How does 4-20 mA output work for +/- 1 mm Gap Transmitter?

Axial Gap or Air Gap Transmitters are transmitters that use the change in DC Voltage of Non-Contact Eddy Current Sensors proportional to Gap. This change is linear between the set full scale. When a Gap Transmitter IRD7100 is set for +/- 1 mm Full Scale - then it provides instantaneous Gap value display, 4-20 mA output, 0-5 V DC output, Forward Alarm, Forward Trip, Reverse Alarm and Reverse Trip per channel.

When the DC Gap Voltage is -2V DC - then the transmitter reads it as -1 mm gap (sensor now closer to target) and when the DC Gap Voltage is -18V DC - then the transmitter reads it as +1 mm gap (sensor now farther away from target). When the DC Gap Voltage is -10V DC - then the transmitter reads it as 0 mm gap (the idle position of the sensor and target)

Scheme for Gap Monitoring using IRD401 and IRD7100

4-20 mA output helps integrate Gap change with DCS/PLC/SCADA and helps user see the readings on their Control Room screen. Here, when the transmitter reads -1 mm gap - then the 4-20 mA output is at 4 mA DC and when the transmitter reads +1 mm gap - then the 4-20 mA is at 20 mA DC. At 0 mm gap - the 4-20 mA output is at 12 mA DC.

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