Conditioning 4-20 Ma signal to output pressure

[poeschel1]

Newbie first post.

I have a pressure transducer that will produce a 4-20 Ma signal.

I can aquire this signal in Labview and display.

How do I condition so the display reads 0-2000 Psi instead of 4-20 Ma?

[Kenny_K]

If you are reading a current meaurement and want to convert that to a pressure, you need to follow some simple math.

Your range on the transducer is 4-20 mA, or a total of 16 mA.  And the range in pressure is 0-2000, so a total of 2000 PSI.  16 mA / 2000 PSI gives you 8 uA/psi.  So for every 8 uA you have another PSI.

Another option is that if you cannot measure current and can only measure voltage, then you can run the current through a resistor and measure the voltage across the resistor. I usually use a 680 ohm resistor, but that depends on the transducer and your input resitance for the measuring device.  You then go through the same process to determine how to convert the voltage to psi.

[AnalogKid2DigitalMan]

Kenny makes a good point on using a resistor to convert current to voltage in case the DAQ card cannot measure current directly. But you also have to consider the maximum voltage input range of your DAQ card. Using 680 ohms with 20mA will yield a voltage of 13.6 volts, most DAQ cards have a max measurement of 10V. A 500 ohm total resitance will convert 20mA to 10V. You should also use precision 1% tolerance resistors for such an application.

[poeschel1]

I actually have a cFP-AI110 module that can read current. I'm playing with NIMax scaling functions.

Hope I'm on the right path.

Thanks for the quick replies.

[Matthew_Williams]

I think the attached will do what you want.  My recollection is that the AI-110 reads in
amps; you may have to adjust the constants if not.

Matt

[mikeporter]

If I recall correctly, MAX lets you define virtual channels with predefined scaling. You simply select 4-20ma current loop and the values that relate to the 4- and 20-ma limits and you're done. The only other thing is if you aren't using a current input device you also have to specify the resistor value. Within your LV program you read from the virtual channel and it is read in the engineering units you defined.

Mike...

[Kodeeswaran]

hai

    i prefer that go and configure the channel scaling in MAX which is easy and in LabVIEW you can display it appropriately