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Calibration
Type Characteristics
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Thermocouple
Insulation, provides
electrical insulation for thermocouple and thermocouple
extension wire. If the insulation breaks down for any reason,
the indicated temperature may be in error. When selecting
insulation moisture, abrasion, flexing, chemical attack,
temperature extremes and any other adverse environmental
considerations must be evaluated. Insulations are rated
for a maximum continuous use temperature and also a maximum
single exposure temperature because after excessive temperatures
have been encountered the insulation may become conductive
or conductive residues may form even though the insulation
remains physically intact. Also do not assumethe temperature
rating as the temperature at the sensing junction of the
thermocouple without evaluating the thermocouple system.
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| Type
T (COPPER vs CONSTANTAN)
is used for service in oxidizing, inert or reducing atmospheres
or in vacuum. It is highly resistant to corrosion from atmospheric
moisture and condensation and exhibits high stability at low
temperatures; it is the only type with limits of error guaranteed
for cryogenic temperatures. |
| Type
J (IRON vs CONSTANTAN) is
used protected or unprotected in vacuum, oxidizing, inert
or reducing atmospheres. Iron element oxidizes rapidly at
temperatures exceeding 1000°F, and therefore heavier gauge
wire is recommended for longer life at these temperatures. |
| Type
E (CHROMEL vs CONSTANTAN)
may be used protected or unprotected in oxidizing, inert or
dry reducing atmospheres, or for short periods of time under
vacuum. Must be protected from sulfurous and marginally oxidizing
atmospheres. Produces the highest EMF per degree of any standardized
thermocouple. |
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Type
K (CHROMELTM vs ALUMELTM)
is used protected or exposed in oxidizing, inert or dry
reducing atmospheres. Exposure to vacuum limited to short
time periods. Must be protected from sulfurous and marginally
oxidizing atmospheres. Reliable and accurate at high temperatures.
(TM Hoskins Mfg Co.)
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| Type
N (NICROSIL vs NISIL) is
used protected or exposed in oxidizing, inert or dry reducing
atmospheres. Must be protected from sulfurous atmospheres.
Very reliable and accurate at high temperatures. |
Fibrous
Insulation,
is either braided or wrapped on the conductors. In general,
fibrous insulations are used for applications where extreme
moisture and abrasion resistance requirements are not prevalent.
Available at moderate cost for upper utilization temperatures
of 900°F (482°C) for fiberglass, 1600°F (780°C)
for high temperature silica fiber, and 2400°F (1315°C)
for ceramic fiber. |
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Type
B (PLATINUM-30%RHODIUM vs PLATINUM-6%RHODIUM)
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Type
S (PLATINUM-10%RHODIUM vs PLATINUM)
Type
R (PLATINUM-13%RHODIUM vs PLATINUM)
Platinum
alloy thermocouples are all recommended for use in inert or
oxidizing atmospheres, or for short periods of time in a vacuum.
Easily contaminated, these elements must be protected from
the effects of reducing atmospheres and contaminating vapors.
Alumina protecting tubes are recommended for directly containing
platinum element. |
| Plastic
Insulation,
is used on comparatively lower temperature applications and
provides good moisture and abrasion resistance. Available
at low to moderate cost for typical upper utilization temperatures
of 220°F (104°C) for PVC and 500°F (260°C)
for teflon and silicon rubber. (PLATINUM-30%RHODIUM
vs PLATINUM-6%RHODIUM) |
Type
C^ (TUNGSTEN 5% RHENIUM vs TUNGSTEN 26% RHENIUM)
Tungsten alloy thermocouples are recommended for use in vacuum,
high purity hydrogen, or high purity inert atmospheres. Very
poor oxidation resistance. (^Not ANSI/ASTM symbol)
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WIRING
ELECTRONIC INSTRUMENTS to conform to national and local
codes does not address the "noise" problems
of electronic instruments. Shielding of thermocouple and thermocouple
extension wire may be necessary but not the only requirement
of reducing noise. Ever since the introduction of electronics
into instruments, noise generated by external relays, switches,
motors, phase fired thyristors, etc., have caused problems
that interfere with the instrument's operation. Now that microprocessors
are being increasingly incorporated into many more varied
instruments, external sources that generate noise pulses that,
in some cases, may render the instrument completely inoperative,
have become crucial to instrument applications. While much
can be done within the instrument to reduce its sensitivity
to external noise, the solution in many cases can only be
resolved by supressing the noise generation at its source. |
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