DAQmx Pressure Reading Issues

[StuartWotherspoon]

Hello! I am having some problems with our set up and labview and was wondering if anyone can help.

We have a test water network with 4 pressure transducers bored and tapped in to the main. I am using a USB 6008.

I have wired these 4 up to an external power source and taken the wires required back to the DAQmx 6008 in order to get the analogue signal in to labview. Since these transducers output in mA I have wired a 250ohm resistor across the input wires (the AI+ and AI-). Now in labview I was looking to acquire signals and chose pressure in order to display this in bar, but it does not detect any inputs.

I have attached an image of our transformer - it is putting out 24DC. Red is +, white is -, black is GND. The wires (pink) in the middle of the tranformer box are the resistors I have wired in.

Does anyione have any advice on how I can get these pressure tranducers to work with labview? Especially in having the output on the graph as a bar reading.

[StuartWotherspoon]

I should say the transducers I am using are these ones: http://uk.rs-online.com/web/p/pressure-sensors/0461335/

[Dennis_Knutson]

Those images don't help. Do you have a schematic that can be attached.

What do you get in MAX with a differential voltage reading?

[crossrulz]

The USB-6008 can only measure up to 10V, 20V differential.  So your 24V will not be measurable.

[StuartWotherspoon]

I will look at getting a schematic up when I am back in the office.

Alternatively to the 6008 is there a better module to be using?

[StuartWotherspoon]

Sorry, I can't seem to edit my post so I am posting again.

I should add that the transducer output is 4 → 20 mA and so would this not convert to a voltage which sits in the range of the usb 6008?

[Dennis_Knutson]

So what is the 24 VDC you are talking about?

[StuartWotherspoon]

Sorry Dennis, I should clarify - it's been a long 3 days learning curve trying to get this to work and I don't like to be beaten.

The transducers have and excitation voltage of 12-36DC which is supplied by an external power source which is providing 24DC.

This 24DC is wired to the transducers which provide an analogue ouput of 4-20mA - I have these wires and a GND returning to the 6008 as per the photographs. At one point in this line I have placed a shunt resistor (250ohm) - this is placed within the box I used to house the power supply and wiring - across the AI+ and AI-.

Now, when I'm in DAQ assistant I can chose current and it gives me a choice of 7 channels. I was hoping to be able to acquire pressure signal. Should I be seeing 7 channels here? I would expect 4 as I have 1 input being _ and - AI and not 7 seperate inputs.

So in labview I'll chose acquire current signal and chose AI0 for talkings sake. Set it up as liner, 4-20ma and it just doesn't want to work of display sensible results. I've played about with it till my eyes have went square but it may be beyond my means.

I'm:

1. Not sure if my wiring is correct (though I believe it to be)

2. Struggling to get any sensible output in labview

[crossrulz]

If you are measuring the current via the shunt resistor, then you need your DAQ to be set up as VOLTAGE.  I = V/R.  So your current is the measured voltage across the shunt resistor divided by the resistance of the shunt resistor.

[StuartWotherspoon]

Even setting up in voltage it seems to discover 7 analogue channels in labiew, where I would suspect there should still be only 4, as I only have 4 instruments.

Also I then have the conundrum of translating that reading in to a pressure and displaying that visually on screen.

I get the feeling that this one may just beat me.

The purpose of the test rig is to research the impact that various changes in the test network has on pressure spikes.