Strain measurements using old Vishay 350a - results seem low

[necktwist]

I am using an old Vishay 350a unit to measure residual strains in zinc castings that are overmolded on steel bar. I glue on the gage, then cut off the casting to see how much it shrinks after the tension is relieved. For some reason, the results are coming up low.

Is there a reason why this model Vishay 350a, or old strain gages tend to read lower when they get old? maybe i have not glued on my gage properly, everything from the early 80's.. gage factor was 2.03 per the packaging.

I am reading 365 micro strain, i expected about double that. this machine could be spot on but i wont know until i get it calibrated (soon).

Any info on these old buggers would be helpful. thanks!

 

[IceStationZebra]

I have used the 350A and love it for it's simplicity. But unfortunately there are some components that go bad over time and are no longer available so they can't be fixed. The quick way to check it is to use a shunt cal and see if it reads the correct amount of apparent strain. Vishay has a good primer on shunt calibration and how to calculate ohms vs. ue.

As far as gages go I don't think they go bad unless the backing material is deteriorating. If they were stored out of sunlight and at reasonable temps they should be OK. I ran into this same question this spring and it turned out my glue was bad.

ISZ

 

[necktwist]

Thanks IceStationZebra! i did the shut cal at 5000microstrain, and it read 4980 so i think its still spot on.. the glue is from grainger its no shrink epoxy.

do you know of a good fast epoxy to use? wish i had somethign that was super glue quick for instant cure to see srains right after casting.

i also noticed the strain values went from 360 upon first cutting off of the casting to over 500 2hrs later.. seems the casting is slowly releasing its residual stresses once the steel core is removed.

I'm also going to measure up some brand new castings to see if they have a lot higher strains. i'll report back. thanks for your help.

 

[IceStationZebra]

I haven't had to deal with castings yet, I normally use super glue on polished steel. But I would be hesitant to use any adhesive not from a strain gage supplier unless you have reliable information that it will work. Your time and material expenses are not worth saving a couple bucks on epoxy.

Google for strain gage epoxy and find out what others are supplying and using. You can also get some information by looking at the MSDS sheets if they are available. I would be worried about excess heat during cure or a chemical reaction with the backing material. And even though they say it is no-shrink, does it expand slightly?

ISZ

 

[necktwist]

Update - the strain gage was fine all along, the adhesive also was good. The issue was that the strain i was measuring was much lower than expected, shows you that these 'thats strange' results are good to find.

the Vishay is performing well. Good question on the epoxy no shrink i just read that on the specification sheet you are right it could be increasing size slightly. I left the unit turned on for 24hrs by accident holding a measurement. it was fine.

i read more strains as low as 50 to 80, and found meidum and high results up to 1600.

I dont know what is considered the real yield point in castings but i must be getting close at 1600.

 

[IceStationZebra]

Glad it worked out. ISZ