Rice Lake Z6 Single-Ended Beam, SS Welded-seal, IP67, OIML C3 - Manual (Page 55)

Load Cell Terms 51
23.0 Load Cell Terms
We know that a load cell is an electromechanical device. It
can be called a transducer as it converts one form of energy
to another—mechanical force or stress to electrical energy. A
load cell has various characteristics that are measurable.
These characteristics are determined by the type of metal
used, shape of the load cell and how well it is protected from
its environment. To understand load cells, there are terms that
you need to become familiar with so you can better match the
load cell to your application.
CALIBRATION - The comparison of load cell outputs against
standard test loads.
COMBINED ERROR - (Nonlinearity and hysteresis) - The
maximum deviation from the straight line drawn between the
original no load and rated load outputs expressed as a
percentage of the rated output and measure on both
increasing and decreasing loads.
CREEP - The change in load cell output occurring over time,
while loaded, and with all environmental conditions and other
variables remaining constant.
CREEP RECOVERY - The change in no load output,
occurring with time, after removal of a load which had been
applied for a specific period of time.
DRIFT - A random change in output under constant load
conditions.
ECCENTRIC LOAD - Any load applied parallel to, but not
concentric with, the primary axis.
ERROR - The algebraic difference between the indicated and
true value of the load being measured.
EXCITATION - The voltage applied to the input terminals of a
load cell. Most load cells have a rated excitation voltage of 10
VDC. There are load cells available that are rated at 15, 20
and 25 VDC and also some that have both AC and DC
excitation ratings.
HYSTERESIS - The maximum difference between load cell
output readings for the same applied load. One reading is
obtained by increasing the load from zero, and the other
reading is obtained by decreasing the load from rated load.
Hysteresis is measured as percentage of the full scale rated
output (% F.S.). Common load cell hysteresis values are .02%
F.S., .03% F.S. and .05% F.S.
INPUT BRIDGE RESISTANCE - The input resistance of the
load cell. It is measured by placing an ohmmeter across the
input or excitation leads. It is usually higher than the output
bridge resistance because of the presence of compensating
resistors in the excitation circuit.
INSULATION RESISTANCE - The DC resistance measured
between the load cell circuit and the load cell structure.
NON-LINEARITY - The maximum deviation of the calibration
curve from a straight line drawn between the no load and
rated load outputs. It is expressed as a percentage of the
full-scale rated output. It is measured on an increasing load
only. Common non-linearity values are .02% F.S. and .03%
F.S.
OUTPUT - The signal produced by the load cell where the
output is directly proportional to excitation and the load
applied. The signal must be in terms such as millivolts per volt
(mV/V) or volts per ampere (V/A).
OUTPUT BRIDGE RESISTANCE - The output resistance of
the cell. It is measured by placing an ohmmeter between the
signal or output leads. Common bridge resistances are 350Ω,
480Ω, 700Ω, 750Ω and 1000Ω.
OUTPUT, RATED - The algebraic difference between the
output at no load and the output at rated load. REPEATABILITY - The maximum difference between load
cell output readings for repeated loadings under identical
loading and environmental conditions.
RESOLUTION - The smallest change in mechanical input
which produces a detectable change in the output signal.
SAFE OVERLOAD RATING - The maximum load, in percent
of rated capacity, which can be applied without producing a
permanent shift in performance characteristics beyond those
specified. A common safe overload rating is 150% F.S.
SENSITIVITY - The ratio of the change in output to the
change in mechanical input.
SHOCK LOAD - A sudden increase in load usually caused by
dropping weight onto the scale. Can cause permanent load
cell damage.
SIDE LOAD - Any load acting 90° to the primary axis at the
point of axial load application.
TEMPERATURE EFFECT ON RATED OUTPUT - The
chang e in rated ou tput du e t o a c h a n g e i n a m b i en t
temperature. It is usually expressed as the percentage
change in rated output per 100°F change in ambient
temperature.
TEMPERATURE EFFECT ON ZERO BALANCE - The
change in zero bal ance due to a change in ambient
temperature. It is usually expressed as the change in zero
balance in percent of rated output per 100°F change in
ambient temperature.
TEMPERATURE RANGE, COMPENSATED - The range of
temperature over which the load cell is compensated to
maintain rated output and zero balance within specified limits.
ULTIMATE OVERLOAD RATING - The maximum load, in
percent of rated capacity, which can be applied to a load cell,
without producing a structural failure.
ZERO BALANCE - The output signal of the load cell with
rated excitation and with no load applied, usually expressed in
percent of rated output.
Figure 23-1 may help you understand important load cell terms.
Figure 23-1.
Rated Output
Calibration Curve
Zero Balance
LOAD
B
OUTPUT
Non-Linearity
{
Combined Error
Hysteresis
Repeatability