Check These Critical Installation Details First
In industrial applications, accurate temperature measurement is essential for process control, product quality, and operational safety.
However, in many real-world projects, engineers often encounter the same frustrating situation:
The instrument is correctly selected, but the measured temperature is still inaccurate.
In most cases, the root cause is not the sensor itself, but installation details that are often overlooked on site.
Based on practical field experience, this article summarizes the most common installation-related issues affecting temperature measurement accuracy—and how to avoid them.
1. Grounded vs. Ungrounded Thermocouple Junctions
In thermocouple installations, whether the hot junction is grounded or ungrounded makes a significant difference.
Grounded junctions, where the hot junction is in direct contact with the thermowell wall, offer:
Faster response time
Reduced thermal lag
Better resistance to electrical noise
Ungrounded junctions provide better electrical isolation but respond more slowly.
⚠ Important consideration:
A grounded thermocouple can only be used if the secondary instrument (transmitter or controller) does not share a common ground with the thermocouple junction. Otherwise, ground loops and measurement errors may occur.
2. Why Thermowells Get Stuck in High-Temperature Furnaces
During maintenance shutdowns of high-temperature furnaces, engineers sometimes find that thermowells cannot be removed.
The main reason is thermal deformation.
At elevated temperatures, long thermowells—especially when installed horizontally—can bend due to thermal stress.
Best practices:
Prefer vertical installation in high-temperature furnaces
If horizontal installation is unavoidable:
Use mechanical supports when insertion length exceeds 1 meter
Always add supports when operating temperatures exceed 700 °C
3. Replacing RTDs with Shorter Insertion Lengths: A Hidden Error Source
When a platinum RTD fails, maintenance personnel may replace it with an element that has a shorter insertion length, assuming it will still work.
This often introduces systematic errors:
Measured temperature appears lower when the process temperature is higher than ambient
Measured temperature appears higher when the process temperature is below ambient
Additionally:
In high-vibration environments, RTD sensing elements are prone to wire breakage
Replacement elements must match original insertion length and mechanical specifications
⚠ If the thermowell inner diameter is too large and the RTD element is too thin, poor thermal contact will result in slow response and inaccurate readings.
4. Why Thermowells Should Not Be Installed at Pipe Elbows
Installing thermowells at 90-degree pipe bends is strongly discouraged.
Reasons include:
Pipe elbows are mechanically weaker than straight sections due to forming and welding
Flow-induced vibration and erosion are more severe at bends
Additional openings at elbows significantly increase failure risk
✔ Recommendation:
Always install thermowells on straight pipe sections with stable flow conditions.
5. Thermowell Installation in Small-Diameter Pipelines
For small-diameter process piping, thermowell selection and orientation are critical.
General guidelines:
Use slimmer thermowells to minimize heat loss to ambient
According to SH/T 3104—2000:
For DN80 pipelines: install at 45° angle
For pipelines smaller than DN80: use a pipe enlargement section to locally increase diameter to DN80 before installing the thermowell
6. Condensation Risks in Chilled Water Temperature Measurement
Armored platinum RTDs are commonly used in chilled water systems to protect sensing elements from moisture.
However, problems occur when the junction box is installed too close to the process connection.
Low process temperature causes cold transfer to the junction box
Moist air inside the junction box condenses
Terminal short circuits and signal instability may occur
✔ Best practice:
Maintain a minimum distance of 150 mm between the process connection sealing face and the junction box.
7. Advantages of Pt500 and Pt1000 RTDs
Pt500 and Pt1000 are thin-film platinum RTDs with higher base resistance than Pt100.
| Type | Resistance at 0 °C | Sensitivity |
|---|---|---|
| Pt500 | 500 Ω | ~1.9 Ω / °C |
| Pt1000 | 1000 Ω | ~3.8 Ω / °C |
Key advantages:
Higher signal amplitude
Improved resolution
Lower requirements for signal conditioning circuits
In applications with moderate accuracy requirements, two-wire configurations can be used, simplifying wiring and reducing cost.
8. Thermowell Insertion Depth: Why “Too Shallow” Always Causes Errors
Insertion depth directly affects measurement accuracy.
Minimum recommended immersion length:
Thermocouples: ≥ 8–10 × thermowell outer diameter
RTDs: ≥ 13 × thermowell outer diameter
If the insertion depth is insufficient:
Heat conducted away from the sensing area causes lower readings
If process temperature is lower than ambient, readings may become artificially higher
✔ Adequate insertion depth ensures proper thermal equilibrium and reliable measurement.
Final Takeaway
In temperature measurement, accuracy is rarely determined by sensor selection alone.
Most errors originate from installation details that seem minor but have major impact.
Before questioning the instrument:
Recheck installation orientation
Verify insertion depth
Confirm mechanical support and environmental conditions
Getting these fundamentals right will eliminate the majority of temperature measurement problems—before calibration or replacement is ever needed.