Thermocouples are sensors that measure temperature and pass that information to a control or monitor. Thermocouples are used in all types of applications, can measure wide temperature ranges, and are offered in a large variety of standard configurations.
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 Fig. 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 junctions. See Fig. 1. Any temperature effects near the instrument can be cancelled 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 metal 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. 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.
Fig. 1 Voltage Difference vs. Degrees F for J Type Thermocouple
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 substrate and then encapsulating it. The electrical resistance of the RTD channges 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 RTD's 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 defined in a wide variety of standard configurations.
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 wihin 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.
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 rate service temperature for that component. Service Temperatures:
Stainleass Stell Tubing/Protection/Mounting Hardware: 1500°F
Stainless Steel Springs: 1500°F
Incon el Springs: 2100°F
Fiberglass Insulated Extension Wire•: 900°F
Junction Box (BX) Connector: 400°F
Plug: 300°F
J Type Thermocouple Junction: 1600°F
K Type Thermocouple Junction: 2500°F
E Type Thermocouple Junction: 1800°F
DIN or NIST RTD: 1607°F
No comments:
Post a Comment