Installation Technology of Thermal Resistors in Industrial Temperature Measurement
The installation process of thermal resistors directly determines the measurement accuracy and response speed in industrial temperature measurement. This paper delves into an often-overlooked yet crucial aspect—the heat transfer issue between the thermal resistor sensor (sheathed layer) and the protective sleeve. By analyzing the severe hazards of air gaps, it expounds on the necessity, methods, and applicable scenarios of filling thermal conductive media, aiming to provide clear guidance for engineering practice.
1. Problems
Core Problem: Air Gaps Are the "Performance Killer" for Low-Temperature Measurement
When an air gap exists between the sheathed layer of a thermal resistor and its protective sleeve, three major problems will arise:
Excessive Conduction Thermal Resistance:
Air is a poor thermal conductor (with a thermal conductivity of only ~0.026 W/m·K). This gap forms a massive "heat insulation layer" in the heat transfer path.
Significant Measurement Lag:
Due to the high thermal resistance, the sensor’s speed of sensing temperature changes slows down dramatically, leading to substantial delays in readings that fail to reflect real-time process fluctuations.
Persistent Low Reading Deviation:
The "heat loss" of the sensor through components such as the mounting base cannot be effectively compensated by the measured medium, resulting in the sensor’s actual temperature remaining consistently lower than the true temperature and causing a stable negative deviation.
Special Note:
This problem is particularly prominent when measuring low-temperature gases (generally referring to temperatures < 150°C, especially within the ambient temperature range). Because heat transfer at this stage is dominated by conduction and convection, while radiative heat transfer—proportional to the fourth power of temperature—plays a negligible role, the negative effects of air gaps cannot be offset.
2. Solutions
The only method to eliminate air gaps and establish a highly efficient heat transfer path is to fill the gap with high-performance thermal conductive media. Below is a detailed description of commonly used thermal conductive media along with standard operating procedures:
Commonly Used Thermal Conductive Media and Their Characteristics
(1) Thermal Grease (Heat Transfer Paste): It is the most versatile and recommended solution, suitable for the vast majority of low-temperature (-50°C~200°C+) and some medium-temperature scenarios.
Advantages: Thermal conductivity is far superior to that of air. Easy to fill and capable of perfectly conforming to microscopically uneven surfaces. Electrically insulating, posing no risk of short circuit. Low cost and simple to apply.
Notes: It may dry out and turn powdery after long-term use at extremely high temperatures, and some products may experience oil separation.(2) Metal Oxide Powder (e.g., Magnesium Oxide Powder): As a traditional method, it is often used in high-temperature applications requiring electrical insulation (similar to the filler material inside sheathed thermal resistors themselves).
Advantages:
Excellent high-temperature resistance.
Good electrical insulation performance.
Disadvantages:
Thermal conductivity is generally inferior to that of high-quality thermal grease.
Difficult to achieve dense filling, which may lead to residual air pockets.
Susceptible to moisture absorption, which can impair insulation properties.
(3) Soft Metal Foil (e.g., Indium Foil): It is suitable for ultra-high-precision laboratory settings or special low-temperature applications with strict low thermal resistance requirements.
Advantages:
Exceptional thermal conductivity.
Good ductility, enabling it to fill tiny gaps.
Disadvantages:
High cost.
Installation requires certain technical expertise.
Potential risk of electrochemical corrosion.
(4) High-Thermal-Conductivity Epoxy Adhesive: It is designed for special installation scenarios that demand permanent fixation and ultra-high thermal conductivity.
Advantages:
Serves both as a fixing agent and a thermal conductor.
High structural strength.
Disadvantages:
Non-detachable, making maintenance and repair difficult.
After curing, mismatched thermal expansion coefficients may induce internal stress.
3. Standard Operating Procedure (Taking Thermal Grease as an Example)
Cleaning: Ensure the outer wall of the sheathed layer and the inner wall of the protective sleeve are clean, dry, and free of oil stains to create conditions for the thermal conductive medium to adhere.
Application: Evenly apply a sufficient layer of thermal grease on the outer surface of the sheathed layer. Do not apply it too thickly; the goal is to fully cover the surface and eliminate most air bubbles.
Installation: Slowly screw or insert the thermal resistor sensor (coated with grease) into the protective sleeve, avoiding air bubble formation during the process.
Inspection: After proper installation, check if a small amount of grease is evenly squeezed out from the sleeve opening. This indicates that internal gaps have been effectively filled.
4. Application Scenario Summary & Decision Guide
Measuring low-temperature gases/fluids (e.g., water, air, coolant; temperature < 150°C): Thermal conductive media must be filled; thermal grease is the first choice. This is the most critical measure to ensure measurement accuracy and fast response.
Measuring medium-to-high temperature gases/fluids (e.g., steam, hot oil; temperature 150°C~400°C): Filling is strongly recommended; use high-temperature-resistant thermal grease. Although radiative heat transfer starts to increase at this stage, filling the medium still significantly improves response speed and consistency.
Measuring high temperatures (e.g., furnace chambers, flue gas; temperature > 400°C): Filling is optional depending on actual conditions. Radiative heat transfer dominates at ultra-high temperatures, so the main concern may be the temperature resistance of the medium itself. High-temperature powder can be used, or follow the sensor manufacturer’s recommendations.
Any temperature measurement in a vacuum environment: Thermal conductive media must be filled or a special design adopted. Conduction is the only heat transfer path in a vacuum; use thermal grease, metal foil, or a gap-free solid contact design. Otherwise, the sensor cannot function properly.
Control loops with extremely high response speed requirements: Thermal conductive media must be filled; thermal grease is the first choice. This is the most cost-effective way to reduce measurement lag and improve control system performance.
Rough indication with low precision requirements: Filling can be omitted (not recommended). Even so, filling the medium still improves the reliability and stability of readings.
