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When it comes to temperature control in industrial applications, the choice of the suitable temperature sensors often narrows to thermistors vs thermocouples.
مقدمة
In February 2024, a woman in South Carolina, United States, lost a hand to an overheating hair dryer. Mary Wilson blamed the appliance for lacking an automatic shut-off mechanism. If it did, she said, then her hand – which doctors had to amputate – would still be intact. The accident ended her long-cherished career as a dog groomer. Wilson’s hair dryer would have turned off automatically if it had a functional temperature sensor.
Having the right temperature sensor can ensure safety and optimal performance. Conversely, a wrong or faulty sensor can impact an application’s overall functionality and even endanger lives. Also,understanding how temperature sensors work is crucial regardless of your application setting, whether it is domestic, industrial or medical.
In this article, we explore the features of thermistors and thermocouples in detail and highlight their various applications for your benefit. In the end, we believe you will gain the requisite knowledge for selecting the most suitable temperature sensor for your application.
Thermistors Vs Thermocouples: What are Thermistors?
Thermistors are temperature sensing devices whose resistance changes with the temperature. They come in two varieties: Negative Temperature Coefficient (NTC) thermistors and Positive Temperature Coefficient (PTC) thermistors.
NTC thermistors
These are sensors whose resistance reduces as the temperature increases. They can be categorized into three types, based on their make and manufacturing process.
Bead Thermistors
- Small size: A range of 0.075 – 5mm.
- Faster response rate: A rapid response rate due to their small size.
- High sensitivity: They are highly sensitive to fluctuations in voltage and current.
- Robust construction: Usually encapsulated in glass to protect the thermistor element from damage and humidity.
- Broad temperature range: Operating across a wide temperature spectrum, usually ranging from -50°C to 300°C.
Disk and Chip Thermistors
- Large size: Ranges from 0.25-25mm.
- Higher current capacity: More capable of managing larger currents and dissipating more power.
- High stability and accuracy: Commercial accuracy down to ±0.05°C.
- Wide ohmic value range: Wide range of resistance values, typically from 1 ohm to 500,000 ohms.
High sensitivity: Their resistance decreases exponentially with increasing temperature.
الثرمستورات المغلفة بالزجاج
- Wide temperature range: Effective over a temperature range of -40°C to 200°C. The temperature range can be extended to 300°C by using nickel leads.
- Small size:Typical sizes range from 0.4 mm to 10 mm.
- Non-corrosive: Prevents the sensor from corrosion and protects it from environmental factors.
- Moisture resistant: Protects the thermistor element from moisture and environmental contaminants, ensuring stable function in harsh environments.
- Fast response time: Thermal response time is typically less than 1 second.
What Are the Applications of NTC thermistors?
NTC thermistors are ideal for use in applications that require exact temperature measurement within a narrow temperature range. Some of the industrial applications include voltage regulation, solar heating systems, oil tanks, surface temperature sensing and volume control.
NTC thermistors are also used in:
- الأجهزة المنزلية including items like ovens, coffee makers, air conditioners, and refrigerators for temperature monitoring and control.
- Consumer Electronics encompassing mobile devices, tablets, and smartphones for temperature compensation and detection, particularly when charging batteries.
- Medical Devices like thermometers and catheters for accurate measurement of temperature.
- LCD Displays to compensate for changes that are induced by temperature, thus ensuring that optimal display performance is maintained over different temperatures.
- LED Lighting for temperature control and thermal protection which prevents overheating and prolongs the LEDs’ lifespan.
- Thermal Printers for controlling the temperature during the printing process. This enhances print quality and the printhead’s longevity.
- Battery Packs to protect them from overcharging and overheating, thereby guaranteeing safety and optimal performance.
PTC thermistors
These are sensors whose resistance increases as the temperature rises. They can be classified into silicon or silistors PTC thermistors, ceramic switching thermistors and polymer PTC thermistors. The classification depends on the manufacturing process, structure and materials.
Silicon PTC Thermistors
- Temperature range: Typical range is -55°C to 150°C, with some models measuring up to 180°C or higher.
- Stability：Silicon is a natural stabilizing material that makes thermistors stable and reliable over time.
- Response speed：Response times range from ~100 seconds to ~1 millisecond.
- Durability： Waterproof for use with liquids and other wet conditions.
Ceramic Switching Thermistors
- Small size: Multi-layer design with dimensions as small as 0.02 x 0.01 inches.
- Fast response time: response times typically range from a few milliseconds to several seconds.
- High sensitivity: Resistance rises when specific temperature thresholds are exceeded.
- Cost-effective: the cost of designing and manufacturing them is usually very low.
Polymer PTC Thermistors
- Fast Response: They can limit current within milliseconds.
- Low Resistance: When a fault occurs (e.g., excessive current), the particles separate, reducing the conductive path.
- Resettable: Once the overcurrent incident is resolved, they can automatically return to a low resistance state once reset.
- Cost-effective: Compared to ceramic PTC thermistors, these thermistors are less expensive to produce.
What Are the Applications of PTC thermistors?
PTC thermistors are suitable for self-resetting fuses which protect circuits against short circuits, overcurrent, and overheating. If there is an excessive current flow, the fuse increases its resistance and thus limits the current. This prevents damage.
