension cable effect, and sensor dimension (size).
