Load cell for creating a weight scale

Discussion in 'The Projects Forum' started by leonhart88, Nov 2, 2010.

  1. leonhart88

    Thread Starter Senior Member

    Feb 23, 2007
    118
    1
    I am trying to find a way to weigh the contents in a small vial (the ones used when people get flu shots or vaccinations), for a automatic vial filling machine I am building.

    I have been looking at load cells...but most of them are for large weights. I need something to measure increments of 1mL (1 gram for water). The load cell also has to be really small in order to fit into a 1 inch opening.

    Does anyone have any experience with this type of stuff? Any suggestions? Also, how exactly would I convert readings from a load cell into weight?

    Thanks,
    Philip
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    check ebay for small digital scales
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,118
    3,042
    Start by reading. For instance, http://en.wikipedia.org/wiki/Load_cell

    Have you considered measuring volume instead? If you really need weight, it's possible that measuring density once (for each lot of liquid to be dispensed) and volume of each squirt, might be an easier strategy to implement.
     
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    Does OP want to make one or buy one.
    Buying is better off if he do not know electronics
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    I used to work on weight-defined process controllers. I would suggest that anything homemade is going to require certification beyond anything you can imagine (if this is for commercial usage).

    A lab electronic balance is probably the best way out. Constructing load cells for minute masses is quite difficult, as the load cells have to be attached to fixtures that deform in predictable ways with the addition of weight. A lab balance uses a controlled current in a coil to drive the weight pan back to a zero position - considerable easier than a tiny load cell. A strain gauge can only elongate or shrink by 5% before suffering permanent damage.
     
  6. leonhart88

    Thread Starter Senior Member

    Feb 23, 2007
    118
    1
    Thanks for the replies everyone.

    Measuring volume would be a nice alternative. However, the liquids we are using are radioactive and have to be sterile. What kind of automation tools would be used to measure volume anyway? Using a photodetector of some sort could be one way to do it (when the volume reaches say the 15mL mark). However, the vial we are filling has to be placed inside a container before filling. The container has a diameter of about 1.5 inches and I am not sure if it would affect this. Here is an example of a similar container:

    http://www.atnuke.com/nuclear/lead/pig.htm

    If we used a flow rate sensor then we could possibly measure the volume dispensed through time...but I am not sure how accurate this will be, and many flow rate sensors need to be in contact with the liquid.

    All the mini scales I have looked at don't fit my size requirements. I am assuming this is because most of them have a display and other circuitry. I'd like to find a OEM scale, which I can wire to a DAQ and acquire the data in a MATLAB program. I am an electrical engineer, but I have more background and knowledge in programming.

    I'm not sure if the levitation idea will acceptable for our design. I wouldn't want the vial to tip over in case the floating magnet tips over or anything. Thanks for the idea however.

    I am currently still looking for load cells and/or mini OEM scales that I could use for this. I haven't had much luck googling anything yet though. Should I try tearing apart a small mini scale? Thanks again for all your inputs!

    Thanks,
    Philip
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,118
    3,042
    Are you familiar with automated dispensing pipettes and the like? These are widely used to deliver precise quantities over many repetitions. There are positive-displacement (plunger) types - usually used for smaller volumes such as your 1 ml - as well as pump systems for, for instance, filling petri dishes or tissue culture flasks with 20-100 mls each. Radioactive and sterile are routine needs in medical, biochem or microbiology labs. Not a problem.
     
  8. leonhart88

    Thread Starter Senior Member

    Feb 23, 2007
    118
    1
    Thanks wayneh. Yes, we actually have a bunch of those.

    However, the prototype of this device has already been built. The device utilizes a peristaltic pump which draws fluid from a "master vial" and dispenses into vials using solenoid pinch valves. It might be easier if I show the attached picture.

    The peristaltic pump is a stepper motor. It's actually very accurate so I originally thought that I could just count the steps needed to deliver 1mL and use that method. However, we have to remove all the tubing and sterilize it and then throw it away after each use (due to some radiation stuff). This removal of tubing causes inconsistencies if I use steps to determine volume.

    I guess now that I have explained the setup more clearly, do you have more suggestions/advice?

    Thanks,
    Philip
     
  9. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    605
    24
    Here is a bit of a wild thought. Stress guages. If you were to place a lever on to one and then the weight if the vial would alter the output of the guage. Not knowing the range of stress guages the length of the lever maybe altered to accomodate the required force.
     
  10. wayneh

    Expert

    Sep 9, 2010
    12,118
    3,042
    I'm not sure this will help, but take a look at http://us.mt.com/us/en/home/products/Laboratory_Analytics_Browse/Density_Family_Browse_main.html.

    There are also bathroom scales that work by a similar principle; measuring the frequency before and after applying a load. The shift is proportional to the load applied.

    Another thought: You have a precision pump but because of the issues you mentioned, you're not really using it's capabilities. Perhaps you could fill a piston/reservoir with the pump and then still use another dispensing method to actually fill the vials?

    Reaching for straws here.
     
  11. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Sterilization is a process to eliminate biological hazard from some equipment. Radiation may cause contamination, but cannot be removed by sterilization.

