SBAA274A September   2018  – March 2023 ADS1118 , ADS1119 , ADS1120 , ADS112C04 , ADS112U04 , ADS1146 , ADS1147 , ADS1148 , ADS114S06 , ADS114S06B , ADS114S08 , ADS114S08B , ADS1219 , ADS1220 , ADS122C04 , ADS122U04 , ADS1246 , ADS1247 , ADS1248 , ADS124S06 , ADS124S08 , ADS125H02 , ADS1260 , ADS1261 , ADS1262 , ADS1263

 

  1.   A Basic Guide to Thermocouple Measurements
  2.   Trademarks
  3. 1Thermocouple Overview
    1. 1.1 Seebeck Voltage
    2. 1.2 Thermocouple Types
      1. 1.2.1 Common Thermocouple Metals
      2. 1.2.2 Thermocouple Measurement Sensitivity
        1. 1.2.2.1 Calculating Thermoelectric Voltage from Temperature
        2. 1.2.2.2 Calculating Temperature From Thermoelectric Voltage
      3. 1.2.3 Thermocouple Construction
      4. 1.2.4 Tolerance Standards
    3. 1.3 Thermocouple Measurement and Cold-Junction Compensation (CJC)
    4. 1.4 Design Notes
      1. 1.4.1 Identify the Range of Thermocouple Operation
      2. 1.4.2 Biasing the Thermocouple
      3. 1.4.3 Thermocouple Voltage Measurement
      4. 1.4.4 Cold-Junction Compensation
      5. 1.4.5 Conversion to Temperature
      6. 1.4.6 Burn-out Detection
  4. 2Thermocouple Measurement Circuits
    1. 2.1 Thermocouple Measurement With Pullup and Pulldown Bias Resistors
      1. 2.1.1 Schematic
      2. 2.1.2 Pros and Cons
      3. 2.1.3 Design Notes
      4. 2.1.4 Measurement Conversion
      5. 2.1.5 Generic Register Settings
    2. 2.2 Thermocouple Measurement With Biasing Resistors Attached to the Negative Lead
      1. 2.2.1 Schematic
      2. 2.2.2 Pros and Cons
      3. 2.2.3 Design Notes
      4. 2.2.4 Measurement Conversion
      5. 2.2.5 Generic Register Settings
    3. 2.3 Thermocouple Measurement With VBIAS for Sensor Biasing and Pullup Resistor
      1. 2.3.1 Schematic
      2. 2.3.2 Pros and Cons
      3. 2.3.3 Design Notes
      4. 2.3.4 Measurement Conversion
      5. 2.3.5 Generic Register Settings
    4. 2.4 Thermocouple Measurement With VBIAS For Sensor Biasing and BOCS
      1. 2.4.1 Schematic
      2. 2.4.2 Pros and Cons
      3. 2.4.3 Design Notes
      4. 2.4.4 Measurement Conversion
      5. 2.4.5 Generic Register Settings
    5. 2.5 Thermocouple Measurement With REFOUT Biasing and Pullup Resistor
      1. 2.5.1 Schematic
      2. 2.5.2 Pros and Cons
      3. 2.5.3 Design Notes
      4. 2.5.4 Measurement Conversion
      5. 2.5.5 Generic Register Settings
    6. 2.6 Thermocouple Measurement With REFOUT Biasing and BOCS
      1. 2.6.1 Schematic
      2. 2.6.2 Pros and Cons
      3. 2.6.3 Design Notes
      4. 2.6.4 Measurement Conversion
      5. 2.6.5 Generic Register Settings
    7. 2.7 Thermocouple Measurement With Bipolar Supplies And Ground Biasing
      1. 2.7.1 Schematic
      2. 2.7.2 Pros and Cons
      3. 2.7.3 Design Notes
      4. 2.7.4 Measurement Conversion
      5. 2.7.5 Generic Register Settings
    8. 2.8 Cold-Junction Compensation Circuits
      1. 2.8.1 RTD Cold-Junction Compensation
        1. 2.8.1.1 Schematic
          1. 2.8.1.1.1 Design Notes
          2. 2.8.1.1.2 Measurement Conversion
          3. 2.8.1.1.3 Generic Register Settings
      2. 2.8.2 Thermistor Cold-Junction Compensation
        1. 2.8.2.1 Schematic
        2. 2.8.2.2 Design Notes
        3. 2.8.2.3 Measurement Conversion
        4. 2.8.2.4 Generic Register Settings
      3. 2.8.3 Temperature Sensor Cold-Junction Compensation
        1. 2.8.3.1 Schematic
        2. 2.8.3.2 Design Notes
        3. 2.8.3.3 Measurement Conversion
        4. 2.8.3.4 Generic Register Settings
  5. 3Summary
  6. 4Revision History

