Page 697 - Plastic Process Equipment, Inc

PRINCIPLES OF THERMOCOUPLES & RTD'S

— THERMOCOUPLES —

GENERAL INFORMATION
Thermocouples are the most widely used devices to measure temperature. In their simplest form they consist of a welded
junction, called the hot junction, between two dissimilar metals (e.g. - iron & constantan, Type J) and a reference junction at
the other end of the lead wires. The hot junction will create an EMF, which can be measured at the reference junction, that
corresponds to a specific temperature. Advantages of using a thermocouple include a wide temperature range, ruggedness,
fast response time, and low cost. PPE Thermocouples are ROHS Compliant.

JUNCTIONS
Grounded - In grounded thermocouples, the conductors are contained in a sheath with mineral insulation, but the actual
hot junction is welded to the end of the sheath (see figure #1). This provides a great deal of ruggedness and environmental
protection without sacrificing too much response time. The disadvantage of this construction is that there is no protection if
stray voltages come in contact with the sheath. Grounded construction is the most commonly used construction in the
Plastics Industry. Unless otherwise specified all PPE thermocouples are grounded.

INSULATION SHEATH
CONDUCTORS HOT JUNCTION (TOUCHES TIP)
FIGURE #1
WELDED TIP
Ungrounded (Insulated) - The ungrounded construction is similar to the grounded construction except the hot junction is
totally contained within the sheath (see figure #2). This provides complete isolation from any foreign voltage that may be
present in the application. The drawback with this construction is response time, as it takes more time for the heat to
penetrate the insulation and reach the hot junction.

INSULATION SHEATH
CONDUCTORS HOT JUNCTION (INSULATED FROM TIP)
FIGURE #2
WELDED TIP

Exposed - In this construction, the conductors actually extend beyond the sheath or there is no sheath at all (see figure #3).
This provides excellent response time, but no protection from the environment or stray voltages. Exposed thermocouples
are relatively fragile.
INSULATION SHEATH
CONDUCTORS HOT JUNCTION (EXTENDS PAST TIP)
FIGURE #3

CALIBRATION (TYPE)
As mentioned earlier, thermocouples are made of two dissimilar metals. The specific metals used determine the type of
thermocouple. Over 90% of thermocouples used in the Plastics Industry are Type J (Iron/Constantan). The remaining are
usually Type K (Chromel/Alumel). Check your machine or temperature control to determine which type you need.

— THERMOCOUPLE vs. RTD —

FACTOR THERMOCOUPLE RTD
COST LOWER HIGHER
ACCURACY GOOD BETTER
RESPONSE TIME FASTER SLOWER
TEMPERATURE RANGE -450°F TO 4000°F -400°F TO 1500°F
DURABILITY GOOD FRAGILE
MEASURING AREA TIP SENSITIVE STEM SENSITIVE

PLASTIC PROCESS EQUIPMENT, INC. www.ppe.com
e-mail: sales@ppe.com
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