NI 6008 DAQ Pressure Transducer reading issues

[prenerk]

Hello Everyone, I am a new user of Labeled and not an expert on electronics, so please bear with me.

I am attempting to read a 4-20 mA signal from a pressure transducer using a NI 6008 DAQ and Labview version 8.6. I'm running  Windows XP professional. I am supplying the transducer, (http://www.omega.com/Pressure/pdf/PX01-I.pdf) with an excitation voltage of 10 V. I have a 235 ohm external resistor across the positive and negative terminals of the DAQ (although I'm not certain that this is correct). Upon starting my VI, I am able to measure a voltage at the DAQ of about .9 V (verified by a voltmeter/ammeter) as well as a current of about .39 mA. 

My issue is the fact that when I supply a pressure to my transducer, I receive no signal whatsoever from the transducer, despite the fact that there is significant pressure that should be giving me a signal. Is this a problem with my setup, the connections, the DAQ (which has been reset in MAX a number of times, and passes the self-check) or the transducer itself? 

Thanks for your time and consideration

[SnowMule]

The inputs on the 600x devices are fairly low impedance.  My guess is that low input impedance is loading the sensor's output.

Build up a voltage follower circuit, wire your sensor's input to that through the resistor and feed the follower's output into the DAQ.

[johnsold]

144000 ohms is not low compared to a 235 ohm resistor.  While not insignificant, it probably produces an error of less than 0.2%. Omega specifies the output of the transducer as 4 to 20 mA +/-0.06 mA which corresponds to 0.3% at full scale.

Most likely the problem is related to the way things are connected.

0.9 V across 235 ohms is 3.8 mA wich seems reasonable.  How did you come up with the 0.39 mA number?

Lynn

[Brian-G · Accepted solution]

Hi prenerk,

You are correct that a resistor needs to be connected across the positive and negative terminals of an AI channel to create a voltage reading from a current.  Here are two troubleshooting steps you can try:

1.  Check to see with an Ammeter if you are detecting a change in current when you apply pressure to the transducer.

2.  Open Measurement & Automation Explorer and use a Test Panel to measure voltage from the device.  This will ensure there is not an error in your LabVIEW code.  Also check the pinout diagram from MAX to see if your leads are connected to the AI channel properly.

If you have not discovered a problem from the above steps we will need to know more about your pressure transducer and the way you are connecting it to the 6008.  We will need the information about the leads of the Omega device as well as how they are connected to the voltage excitation source and 6008.  Let us know how it goes!

Brian

[johnsold]

I wonder if the problem is that the excitation voltage is too low?  The Omega datasheet specifies a minimum excitation of 10 V.  If the 10 V is near the minimum required drop across the transmitter, the 10 V source mentioned in the original post could be too low.  At 20 mA there would be just over 5 V across the transducer.  I have seen ssytems where not enough compliance voltage was available.

Try it with a 15 to 30 V excitation source.

Lynn

[prenerk]

Hello Everybody,

I followed the steps that you outlined in tracing the voltage through the system I had set up.I followed the DAQ wiring diagram and correctly wired the system. It was a bit tricky because the sensor is a 3 wire lead, but the system I am using has a 4 wire attachment. At any rate, I used the ammeter and finally detected the signal when there is pressure on the line.

I have a few questions though:

1. Will the fact that I am not using the default 249 ohm shunt resistor affect the output as detected in labview?

2 Is the conversion of the signal from 4-20 mA as simple as:

(PSI)= (signal[mA]-4mA)*(2000 PSI/16 mA)

Thanks again for everybody's help, you have been of great assistance!

[johnsold]

I am glad you are making progress.

1. The resistor value does not matter as long as you do not violate the compliance voltage limits.  The voltage at the DAQ input will be lower of course.

2. That conversion seems reasonable for 2000 psi full scale transducer.  Since you are measuring voltage and not current, you also have a 150 ohm*current factor in there as well.

Lynn