“Truck scales are calibrated for the much
heavier weights of the big rig trucks. Such scales operate best in
their mid-range, therefore inaccuracies occur when weighing RVs,
which may be only 1/3 the weight or much less of a loaded
semi-tractor/trailer combination.”
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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. Further, scale 2 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.”
Where do these scales weigh in?
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.
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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)
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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)
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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)
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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 the sake of safety, weigh your rig.
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 St.
