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InterviewSolution
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Why Most Of Analog O/p Devices Having O/p Range 4 To 20 Ma And Not 0 To 20 Ma? |
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Answer» 4-20 mA is a STANDARD range used to indicate measured values for any PROCESS. The reason that 4ma is chosen instead of 0 mA is for fail safe OPERATION .For example- a pressure instrument GIVES output 4mA to indicate 0 psi, up to 20 mA to indicate 100 psi, or full scale. Due to any problem in instrument (i.e) broken wire, its output REDUCES to 0 mA. So if range is 0-20 mA then we can differentiate whether it is due to broken wire or due to 0 psi. 4-20 mA is a standard range used to indicate measured values for any process. The reason that 4ma is chosen instead of 0 mA is for fail safe operation .For example- a pressure instrument gives output 4mA to indicate 0 psi, up to 20 mA to indicate 100 psi, or full scale. Due to any problem in instrument (i.e) broken wire, its output reduces to 0 mA. So if range is 0-20 mA then we can differentiate whether it is due to broken wire or due to 0 psi. |
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