Wise to use a 10k resistor between sense resistor and Arduino pin (safety), and a 100n cap from Arduino pin to ground.Let us know if you need a diagram.Leo..
250ohm means you're going to use default Aref, and rely on the stability of the Mega's 5volt rail.Bad idea.The Mega has in internal 2.56volt Aref (called in code), which is potentially much better.That means the sense resistor should be ~125ohm.That also would eleminate the need for a 5.1volt zener, which is also a bad idea (temp dependent non-linearity).Wise to use a 10k resistor between sense resistor and Arduino pin (safety), and a 100n cap from Arduino pin to ground.Let us know if you need a diagram.Leo..
Yes, if you run the 4 - 20 mA output current through a 250 Ohm resistor you will get 1 - 5 volts which you can read with the Mega's analog to digital converter and get a reading from 204 - 1023. 0 pressure = 204, 200 PSI = 1023. Or pressure = (ADC reading - 204) / 818 * 200. You should put a current limit resistor in series with the analog input pin, value depending on the sensor's supply voltage (Vsupply / 0.0009) or a 5.1V Zener diode between input pin and GND.EDIT: You can get close to 250 Ohms by putting a 3300 Ohm in parallel with a 270 (both are standard values).
@Wawa:Why is a Zener not good? I've only used them in PS type circuits.
Are you using the diagram from post#5, with a 51ohm sense resistor?Then you can use a 12volt regulated supply for the sensor, and share supply ground (-) with Arduino ground.12volt supply (+) to sensor (+), pin1.Sensor (-) pin2 to the 51ohm resistor (current loop (+) in the diagram from post#5).Post a diagram, so we can check if you understand it.Leo..