17-12-2012, 04:08 PM
Recorders
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Abstract
Every aspect of our lives, both at home and at work, is influenced by
temperature. Temperature measuring devices have been in existence for centuries. The principle of operation is ageless as the device itself. In this module we will look at the theory and operation of some temperature measuring devices commonly found in a generating station. These include thermocouples, thermostats and resistive temperature devices. Thermocouples (T/C) and resistive temperature devices (RTD) are generally connected to control logic or instrumentation for continuous monitoring of temperature. Thermostats are used for direct positive control of the temperature of a system within preset limits.
Introduction
A Recorder is a measuring instrument that displays a time-varying signal in a form easy to examine, perhaps long after the original signal has ceased to exist. In other words a recorder is a device whose function is to record the value of a quantity as it is being measured. It records Electrical and Non electrical quantities as function of time. Currents and Voltages can be recorded directly while any non-electrical quantities are recorded indirectly by first converting them into the form of electrical signal with the help of sensors or transducers. All recorders are not measuring or analytical instruments, for instance recorder used for speech and music. In this seminar, I am going to give detailed information on three basic recorders; Analog Recorder, Temperature Recorder, Audio Recorder.
Resistance Temperature Detector (RTD)
Every type of metal has a unique composition and has a different resistance to the flow of electrical current. This is termed the resistively constant for that metal. For most metals the change in electrical resistance is directlyproportional to its change in temperature and is linear over a range oftemperatures. This constant factor called the temperature coefficient of electrical resistance (short formed TCR) is the basis of resistancetemperature detectors. The RTD can actually be regarded as a highprecision wire wound resistor whose resistance varies with temperature. Bymeasuring the resistance of the metal, its temperature can be determined.Several different pure metals (such as platinum, nickel and copper) can beused in the manufacture of an RTD. A typical RTD probe contains a coil ofvery fine metal wire, allowing for a large resistance change without a greatspace requirement. Usually, platinum RTDs are used as processtemperature monitors because of their accuracy and linearity.To detect the small variations of resistance of the RTD, a temperaturetransmitter in the form of a Wheatstone bridge is generally used. The circuit compares the RTD value with three known and highly accurate resistors.
Thermocouple (T/C)
A thermocouple consists of two pieces of dissimilar metals with their ends
joined together (by twisting, soldering or welding). When heat is applied to
the junction, a voltage, in the range of milli-volts (mV), is generated. A
thermocouple is therefore said to be self-powered. Shown in Figure 3 is a completed thermocouple circuit
The voltage generated at each junction depends on junction temperature. If
temperature T1 is higher than T2, then the voltage generated at Junction 1
will be higher than that at Junction 2. In the above circuit, the loop current
shown on the galvanometer depends on the relative magnitude of the
voltages at the two junctions.
In order to use a thermocouple to measure process temperature, one end of
the thermocouple has to be kept in contact with the process while the other
end has to be kept at a constant temperature. The end that is in contact with
the process is called the hot or measurement junction. The one that is kept
at constant temperature is called cold or reference junction.