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why do we use 4-20 ma current why cant o-20ma
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Answer Posted / s.chandra mowli
for example take a temperature process
0 degree it must indicate -0 ma
suppose if there is any fault in the circuit then for any slight increase in temperature it indicates only zero
in order to differentiate the malfunction of instrument
we use live zero and dead zero
the instrument is biased to 4 ma to indicate the proper function of the the instrument that is Live zero
if it indicates 0 then is a malfunction in instrument that is Dead Zero
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what is output of FOUNDATION FIELD BUS transmitter
Q1: Consider part of a control loop, which excludes the transmitter, consisting of a process, a controller and a control valve which may be represented by two dead times of 0.5 min each and three exponential lags of 0.8 min., 1.0 min. and 1.5 min. respectively. We wish to express this system as an overall first order plus dead time (FOPLD) model ie gain, time constant and process dead time. (We will see later that this is often done, to simplify controller tuning). For this exercise, gain is considered to be 1.0. (A) If the transmitter is a flow transmitter whose behaviour can be described by a dead time of 0.2 min. and an exponential lag of 0.5 min. in terms of the overall dead time and overall first order lag how can the system behaviour be approximated ? Overall dead time = Overall time constant = (B) If the transmitter is a temperature transmitter with a temperature sensor in a protecting well whose behaviour can be described by a dead time of 0.7 min. and an exponential lag of 15 min. how can the overall system behaviour be approximated now? Overall dead time = Overall time constant =
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