1.2.4 Tolerance Standards

Temperature measurement accuracy and range depend on the type of the thermocouple used and the standard followed by the manufacturer. The International Electrotechnical Commission standard outlined in IEC-EN 60584 contains the manufacturing tolerances for base metal and noble metal thermocouples. A parallel standard used in the United States from the American Society for Testing and Materials is described by ASTM E230. Table 1-4 shows the tolerance of different thermocouples based on different standards and tolerance classes.

Table 1-4 Thermocouple Tolerance Class Information
Thermocouple
Type		Tolerance ClassTemperature
Range (°C)		Thermocouple Error (°C)
(Larger between two columns)
JIEC-EN
60584-2		Class 1–40 < T < 750±1.5°C±(0.004 · |T|)
Class 2–40 < T < 750±2.5°C±(0.0075 · |T|)
Class 3–––
ASTM E230
ANSI MC96.1		Special0 < T < 750±1.1°C±(0.004 · |T|)
Standard0 < T < 750±2.2°C±(0.0075 · |T|)
KIEC-EN
60584-2		Class 1–40 < T < 1000±1.5°C±(0.004 · |T|)
Class 2–40 < T < 1200±2.5°C±(0.0075 · |T|)
Class 3–200 < T < 40±2.5°C±(0.015 · |T|)
ASTM E230
ANSI MC96.1		Special0 < T < 1250±1.1°C±(0.004 · |T|)
Standard–200 < T < 0
0 < T < 1250		±2.2°C
±2.2°C		±(0.02 · |T|)
±(0.0075 · |T|)
TIEC-EN
60584-2		Class 1–40 < T < 350±0.5°C±(0.004 · |T|)
Class 2–40 < T < 350±1.0°C±(0.0075 · |T|)
Class 3–200 < T < 40±1.0°C±(0.015 · |T|)
ASTM E230
ANSI MC96.1		Special–200 < T < 0
0 < T < 350		±0.5°C
±0.5°C		±(0.008 · |T|)
±(0.004 · |T|)
Standard–200 < T < 0
0 < T < 350		±1.0°C
±1.0°C		±(0.015 · |T|)
±(0.0075 · |T|)
EIEC-EN
60584-2		Class 1–40 < T < 800±1.5°C±(0.004 · |T|)
Class 2–40 < T < 900±2.5°C±(0.0075 · |T|)
Class 3–200 < T < 40±2.5°C±(0.015 · |T|)
ASTM E230
ANSI MC96.1		Special–200 < T < 0
0 < T < 900		±1.0°C
±1.0°C		±(0.005 · |T|)
±(0.004 · |T|)
Standard–200 < T < 0
0 < T < 900		±1.7°C
±1.7°C		±(0.01 · |T|)
±(0.005 · |T|)
SIEC-EN
60584-2		Class 10 < T < 1600±1.0°C±[1 + 0.003 · (|T| – 1100)]
Class 2–40 < T < 1600±1.5°C±(0.0025 · |T|)
Class 3–––
ASTM E230
ANSI MC96.1		Special0 < T < 1450±0.6°C±(0.001 · |T|)
Standard0 < T < 1450±1.5°C±(0.0025 · |T|)

As an example, Figure 1-4 graphically shows the error of a type-K thermocouple with the IEC-EN 60584-2 tolerance classes. At higher temperatures, the thermocouple error becomes significantly greater.

GUID-0FAA10F8-66D6-4AEB-AF53-81F9A0C43F06-low.gifFigure 1-4 Type-K IEC-EN 60584-2 Tolerance Class Errors

Thermocouples show a wide range of error dependent on the tolerance class. However, few of these thermocouples have error tolerances better than ±1°C. For this reason, RTDs are preferred for applications requiring higher precision and accuracy. It common to use 16-bit ADCs for thermocouple measurements and 24-bit ADCs for RTD measurements.