Product guide

8
THERMOCOUPLE ▼
Thermocouple Selection Guide
Probe Sheath/Body Materials
Inconel
®
600 Sheath is ideal for severely corrosive environments
and elevated temperatures. Resists progressive oxidation.
Maximum operating temperature: 2100°F (1148°C) continuous;
2500°F (1371°C) intermittent.
304 SS Sheath is for general-purpose use, is corrosion-resistant,
and good for food service and biological applications. Maximum
operating temperature: 1650°F (898°C) continuous; 2550°F
(1398°C) intermittent.
316 SS Sheath has higher corrosion resistance than 304 SS.
Withstands some strong acids. Maximum operating temperature:
1650°F (898°C) continuous; 2500°F (1371°C) intermittent.
SS Sheath with Coating of PTFE or PFA with grounded junction
is ideal with corrosive liquids and atmospheres. Longer response
time. Temperatures to 500°F (260°C).
Polymer Body Probes are available in a variety of polmyers
including Kapton
®
and PTFE. These provide excellent flexibility
and often good chemical resistance. Be sure to consult a chemical
compatibility table when selecting a probe for your application.
Thermocouple Probe Junction Types
Sheaths with small diameters have faster response times;
sheaths with larger diameters have longer life and are better
for measuring higher temperatures.
Exposed Junction has the fastest response
time—ideal for measuring rapid temperature
changes. Clear coating on most models
provides a humidity barrier for the
thermocouple. Do not use with corrosive
fluids or atmospheres.
Ungrounded Junction has a welded junction
insulated from the protective sheath and is
electrically isolated. Longer response time; use
for conductive solutions or where isolation of
the measuring circuitry is required.
Grounded Junction has a junction welded to
tip of sheath. Wires are completely sealed
from contaminants. Good response time.
Physical Characteristics of Thermocouples
Type J Thermocouple
Wire insulation color:
+ = White
– = Red
Wire material:
+ = Iron
– = Constantan
Properties:
+ = Strongly magnetic
Atmosphere for exposed junction:
Reducing
Type T Thermocouple
Wire insulation color:
+ = Blue
– = Red
Wire material:
+ = Copper
– = Constantan
Properties:
+ = Copper color
Atmosphere for exposed junction:
Mildly oxidizing and reducing or with moisture
Type E Thermocouple
Wire insulation color:
+ = Purple
– = Red
Wire material:
+ = Chromel
– = Constantan
Properties:
+ = Greater stiffness
Atmosphere for exposed junction:
Vacuum, inert mildly oxidizing or reducing
General Considerations
Extend Your Thermocouples up to 2000 feet without signal loss.
Extension wire must be the same type as the thermocouple.
System Error becomes important when you select a probe and
meter to make a complete temperature measurement system. For
example: a meter has an accuracy of ±0.7°F; probe error for the
type T probe with metal sheath, straight cable, and stripped ends
will have an error limit of ±1.8°F at 400°F. Therefore, the probe-
meter system accuracy will be (±0.7) + (±1.8) = ±2.5°F at 400°F.
NIST Traceability is required for many applications. In order
to make an item traceable to NIST standards, the item and the
standard are exposed to the same conditions, the readings are
noted, and the difference between the readings is recorded on a
NIST certificate. When taking future readings with the item, the
value on the certificate must be added or subtracted from the
measured value.
Type K Thermocouple
Wire insulation color:
+ = Yellow
– = Red
Wire material:
+ = Chromel
– = Alumel
Properties:
+ = Moderately magnetic
Atmosphere for exposed junction:
Clean oxidizing
Grounded junction
Ungrounded junction
Exposed junction
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