Thermocouple detector circuits

As was shown in Figure 3.159, cryogenic temperatures can be detected by integrated circuit diodes types K, T, and E thermocouples (TCs) class A and B resistance temperature detectors (RTDs) acoustic and ultrasonic thermometers germanium and carbon resistors and paramagnetic salts. As TCs and RTDs will be discussed in separate subsections, here the focus will be on the other sensors. 

Figure 16.27 in practice approximates the error only for instruments with Johnson or thermal noise-limited detectors, such as photoconductive detectors like CdS or PbS detectors (400 to 3500 nm) or thermocouples, bolometers, and Golay detectors in the infrared region. Johnson noise is produced by random thermal motion in resistance circuit elements. 

The resistance ratio for Chromel-P thermocouple wire was determined with the circuit shown in Fig. 1. A fixed current was supplied to the specimen with a stabilized power supply (Trygon Model H20-1.5) having remote sensing. Two laboratory-type rheostats were used to give additional fine current adjustments the specimen current was maintained at a selected value of 0.10000 amp by adjusting the potential across the standard 1-ohm resistor, Rs, to 0.10000 v. The potential across the standard resistor and the specimen coil was determined with a potentiometer (L N K-3), a null detector (L N Model 9834-1), and an Eppley standard cell. 

Another component in the circuit is a temperature sensor. The temperature sensor measures the actual temperature in the zone, converting it to a signal that is sent to the controller. The sensor may be in contact with metal or polymer. There are various types of sensors, including thermistors and infrared detectors, but the most common type is the thermocouple. 

Intrinsic safety circuit use (in %). IP, ingress protection RTD, resistance temperature detector T/C, thermocouple. 

IS circuits can be intetfaced with almost all types of instrument types and I/Os, namely, tesistance temperature detector (RTD), Thermocouple, mV, mA loop. Digital I/Os, and Fieldbus systems. 

Resistance temperature detector (RTD) input Like the thermocouple, in the case of an RTD, IS barriers prevent excess energy from possible faults on the safe side, say from the logic solver, from reaching the hazardous area. A typical RTD input IS circuit is depicted in Fig. X/3.7.2-2. 

High and low limit checks for transmitters have been in practice for quite some time, even when discrete instmmentations were in use. Now with the DCS it is very easy to monitor out of limits for the transmitter and open-circuit and short-circuit tests for sensors like resistance temperature detectors and thermocouples. Most of the transmitters are monitored for out of span (e.g., <4 or >20 mA). Also since smart transmitters have a diagnostic system, they also can detect faults and isolate them, that is, the output of a faulty transmitter could be inhibited generating an alarm. The transmitter is connected via HART/Profibus/fieldbus, and such detections are more explicit and well reported in the system. Also there exists a facility for the operator to select any transmitter manually. 

Thermocouples, The most commonly used detectors in analytical infrared instruments have been thermocouples. These thermoelectric detectors depend on the Peltier effect, which is the generation of a voltage in a circuit containing two dissimilar metals or semiconductors when the junctions are at different temperatures. 

These coolers use the Peltier effect, which can be described as the inverse of the thermocouple effect When a current flows through a circuit with dissimilar metals, one junction heats and the other cools. By cascading junctions, temperatures of just under 200 K can be achieved with a small heat load. (They are also used to provide a stable near-room temperature environment for uncooled detectors.)... 

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