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Spectrum Controls 1769sc-IF8u User Manual

Page 92

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Compact IO

Universal Input Module

manganese content to achieve the most stable performance. The use of
special Ni-Cr based alloys for sheathing to improve the chemical and
physical compatibility with the thermoelements also has been investigated
by Burley [54-56] and by Bentley [57-60].

Neither thermoelement of a type N thermocouple is extremely sensitive to
minor differences in heat treatment (provided that the treatment does not
violate any of the restrictions mentioned above). For most general
applications, they may be used with the heat treatment routinely given by
the wire manufacturer. Bentley [61,62], however, has reported reversible
changes in the Seebeck coefficient of type NP and NN thermoelements
when heated at temperatures between 200°C and 1000°C. These impose
limitations on the accuracy obtainable with type N thermocouples. The
magnitude of such changes was found to depend on the source of the
thermoelements. Consequently, when the highest accuracy and stability
are sought, selective testing of materials, as well as special preparatory
heat treatments beyond those given by the manufacturer, will usually be
necessary. Bentley’s articles [61,62] should be consulted for guidelines and
details.

ASTM Standard E230-87 in the 1992 Annual Book of ASTM Standards
[7] specifies that the initial calibration tolerances for type N commercial
thermocouples be ±2.2°C or ±0.75 percent (whichever is greater)
between 0°C and 1250°C. Type N thermocouples can also be supplied to
meet special tolerances that are equal to approximately one-half the
standard tolerances given above. Tolerances are not specified for type N
thermocouples below 0°C.

The suggested upper temperature limit of 1260°C given in the ASTM
standard [7] for protected type N thermocouples applies to AWG 8 (3.25
mm) wire. It decreases to 1090°C for AWG 14 (1.63 mm), 980°C for
AWG 20 (0.81 mm), 870°C for AWG 24 or 28 (0.51 mm or 0.33 mm), and
760°C for AWG 30 (0.25 mm). These temperature limits apply to
thermocouples used in conventional closed-end protecting tubes and they
are intended only as a rough guide to the user. They do not apply to
thermocouples having compacted mineral oxide insulation.

Type R

Type R

Type R

Type R

Type R

T

TT

T

T h e r

h e r

h e r

h e r

h e r m o c o u p l e s

m o c o u p l e s

m o c o u p l e s

m o c o u p l e s

m o c o u p l e s

This section describes Platinum-13 percent Rhodium Alloy Versus
Platinum thermocouples, called type R thermocouples. This type is often
referred to by the nominal chemical composition of its positive (RP)
thermoelement: platinum-13 percent rhodium. The negative (RN)
thermoelement is commercially-available platinum that has a nominal
purity of 99.99 percent [21]. An industrial consensus standard (ASTM
E1159-87) specifies that rhodium having a nominal purity of 99.98 percent
shall be alloyed with platinum of 99.99 percent purity to produce the
positive thermoelement, which typically contains 13.00 ± 0.05 percent
rhodium by weight. This consensus standard [21] describes the purity of
commercial type R materials that are used in many industrial thermometry