# 4-20mA pressure sensor keeps reading -75PSI output

Hi all,

I'm trying to connect my 4-20mA to my Arduino board. I have the positive of my power supply connect with the positive wire of my pressure sensor. Then I have the negative wire of my pressure sensor connect to one side of a 250ohm resistor (R1). Then on that same node, I connect the resistor with another 250 ohm resistor (R2) in series for safety purpose, then connect the other side of R2 to pin A0. The ground from my power supply connect to the other side of R1, and on the same node, it connects to GND on the Arduino board (so one side of R1, ground of the power supply, and GND on the Arduino board all connect together on one node).

Below is my code:
const double pressureInput = A0; //select the analog input pin for the pressure transducer
const double pressureZero = 204.8; //analog reading of pressure transducer at 0psi
const double pressureMax = 1024; //analog reading of pressure transducer at 100psi
const int pressuretransducermaxPSI = 300; //psi value of transducer being used

pressureValue = ((pressureValue-pressureZero)*pressuretransducermaxPSI)/(pressureMax-pressureZero);

My pressure sensor range is 0-300 PSI.

Word descriptions of electronic circuits are often lacking. Schematics are the language of electronics.

Here's a sketch of my schematic.

Also, the way I calculated pressureZero is 1024(1)/5. PressureMax is 1024(5)/5.

All power sources produces volt..... How many?

sensor data ?

The R2 should be some 20 kOhm to 100kOhm to be protective.

thank you all for the responds. i don't have a datatsheet for my sensor, its range is 0-300 PSI and it produces 4-20mA output signal (it's a Prosense). i currently don't have any other resistor but i'll change it once i have some more.

when i measured the input voltage going into pin A0, it was very very small. then i measured the output amps from the sensor and it was also very small. the power supply did give out 24V to power up the sensor so i don't know why my signals were so small.

Does the sensor really work from 5 volt? It looks low to me.

picture of the sensor , name ?

Try a search for "Prosense pressure 4-20 arduino" . Maybe see if anyone else has used a similar sensor.

it's a Prosense sptd25-20-0300H

and to answer railroader, i have a 24V power supply (like the schematic i posted up there) connect to the sensor to power it.

Connect as circuit post #5 and measure the voltage on A0

It appears that Prosense makes sensors that give outputs either as current or voltage. It looks to me as though you have one that gives current, but you're trying to measure as if it were voltage.

i don't really understand that circuit, can you explain more please.. again, i'm fairly new
it is giving out current output, that is why i have it connects with the 250 ohm resistor to convert it to voltage.

How current is measured ?
We measure a voltage drop on known resistance.

250 Ohm x 4 mA =1000mV = 1V
250 x 40 = 10V

This is a voltages you should measure on A0, do not connect to arduino yet, just use voltmeter.

i got 0.079V when there's no pressure coming in. and it didn't change when i applied pressure either.. why did you do 40? isn't it supposed to be 20mA, so it'll give a 5V signal when it reaches max pressure?

you right 20 mA

That wiring in #5 looks totally wrong to me. The reference resistor, 250, or 270 Ohm ought to be connected to gnd, sensor + to its power supply + and the middle point to A0 via a 27 - 47 kOhm serial resistor.

Minimum voltage for sensor is 8.5V, apply 24V and do measurement.