PTC thermistors are also used to:
- Limit inrush current in industrial inverters, motor starters and onboard chargers.
- Trigger or control machines in automated assembly systems and packaging lines.
- Regulate temperature in industrial applications such as automotive components, computing devices, medical equipment, and HVAC (Heating, Ventilation and Air Conditioning) systems.
- Safeguard telecommunication applications against overvoltage and overcurrent that can result from surges caused by lightning strikes, induction or direct contact between phone lines and power lines.
Thermistors Vs Thermocouples: What are Thermocouples?
Thermocouples are temperature sensors that consist of two different metals joined together at each end. When there is a change in temperature at this joint, it generates a voltage in the thermocouple’s electrical circuit, which, in turn, can be used to measure temperature.
Furthermore, there are different types of thermocouples, with the most common ones being Type K, Type J, Type T, and Type E.
- Type K Thermocouple is constructed from alumel and chromel and operates effectively within a temperature range of -200°C to 1,372°C. It is one of the most commonly used thermocouples, particularly well-suited for applications like power plants, HVAC systems and industrial processes.
- Type J Thermocouple comprises constantan and iron and features a temperature range extending from -40°C to 750°C. Its primary use includes furnaces, ovens and food processing.
- Type T Thermocouple consists of constantan and copper and can measure temperatures ranging from -200°C to 350°C. It is often used in low temperature applications such as cryogenics and refrigeration.
- Type E Thermocouple is made from a combination of chromel and constantan. It functions effectively within a temperature range of -200°C to 900°C and is predominantly utilized in applications such as aerospace, petrochemical and automotive industries.
Thermistors Vs Thermocouples: Comparison
While thermistors and thermocouples are both temperature sensors, they differ in various aspects as outlined in the table below.
| ميزة | Thermistor | Thermocouple |
| Operation | Measures temperature change through resistance. | Measures temperature change through voltage generation. |
| نطاق درجة الحرارة | Limited temperature range of typically -50°C to 150°C. | Broad temperature range of -200°C to 2000°C. |
| دقة | Higher accuracy within a specified range. | Varies from moderate to high depending on the specific type. |
| Sensitivity | High sensitivity and thus can quickly detect subtle temperature changes. | Moderate to low sensitivity due to their wide temperature range. |
| Response Time (Based on Size and Packaging) | Faster response time because of their small mass. Typically 0.2-10 seconds | Slower response time although this is usually adequate for most industrial applications. Typically 0.12-10 seconds |
| Durability | Less durable and easily affected by the conditions in which they operate. | Can endure harsh environments such as corrosive surroundings and thus are long lasting. |
| Construction Material | Made from cobalt oxides, nickel, manganese or semiconductors. | Made from different metal alloys that include iron, copper, constantan, chromel, rhodium and platinum. |
| Cost | Cheaper due to their materials and simple make | Expensive because of their complexity and additional circuitry. |
| Standardization | Non-compliant with international standards. They are therefore not interchangeable across different manufacturers. Manufacturers have the prerogative of determining the designs and specifications. | Comply with international standards and thus can be used interchangeably across various manufacturers. |
| Power Source | External | Self-powered. |
How to Choose the Best Solution for Industrial Use
Temperature sensors in industrial applications can affect product quality, process control and operational costs. It is therefore crucial to choose the appropriate sensor for your unique requirements. Here are some factors to consider:
Temperature range: The temperature sensor should have the capacity to function within the expected temperature range. For example, the temperature of a fully warmed up automobile engine block can exceed 100°C and a thermocouple would be the best to use.
Accuracy: While some applications can function within a wider range, others require more precision. For example, food processing and chemical industries may require higher accuracy compared to metal processing application.
Durability and Cost: If the sensor is to be used in harsh conditions such as corrosive atmospheres and high temperatures, then a thermocouple is the best. The sensor’s overall cost-effectiveness can be determined by evaluating its initial cost in relation to the long term benefits.
وقت الاستجابة: Some applications such as industrial baking ovens may have a slower response time since their temperature does not change quickly. But for applications such as water heaters, a fast response time is necessary for efficiency of the device and user’s safety.
خاتمة
Understanding the characteristics and various types of temperature sensors – whether thermistors or thermocouples – is essential for selecting the appropriate one for your application. Thermistors are well-suited for use in applications that need accurate measurement of temperature. Their high sensitivity, precision as well as affordability make them ideal for a wide range of industries such as automotive and HVAC systems. On the other hand, thermocouples are suitable for environments that feature high temperatures due to their durability and wide temperature range. Their primary use includes applications such as power plants and furnace monitoring systems.
Further development in the microfabrication and design of thermistors could result in even smaller and more integrated sensors, allowing their use in limited-space applications. More research into fabrication techniques and new materials may improve the durability and precision of thermistors, thereby making them suitable for more demanding applications. Investigating new thermocouple materials that provide enhanced precision, durability and stability in adverse environments may wide the scope of their applications. The development of more accurate and resilient manufacturing techniques may lead to more superior thermocouples with enhanced performance.
في DTTTEK, we specialize in the manufacture and supply of both thermistors and thermocouples, in their varieties. We also offer customized solutions for different temperature sensor needs and applications. For more information, including personalized guidance on choosing the right sensor, please get in touch with us.
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