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Read Temperature by Measuring Millivolts

Posted on October 4th, 2016 by vapro

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 Thermocouple an-htp08.pdf
Type R Thermocouple an-htp09.pdf
Type S Thermocouple an-htp10.pdf
Type T Thermocouple an-htp11.pdf
Type K Thermocouple an-htp12.pdf
Type C Thermocouple an-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 LETTER LEG* METALLIC COMPOSITION MELTING POINT USABLE TEMPERATURE RANGE TOLERANCES (THE GREATER OF BASE OR % OF READING) REFERENCE TABLE DATASHEET NUMBER
°F °C (LONG TERM) **STANDARD PREMIUM
B P PLATINUM +30% RHODIUM 3320 1825 400 TO 3050 ° F ± 0.5% NOT SET AN-HTP08
     
N PLATINUM +6% RHODIUM 200 TO 1680 °C
C *** P (TUNGSTEN +5% RHENIUM) 4500 2480 30 TO 4200 ° F NOT ESTABLISHED SEE IPTS-90 N.A. AN-HTP13
     
N (TUNGSTEN +26% RHENIUM) 0 TO 2300 ° C
E P CHROMEL®, 2230 1220 -300 TO 850° F ± 1.7° C ± 1.0 ° C or N.A.
         
N CONSTANTAN -200 TO +450° C o r± 0.5% ± 0.4%
         
J P IRON 2230 1220 30 TO 700° F ± 2.2° C ± 1.1 ° C or 0.4% N.A.
       
N CONSTANTAN 0 TO 400° C or ± 0.75%
       
K P CHROMEL, 2550 1400 -300 TO 1800° F ± 2.2° C ± 1.1° C or ± 0.4% AN-HTP12
       
N ALUMEL, -200 TO 1000° C or ± 0.75%
       
N P NICROSIL**** 2440 1340 30 TO 1800° F ± 2.2° C ± 1.1° C N.A.
         
N NISIL 0 TO 1000° C or 0.75% or ± 0.4%
         
R P PLATINUM +13% RHODIUM 3215 1770 400 TO 2700° F ± 1.5° C ± 0.6° C AN-HTP09
         
N PURE PLATINUM 200 TO 1500° C or ± 0.25% or ± 0.1%
         
S P PLATINUM +10% RHODIUM 3215 1770 400 TO 2700° F ± 1.5° C ± 0.6° C or ± 0.1% AN-HTP10
       
N PURE PLATINUM 200 TO 1500° C or 0.25%
       
T P COPPER 1980 1080 -450 TO 660° F ± 1.0° ± 0.5° C or ± 0.4% AN-HTP11
       
N CONSTANTAN, -270 TO 350° C or 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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