My attempt to weigh the trailer - it begins

Discussion in 'Tow Vehicles, Hitch & Towing' started by Anthony Hitchings, Feb 14, 2020.

  1. Anthony Hitchings

    Anthony Hitchings Well-Known Member

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    Key Materials needed: bridge-type strain gage load cell ($30), Digital volt meter (in hand), 12V source (easy) , a spreadsheet, two or threes temporary supports, a tape measure, maybe a couple of bolts (or not).

    This should be very simple. Its using what I was taught over 5 decades ago (wheatstone bridge technique).

    And yes, I am an Engineer (tho not electrical).

    Stay tuned to see how it works out.
     

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  2. NMroamer

    NMroamer Well-Known Member

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    Familiar with the wheat stone bridge theory. Curious to see your data.
     
  3. Anthony Hitchings

    Anthony Hitchings Well-Known Member

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    load cell is now on the premises :)
     

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  4. Anthony Hitchings

    Anthony Hitchings Well-Known Member

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    so I tried using it to weight me - my DVM needs an extra digit - it is too coarse

    see attached data

    Tomorrow (Saturday) I will see if I can double the excitation(supply) voltage; I'm not sure that possible with the dual channel power supply in question, which I built decades ago (each channel is 0-15V, and today's test used only one channel).

    I just now also ordered a 0-20 mA panel meter (analog) - that ill help me see where the low end readings really are.
     

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    Last edited: Feb 14, 2020
  5. Anthony Hitchings

    Anthony Hitchings Well-Known Member

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    UPDATE (Saturday morning): for the test using just my clothed body weight, I doubled the excitation voltage to 30V, to make up , sort of, for the lack of sensitivity on my DVM. The resulting computed wight (165) is now closer to my actual weight (178) - though still limited by the lack of a 4th digit on my DVM. This DVM limit will be less significant when measuring the actual wheel loads. I can do the tongue weight using my more accurate digital bathroom scale.
     

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  6. tombiasi

    tombiasi Well-Known Member

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    Interested in following this. The extra digit on the DVM is not going to fix the 165 vs. 178. What are you planning for that.
     
  7. Anthony Hitchings

    Anthony Hitchings Well-Known Member

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    Yes, the extra digit on a DVM would help, because I am not yet at the level of accuracy for the missing digit, which is why the two values do not agree. I got half an extra digit, virtually, by doubling the voltage. If I further doubled the excitation voltage I would get an extra quarter digit, but doing so may blow the strain gauges inside the load cell. Worst case scenario is to add a preamplifier.

    I will return to the project next week.
     
  8. tombiasi

    tombiasi Well-Known Member

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    OK Ill be watching
     
  9. Anthony Hitchings

    Anthony Hitchings Well-Known Member

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    Update: I worked thru the formulae to get weight and lateral CG from loads measured under the springs, which is a good location as my axles has perches top and bottom (due to the SOA lift). Attached is the result using guesstimated (readings) - as on of several preparations to doing a real weighing
     

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  10. Anthony Hitchings

    Anthony Hitchings Well-Known Member

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    RESULTS ARE IN

    Trailer is a possibly bit heavier than I assumed, but I do have some heavy gear inside.
    Tongue weight is too low.

    Max error is 8% at low loads , but more likely 4% or less at higher loads since I could not check (!) calibrate the system in a high load test rig. My limiting factor is the resolution of my DVM.

    Of course I messed up the first attempt by forgetting to raise the stabilizers before measuring!
     

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    Last edited: Mar 11, 2020
  11. Anthony Hitchings

    Anthony Hitchings Well-Known Member

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    The load cell has a loose bolt - bolt has a rounded head, and there are two washers loose under the rounded bolt head. This locates the load cell under my axle's lower perch.
     

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  12. Anthony Hitchings

    Anthony Hitchings Well-Known Member

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    Calibration review, again using my body weight. If the missing digit on my DVM had lead to a reading of .0095 the calculated load (from me standing on the cell) would be 175 Lbs, today, and today my weight is 173.6 Lbs on the digital bathroom scale. So the system may be reading as true as it can, given my missing 4th digit of resolution. That .0005V corresponds to 9 Lbs. So that error times each spring location would be 18 Lbs total - not much.

    So I think that I have a very usable system (its dead easy to use on my Aliner axle) that will read to within about 20 Lbs of the true weight.

    It cost me $30 for the load cell and $13 for some minor parts, I had everything else lying around.

    You need a low profile floor jack (usually aluminum), which I already had.
     

