I am trying to connect a 1/4 bridge strain gauge to USB-6008 and I cannot figure out a way to make it work. Please help.

[CDawgttu]

I am using the NI Datalogger that came with the software and I have tried every combination of wire placement and Terminal input configuration.  I also have a 2 wire thermocouple that I have tried to hook up recently.  I see that the NI Datalogger has a thermocouple type list but I cannot get any reading with this either.  Please help me get these working, I am evaluating the USB-6008 and trying to determine if it is applicable for a research project.

[Process_Systems]

http://zone.ni.com/devzone/conceptd.nsf/webmain/2D5849B3108E416B86256DA2007A8E7B Try the KB at this link. It will give you an idea of strain gauge measurement basics. You can also do a search from the NI home page for strain gauge to get much more info. These KB's can explain what you must do much clearer than I ever could. Also, you can do a search for thermocouple measurements as well. You will find that the voltage output from a T/C is VERY small, not enough to be usable without signal conditioning/amplification with the 6008. Do yourself a favour and so some reading. I think it will answer most of your questions. Good luck, Dave

[CDawgttu]

Thank you for the reply, but I have already looked at this resource.  I have a USB 6008 and none of the literature explains how to hook a strain gauge to it for reading.  I have used a completion circuit in an attempt to solve my problem.  I can read the strain gauge using other equipment but I am trying to use this piece of equipment.  I am begining to wonder if the 6008 cannot read strain gages. 

Thanks for the help with the T/C, I will try it later today.

[Otis]

Hi CDawg,

While you could technically use the USB-6008 for measuring a Strain Gauge it is definitely not the recommended method. The reason is because the 6008 does not have proper gain adjustment for Strain Gauges and the resolution of your measurements will likely be too low to read any viable information.  If you do the math you can see that if the minimum range for the 6008 is ±1 V and the resolution is 12 bits, then the finest measurement you can see is:
CW = Range /(Amplification * 2^Resolution) = (1-(-1))V / (1 * 2^12) = 2/4096 = 488 µV

This may sound like a fairly high resolution, but the output of a strain gauge may only be a few mV, so this could represent a large code width for your measurement. 

Translation: Even if you try to read strain with the 6008 your measurements will not be very precise.

If you still want to use the 6008 though, then you need to make sure that you connect it as you see below:

You will probably want to do the Two-Wire method:

• R1 - Source to AI-
• R2 - AI- to AIGnd
• R3 - AI+ to AIGnd
• R_Strain - Source to AI+

As long as the leads are relatively short, R_L (Lead Resistance) should be negligible

[Otis]

I just saw that you also had a few questions regarding Thermocouples with the USB-6008.  There is actually already a great Knowledge Base on the topic:

KnowledgeBase: Thermocouple Measurements with the USB-6008 or USB-6009

[jonw9]

I will try to tag along on an existing thread.

I have a strain gage (4-wire) that I want to read.  i do not necessarily need the actual force, just the change in voltage.

We have a couple modules, like the 6008/6009 series, but i have read that they don't work well in this application, due to amplification.

We also have a thermocouple unit (not sure of the P/N), would this be better to use for this application?  Would it be suitable use, I don't want to damage something.

Unfortunately, the forces are only 25 or so pounds on a 250 pund gage, so the excitation will not be very large.

I can check the actual part number when i get into the lab tomorrow, but wanted some insight before I get started.

[Chris_K_]

Hi Jonw9,

Thank you for using our discussion forums.

It is difficult to tell if you can perform strain gauge measurements with the TC module that you have without knowing exactly which one it is.  Please let me know the PN and I should be able to be of more assistance.  Basically if your input range is able to handle the range that your strain gauge will output within, and the resolution on the device will be able to capture the information properly, then you will be able to use it for this sort of measurement.  Note that we do have strain gauge modules that will be better suited for this sort of measurement.  The 9219 and the 9237 are better for strain measurements.  They offer built in bridge completion, and excitation which make them ideal for strain measurements.  See the following link for more information. 

[MoyerJ]

would it be possible to use the 6008 to read a strain gauge if you used a signal amplifier for the strain gauge?