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  • Measuring force

    I think I know the answer to this but am looking for some confirmation that I am not missing something very obvious. I am looking to make a sort of quick and easy device to measure an applied force. Pretty much a load cell for a press sort of application. I know the usual setup is a suitable oil filled hydraulic cylinder with a gauge attached: squish, read pressure, calculate force using piston area. Simple and straightforward.

    Lets say I use the same arrangement except instead of an oil filled system I have a sealed system with just air in it. As the air is compressible the cylinder piston will have lots more spring to it than an oil filled system, but in my case that may not matter. I am looking to measure relatively low forces (5-20 Kg at most) but would like the measurement to be pretty accurate and maybe more importantly, repeatable.

    Question is this: Assuming a 1 square inch piston area, if I measure 10 psi, I have 10 pounds on the piston, right? Does the fact that the working fluid is compressible change the overall way this works or skew the results in some way? I am actually embarrassed to ask this as it seems to me really simple physics going on here, but have the nagging sense that I am overlooking something simple and obvious.

  • #2
    The physics is the same. At very low pressures seal drag may be significant. A strain gage load cell might be more accurate, though more expensive and complicated, too.

    Apparently they make USB load cells now, but they appear to be spendy.

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    • #3
      Originally posted by MTNGUN View Post
      The physics is the same. At very low pressures seal drag may be significant. A strain gage load cell might be more accurate, though more expensive and complicated, too.

      Apparently they make USB load cells now, but they appear to be spendy.
      Thanks. I may go the load cell route if the overall thing works. There are some inexpensive load cells (under a few dollars from the direct from china places and ebay) that could be used. That's a separate project and as electronics is what I sort of do for a living, I can deal with that if I end up going that route. I just want to do some easy testing with stuff I likely have around before getting into a more complex project, and started second guessing my logic for some reason.

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      • #4
        An FSR seems like it would be cheapest and easiest: https://www.adafruit.com/product/166...Fc5bfgodITMEBw

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        • #5
          Would a simple scale calibrated in pounds, oz., kg. etc not be an option?
          I guess it depends on what the force is and how to interface it with a "readout".
          Home, down in the valley behind the Red Angus
          Bad Decisions Make Good Stories​

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          • #6
            Originally posted by elf View Post
            An FSR seems like it would be cheapest and easiest: https://www.adafruit.com/product/166...Fc5bfgodITMEBw
            I'm not sure that this matches the OP's needs: From the link.....
            "FSR's are basically a resistor that changes its resistive value (in ohms Ω) depending on how much its pressed. These sensors are fairly low cost, and easy to use but they're rarely accurate. They also vary some from sensor to sensor perhaps 10%. So basically when you use FSR's you should only expect to get ranges of response. While FSRs can detect weight, they're a bad choice for detecting exactly how many pounds of weight are on them."

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            • #7
              A heavy die spring and a dial indicator. F= kx.

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              • #8
                Originally posted by Willy View Post
                Would a simple scale calibrated in pounds, oz., kg. etc not be an option?
                I guess it depends on what the force is and how to interface it with a "readout".
                Yeah, anything easily translated into Kg would be fine. This will be used to apply a known force for a short amount of time, think hardness tester sort of application. 5 to maybe 15Kg covers the range needed, if it reads out in pounds or whatever, the conversion is simple and easy. Per Erich, I thought about a die spring with a scale sort of arrangement, and that could work I think. I looked at the FSR's but I suspect they are not quite precise enough.

                Thanks for the ideas.

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                • #9
                  I would go the strain gauge route, the piston stuff is just messing things up with friction and the pressure sensor is a strain gauge anyway....
                  A strain gauge from a weight scale is the cheapest option:
                  https://www.sparkfun.com/products/10245
                  If you're an electrical guy you can figure out the rest.

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                  • #10
                    Simple solution maybe to setup a bathroom scale (or the components from it) on the base of your press; put the item being pressed on top of the scale.

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                    • #11
                      Yes, gas or liquid, the formulae are the same.

                      Pressure gauges and spring scales probably have similar accuracy. You can probably purchase better and worse versions of each, but 1% will probably be expensive and harder to find. 5% or 10% should not be difficult or overly expensive.

                      I am not familiar with strain gauges or pressure cells, but you will probably have to calibrate them with known weights.

                      A tap or three on the cylinder will do wonders to eliminate the effects of friction between the cylinder and piston.



                      Originally posted by MTNGUN View Post
                      The physics is the same. At very low pressures seal drag may be significant. A strain gage load cell might be more accurate, though more expensive and complicated, too.

                      Apparently they make USB load cells now, but they appear to be spendy.
                      Paul A.

                      Make it fit.
                      You can't win and there is a penalty for trying!

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                      • #12
                        Originally posted by alanganes View Post
                        Lets say I use the same arrangement except instead of an oil filled system I have a sealed system with just air in it.
                        Air is a compressible fluid, and so there will be Temperature effects as the Pressure/Volume changes occur. When you compress a cylinder of air it heats up. When it expands it cools down. As the Temperature of the air in the cylinder equilibrates with the environment the Pressure will change. Your readings will depend on how fast you read the Pressure gauge after the compression.

                        The size of this effect will depend on a lot of factors, including rates of measurements, rate of heat transfer from/to the cylinder, relative change in volume / pressure in the cylinders, etc. Seems fraught with possibilities for error.

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                        • #13
                          Originally posted by ikdor View Post
                          I would go the strain gauge route, the piston stuff is just messing things up with friction and the pressure sensor is a strain gauge anyway....
                          A strain gauge from a weight scale is the cheapest option:
                          https://www.sparkfun.com/products/10245
                          If you're an electrical guy you can figure out the rest.
                          That may be the end game. Actually these load cells are ridiculously cheap, probably because they get made by the boatload for things like electronic bathroom scales:

                          EBAY link

                          These look to be pretty much identical to the sparkfun ones, or very similar. One thought I did have that our favorite cheap tool store sells an electronic bathroom scale for ~15 bucks. From what I gather it has four of these and of course all of the electronics to condition and display the weight. Might be easiest and cheap to just buy one to scavenge the guts...

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                          • #14
                            Originally posted by Norman Bain View Post
                            Simple solution maybe to setup a bathroom scale (or the components from it) on the base of your press; put the item being pressed on top of the scale.
                            This may be the "proof of concept" approach to see if this all works as envisioned.

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                            • #15
                              Originally posted by Dan_the_Chemist View Post
                              Air is a compressible fluid, and so there will be Temperature effects ...

                              The size of this effect will depend on a lot of factors, including rates of measurements, rate of heat transfer from/to the cylinder, relative change in volume / pressure in the cylinders, etc. ...
                              It could affect the force by affecting the pressure, whatever. But it doesn't matter as far as determining the force by reading pressure. Force & pressure are simply & always related by the piston area. Period.

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