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HOW TO DETERMINE TEMPERATURE BY MEASURING THE OUTPUT MILLIVOLTAGE OF A THERMOCOUPLE

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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 junction.
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

(LONG TERM)

TOLERANCES (THE GREATER OF BASE OR % OF READING)

REFERENCE TABLE DATASHEET NUMBER
° F
° C
** STANDARD
PREMIUM

B

P

N
PLATINUM +30% RHODIUM

PLATINUM +6% RHODIUM

3320

1825

400 TO 3050 ° F

200 TO 1680 ° C
± 0.5%
NOT SET

AN-HTP08

C ***
P

N
(TUNGSTEN +5% RHENIUM)

(TUNGSTEN +26% RHENIUM)

4500
2480
30 TO 4200 ° F

0 TO 2300 ° C
NOT ESTABLISHED SEE IPTS-90
N.A.
E
P

N
CHROMEL®,

CONSTANTAN

2230
1220
-300 TO 850° F

-200 TO +450° C

± 1.7° C

o r± 0.5%

± 1.0 ° C or

± 0.4%

N.A.
J
P

N
IRON

CONSTANTAN

2230
1220
30 TO 700° F

0 TO 400° C

± 2.2° C

or ± 0.75%

± 1.1 ° C or 0.4%
N.A.
K
P

N
CHROMEL,

ALUMEL,

2550
1400
-300 TO 1800° F

-200 TO 1000° C

± 2.2° C

or ± 0.75%

± 1.1° C or ± 0.4%
N
P

N
NICROSIL****

NISIL

2440
1340
30 TO 1800° F

0 TO 1000° C

± 2.2° C

or 0.75%

± 1.1° C

or ± 0.4%

N.A.

R

P

N
PLATINUM +13% RHODIUM

PURE PLATINUM

3215

1770

400 TO 2700° F

200 TO 1500° C

± 1.5° C

or ± 0.25%

± 0.6° C

or ± 0.1%

AN-HTP09

S
P

N
PLATINUM +10% RHODIUM

PURE PLATINUM

3215
1770
400 TO 2700° F

200 TO 1500° C

± 1.5° C

or 0.25%

± 0.6° C or ± 0.1%

T

P

N
COPPER

CONSTANTAN,

1980

1080

-450 TO 660° F

-270 TO 350° C

± 1.0°

or 0.75%

± 0.5° C or ± 0.4%

AN-HTP11

 

* 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.

 

 

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