Is there an industry standard or performance expectation for 2 wire 4-20mA output devices (actuators, positioners) for signal levels below 4mA? For example, many field transmitters incorporate special signal indications outside the 4-20mA interval for alarm levels or diagnostic indications.  And I am aware that for valve positioning driving sometimes DCS systems incorporate driving below 4mA (or above 20mA) to assure valve seating at the endpoints.

>>>>>>>>>>>>>>>>>>>>needle valve  

No "Standard" for less then 4 ma.    For valves,  nothing less then 4 ma,   but you can get more information from the positioner using HART or other inteligent communication if the positioner equipped with the said feature.
  For transmitters,  usually when 4 or 3 wires transmitters there are a loy of options - not really standard.  we use gas detectors and fire detectors when the 4 to 20 mais the instrument readout and there are few step of lower reading for transmitter problem indication.

1st, "Is there an industry standard or performance expectation for 2 wire 4-20mA output devices (actuators, positioners) for signal levels below 4mA?"

This answered by Postdanw2, Yes, there is a Standard laid down by NAMUR and Nearly all european, japanese manufacturers adopt NAMUR's recommendations use 3.6mA for the fail safe 'fail-low' signal. These levels are used as common alarms indications, where smart transmitters are transmitting analouge signals (not part of a bus system). If a smart transmitter outputs a "failure" sinal of 3.6mA, then a techie can connect a communication to dianose the fault.



2nd , "I am aware that for valve positioning driving sometimes DCS systems incorporate driving below 4mA (or above 20mA) to assure valve seating at the endpoints.  

The calibration of the valve positioner to the valve position will determine the effectiveness of the shutoff and seating at the end points (assuming a healthy plug and seat). Digital positioners can these days "self calibrate" by driving to the physical limits of the valve before referencing them to 4-20mA. They do this by applying minimum and maximun air pressure to the actuator. This ensures that include the maximum force available to them as part of the calibration.

Positioner self calibration is very diferent from a techie calibrating 4-20mA to give a pressure output of 3-15psi (or what ever bench pressure the valve is set to). As typically he/she will use sight and touch to determine the full stroke of the valve.

The practice of driving beyond 4-20mA for this second point ignores the true purpose of the valve positioner.

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2011-01-30

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