Z-51Z-51

Comparison of Time Constant* vs. OverallOutside Diameter of Bare Thermocouple Wiresor Grounded Junction Thermocouples In AirTime constants calculated for air atroom temperature and atmosphericpressure moving with velocity of65 feet per second for thermocouplesshown in Figures #1 and #2.For beaded-type and ungroundedjunctions (Figures #3 or #4), multiplytime constants by 1.5.Time constant of thermocouple madewith exposed butt welded 0.001 inchdia. wire = .003 sec.

D DBARE WIREButt Welded

Fig. #1

GROUNDEDJunctionFig. #2

D DBEADED-TYPEThermocouple

Fig. #3

UNGROUNDED-TYPE

ThermocoupleFig. #4

* The Time Constant or Response TIme is definedas the time required to reach 63.2% of an instantaneoustemperature change.

TIM

E CO

NSTA

NT

- SEC

OND

S

TIM

E CO

NSTA

NT

- SEC

OND

S

WIRE OR SHEATH DIAMETER - INCHES D

Time constant of thermocouple made withexposed, butt welded0.001 in. dia. wire= .003 sec.

.001.002 .004 .006 .008 .010 .012 .014 .016 .018 .020 .022 .024 .026 .028 .030 .032 .034

1.2

1.1

1.0

.9

.8

.7

.6

.5

.4

.3

.2

.1

0.0

2.1

2.0

1.9

1.8

1.7

1.6

1.5

1.4

1.3

1.2

1.1

1.01/64 in. 1/32 in.

TIM

E CO

NSTA

NT

- SEC

OND

S

TIM

E CO

NSTA

NT

- SEC

OND

S

WIRE OR SHEATH DIAMETER - INCHES D

.03125 .0625 .09375 .125 .15625 .1875 .21875 .250 .28125 .3125 .375 1/32 1/16 3/32 1/8 5/32 3/16 7/32 1/4 9/32 5/16 3/8

11.0

10.0

9.0

8.0

7.0

6.0

5.0

4.0

3.0

2.0

1.0

110

100

90

80

70

60

50

40

30

20

101.8 sec.

Figure M Sheath Diameter 132" to 38 "

Because of space limitations,time constant curve is dividedinto 4 separate curves.

Note:These comparisons apply to either bare butt-welded or grounded junction thermocouples. If the thermocouples are the beaded type orungrounded, the time constant is longer. These times are only approximate and are provided for comparison purposes only. Multiply valuesfrom Time Constants by 1.5 for junctions shown in Fig. #3 and Fig. #4.

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