Temperature Sensors
 
Introduction
Sensors measure temperature and pass that information to a control or monitor. Dwyer offers three types of sensors: thermocouples, resistance temperature detectors (RTDs), and thermistors.

Thermocouples
Thermocouples are made of two dissimilar metal wires joined at their measuring end forming the "measuring junction" also known as the "hot junction". A small voltage, known as the Seeback voltage, is created at a junction of dissimilar metal alloys. This voltage changes as a function of temperature, See Figure 1. The control or monitor measures this small voltage and converts it to a temperature signal. Modern instrumentation also measures the temperature where the thermocouple is connected to the instrument. This is the reference junction. See Figure 2. Any temperature effects near the instrument can be canceled out leaving an accurate reading of the process to be measured.

A thermocouple may be directly connected to a control or monitor. Extension wires, if used, must be of the same materials as the thermocouple wires. Thermocouples designed with their measuring junctions in contact with new surfaces are known as grounded junction thermocouples. These are the most common, generally have the fastest response times, and are the most economical. Ungrounded junction thermocouples offer the advantage of electrical isolation. Dwyer manufactures both types of thermocouples. Thermocouples are generally more rugged and less expensive than other sensor types. All Dwyer thermocouples are manufactured using industry standard alloys and meet astringent ANSI standards. This assures interchangeability with other standard thermocouples without special instrument recalibration. Dwyer thermocouples are used in all types of applications, can measure wide temperature ranges, and are offered in a large variety of standard configurations.

RTDs
RTDs are usually platinum wire wound on a glass or ceramic bobbin and sealed with a coating of ceramic or glass. They can also be made by depositing platinum as a film on a substitute and then encapsulating it. The electrical resistance of the RTD changes as a function of temperature. Circuitry similar to a Wheatstone bridge is built into controls designed for use with RTDs. Constant current into the bridge produces an output voltage that varies with temperature. Lead wire resistance can significantly affect the RTD measurement. This is typically corrected using a third (compensating) lead wire. See Figures 3 and 4.

Extension wires used with RTDs may be plain copper wire. RTDs are generally more accurate and more stable over time than thermocouples. Dwyer RTDs are built to rigorous DIN (most common) or NIST standards and are offered in a wide variety of standard configurations.

The third wire in the 3-wire RTD compensates for the wire lead resistance and its temperature change.


Thermistors
Thermistors have a semiconductor material which changes its electrical resistance as a function of temperature. Extension wires used with thermistors can be plain copper wire. Thermistors offer accuracy similar to RTDs within narrow temperature ranges near ambient temperature. They also generally offer faster response times. Since thermistor standards vary, care must be taken to match the instrumentation to the sensor.


Ordering Sensors:
Sensors are constructed with various types of protection/mounting hardware, extensions, and wire terminations. The sensor types and their temperature ranges are shown in the table below. See "Temperature Limits" below for maximum service temperatures applicable to the protection tube, mounting hardware, wire extensions, etc.

This section shows only a limited selection of the available sensors. The sensors are organized by hardware type. Most hardware can house any type thermocouple, RTD, or thermistor. Terminations are usually either lug type or standard plugs, but many other types are available. Various 'head enclosures' are also available. Dimensions can be custom designed to meet your specifications. The selections listed are the most popular configurations. Please ask your sales representative about other possible selections.
Thermocouple
Types
Wire Type Temperature
Range (°F)
Temperature
Range (°C)
J
K
E
T
R
S
B
Iron/ Constantan
Chromel/Alumel
Chromel/Constantan
Copper/Constantan
Plat. 13% /Rhod. Plat.
Plat. 10% /Rhod./Plat.
Plat. 6% /Rhod./Plat.30% Rhod.
32 to 1400
320 to 2300
-300 to 1600
-300 to 700
32 to 2700
32 to 2700
1600 to 3100
0 to 760
0 to 1200
-184 to 871
-184 to 371
0 to 1482
0 to 1482
871 to 1704
Thermistor Types
Cal. 100
Cal. 101
Cal. 109
2K @ 25°C
5K @ 25°C
100K @ 25°C
-60 to 150
50 to 250
300 to 600
-51 to 66
10 to 121
140 to 315
RTD Types
DIN (100Ω@0°C α=.00385 Ω/Ω/°C)
NIST (100Ω@0°C α=.00392 Ω/Ω/°C)
Nikel (120Ω@0°C α=.00672 Ω/Ω/°C)
-420 to 1500
-420 to 1500
-150 to 600
-200 to 875
-200 to 875
-101 to 315

Temperature Limits:
Sensor selection depends on two separate temperatures: process temperature and connector temperature. Make sure the local temperature at each component does not exceed the maximum rated service temperature for that component. Note that the extension wire must withstand the process temperature. All Dwyer thermocouple and RTD assemblies (including extension wire) are designed for process temperatures to at least 900°F. Please consult factory if higher service temperatures are needed.

Service Temperatures:

Stainless Steel Tubing/Protection/Mounting Hardware
Stainless Steel Spring
Inconel® Spring (Dwyer Standard)
Fiberglass Insulated Extension Wire (Dwyer Standard)
Junction Box (BX) Connector
Plug
J Type Thermocouple Junction
K Type Thermocouple Junction
E Type Thermocouple Junction
DIN or NIST RTD
1500°F
1500°F
2100°F
900°F
400°F
300°F
1600°F
2500°F
1800°F
1600°F