    If your tubing is neutral, simply flushing the length with some volume of distilled water should do that trick. Some research to discover how such processes are done in labs everywhere might lead to some better understanding.
     
  12. leonhart88

    Thread Starter Senior Member

    Feb 23, 2007
    118
    1
    Thanks for your inputs everyone, I'll take a look at what you suggested.

    windoze killa: Aren't strain/stress gauges typically used in load cells?

    wayneh: I thought of calibrating it like that...filling a random vial and determining the # of steps required for a certain number of mL. Unless I can make this fully automated and easy to use, I don't think the chemists would like it. The machine is going to be run approximately 3 times every day, which means it would have to be calibrated every time (which might be annoying to end users who don't really understand the situation). I think that if I cannot find a way to measure the weight/volume, I will have to take this approach.

    beenthere: Thanks for your advice. However, the chemists (and other higher-ups) have told me that this is the way the process needs to be done. I don't have a chemistry or biology background, so I can't really argue with them. I'm sure there is a reason for doing so that I probably don't understand. I have to find a way to work with what I've been assigned unfortunately.

    Has anyone worked with this small force sensor?

    http://www.tekscan.com/flexiforce/flexiforce.html

    I'm not entirely sure if it will be sensitive enough to measure grams...I have contacted the company though. I don't really understand why something like a small OEM scale is so difficult to find. There are tons of cheap mini/pocket scales out there which do what I want. I just need something a bit smaller and without the LCD display that they usually come with (so I can hook it up serially to my PC).

    Thanks again for all your help.
     
  13. leonhart88

    Thread Starter Senior Member

    Feb 23, 2007
    118
    1
    The company has just told me the flexiforce sensor is not sensitive enough...many other load cells I've found don't really fit my application either (and are very expensive).

    I am thinking about purchasing a pocket scale and taking it apart to try to get a better understanding of how they work. I believe they use strain gauges with wheat stone bridges (I have learned about these at school). Would it be possible to just purchase a strain gauge and manufacture a custom scale top to place onto the strain gauge?

    Thanks.
     
  14. wayneh

    Expert

    Sep 9, 2010
    12,118
    3,042
  15. leonhart88

    Thread Starter Senior Member

    Feb 23, 2007
    118
    1
    wayneh: Yes, I've sent them an e-mail already asking about some of those load cells. Hopefully they will get back to me soon.

    Alberto: Yes, I would have to recalibrate every time. I don't really understand what you mean by one tube per vial and a multiple head. We have a very small chamber for the device, so we can't have any fancy robotic parts. Calibrating the systems every time is my last resort. This is because the tubing set-up is really complicated...there are certain ways of connecting and assembling the tubing that I have no control over, so finding a way to automate the calibration will be really difficult. Of course, I am still thinking about this.

    I've contacted a bunch of companies and am now waiting for feedback from them. Hopefully I will get a lead soon...thanks again for the help!
     
  16. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    605
    24
    Yes. Is this not what you are looking for?

    As this stands it won't be sensitive enough but if you apply the lever theory/analogy you could make it sensitive enough. I don't know the formula but if you apply 1gm to a 12cm plate and have a button pressing on the sensor at the 2cm mark I am sure that will multiply the force. You just need to work out how long and where the pivot point must be.
     
  17. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    605
    24
    PS. Grab one and make it smaller. Rip it apart and only use the bits you need.
     
  18. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,689
    901
    In the US, our regulations are such that radioactive biological waste must be sterilized (autoclaved) before discard as radioactive waste. That may not make a lot of sense for very low-level radiation and low-level potential pathogens, but one cannot get around the regulations. Some local ordinances and practices are even more strict than Federal regulations. One such idiocy is that normally sterile fluids (depending on a variety of factors) may need to be autoclaved before discard.

    What sort of precision and accuracy do you need?
    What is the actual volume being dispensed?
    How consistent are the vials? (Will you need to do a tare weight?)

    John

    Edit: Can metal contact the liquid? I am thinking of something like a fill needle and a level-sense needle.
     
    Last edited: Nov 5, 2010
  19. leonhart88

    Thread Starter Senior Member

    Feb 23, 2007
    118
    1
    windoze killa: Yes, those are the ones I think I am looking for. I think the load cell that is most suited to my application is a single point bending beam load cell. Thanks. And I am planning to rip a scale apart as well.

    jpanhalt: I need to dispense around 15mL. My computer program needs to know when this 15mL has been reached, so if I'm using a scale, I'd like to be able to measure 1 gram (1mL for water) or at best, 0.1 grams. I will have to do a tare. I believe the needle can touch the liquid since it will be sterile...is there a special type of needle I could use?

    Alberto: Thanks for the link, I understand what you mean now. Currently, the device is a prototype, but I will keep that design in mind for the future.

    I'm leaning towards using a single point bending beam load cell and a load cell evaluation board from Texas Instruments.

    I'm curious as to how the flexicon peristaltic pumps stay accurate, even when tubing is switched and replaced. Some of their machines don't seem to have any weighing mechanism or anything. Maybe my peristaltic pump is inferior?

    Thanks everyone.
     
  20. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,689
    901
    If the needle ( stainless?) can touch the liquid, why not just use conductivity to determine fill? John
     
Loading...