Reading a Strain Gauge 1

[metalman8357]

Hi all,

I'm a materials engineer working on an R&D project and we do not currently have an electrical engineer on staff. All I'm looking to do is order a few standard strain gauges, and I need a way to measure the strain on each gauge in real time. I'm just looking for the bare equipment necessary to plug the strain gauge into so that I can read the strain readings on a digital meter. So far I've come up with the following two products (from googling), and I'm wondering if these will suffice. Keep in mind I have no knowledge of how to use these, so the more user friendly the better. Any suggestions would be greatly appreciated, if you know of any other great products.

1.)

http://www.omega.com/Pressure/pdf/dp25b.pdf

(the DP25B-E)
2.)

http://www.omega.com/Pressure/pdf/BCM-1.pdf

Thanks,
M

 

[btrueblood]

Those two units should work. There are also some white papers and such on the website for that company, that explain how strain gages work and methods for attaching them. I'd also look to download the instruction manual or data sheet for the two devices you linked to, and read them.

 

[MikeHalloran]

No. You need the DP25B-S, which provides 'excitation' (small DC voltage) for the strain gages and reads the (tiny) resistance changes with strain.

I'm not convinced you need the BCM-1 at all.
( Sparkys are enamored of Wheatstone bridges, which in the strain gage world are used, with four gages per readout, to magnify the gage signal. E.g. on a symmetrical beam, two gages with positive strain and two gages with negative strain are wired in a bridge to rougly quadruple the gage output. The bridge also allows insertion of a pot to zero the bridge. ... which may be necessary if the beam and the gages are not precisely symmetrical. The bridge also has the unfortunate side effect of canceling out some strains that you might like to see. )
The BCM-1 sort of artificially makes one gage appear to be a bridge of four, or two gages appear to be a bridge of four, and provides a zero adjust.
I'd rather use one readout and one gage per gage point.

You should also be aware that installing strain gages so that they work well and reliably requires insane attention to detail, e.g. cleaning the surface several times, abrading it in a precise manner, applying just the right amount of cyanoacrylate, clamping the gage with just the right amount of force while the glue sets, using just the right clamp pad so you don't cut the gage, tying down the lead wires, and applying just the right special tar/epoxy over the entire gage point to protect it from environmental influences like people's breath, and so forth. The gage manufacturers will be happy to provide a procedure and recommendations, which you should study.

... but if at all possible, I suggest you try to set up your measurements so you can use a pre-gaged, pre-sealed load cell and let someone else worry about the super-nit-picky stuff.






Mike Halloran
Pembroke Pines, FL, USA

 

[metalman8357]

We are looking into possibly using strain gauges to monitor strain in the field for 20+ years in hard to reach places. My initial testing was just to purchase some strain gauges and meters and determine the best location to place the gauge so that it matches up with readings given by our extensometer during tensile testing. Mike Halloran, you're saying that I can get away with just purchasing the DP25B-S for reading the strain in a single gauge?

 

[metalman8357]

Also, I'm not too worried about assembly. If attention to detail is needed, we can accomplish this.

 

[MikeHalloran]

If I'm reading the catalog page right, you need only one DP25B-S to read one strain gage. If Omega disagrees, ask them to explain why you need more.

Also ask Omega for suggestions about monitoring for 20 years; that's a whole different ball game.

I worked in a shop that made load cell beams as part of other products. They used women exclusively to install strain gages, and it took the women a couple of years to get up to speed on the techniques, and the rigor required to make the installed gages last through the warranty period.

I salute your enthusiasm, but right away, you need to use whatever your process will be to install a statistically significant number of strain gages on test pieces, and see if you can make the installed gages behave consistently for even a month. If you can do that, maybe you have a chance. More likely, you'll be shopping for preinstalled weatherproofed gage modules that you can just bolt on.



Mike Halloran
Pembroke Pines, FL, USA

 

[BrianE22]

Monitoring for 20+ years? That sounds tough. I assume you want to measure down to DC? I'd be worried about things changing through the years (the adhesive in particular).