The 4-20 mA represents the sensor's measurement span. The 4 mA set point is typically placed near the bottom of the empty tank, or the greatest measurement distance from the sensor. The 20 mA set point is typically placed near the top of the full tank, or the shortest measurement distance from the sensor. The sensor will proportionately generate a 4 mA signal when the tank is empty and a 20 mA signal when the tank is full.
The old 10-50 mA standard lost the battle to 4-20 mA due to the following benefits: 1) The same two-wires carry both the sensor power and signal, 2) 4-20 mA is easily converted to a 1-5 VDC input via a 250 Ohm, _ Watt resistor or 1-10 VDC input via a 500 Ohm, _ Watt resistor, 3) 4-20 mA is low current and therefore has a low heating loss through the signal wire, and 4) with shielded cable, 4-20 mA is noise resistant to radiated and inducted interference.
The sensor can be calibrated in about a minute using a 24 VDC power supply, tape measurer and flat reflective target such as a wall or liquid in the tank. To calibrate the 4 and 20 mA set points, attach the calibration wire to the appropriate power supply leg (see manual), and hold the sensor at the desired range to the target until the blue LED blinks.
It's often more convenient to target calibrate the sensor against a wall or other flat reflective surface using a tape measurer for 4 mA and 20 mA set point distance.
While the 4 mA and 20 mA set point calibration is not fundamentally required, it's generally recommended because it optimizes the sensor's installed performance.
The sensor can be reverse calibrated such that the 20 mA set point is empty and the 4 mA set point is full. Reverse 20-4 mA calibration can also help optimize the sensor's response time in applications with a fast rate of level change.
The 20 mA tank full set point should be placed at a distance that's greater than or equal to the riser height plus half the sensor's dead band. For example, based on an LU13 sensor, if the riser height is 6 and the dead band is 4" then the 20 mA full set point would be placed at 8"."
The 4-20 mA span can be calibrated by measuring the current consumption alone with a digital multi-meter or display. During the initial moments of calibration, the sensor draws 22 mA while it's acquiring the target distance. At the time that the sensor places the set point, the current will jump down to the appropriate 20 mA or 4 mA set point and the blue LED will flash.
Yes, as long as the total loop resistance is less than 900 Ohms.