Strain measurements using old Vishay 350a - results seem low
Strain measurements using old Vishay 350a - results seem low
(OP)
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!
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!





RE: Strain measurements using old Vishay 350a - results seem low
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
RE: Strain measurements using old Vishay 350a - results seem low
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.
RE: Strain measurements using old Vishay 350a - results seem low
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
RE: Strain measurements using old Vishay 350a - results seem low
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.
RE: Strain measurements using old Vishay 350a - results seem low