I am having trouble Omega PX4200 Pressure Transducers to where I can get a reading from Labview

[DevM]

Hi, 

I am an extremely new user to LabVIEW, and I am currently trying to wire in an Omega PX4200 4-20mA pressure transducer into an NI USB-6009 Data Acquisition Device. I have the power sources as a SOLA SDN 5-24-100P, and I am using a 3-wire system. I have the Black wire going to the SOLA on a +, the ground wire going into the GND on the NI USB-6009, and the white wire going to the AI0. Currently, I am unable to generate a meaningful signal either in the measurement and automation explorer or LABVIEW Signal Express 2010. 

If anyone has any idea on what I am doing wrong, please reply to this thread. 

Thanks,

Dev

[RavensFan · Accepted solution]

The USB 6009 measures voltage.  The pressure transducer puts out a current signal.  You need to add some kind of signal conditioning.  At the minimum, you need to use a resistor across the terminals so that you can convert the 4-20 mA signal the transducer puts out to a voltage.  To convert a 20mA signal to 10V, you'd need a 500 ohm resistor.

[DevM]

I am getting readings from the transducer, but the wiring guide generated by Labview Signal Express doesn't seem to be correct. Any suggestions?

[RavensFan]

You'll have to show a screen shot of the wiring guide so we can see how it is not correct.

[DevM]

Here is a photo of my wiring scheme. I'm not entirely sure where to place the resistor, so if you can tell me exactly where it should go, that would be very helpful. 

[RavensFan]

First question.  Do you have your wire colors in the picture backwards?  It looks like you are using a red wire for your negative side of the power supply and a black wire for the positive.  I am getting this by comparing your power supply picture to a Sola supply I have and comparing to your schematic.  While swapping colors won't hurt anything directly, it just confuses the issue more and could make it that much more likely you wind up shorting something out.

Second,  why do you have the 0V of the power supply going to analog input 0?  You don't have a ground return path from your sensor except through the AI0 input to the DAQ cards ground.  That is a high impedance path (since it is intended to measure voltage, not pass high current.)  It is even possible you may have just damaged your channel or even your entire USB DAQ module.  Hopefully, you're lucky and you didn't.

In your schematic, the wire coming from the 0V of the power supply should be tied to the ground channel of the DAQ card and the ground of the sensor.  Nothing should connect to AI0.  Put the resistor between AI4 and GND.

Make sure you use red wires for +24V and black for 0V so that you don't get confused and short something out.  Also, your red wire has a lot of bare wire exposed.  And you have a bare wire tied to ground right near it.  Clean up your wire terminations so that you don't risk a short between +24V and Gnd causing you to blow up a power supply.  Wire should be stripped only just enough to get into the terminal strip.  Any excess exposed bare wire is too risky that it will touch something else you don't want it to.

If you are not familiar with electrical wiring and how to connect sensors, I highly recommend that you find someone in your lab with experience to review your circuit before you power anything up.

[DevM]

Here is my updated wire scheme. I am getting a 10.5 volt input on AI0. Not really sure if this means anything. Attached are the images. 

Unfortunately, we currently don't have anyone with a great deal of wiring experience to help me at the moment. 

[RavensFan]

You still need to fix your wire colors.  Red is positive.  Black is negative.

And it looks to me from your picture that you have the resistor across the +24VDC and GND of the power supply which is not what I told you to do.  I told you to put the resistor between AI4 (now AI0) and ground.

Let's backup and see what you are actually getting signal wise.

Disconnect the DAQ card and the resistor.  Set your multimeter to mA current and measure the current across the output signal of the sensor and ground.  (Of course leave the +24VDC RED wire connected to the Power input of the sensor, and the 0V/ground BLACK wire from the 0Vside of the power supply to the low side of the sensor.  How many mA are you reading?

Now put the resistor in place of the meter.  Set your meter for Volts, and change the probes.  How many ohms is your resistor?  Measure the volts across the resistor.

Now connect the output of the sensor side of the resistor to AI0 of the DAQ card.  Connect the ground side of the resistor to GND on the DAQ.  Is your DAQ set up for single ended mode?  Now the DAQ card is reading the voltage across the resistor in place of the multimeter.  What is the voltage the DAQ card is reading?

[RavensFan]

Here is what your diagram should look like.  Be sure to compare it to the transducer's manual.

Also, in your picture.  What kind of wire do you have going to the sensor?  It looks to me like a 2-wire with a shield or drain wire.  This may work for you, but you really shouldn't be using the shield or drain wire for current carrying signals. You run the risk of electrical noise interfering with the signal.

Your should be using a true 3 conductor wire.

[DevM]

Thank you for all your help, we finally got it working today. 

I have a follow up question, do you know of any pressure transducers that hook up to the NI DAQ that operate better than the Omega transducers. 

Again, thanks a lot for your assistance.