Why 4 20ma to 0 20ma

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Start networking and exchanging professional insights Register now or log in to join your professional community. Follow Why we use ma rather then ma in control loop give 2 reasons?

Upvote 1 Views Followers Write an Answer Register now or log in to answer. Zero detection circuitry has to be added to field transmitters, 3. Induced Eddy currents will cause false readings. Upvote 4 Downvote 0 Reply 0. Upvote 3 Downvote 0 Reply 0. Contractor of Saudi Aramco 5 years ago. Upvote 1 Downvote 0 Reply 0. Wire break detection Induced eddy current wont effect at 4ma. Upvote 0 Downvote 0 Reply 0. Another factor it is easy to scale MA to v Using ohm resistance. Any process parameter such as Temperature, Pressure, Flow, Density is sensed, measured by a sensor and is converted to an electrical signal in the form of voltage.

The voltage from the sensor is sent to the transmitter to convert in a standard form of the current output signal. Live Zero i. It is easy to detect that the signal wire is not broken. If the range zero were truly zero mA, then there would be no current, and therefore no power to drive the field instrument.

Field instrument would require 3 or 4 wire devices. Additional wiring costs come into account. Another reason for choosing 4 mA is when the transmitter was first introduced, it was working on pneumatic pressure signal for transmission. Later, electronics chips were introduced those require a minimum of 3 mA of current to function. So, a margin of 4 mA is taken as reference. Scale and Calculate : with mA signal it is easy to scale and calculate the process parameter.

Calculate percentage of signal corresponding to mA signal. For conversion of current from mA to V needs a ohms resistance. Reason for 20 mA: 20 mA is used as maximum because the human Heart can withstand up to 30 mA of current only. So, as a safety point of view, 20 mA is chosen. Very long cables have resistance, that causes a voltage drop.