Answer / carl_ellis

1-5Vdc is not a standard, it is a widely used convention for 4-20mA current loops.

A 250 ohm resistor drops a 1-5 Vdc voltage proportional to the 4-20mA current.

DCS analog inputs historically used 250 ohms for 4-20mA analog input resistance and over time that value has become quite common.

But I have seen 1 ohm, 4.7 ohms, 10 ohms, 12 ohms, 24.9 ohms, 50 ohms, 62.5 ohms, and 100 ohm resistances for 4-20mA current input.

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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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