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  13. Spridle

    Spridle Well-Known Member

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    So this turns pressure into voltage, which you convert into weight? Are you measuring three points?
     
  14. Anthony Hitchings

    Anthony Hitchings Well-Known Member

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    Its an electrical wheatstone bridge strain gage assembly integrated into the steel load cell, pre calibrated; all I have to do is apply voltage across one pair of bridge wires and measure a lowish millivolt reading on the other pair of bridge wires, and multiply by the constant to get weight in kiligrams. I measured at both the spring perches and at the nose of the tongue hitch socket.

    Left and right values are the same - pure coincidence and partly due to limited reasolution of my digital voltmeter.

    No hydraulics (pressures) are involved.
     
  15. wmgeorge

    wmgeorge Member

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    To bad you don't live in a farming area, I just took mine to the local grain elevator scale.
     
  16. Anthony Hitchings

    Anthony Hitchings Well-Known Member

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    UPDATE: the millvoltmeter module arrived, so I tested it in the garage, to check operation. It seems to be working OK. This module has one more digit than my DVM has (so it should solve my "missing digit" problem).
     

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  17. Anthony Hitchings

    Anthony Hitchings Well-Known Member

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    UPDATE:
    Fixed error in spreadsheet, and got a millivolt panel meter and battery clip for Ryobi 18VOne+ system. I had to use separate power sources for the load cell (12V SLA cell from not-used dedicated breakaway battery) and for the panel meter (18V Ryobi battery). The measured weight etc seems reasonable for what's in and not in the trailer.

    At 1583 Lbs partially loaded, by the time we add the Engel frig and load it up, and add some water and food, and a few odds and ends, we would be right at 1700 Lbs.

    I also need to keep an eye out for too much tongue weight, because I don't want to push the rear of the Jeep down too far.

    FYI - at first I checked Ryobi 18V battery voltage on my DVM 20V range and got "1". ???
    Changed the DVM to 200V range and got 20.6 volts (better than 18 :)
     

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  18. Anthony Hitchings

    Anthony Hitchings Well-Known Member

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    UPDATE:

    my load cell system is working, and I measured it at 96% calibration factor by using my system to weigh myself and compare it to my bathroom scale.

    Due to recent issues with Aliner axles and springs, we took everything out of the Aliner and reweighed it, and re-estimated the expected weight. The propane tank was on the tongue (propane tank was included by Aliner, based on the Manuf. placard).

    Weighing result is 311 Lbs more than expected. This is a heck of a LOT relative to the claimed unloaded with 895 Lbs. (Its 33%)

    Since "expected" is based in (a) Manuf. Plate and (b) items I changed or added, and the latter is known in detail
    the Manuf. Weight is suspect.

    What might be an issue?
    Manuf. may have excluded the one original battery (even if he included the battery holder) - this is around 50 Lbs - if it was not supplied back in 2003
    Manuf. may have excluded the portable galley (galley = sink and cooktop in hokey plastic frame) approx 29 Lbs - as it is a movable item
    Manuf. may have excluded cushions - as they are movable items (not much weight in this category)
    There might be one heck of a lot of moisture trapped in the roof foam (I doubt it, since the roof lifts easily).

    Dometic Frig was "built in" so it should have been included in the weighing ( 49 Lbs)

    At this time its not obvious, to me, what might be a cause of the (remaining) large difference.
     

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  19. Econ

    Econ Well-Known Member

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    On the 2014 Expedition CNW shows a battery and 2 propane tanks on the front of the brochure. They only include the weight of one empty propane bottle in published tongue weight. You need to have CNW define the terms.

    CNW plays games with the numbers as they go for the non pickup TV market and need to keep the numbers low.

    Good luck
     
  20. Anthony Hitchings

    Anthony Hitchings Well-Known Member

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    My spreadsheet of adds and subtracts plus the OEM weight placard suggest a current mostly empty weight of 1,025 Lbs. ("axle plus tongue" weight). We essentially took everything (including tables, galley etc.) out of the trailer and added a very few known weight items back in.

    My own load cells (used at 100% rating) and also the CAT scale says the load on my axle is 1,185 and 1,200 Lbs respectively. Very good agreement, since the CAT scales are meant to be within 20 Lbs.

    So in round numbers there is a 200 Lb excess weight. Its not huge - but it sure makes a joke of the load placard on the trailer.

    This mismatch in weight is not enough to have flattened my springs (twice). But its not helpful.
     

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