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 Type J 
 Temperature sensors 

Reliable thermocouple for industrial temperature monitoring, suitable for moderate ranges up to 750 °C in controlled environments.

 Maximum precision
+/- 1,5°K

 Minimum temperature
-210°C

 Maximum temperature
+760°C

 Minimum dimensions
0,5 x 50 x 3

 Response time
Medium

 Internal Resistance
Medium

 Linearity
High

Price
Medium

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

What is a Type J thermocouple ?

The type J thermocouple is one of the most widely used models in industry.

Based on an iron/constantan pair, it offers a good balance between cost, robustness, and accuracy.

Designed for general applications, it works well on industrial machines, heating systems, or moderate temperature manufacturing processes.

It is particularly valued for its stable response and ease of integration, especially in environments where the type K would be too sensitive to oxidation.

Operating principle

As with all thermocouples, the measurement relies on the Seebeck effect:

a voltage is generated between the hot and cold junctions.

E = Sj × (T_hot - T_cold)

For type J:

S_J ≈ 55 µV/°C at 0 °C
→ about 55 mV at 1,000 °C, but the recommended range stops at 760 °C to avoid iron oxidation.

Technical specifications

Parameter Typical value
Measurement range −210 °C → +760 °C
Sensitivity 55 µV/°C to 0 °C
Tension at 100 °C ≈ 5,27 mV
Tension at 500 °C ≈ 26,64 mV
Tension at 750 °C ≈ 42,92 mV
Oxidation resistance Low (iron oxidizes easily)
Response time (cable sheath Ø3 mm) 1 to 4 s
Reference standard IEC 60584-1, ASTM E230

Voltage/Temperature Curve

(Reference: Cold junction at 0 °C — IEC 60584-1 standard)

The J thermocouple exhibits a nearly linear curve up to about 600 °C, followed by slight non-linearity beyond that.

Its output voltage remains moderate but stable, making it ideal for typical industrial environments.

📈 General behavior:

  • From −200 to 0 °C → negative voltage (~−8.1 mV to 0 mV)
  • From 0 to 760 °C → positive voltage (~0 to +42.9 mV)
  • Average slope: ≈ 55 µV/°C

💡 Drift may occur after long thermal cycles, due to oxidation of the iron wire.

Compatibility / Compensation

The J type requires cold weld compensation, typically provided by an integrated sensor or a specialized amplifier (MAX31855-J, AD8495-J).

It is compatible with most industrial measuring instruments, making it a reliable choice for common medium temperature applications.

Application areas

⚙️ General industrial processes (machines, engines, light furnaces)

🧰 Maintenance, heating, air conditioning, component testing

🔩 Test benches, mechanical testing, production environment

🧪 Cost-effective laboratory instrumentation




Should I choose a Type J thermocouple?

Strengths points

  • ⚙️ Robust and versatile
         → Solid, simple, and low-cost — an excellent choice for industrial installations at moderate temperatures.
  • 💰 Good value for money→ It is the ideal thermocouple when you are looking for reliability without breaking the budget.
  • 🔧 Easy to integrate
         → Compatible with most market regulators and acquisition modules, it installs without specific adaptation.
Type J Thermocouple sensors

Weaknesses points

  • 🔥 Limited beach
         → It cannot withstand very high temperatures: beyond 760 °C, iron oxidizes and the measurements become unstable.
  • 🌬️ Rapid aging in an oxidizing atmosphere
         → The conductor rusts quickly, especially without a suitable protective sheath.
  • 📉 Slight drift over time
         → The repeated thermal cycles modify the characteristics of the wire, affecting long-term accuracy.

Useful information

Here is some useful information regarding Type J thermocouples.

(Reference: cold junction at 0 °C — IEC 60584-1 standard)
Temperature (°C) Tension (mV) Temperature (°C) Tension (mV)
−200 −8,095 200 10,777
−100 −3,554 400 21,846
0 0,000 600 33,102
100 5,268 760 42,919
200 10,777 — —
💡 Type J generates approximately 42.9 mV at 760 °C, making it a powerful sensor at a low cost.

Class Tolerance (K) Usage area
 Description
Class 1 ±1,5 K ou ±0,4 % −40 → +750 °C High precision
Class 2 ±2,5 K ou ±0,75 % −210 → +760 °C Industrial standard
Class 3 ±2,5 K ou ±1,5 % −210 → 0 °C Cryogenics / low temperatures
🔹 Class 2 is the most commonly used for standard industrial applications.

Example 1 – Calculation of the generated voltage

Measured temperature: 400 °C

Cold weld: 0 °C

E = 23,019 mV

If the cold weld is at 25 °C, the correction voltage is ~1.277 mV:

E_corrected = 23.019 - 1.277 = 21.742 mV

✅ Actual voltage ≈ 21.74 mV


Example 2 – Calculating temperature from a measured voltage

Measured voltage: 28.173 mV (cold junction at 0 °C)

→ By consulting the IEC table:


T=500°C

✅ Measured temperature ≈ 500 °C


The type J thermocouple produces a voltage between -8 and +43 mV depending on the temperature. Its signal is weak and must be amplified, compensated, and converted before processing.

🔹 Typical components

Component
 Function
Thermocouple type J (Iron / Constantan)
 Generate the Seebeck voltage
Differential amplifier (e.g., AD8495-J, INA333)
 Amplify the signal from a few mV to a few V
Compensation sensor (NTC, integrated sensor, etc.)
 Correct the temperature of the cold weld
ADC 16–24 bits Convert the amplified voltage into digital data
Microcontroller / Data Acquisition System
 Calculate the temperature using a table or linearization equation
🔹 Functional diagram (ASCII)


[HOT JUNCTION]──(Thermocouple J)──[AMPLIFIER]──[ADC]──[µCONTROLLER]
                                      │
                                      └──(Compensation sensor)
💡 Type J being compatible with most thermocouple converters (MAX31855, AD8495, LTC2983), it is often used for cost-effective and robust systems.

 We integrate any sensor into any probe 

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PCBA design

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