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Read Temperature by Measuring Millivolts

Thermocouple Reference Tables

The N.I.S.T. (National Institute of Standards and Technology) is the U.S. standards setting agency. They have determined the output millivoltage of all type thermocouples, at all temperatures, within their range. The resulting tabulations are called “Thermocouple Reference Tables” and the thermocouple output millivoltage is shown for each degree of temperature.

Thermocouple Reference Tables
Type B Thermocouplean-htp08.pdf
Type R Thermocouplean-htp09.pdf
Type S Thermocouplean-htp10.pdf
Type T Thermocouplean-htp11.pdf
Type K Thermocouplean-htp12.pdf
Type C Thermocouplean-htp13.pdf

The junction of each type thermocouple produces a specific millivoltage across it at a specific temperature. A thermocouple consists of two junctions connected in opposition. One is the measuring junction and the other is the reference junction. VD is the millivoltage resulting from the difference between the millivoltages generated by the two opposing junctions. VD is the millivoltage read when a meter is connected across the thermocouple as shown below.

How to Determine the Measuring Junction Temperature

  1. Measure the “VD” millivoltage as shown above.
  2. Measure the actual temperature of the reference junction with a thermometer.
  3. Go to the table for the thermocouple being used and look up the millivoltage produced at that temperature.
  4. Add that millivoltage to the millivoltage measured as “VD” to get a total.
  5. Find that millivoltage total in the reference table. The corresponding temperature is the temperature of the measuring junction.

Example #1 – Type “T” Thermocouple

Measured “VD” = 3.41 mV

Reference Junction Temperature = 22°C (71.6°F)

  1. From the table; 22°C = 0.87 mV.
  2. Adding 0.87 mV to 3.41 mV = 4.28 mV.
  3. Finding 4.28 mV In the table; the corresponding temperature is 100°C (212°F) and is the temperature of the measuring function.

Example #2 – Type “T” Thermocouple

Measured “VD” = 4.47 mV

Reference Junction Temperature = -5°C (23°F) (lower than the table reference of 0°C)

  1. From the table; 5°C = -0.193 mV
  2. Adding -0.193 mV to +4.47 mV = +4.28 mV
  3. Finding 4.28 mV In the table; the corresponding temperature is 100°C (212°F) and is the temperature of the measuring junction

Metric/English Scale Conversion °C = °F – 32 °F = 1.8°C + 32

ANSI LETTERLEG*METALLIC COMPOSITIONMELTING POINTUSABLE TEMPERATURE RANGETOLERANCES (THE GREATER OF BASE OR % OF READING)REFERENCE TABLE DATASHEET NUMBER
°F°C(LONG TERM)**STANDARDPREMIUM
BPPLATINUM +30% RHODIUM33201825400 TO 3050 ° F± 0.5%NOT SETAN-HTP08
   
NPLATINUM +6% RHODIUM200 TO 1680 °C
C ***P(TUNGSTEN +5% RHENIUM)4500248030 TO 4200 ° FNOT ESTABLISHED SEE IPTS-90N.A.AN-HTP13
   
N(TUNGSTEN +26% RHENIUM)0 TO 2300 ° C
EPCHROMEL®,22301220-300 TO 850° F± 1.7° C± 1.0 ° C orN.A.
     
NCONSTANTAN-200 TO +450° Co r± 0.5%± 0.4%
     
JPIRON2230122030 TO 700° F± 2.2° C± 1.1 ° C or 0.4%N.A.
    
NCONSTANTAN0 TO 400° Cor ± 0.75%
    
KPCHROMEL,25501400-300 TO 1800° F± 2.2° C± 1.1° C or ± 0.4%AN-HTP12
    
NALUMEL,-200 TO 1000° Cor ± 0.75%
    
NPNICROSIL****2440134030 TO 1800° F± 2.2° C± 1.1° CN.A.
     
NNISIL0 TO 1000° Cor 0.75%or ± 0.4%
     
RPPLATINUM +13% RHODIUM32151770400 TO 2700° F± 1.5° C± 0.6° CAN-HTP09
     
NPURE PLATINUM200 TO 1500° Cor ± 0.25%or ± 0.1%
     
SPPLATINUM +10% RHODIUM32151770400 TO 2700° F± 1.5° C± 0.6° C or ± 0.1%AN-HTP10
    
NPURE PLATINUM200 TO 1500° Cor 0.25%
    
TPCOPPER19801080-450 TO 660° F± 1.0°± 0.5° C or ± 0.4%AN-HTP11
    
NCONSTANTAN,-270 TO 350° Cor 0.75%
    

* P=Positive Leg N = Negative Leg

**”Standard” grade wire is sufficiently accurate for most applications. The purity and composition of “premium” grade wire is more closely controlled, and its millivoltage output is closer to the NIST standard chart and therefore reads somewhat more accurately than the “standard” grade does.

*** Not an ANSI symbol, but is commonly used as a designated name; also sometimes referred to as a Type “W”.

****Trademark of Hoskins Mfg

NOTE: Individual T/C units may be calibrated by measuring their output at several known temperatures and preparing an error correction chart. This chart is used to eliminate any deviation from the “standard” output millivoltage versus temperature readings inherent in this particular thermocouple. The result is known as an “NIST” traceable thermocouple.

Read Temperature by Measuring Millivolts

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