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 Type D 
 Temperature sensors 

Refractory thermocouple designed for very high temperatures up to 2300 °C in controlled industrial environments.

 Maximum precision
+/- 1,0°K

 Minimum temperature
0°C

 Maximum temperature
+2300°C

 Minimum dimensions
0,5 x 50 x 5

 Response time
Low

 Internal Resistance
High

 Linearity
High

Price
High

What is a Type D thermocouple ?Operating principleTechnical specificationsVoltage / Temperature CurveCompatibility / CompensationApplication areas

What is a Type D thermocouple ?

The type D thermocouple is a sensor for extreme temperatures, similar to type C, but made of a different tungsten-rhenium alloy:

  • W-3%Re / W-25%Re.

This composition provides a slight improvement in linearity and better mechanical stability at very high temperatures.

Used in vacuum or inert gas environments, it is essential for measurements beyond 2,000 °C where platinum-based sensors are unusable.

Operating principle

The type D exploits the Seebeck effect, creating a voltage proportional to the temperature difference between the two junctions.

E = S × (T_hot - T_cold)

For type D:

S ≈ 19 µV/°C at 1,000 °C

The signal is comparable to that of type C, with a slight thermal drift reduced thanks to its optimized alloy.

Technical specifications

Parameter Typical Value
Measurement range 0 °C → +2 300 °C
Sensitivity 19 µV/°C at 1 000 °C
Tension at 1000 °C ≈ 14,3 mV
Tension at 2000 °C ≈ 28,6 mV
Tension at 2 300 °C ≈ 32,3 mV
Oxidation resistance Null in open air (use under inert or vacuum atmosphere)
Response time (6 mm diameter sheath) 5 to 12 s
Reference standard ASTM E230

Voltage / Temperature Curve

(Reference: Cold junction at 0 °C — ASTM E230 standard)

The type D curve is very consistent, with a slightly lower voltage than type C but better structural stability beyond 2,000 °C.

📈 General behavior:

  • From 0 to 2,300 °C → increasing voltage (~0 to +32.3 mV)
  • Average slope: ≈ 19 µV/°C at 1,000 °C
  • Drift <0.05% after 100 h at 2,000 °C

💡 Type D offers remarkable accuracy and longevity in conditions that few sensors can withstand.

Compatibility / Compensation

The type D requires a specialized acquisition chain, similar to that of type C.

It must never be used outdoors, under penalty of immediate oxidation.

It is used with high-precision differential converters, often integrated into vacuum furnace control systems.

Application areas

🚀 Turbines, rocket engines, and space propulsion

⚙️ Industrial arc furnaces, refractory metal melting

🧪 Materials physics laboratories and high vacuum

🔬 Instrumentation for controlled atmosphere environments





Should I choose a Type D thermocouple?

Strengths points

  • 🚀 Extreme specialist
         → Operates unwaveringly at over 2,000 °C, ideal for fusion, plasma, or high vacuum applications.
  • 🧱 Ultra-stable construction→ Its W3Re/W25Re alloy is more resistant to deformation and crystallization than other tungsten-based compositions.
  • 🧪 High reproducibility of measurements
         → Offers better linearity and minimal drift over long periods.
Thermocouple Type D sensors

Weaknesses points

  • 🌬️ Unusable in the open air
         → Immediate oxidation of tungsten at 500 °C — exclusive use under vacuum or inert atmosphere.
  • 💸 High price and niche use
         → Designed for research and critical applications, it remains too expensive for common industrial use.
  • ⚙️ Mandatory dedicated electronics
         → Requires a high-temperature calibrated measurement chain, not compatible with standard type K or N circuits.

Useful information

Here is some useful information regarding Type D thermocouples.

(Reference: cold junction at 0 °C — ASTM E230 standard)
Temperature (°C) Tension (mV) Temperature (°C) Tension (mV)
0 0,000 1 400 19,48
400 5,00 1 600 22,01
800 9,85 1 800 25,03
1 000 14,30 2 000 28,60
1 200 16,95 2 300 32,30
💡 Type D tension is slightly lower than type C, but more consistent and stable beyond 2,000 °C.
Class Tolérance (K) Usage area
 Description
Standard (C68) ±1,0 K ou ±0,25 % 0 → +2 300 °C High precision
No IEC class
 — — Non-standardized according to IEC 60584
🔹 Type D does not have an official European classification; its use remains limited to research and specialized environments.

Example 1 – Calculation of the generated voltage

Measured temperature: 1,800 °C

Cold weld: 0 °C

E = 25,03 mV

Cold welding at 25 °C → correction voltage ≈ 0.27 mV :

E_measured = 25,03 - 0,27 = 24,76 mV

✅ Actual voltage ≈ 24.76 mV


Example 2 – Calculating the temperature from a measured voltage

Measured voltage: 28.60 mV (cold weld at 0 °C)

→ By consulting the ASTM table:

T=2000°C

✅ Measured temperature ≈ 2,000 °C

The type D thermocouple generates a low but stable voltage in extreme environments.

It requires isolated electronics that are resistant to disturbances and calibrated for very high temperatures.

🔹 Typical components

Component
 Function
Thermocouple type D (W3Re / W25Re) Generate the Seebeck voltage
Isolated differential amplifier
 Amplify and protect the signal
Compensation sensor (high stability)
 Correct the cold junction
ADC 24 bits Digitize the amplified signal
High Temperature Acquisition System
 Calculate the temperature according to the ASTM table
🔹Functional diagram (ASCII)
[HOT JUNCTION]──(Thermocouple D)──[ISOLATED AMPLIFIER]──[ADC]──[µCONTROLLER]
                                      │
                                      └──(Compensation sensor)
💡 Type D is reserved for extreme applications where no other thermocouple survives: vacuum, plasma, fusion, propulsion.

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