strain Gauge recommendation

Discussion in 'Physics' started by hunterage2000, Dec 3, 2011.

  1. hunterage2000

    Thread Starter Active Member

    May 2, 2010
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    Hi can someone please recommend a strain gauge to measure mass flow rate of a pneumatic conveying system. The full bridge circuit strain gauges are to be placed on a cylindrical cantilever beam with a length of roughly 160-170mm and a diameter of 36mm and made of iron.

    From what I have read copper-nickel alloy has the best combination of desirable characteristics such as:

    linear strain sensitivity in the elastic range (around 2)
    high resistivity for a smaller size
    low hysteresis for accuracy and repeatability
    low temperature co-efficient for good temp compensation (not sure if this applies as average temp is 20.5c)

    Polyimides is the most used backing material for general purpose use.

    Can someone recommend one for me? Im not sure if this information is enough so ask if needed.
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Unless you are wanting to make this from scratch the folks at Omega are the ones to ask. They sell just about anything you would need, and would be happy to make your technical specs to their products.

    I've made strain gauges from scratch. Did not design them, but when our custom designs were trashed due to accidents I had to build new ones. Painstaking process, but doable with care.
     
  3. hunterage2000

    Thread Starter Active Member

    May 2, 2010
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    Thanks I will have a look at the site. As someone who made strain gauges from scratch do you know if there is a universal selection criteria for choosing a strain gauge?
     
  4. Wendy

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    Mar 24, 2008
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    You can buy a wide selection of "stickers", little mylar tabs with precision resistors etched on them. You glue the sticker on the strain gauge with special epoxy (supplied from the same vendor). Every thing has to be extra clean. I used our IPA (isopropyl alcohol) degreasers, which used both U/S and boiling alcohol, to clean the mechanical pieces.

    Here is an example of what I made...

    [​IMG]

    from this post...

    http://forum.allaboutcircuits.com/showthread.php?t=53764&highlight=strain+gauge

    http://forum.allaboutcircuits.com/showthread.php?t=33970&highlight=strain+gauge
     
  5. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Don't know how much you're willing to spend, but we get all our strain guage/load cell/conditioning/meter equipment from Interface Advanced Force Measurement. They have application engineers that will come up with an optimal system for any application. not cheap.
     
  6. Wendy

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    Omega will recommend their product line matches for free. Strain gauges are not really complicated. They are not easy to use sometimes, due to bouncing voltages, but easy to make and install.

    If you understand Whetstone Bridges, you understand most of what makes a strain gauge.
     
  7. strantor

    AAC Fanatic!

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    food for thought: I always make a high dollar recommendation whenever something sounds like an industrial application. Reason being, most people on this site will be more than helpful in working out a DIY solution for you, but often times in industry "DIY", or "DIY-appearing" solutions are frowned upon. Also, if ever the equipment malfunctions in the future, eyes will be turned on you, whether your DIY device caused the malfunction or not. Its sometimes better to pay the big bucks and have a (licensed, certified, official) 3rd party come in and do it; takes the heat off of you if there's ever a problem in the future. That being said, I don't always follow the advice I just laid out, and its your deal.
     
  8. Wendy

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    Thing is, mine was an industrial application, Omega is an industrial provider (their stuff is not cheap), and I am a professional.

    You do not need outside consultants to do the job right. The right hardware and knowledge of how to do it will suffice.

    If in doubt, ask for help from an engineer, that will spread the blame.
     
  9. strantor

    AAC Fanatic!

    Oct 3, 2010
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    yeah that's where knowing your limits comes in. That's why I said I sometimes don't follow that advice. Its a judgement call. Most things I know I am competent to do, so I do them, and so far without incident. Other things, I weigh the outcomes and take a different route, especially if it something critical which could cause severe losses or injury if failed. We never know the competency of someone who comes here, so I always try to plant that seed. People should recognise when/if they are in over their head, and if they are, having a consultant or contractor come in is a safe way out. hunterag2000 I hope you do not take this mean I think you are incompetent
     
  10. strantor

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    As an example of "I don't always follow that advice", regarding "DIY" things in industrial applications; my first post here on AAC I was looking for help designing a fault tester to be integrated into a cable production machine and monitor a cable product during production and shut the machine down. It functions marvelously, but I still worry about it. I wonder what is going to happen one day a year or ten years down the road if it is found to be faulty. It gives me a little stress when I think about it, but maybe I should just be a little more confident. I was super stressed out the first week it was in service. That project, and the stress it caused me is the reason why I make the recommendations that I do.
     
  11. Wendy

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    Like I said, look for an engineer to review it, then get his signature.
     
  12. strantor

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    Thats a really good idea that I hadn't considered.

    Would the engineer need to do extensive R & D testing on it before signing off? If you were an engineer who already had his own projects going on, and then some random dude from the maintenance department came dropped a kludge-looking device on your desk, would you even look at it? or would you be intrigued and excited to help? The way I see it, I would be asking for a favor.
     
  13. Wendy

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    That would, of course, be up to the engineer. Surely you have production engineers? My plant does, their job is just for things like that. You could even go to management, and let them make the call.

    Heinlein once said of producers and scripts, "They like to pee in it, they like the taste better". Or words very close to that effect.
     
  14. strantor

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    well, I had to read the job description of a production engineer to answer this:
    In that sense, not really. The lead engineer is a thermoplastics engineer, then there are some engineers under him who are electrical & thermoplastics engineers & one mechanical engineer. The big guy has a management degree but he's not an engineer. All the engineers are basically devoted only to improving & designing new cables. Improving the process (which I take would be the manufacturing engineer's job if we had one) has to be a joint effort between the engineers, the operators, and us maintenance guys. When something goes wrong, the big guy comes down and rather than analyzing the problem and ways to prevent it, he delegates this to a few of the engineers and then goes back to wherever he was. This is just in the cables-making plant, but we have other facilities next door & down the street (same company) producing electronics and I could probably find a good candidate there.
    I did go to management with my project; I demonstrated it to a group of engineers and management and they liked it and gave me the go-ahead.
     
  15. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    Yes, Omega are the ones to visit for excellent basic information. One of the first things that you'll learn, it that the packaged strain removes most of those questions you have, and turns your attention to the more important details. The first one being the substrate upon which the gauge is mounted. I have slapped strain gauges on just about anything I can reach, from bicycle pedals to top drive quills spinning over a mile of drill stem. By and far the most usefull arrangements have come about by 'guiding' the measurable stress through a calibrated link, upon which the strain gauge(s) are mounted. Bill's image illustrated this in what you'd find in a typical load cell. The link must bear the expected range of stress without pemenant deformation, yet provide measureable yield over the same range. Mechanical arrangements that are unable to 'guide' the stresses are way worse in the measurement world, then any nonlinear gauging. In torsional measurements, 'insitu' type links are often the norm and will require multi dimensional gauging.

    I view gauging as 'exploring my surroundings'. I don't need an Engineer for that.
     
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