Polyimide film heaters (Kapton heaters) are widely used in battery warming, optics de-icing, sensors, medical devices, 3D printers, and aerospace electronics. But the performance of the heater depends heavily on one component that is often overlooked, that is temperature sensors.
There are two common options used in flexible heaters: RTDs (Resistance Temperature Detectors) and thermocouples. Both can be embedded seamlessly into polyimide heaters — but which one is actually better? What are differences between rtd and thermocouple and when to use them? Today, Danyu team will introduce them in details and hope this is helpful for you. Let’s keep reading!
RTD stands for “Resistance Temperature Detector.” It measures heat by tracking how the electrical resistance of a metal changes with temperature. Most RTDs used in polyimide heaters are made of platinum.
The most popular type is the PT100 or PT1000, where “PT” means platinum.
Key characteristics of RTDs:
RTDs are commonly built into polyimide heaters used in:
A thermocouple sensor is formed by joining two different metals, which create a voltage when they experience heat — known as the Seebeck effect.
Common thermocouple types:
Key characteristics of thermocouples:
Thermocouples are often chosen for:
There are several key differences between an RTD vs thermocouple setup. The table below summarizes the main points:
| Feature | RTD | Thermocouple |
| Working principle | Resistance change of a metal | Voltage generated from two metals |
| Accuracy | High (±0.1–0.3°C typical) | Medium (±1–2°C typical) |
| Temperature range | –50°C to ~250–300°C | –200°C to 1200°C+ |
| Response time | Moderate | Very fast |
| Stability | Excellent | Moderate (drifts over time) |
| Output signal | Resistance (Ω) | Voltage (mV) |
| Cost | Higher | Lower |
| Best use case | Precision control | High-temperature or dynamic heating |
The difference between RTD and thermocouple usually depends on accuracy vs. range. RTDs win in precision, while thermocouples win in speed and high-temperature ability.
RTDs measure temperature by detecting how a metal’s electrical resistance increases with heat. A stable constant current is applied to the RTD element (usually platinum), and as the resistance changes, the controller converts the voltage drop into an accurate temperature reading. Because the resistance–temperature relationship of platinum is very linear and predictable, RTDs provide high accuracy and excellent long-term stability.
Thermocouples, by contrast, operate based on the Seebeck effect. When two dissimilar metals are welded together, the temperature at the hot junction generates a tiny voltage that varies with temperature. The control device reads this millivolt-level output and converts it into a temperature value. This direct voltage-generation mechanism allows thermocouples to achieve very fast response and extremely wide temperature ranges, though with lower accuracy compared with RTDs.
In short: RTDs use resistance change, ideal for precision control; thermocouples use voltage generation, ideal for high temperature and fast response applications.
RTD sensors are more accurate than thermocouples, here is a simple comparison table between them:
| Parameter | RTD (PT100 / PT1000) | Thermocouple (Type K) |
| Typical Accuracy | ±0.1°C to ±0.3°C | ±1°C to ±2°C |
| Long-Term Drift | Very low | Moderate |
| Repeatability | Excellent | Good |
| Stability Over Many Heat Cycles | High | Medium |
| Suitability for Precision Control | Strong | Average |
Different sensors work better in different ranges.
A polyimide film heater rarely needs more than 250°C. Polyimide begins to degrade above that. This means an RTD already fits most scenarios for electronics, batteries, displays, and sensor housings. But for systems exposed to extreme heat or rapid spikes, a thermocouple may offer better survival.
| Metric | RTD (Thin-Film) | Thermocouple (Exposed Junction) |
| Response Speed | Medium-Fast | Very Fast |
| Thermal Mass | Slightly higher | Very low |
| Time to Sense a Sudden Heat Spike | Slower | Faster |
| Time to Reach Stable Reading | Very stable | May fluctuate slightly |
| Best Use Case | Precision temperature control | Rapid temperature swings |
When embedded in thin polyimide heaters, both RTDs and thermocouples can achieve millisecond-level response, more than sufficient for most applications.
A thermocouple generally reacts faster because its sensing junction has very low thermal mass, which helps it register temperature shifts almost immediately after the surrounding environment changes. This rapid response is valuable in systems where heat rises or falls quickly, especially in compact assemblies that cannot tolerate long delays in feedback.
Thin-film RTDs can also provide strong response performance, but they still carry slightly more mass than a thermocouple junction, and that added material slows down heat transfer by a small amount. The difference may be minor in steady environments, yet in applications with abrupt thermal swings, the faster behavior of a thermocouple helps controllers maintain tighter control.
When a design emphasizes stability rather than speed, an RTD still delivers more consistent long-term readings, but for dynamic conditions, the thermocouple remains the quicker option.
The output signal shapes the entire control system.
RTD output:
Thermocouple output:
Both outputs work for polyimide heaters. But for precision control, RTDs offer a more predictable signal.
Choosing between thermocouple vs RTD depends on the design goals:
Use an RTD when:
Use a thermocouple when:
For most polyimide film heaters, the RTD fits better because the heater itself works in a moderate range and needs stable control.
When picking a sensor for a polyimide heater, the real question is not just RTD vs thermocouple. It is how the heater will behave in your device. Consider these points:
1. Accuracy needs
If your design requires smooth, steady temperature control, an RTD gives better consistency. This is important for optical systems, battery warmers, medical diagnostics, and lab instruments.
2. Thickness limits
Polyimide heaters are thin. RTDs and thermocouples must match this profile. Thin-film RTDs blend well. Micro-junction thermocouples also fit, but they need careful placement.
3. Wiring and EMI
Long cables or high-noise environments favor RTDs. Their resistance output is easier to stabilize.
4. Temperature span
If the heater reaches 250°C or less, RTDs cover the range with ease. If a system must survive extreme spikes, a thermocouple gives more margin.
5. Cost considerations
Thermocouples cost less, especially in high-volume builds. But many engineers accept the added cost of an RTD to gain stability and control.
At Danyu electronics, we build custom polyimide film heaters with embedded RTDs or thermocouples. Our team helps customers match sensor types with material thickness, insulation layers, bonding films, wire harnesses, and controller requirements. All heaters follow strict quality control under ISO9001, ISO13485, and IATF16949 to keep performance consistent across batches.
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