The above quote is not uncommon and similar comments can be found written on most any RV related forum.
So the question is, “Is there really any truth to this assertion?”
I set out to find the answer to that question and the task was not that simple. In order to get to that answer, it required a better understanding of the construction of a scale.
My first call was made to Fairbanks Scales and I talked to one of their engineering techs, Jim. Fairbanks Scales manufactures the scales and associated software for CAT Scale. Jim explained that all their scales are tested and calibrated by a technique referred to as “weight buildup.” He went on to explain that they start with 3,000 pounds and continue adding weight and calibrating each range until they reach the capacity weight of the scale. Then he stated that with this design and calibration process the tolerance level between weighing a car versus a heavy truck would be relatively small. Jim also said that once the scales are turned over to the buyer, the buyer may have a completely different calibration procedure in place and recommended I contact CAT Scale.
I then contacted Heather at CAT Scale. Heather stated that their calibration procedure was proprietary and could not discuss any of the details, but she did provide the capacity and accuracy details. Similar to Jim’s comment above, Heather said that CAT Scale can weigh cars as well as heavy truck loads within the required federal guidelines. I asked, “What are those guidelines?” She then informed me that their scales meet or exceed HB 44.
Handbook 44 is the federal requirements established by the National Institute of Standards and Technology (NIST). Handbook 44 is the Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices. After downloading Handbook 44, I quickly realized that this 497 page manual was way over my head. I did make an effort to look for some answers but resorted to calling NIST.
A week later, I received a three-page e-mail from John who is the Weights and Measures Coordinator at NIST. John was very thorough and provided some examples. The challenge became in interpreting the information and condensing it into some simple paragraphs.
One of the aspects associated with scale accuracy is the scale’s resolution. Scale resolution is the number of divisions on the scale within the weight range capacity. For example: Two scales have a capacity of 10,000 pounds. Scale 1 divisions are 200 and scale 2 divisions are 1,000. That means that scale 1 indicates 50 pounds of weight for each division. Scale 2 indicates 10 pounds of weight for each division. Scale 2 is more accurate and will read within plus or minus 5 pounds. Scale 1, being less accurate, will have a plus or minus range of 25 pounds per 50 pound division.
There are some additional accuracy issues related to whether the scale is mechanical (analog) or digital. The user reading an analog dial is potentially capable of reading a value that falls somewhere in between two adjacent gradations. This is not the case with digital indicating scales where it is left to the function of the indicating element of the scale to “decide” which digital increment is closest to the weight sensed by the scale.
Another interesting tidbit - regardless of scale capacity, an object that weighs 5000 pounds will have less potential error than an object weighing 500 pounds on the same scale. The potential of error partially depends on the number of consistent divisions given within the weight capacity of the scale. John supplied a few good examples but they are too lengthy to explore here.
With the advancement of the digital age, most scales are now designed to operate using one scale division for loads in a lower range, and a larger scale division when operated in a higher range. This design helps to minimize the effect of rounding when a wide range of loads are capable of being weighed by a single device.
In John’s summary, he states: “Truck scales, if calibrated, used and maintained properly, will be capable of providing accurate values for loads of various degrees of weight. [This is especially true of the recent digital models.] However, a weight value obtained on the scale (with a consistent scale division size) for a smaller or lighter load will include a greater degree of uncertainty as compared to a weight value obtained on the scale for a heavier load.”
Shown below are specifications for two portable scales that are commonly used for weighing RVs and the specifications for the most common commercial truck scale. By examining the specifications you’ll notice how the different scale divisions indicate a greater or lesser potential of error. And for digital scales, the potential of error also depends on the quality of the weighing element and the associated scale software.
Haenni Instruments of Switzerland
(Analog)
Loadometer Corporation, Model WL101 Capacity: 20,000 pounds
Division: 50 pounds Accuracy at first calibration
±50 lb (2,500 lb)
±100 lb (10,000 lb)
±150 lb (20,000 lb) Accuracy in operation
±100 lb (2,500 lb)
±200 lb (10,000 lb)
±300 lb (20,000 lb)
LCD Ultraslim Wheel Load Scale
(Digital)
Massload
Capacity: 20,000 pounds,
Division: 20 pounds Static Accuracy: 0.5% Full Scale
±12.5 lb (2,500 lb)
±50 lb (10,000 lb)
±100 lb (20,000 lb)
CAT Scale
(Digital)
Fairbanks Scales
Capacity: 200,000 pounds,
Division: 20 pounds Accuracy: HB 44, Class III L
±20 lb (2,500 lb)
±20 lb (10,000 lb)
±40 lb (20,000 lb)
After examining the examples above and accounting for the differences between analog and digital scales, the quote at the beginning of this article may be accurately stated only if all scales were analog scales with a consistent scale division size or poorly designed digital scales. With this new education provided by this article on scale construction and accuracy capabilities, RVers can confidently weigh their rig at most certified truck scale facilities expecting acceptable weight accuracy.
For your own safety and those traveling with you and the safety of those traveling on the road near your moving RV, periodically weighing the RV is strongly recommended. The best choice is to obtain a weight safety report from a provider that weighs each wheel position. It is unlikely, if not impossible, to accurately weigh each wheel position at a truck scale. Below are some website contacts that provide wheel by wheel weighing. If the timing and place is inconvenient, don’t wait. Go to the nearest certified truck scale and weigh your rig. The premier self-service weight safety report providing 13 points of safety measures can be obtained online at Fifth Wheel Street.
RV Safety & Education Foundation (RVSEF) (WL101) The safety education leader in the RV world offers wheel by wheel RV weighing services.
Weigh-It Portable RV Scales (WL101) Weigh-It Portable RV Scales operate during the summer months in Sault Ste Marie, Ontario, Canada at the KOA Campground. During the winter months they are on the road weighing in the USA.
SmartWeigh (WL101) by Escapees RV Club (Available to all RVers) Includes permanent locations at Livingston, TX, Congress, AZ and Bushnell, FL.
Weigh To Go, LLC (LCD Ultraslim) *Under new ownership as of 2024.
Copyright © 9/17/12, Dave Gray, Reprint by permission only.
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