9237 actual excitation voltage

[AllieL]

Hi,

 I am using NI 9237 for load cell measurements (full-bridge) and have the excitation value set to 10V. However, I am reading only 8 volts at the load cell's excitation wires. Since 10V is being used for scaling, my measurements are off. Is there a way to offset this? (Also, when Vex is set to 8V, I read 5V at the pin). I am using RJ50 cable and jack.

Thanks in advance for any assistance.

[DBerry]

Are you measuring the voltage at pins 6,7 (Ex+, Ex-) of the 9237 module without the load cell attached?  I would start troubleshooting without the load cell attached and check the output directly at the module (or at least at the pins of the RJ50 cable).

Dan

[AllieL]

Thanks, Dan. I read 10V at the pins with no load cell attached. I don't have RS connections, so I am wondering what the DAQ uses for scaling the voltage - 10V or 8V?

[DBerry]

It will use the 10V, or whatever you specify using the Daqmx/Daq Assistant funciton.

[AllieL]

That was my concern. Is there a way to configure so the measurements indicate what the load cell is actually seeing? Thanks again for your help

[DBerry]

I'm not sure what you mean.  If the NI9237 is providing 10V of excitation at the pins then that is what the load cell is seeing.  Under what circumstances did you measure 8V and at which pins?

Dan

[MarkGrot]

If you are specifying a 10V excitation voltage and seeing 8V, you are likely exceeding the 150mW the NI 9237 can provide.  See the "Excitation Voltages" section in the NI 9237 Operating Instructions.  The module will lower the voltage to keep it the excitation power under 150mW.  If the lowered excitation voltage won't work for you, you can either specify a lower excitation or supply your own using external excitation.

[DBerry]

Good point from Mark regarding power consumption.

Do you know what the bridge resistance of your load cell is?

[RavensFan]

Any reason you are using 10Vexc?  Why not try 5V or 2.5 V for your excitation voltage?

[AllieL]

I think Mark is right. When I drop Vex down to 5V, it reads 5V at the pin. The manufacturer's recommended excitation is 10V (input resistance ~395Ohms, output resistance ~350Ohms), which is why I started with that. Am I okay to assume that the resolution of the 9237 will make up for the lower excitation voltage?  I need precise measurements and I'm only working with 1.281mv/V FSO. 

Thanks to all for your help