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Hi Frnds , If Suppose 1 Pair cable of 1.5 sq mm is used for
Transmitter and instead of 1.5 sq mm if i used 0.5 sq mm
cable will the resistance change will occur and does it
affect our transmitter reading. plz mail ur answer to
ntnkamble@gmail.com plz immediately mail ur reply
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Answer Posted / carl_ellis
It depends on how much voltage the field transmitter needs to operate and the total loop resistance of the loop circuit .
Every 2 wire loop powered transmitter has a specification for the voltage required to drive a given loop resistance. Sometimes that is a formula, sometimes a load line graph, sometimes a statement like "minimum liftoff voltage is 17.7V", or rarely, a vague statement of maximum loop resistance (common with active analog outputs, rare with loop powered transmitters)
Excessive wire resistance in a 2 wire loop powered transmitter circuit can affect the operation by
- preventing the transmitter from turning on and operating
- allowing partial operation at lower current outputs, but failing to operate at higher current outputs. That failure can be either an output that just never exceeds a certain value or a transmitter that cycles on-off-on-off an never initializes.
If there is sufficient voltage to run drive the total loop resistance, then the transmitter could function normally.
You need to calculate the wire resistance over its total wire run length (out and back) and know what the receiver's analog input resistance is. Then look up the specification for how much resistance the transmitter's can drive and see whether your loop resistance is lower than the maximum.
Carl Ellis
Measure First
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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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