Measuring differential current on NI USB-6255 with DAQExpress

[milancurcic]

Hi NI Community,

I'm trying to log a current signal from a pressure transducer. I'm using DAQ Express and the Analog Input

My problem is that the expected current range from the sensor is 4-20 mA. However the current that I read on AI is pegged at ~42 mA. See the attached screenshot.

However, I'm new to National Instruments devices and software and wonder if there's an important step I'm missing, whether regarding the wiring or DAQExpress.

Things that I triple-checked and had reviewed and verified by a colleague:

* I'm providing sufficient input Voltage (12-30 VDC)

* I wired correctly (wiring diagram on page 3 of https://assets.omega.com/manuals/M5720.pdf)

* The output is outside of the specified output range for this sensor.

Any ideas on troubleshooting this would be very helpful.

Thank you! 

[ZYOng · Accepted solution]

USB-6255 is a multifunction I/O module and does not support measuring current directly. You need to use a shunt resistor and measure the voltage across the resistor. Measuring Current with the NI Analog Voltage Input Channels

In the DAQ Express, you specify a shunt resistance value of 249 ohms. Hence given the input voltage of USB-6255 is +/- 10 V, maximum current is

I = V/R

  = 10/249

  = 0.04016 A

Hence the max and min current in DAQ Express is +/- 40.16 mA.

USB-6255 has a maximum working voltage of 11 V for analog input, hence in actual operation, it can return a current value of up to 44.18 mA.

[milancurcic]

Thank you, very helpful. We were able to measure the expected signal output on another device that has a built-in shunt resistor. I'm a novice in electronics so it wasn't apparent to me that I needed a resistor. I'll get some and follow the resource doc you linked to measure current via voltage input as you suggested.