Problem with strain measurement using NI cDAQ-9178 with NI 9237 Strain Module installed

[New_But_Learning]

I have a NI cDAQ-9178 chasis with NI 9237 (Strain Module) installed.  I am using the NI 9944 120 Ohm completion module.  I've set-up a data channel in SignalExpress for 120 ohm gage resistance, Gage Factor of 2, internal Vex, 2.5V Vex, Quarter Bridge I configuration, and wired my 120 ohm strain gage as shown for the quarter bridge I configuration.  I checked the voltage across my strain gage and it is 1.25 V as expected.  The problem is I'm not seeing response from the data channel.  When I calibrate the channel, and/or display the channel on a data view chart the measured strain value is a constant value of 52.58 millistrain and never changes.  If I shunt my strain gage with a cal resistor there is no change in the reading.  What am I overlooking?

[Brooks_C]

Hello New_But_Learning,

Given your strain gauge configuration parameters, it sounds like you're reading a full scale negative value.  Take a look at the Quarter Bridge I scaling equation in this developer zone article:

Strain Gauge Configuration Types

The scaling equation is:

Strain = [(-4* Vr) / (GF (1 + 2*Vr)] * (1+ (Rl/Rg))

I'm assuming you configured Rl as zero ohms, so the second term becomes 1.  If we solve this equation for the voltage ratio in V/V we get the following:

0.05258 Strain = -4 * Vr / (2* (1+ 2*Vr))

then,

Vr = (0.05258/-2)/ (1 - 2 * (0.05258/-2))

and finally,

Vr = -0.02498 V/V

Since the full scale input range of the NI 9237 is ±0.025 V/V then this indicates that you're reading a full scale negative value.  This would be the expected value for an open channel since the quarter bridge completion resistor would put AI+ down to EX- if a gauge was not connected as shown in the Quarter Bridge I wiring diagram.

I would start by checking your wiring to make sure that it corresponds to the diagram shown on the NI 9944 120 Ohm completion module.  If that looks okay then I would disconnect the strain gauge and measure the gauge's resistance with a hand held DMM.  You may find that the gauge was damaged during installation and it may be an open circuit now (or you may just find that it has a resistance considerably larger than 120 ohms that has put the bridge out of balance).

Cheers,

[New_But_Learning]

As simple as it is, I had an error in my wiring of the NI 9944.  Thanks for the feedback!