Dave Kamp of Iowa 80 Group (CAT Scale) responds to our Ultimate Truck Camper Payload Match Challenge with important insights about CAT Scales and our surprising results. This is a must read for any RVer who uses CAT Scales.
We were delighted when Dave Kamp of Iowa 80 Group contacted us with critical information about how CAT Scales operate, and exactly why we got the CAT Scale results we did during our Ultimate Truck Camper Payload Match Challenge.
Dave also invited us to examine the inner workings of a CAT Scale installation. We are excited to take him up on that invitation when our paths cross. Anything we can do to further truck camping and RV safety remains a top priority for us.
The following is Dave Kamp’s response to our, “Ultimate Truck Camper Payload Match Challenge”. If you haven’t already read that article, start there.
“I just stumbled upon your capacity challenge article and there are some things you should understand about our scales to understand why your scale weights appear as they do.
First, our scales are Certified Legal For Trade. In the United States, that means they are designed, built, programmed, documented, tested, and maintained per NIST (National Institute of Standards and Technology) Handbook 44. All operational characteristics for LFT scales apply accordingly.
LFT scales must read in 10000 increments between zero and full capacity. Our scales’ 200,000lb gross capacity predicates that the scale indications occur in 20lb increments. They will never read 10, 30, 50, 70, 90, 5, 1, or 0.5lb.
They always round to the nearest increment using HBK44’s formulation. This formulation is 1/10th of an increment. In case of our 200,000lb/20 increment circumstance, if any of the three scales (Steer, Drive, or Trailer scale) land on the 10lb mark, the scale will round either to 0 or 20, based on the directly most recent deviation of one pound. This means, if it saw 11lbs before 10, it rounds to 20. If it saw 9 lbs prior to going to 10, it shows 0.
As for deviation of weights between steer and drive, you noted that although gross was same, steer and drive changed. That was not due to position on the scale. It was simply because the scale’s motion toggled both Scale 1’s rounding (steer), and Scale 2’s rounding (drive) in response to motion still occurring on the scale.
The scale is sensitive enough to recognize motion such that it must, at ‘capture’ moment, take a data snapshot, and interpret those points. It saw, at those moments, scale weights which were much finer than the 20lb increment required by NIST HBK44, and rounded appropriately, but what appeared slightly higher (just two pounds) on one end, was offset by what appeared slightly lower (the same two) the other way.
When I calibrate our scales, I place a 20,000+ lb motorized weight cart over each of the scales’ load cells. Most of our scales have either 14 or 16 load cells. Then I adjust the calibration factors for each one to identify that strain gauge’s sensitivity to zero error; dead on, for that cart.
Once I’ve adjusted the calibration of each cell, I then pull on an unknown, but heavy weight – typically a 35,000+lb truck – onto each scale, and take a reading. Then I apply my test cart’s weight, and observe the rise. It should be commensurate with the actual weight of the cart. This is called strain test.
We then do a linearity test (aka ‘buildup’, but we usually do it ‘backwards’). This involves placing just the loaded test cart on the scale, and lifting off groups of weights in regular intervals. At each interval, we observe the weight fall. It should reduce commensurate to the quantity of certified test weight removed.
When you weigh your truck, and perform your bathroom-scale comparisons, it is very important to understand the difference between your certified weight ticket, and your bathroom scale. It doesn’t matter how many decimal places of precision your bathroom scale indicates.
You’re comparing its non-certified/non-calibrated/non-tested indication of a small-capacity instrument, to that of a certified, calibrated, tested indication of a large capacity instrument. Neither is ‘wrong’, but one cannot measure with a micrometer, mark with chalk, and cut with an axe, and consider something awry in the result.
Your truck’s advertised weight, advertised capacity, GVWR, and what it actually weighs, are all totally different things. Auto manufacturers don’t make it their point to weigh and advertise that indication for each individual vehicle. Any option or provisioning makes that number a variable.
The GVWR, for instance, is an engineering benchmark value by which axle types, spring types, chassis construction (etc.) are all focused upon for meeting the needs of a particular customer base.”
– Dave Kamp, Iowa 80 Group
