Creating a scale based on 100k shunt value

[jlduchar]

Hello all,

I'm still getting used to Labview data acquisition and need help creating a scale for a bridge based torque transducer when given a 100k shunt value.

At my company, we have a dedicated calibration lab that does the calibration for all of our transducers, sensors, etc. Included with the calibration sticker is the 100k shunt value. I would like to leave the calibration to them rather than try and calibrate myself using NI Max.

For example, here are the specs I am given:

100k shunt value = 131.96 ft. lbs

Full scale output = 2mV/V @ 300 ft. lbs

350 ohm bridge

I want to convert mV readings to ft. lbs. I understand I could use DAQmx AI Torque Bridge function to create a linear scale using two points, polynomial, or a table. It would look something like

First Electrical Value - 0 mV/V

Second Electrical Value - 2 mV/V * Vex

First Physical Value - 0 ft. lbs

Second Physical Value - ~300 ft.lbs

but we all know that nothing in this world is perfect and that there will be some error associated with this, that is why I want to rely on the 100k shunt value. I want to trust in our calibration group's techniques. Please help me convert the above 100k shunt value to a usable scale. 

For reference, I am using cDAQ-9174 and NI 9237 to read AI torque/load. 

Thanks,

jlduchar

[RTSLVU]

Now I have never worked with a torque bridge or cDAQ but I can't imagine the shunt scaling would be any different than scaling a current shunt.

Here is VI I wrote for our ATE stations granted it's probably a little more complicated than you need but it should give you the general idea. 

This VI gets it's values (Rshunt, Amps, mV) from a config file and the scaling factor is then entered into our data loggers (Agilent 34970A) and configures it to record Amps directly instead of millivolts.

Oh yeah, Rshunt is the measured resistance value of the shunt our calibration house gives us.

[jlduchar]

Appreciate the response. However, I do not know the shunt output in mV, only the shunt output in physical units, which is ft. lbs. Using your example, I don't know the denominator of my "ratio". Should I try to get calibration papers and see if they give mV outputs at the zero and shunt. Then use these two points to create a scale? I was hoping to only use the 100k value in ft. lbs to create a scale but it seems that I need the mV output to do this.

[RTSLVU]

Have you tried measuring it?

Take a few points of known foot pounds and measure the voltage across the 100k resistor.