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Pt 100 means its platinum and temperature is 0 oC at 100 ohms resistance
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| Pt 100 means its platinum and temperature is 0 o C at 100 ohms resistance. Measurement of unknown resistance The electrical circuit used for temperature measurement is a Wheatstone bridge. The bridge converts the RTD’s change in resistance to a voltage output. This circuit uses four separate electrical resistors, one of which is the RTD. The bridge is initially balanced, with voltage output equal to zero, because all four resistors are equal. If the resistance of the RTD changes, due to a temperature changes, the bridge becomes unbalanced, resulting in a voltage output other than zero. A.RTD BRIDGE CIRCUIT B. RTD RESISTANCE VS VOLTAGE GRAPH 17 Field Instrumentation GEN Rev) In two conductors RTD, these conductors become part of resistance being measured, so ambient temperature variation will result in error of net temperature. A TWO WIRE RTD CIRCUIT Three conductor RTD cable is mostly used to minimize the effect of ambient temperature variations on cable. In three lead circuit, Land L are in opposite arms of the bridge. The change in L is equal to L. Because L is in series with the input voltage, the bridge output voltage is unaffected. Four conductor RTD is used to reduce temperature effects on cable. This circuit is used in a system with long lead wires whose temperature varies greatly during measurement. 18 Field Instrumentation GEN Rev) Resistance elements are usually long springlike wires enclosed in a metal sheath. The platinum element is surrounded by a porcelain insulating material that prevents a short circuit from developing between the wire and the sheath. The sheath is made of Inconel, an alloy of nickel, iron, and chromium. This material has excellent corrosion resistance and can be used in extremely harsh environments for long periods of time without deteriorating. The thermo well protects the RTD from any contamination or corrosion caused by the gases or liquids being measured. A heavy metal head, made of cast iron or aluminum, covers the terminal block containing the electrical connections between the RTD and Wheatstone bridge. The main advantage of RTD is stability, linearity and accuracy. Download 3.37 Mb. Share with your friends:
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