looking to use foil strain gauges with a AM16/32 multiplexer and CR3000. useing single gauges so quarter bridge and assume I need to use 120 ohm completion resistors at the logger.
not sure how to wire these up or program.
Totally stuck.
any one have any ideas?
Go to URL:
http://www.campbellsci.com/4wfb120
for information on our Terminal Input Module (TIM) that supplies the completion resistors for the wheatstone bridge.
Click on the manual link and go to page 3. Figure 4.1 show how to wire it up.
You will need one of these for each strain gage (place on face of the multiplexer, not on the the logger).
Placing relays internal (on face of logger with multiplexer relays internal of wheatstone bridge) a Wheatstone bridge strain system is discouraged. Any change in resistance of the relay contact would have a corresponding change in the bridge output voltage.
Changes in contact resistance can be induced by temperature fluctuations, oxidation, environmental conditions, and normal wear of contact surfaces. The specification for the relays that we use state that initial contact resistance will be less than 50 mOhms (AM416) or less than 100 mOhms (AM16/32). There is not a specification for change in contact resistance for the relays because there are so many variables that affect contact resistance. I do have test reports for various test conditions that show contact resistance changing over time by 10 to 20 mOhms. These tests were performed using static test temperatures.
When strain gauges are used in the Wheatstone bridge, small changes in contact resistance result in large apparent strains. If two relays vary by 10 mOhms, the resultant apparent strain would be about 80 microstrain when using 120 ohm strain gauges in a one active element system.