
Let's start with the basic assumption that any cell that needs
calibrated should be bolted down by the end user of the equipment prior
to being sent anywhere for calibration. When you send a load cell that
requires bolting, you should not expect the calibration results to be
valid for your testing needs.
If you are not testing the load cell in the machine it is being used in,
you will need to account for additional errors due to the following;
mounting considerations, variation between different bolts, material in
the base, surface finish on the base, hardness, stiffness, alignment,
flatness, bending and variations from using different bolting sequences
may contribute to the uncertainty. Lets us not forget the torque
wrench that may be accurate to 4 % of applied torque that was used to
torque the bolts. This assumes a torque wrench was even used.
I hope by now most of you are starting to understand that this type of
calibration has several major possible uncertainty contributors that all
need to be addressed. Even if all of these potential errors are
quantified, my bet is an R & R study has not been performed between
the technicians installing the load cell in the machine, and the
technicians in the laboratory performing the calibration. Is there a
significant difference between these technicians? I'm going to go on
record and say " You Betcha" unless a very rigid testing procedure was
documented and you have SPC charts to prove it. My assumptions are that
any lab going to these great lengths to quantify all of these errors,
has already realized that there is probably a better way to ensure more
repeatable results.
Morehouse is always going to suggest purchasing the proper equipment
that will allow for calibration of the load cells in the machines they
are being used. There are two very good standards that will give you
the detailed instructions on how to use a load cell system to calibrate
these load cells in place. These standards are ASTM E4 and ISO 7500.
Morehouse can provide a complete load cell system with ISO 17025
accredited calibration in accordance with ASTM E74 or ISO 376 to perform
in place calibrations of load cells in the machines they are being
used. If you are working to ASTM standards that references ASTM E4 as
a calibration requirement, it is important to note that the entire
system needs to be calibrated, if removed. When equipment is not
available to allow for in place calibrations, please remember to account
for the various uncertainty contributors listed above in addition to
environmental conditions, the uncertainty of the reference standards
used to perform the calibration, the resolution of the device, the
stability of the instrumentation and the reproducibility and
repeatability of the measurement process.
For most load cell manufacturers, the removal of the load cell from the base voids the warranty
Everything we do, we believe in changing how people think about force and torque calibration. Morehouse believes in thinking differently about force and torque calibration and equipment. We challenge the "just calibrate it" mentality by educating our customers on what matters, what causes significant errors, and focus on reducing them. Morehouse makes our products simple to use and user friendly. And we happen to make great force equipment and provide unparalleled calibration services.