FAQ

When selecting a thermocouple, accuracy is a critical factor. Two common terms used to define thermocouple accuracy are Accuracy Class and Special Limits of Error (SLE). Here’s how they differ:

1. Accuracy Class

• Defined by international standards such as IEC 60584-2, each class has specified allowable deviations in temperature measurement based on the thermocouple type..
• Thermocouples are categorized into different classes based on their accuracy levels:
  • Class 1 – Highest accuracy (tightest tolerances).
  • Class 2 – Standard accuracy (most commonly used).
  • Class 3 – Lower accuracy, typically for extreme low temperatures.
• Accuracy is typically expressed as a percentage of the temperature range or in degrees (°C or °F).

2. Special Limits of Error (SLE)

• Defined by ASTM E230 / ANSI MC96.1 (commonly used in the U.S.).
• SLE thermocouples are manufactured to tighter tolerances than standard thermocouples.
• SLE thermocouples provide twice the accuracy of standard thermocouples within a defined temperature range.

• Commonly specified when higher precision is required in industrial and laboratory applications. 

   

Key Difference:

• Accuracy Class (IEC-based) refers to defined accuracy levels under international standards.
• Special Limits of Error (ANSI-based) refers to a tighter accuracy tolerance than standard thermocouples in U.S. standards.

Which One Should You Choose?

Use Class 1 thermocouples or SLE thermocouples when high precision is required.
Class 2 or standard thermocouples are sufficient for general industrial applications where ultra-high accuracy is not necessary.

Why Proper Cable Extension Matters

Extending an LVDT (Linear Variable Differential Transformer) sensor cable requires careful planning to prevent signal degradation caused by noise, impedance mismatch, or voltage drops. Here are best practices to ensure a successful extension:

1. Use Shielded Twisted Pair (STP) Cable

• Choose a shielded, twisted-pair cable to reduce electromagnetic interference (EMI) and crosstalk.
• For AC LVDTs, use one twisted pair for excitation lines and another for signal return.
• For DC LVDTs, use twisted pairs for power, ground, and signal lines to maintain signal quality.

2. If practical, match Cable Specifications

• Use an extension cable that matches the gauge, impedance, and capacitance of the original cable.
• Select a low-capacitance cable to preserve signal integrity over long distances.
• An exact match isn’t always required, but closer specifications reduce the risk of signal issues.

3. Limit Cable Length

• Keep extensions as short as possible to minimize signal degradation.
• AC LVDTs: Long cables can impact excitation voltage due to impedance, with a typical max range of 50–100 feet.
• DC LVDTs: Cables longer than 100 feet may introduce voltage drops in the power supply or signal lines.

4. Consider Signal Conditioning at the Source

• For long cable runs, place a signal conditioner or amplifier close to the LVDT sensor to convert the signal to a more robust form (e.g., 4–20 mA or ±10 V) that is less susceptible to noise.

5. Grounding and Isolation

• Ensure proper grounding of the sensor, cable, and signal conditioner to prevent ground loops.
• If ground loops are a concern, consider using isolated signal conditioners to improve signal stability.

6. Maintain Shield Continuity

• Connect the cable shield to signal ground at one end only (typically at the signal conditioning or receiving end) to avoid ground loops.
• Ensure that the shield remains continuous across the entire extension with no breaks or floating segments.

7. Avoid Running Near High-Voltage Lines

• Keep LVDT cables away from power cables, motors, and high-EMI sources like relays and VFDs (Variable Frequency Drives).

8. Use Cable Trays or Conduits

• Protect cables in industrial environments using cable trays, conduits, or armored cable to prevent physical damage and EMI interference.

9. Use Proper Connectors

• Use soldered or crimped connectors designed for signal integrity and environmental protection.
• For industrial or outdoor environments, consider using IP-rated connectors for additional protection.

10. Verify and Test After Installation

• After extending the cable, use an oscilloscope or data acquisition system to verify the signal quality.
• Check for added noise, distortion, or signal offset before putting the system into operation.

Conclusion

By following these best practices, you can extend an LVDT sensor cable while maintaining accuracy and reducing noise interference.