Think back to the last heating system your engineering team designed. When did you consider the integration of temperature sensors like RTD sensors, thermocouples, or thermistors? If it was near the end of development, you're not alone. Many system designers treat sensors as an afterthought, which can lead to costly redesigns and project delays.
Planning for sensors at the outset allows for proper placement, optimized performance, and protection of wiring and components. Early integration also ensures compatibility with controls like programmable logic controllers (PLCs) and process controllers.
Understanding Temperature Sensors: RTD Sensors, Thermocouples and Thermistors
Choosing the right temperature sensor is essential to your system’s reliability, accuracy and safety. Three of the most common types include:
Each type offers unique advantages and limitations, making them suitable for different applications and temperature ranges.
Thermocouples: Rugged Sensors for High-Temperature Environments
Thermocouples are ideal for high-temperature industrial applications such as:
-
Heat treatment systems
-
Semiconductor and glass processing
-
Metalworking furnaces and autoclaves
-
Aerospace exhaust monitoring
These sensors can measure temperatures up to 4,200°F and are designed using two dissimilar metals encased in ceramic insulation. Their fast response time and flexible design allow for precise placement even in small or complex spaces.
Advantages:
-
Temperature range: Up to 4,200°F
-
Fast response time
-
Compact and flexible
-
High durability and vibration resistance
Considerations:
-
Less accurate than RTDs or thermistors
-
Prone to signal drift over time
-
Requires specialized electronics for signal interpretation
Thermocouples are often used in heaters, especially when the sensor is embedded within the heating element.
RTD Sensors: High Accuracy and Repeatability
Resistance temperature detectors (RTD sensors) are known for their exceptional accuracy and repeatability. Typically made of platinum, these sensors are used in:
-
Food and beverage manufacturing
-
Biotech and pharmaceutical development
-
Environmental chambers and chemical processing
-
Semiconductor fabrication
RTDs generally measure temperatures up to 1,200°F. Their platinum element is wrapped in a ceramic sheath, making them ideal for controlled environments where precision is critical.
Advantages:
-
Excellent accuracy and long-term stability
-
Minimal signal drift
-
Ideal for process control applications
Considerations:
-
Rigid, non-flexible design
-
Requires sufficient immersion to avoid inaccurate readings (stem loss)
-
More expensive than thermocouples
RTD sensors are a reliable choice for applications requiring precise thermal monitoring, such as deep fryers in food processing systems or quality control in biotech labs.
Thermistors: Sensitivity for Low-Range, High-Precision Applications
Thermistors are temperature sensors made from a blend of metal oxides that allow for extreme sensitivity over narrow temperature ranges, typically below 500°F. They are commonly found in:
-
HVAC systems
-
Medical devices like blood analyzers
-
Automotive dashboard displays
Unlike RTDs, thermistors exhibit nonlinear resistance changes, resulting in a steep curve in response to temperature. While this limits their range, it makes them ideal for detecting small temperature changes very quickly.
Advantages:
-
High sensitivity in narrow temperature bands
-
Cost-effective and compact
-
Quick response times
Considerations:
-
Prone to drift at high temperatures
-
Limited durability under vibration
-
Lower maximum temperature threshold (generally up to 500°F)
Thermistors are not suitable for extreme environments but offer exceptional performance in tightly controlled conditions.
How to Select the Right Temperature Sensor
When choosing between RTD sensors, thermocouples, thermistors, consider the following:
-
Temperature range: Thermocouples handle the highest temperatures; thermistors, the lowest.
-
Accuracy and stability: RTD sensors are the most precise and stable over time.
-
Environmental conditions: Thermocouples withstand harsh environments, including vibration and moisture.
-
Size and flexibility: Thermocouples can be ultra-thin and bent; RTDs and thermistors are more rigid.
For instance, when integrating a sensor into a heater, thermocouples are often recommended for their durability and ability to withstand extreme internal heat.
Sensor Placement: Getting Accurate Readings
Correct placement is essential for accurate temperature readings. Consider the following:
-
Immersion depth: In fluid systems, place the sensor deep enough to avoid ambient temperature influence.
-
Environmental isolation: Protect sensors from air drafts, radiant heat, or excessive vibration.
-
Accessibility: Avoid placing sensors where they may be damaged during operation or maintenance.
Whether you’re installing thermocouples in a pipe or RTD sensors in a fryer, the location determines the sensor’s effectiveness.
Maintenance and Replacement of Temperature Sensors
With proper installation and operation, RTD sensors, thermistors and thermocouples can last between 3 and 25 years. However, several factors can reduce lifespan:
-
Overheating beyond the rated limit
-
Moisture ingress in humid environments
-
Mechanical damage from vibration or impact
Routine inspections and scheduled sensor replacements can prevent downtime and costly process interruptions—especially in high-stakes environments like semiconductor fabs or large-scale manufacturing plants using electric heaters.
Redundant Sensors Improve Safety and Reliability
To protect against sensor failure, many systems include redundant sensors or high-limit shutoffs. For example:
-
A deep fryer equipped with an RTD sensor may include a secondary high-limit thermocouple to shut down the heating element if oil temperatures exceed safe levels.
-
Refineries and remote industrial systems may use dual sensors to reduce the need for service calls.
This redundancy ensures system safety and continuous operation even in the event of sensor failure.
Work With the Experts in Temperature Sensor Solutions
Whether you're integrating thermocouples in a high-temperature furnace, selecting RTD sensors for biotech processing, or designing HVAC systems with thermistors, Watlow’s experts can help.
Our team will guide you through the selection, placement, and integration process to ensure optimal performance, safety, and ROI. Contact a Watlow representative today to get started with a customized sensor solution that fits your application needs.