Download Load Cell Troubleshooting

VPG TRANSDUCERS
Load Cells
Application Note VPG-08
Load Cell Troubleshooting
Scope
Load cells are designed to sense force or weight under a
wide range of adverse conditions; they are not only the
most essential part of an electronic weighing system, but
also the most vulnerable.
Load cells m ight be damaged because of (shock)
overloading, lightning strikes or heavy electrical surges
in general, chemical or moisture ingress, mishandling
(dropping, lifting on cable, etc.), vibration or internal
component malfunction. As a direct result the scale or
system might (zero) drift, provide unstable/unreliable
readings or not register at all.
This application note is written to assist our customers
with potential load cell problems. It describes basic field
tests which can be performed on site, and provides the
information necessary to interpret the results.
Proper field evaluation is absolutely critical to prevent
similarly induced damage in the future! Under no
circumstances should fault location, as described below, be
attempted on load cells installed in a hazardous area!
In General
Carefully check the system integrity before evaluating the
load cells:
Document Number: 11867
Revision 14-Dec-2011
The following test equipment is required to properly
evaluate a load cell:
• A high quality, calibrated, digital volt- and ohmmeter
with a measuring accuracy of ±0.5Ω and ±0.1 mV, to
measure the zero balance and integrity of the bridge
circuit.
• A megohm meter, capable of reading 5000 MΩ with
an accuracy of 500 MΩ at 50 volts, to measure the
insulation resistance. Do not use megohm meters which
supply more than 50 volts to the load cell, in order to
prevent permanent damage!
• A means to lift the dead load (weighbridge, tank,
hopper, conveyor, etc.) off the load cell to be able to
measure the zero balance or to remove the load cell(s),
i.e. a crane, hydraulic jack, etc.
Load cells are produced according to specifications and
tolerances which are described in the applicable data
sheet. More detailed information can be found on the
calibration certificate which is packed with each load
cell. The calibration certificate mentions the exact values
for the input and output resistance, insulation resistance,
zero balance, rated output and the correct wiring code;
it provides an important reference for the values which can
be measured and should be f iled with the system
documentation set.
For technical support, contact in Americas [email protected],
in Europe [email protected], in China [email protected],
in Taiwan [email protected]
www.vpgtransducers.com
1
A PPL I CAT I O N N OT E
• Check for force shunts (might be caused by dirt,
mechanical misalignment or accompanying
components such as stay- or check rods).
• Check for damage, corrosion or significant wear in the
areas of load introduction.
• Check cable connections to junction box and
indicator.
• Check the measuring device or indicator with an
accurate load cell simulator.
Visually inspect the load cells before performing the
tests as described on the following pages. Pay particular
attention to signs of corrosion (especially around the
critical gauge area), the integrity of the cable (might be
compromised due to cuts, abrasions, etc) and the condition
of the cable entry.
Test #1
Zero Balance
Test #2
Insulation
Resistance
OK
VPG-08
OK
Te
Br
Inte
VPG Transducers
Wrong
Wrong
Load Cell Troubleshooting
TEST PROCEDURES AND
ANALYSIS
Mechanical
Moisture
or
chemical
overload
Test Procedures
andfor
Analysis
The diagram below represents a proposed
sequence
testing load cells after
a particular system
ingress
thea proposed
fault location
byfor
moving
relatively
deadweight
over each load
cell,
The malfunction.
diagram below Isolate
represents
sequence
testing a
load
cells aftersmall
a particular
system malfunction.
Isolate
the
faultor
location
by moving a relatively
small
by disconnecting
load cell
by deadweight
load cell. over each load cell, or by disconnecting load cell by load cell.
R1kΣ
Sudden change
in Zero point
Unstable readings,
random change
in Zero point
Scale reads overload,
incorrect or not at all
Erratic readings
when load is
appliedcircuit
or removed
Short
Brok
or com
to housing / screen
Test #1
Zero Balance
Test #2
Insulation
Resistance
OK
OK
Test #3
Bridge
Integrity
OK
TEST #1: ZERO BALANCE
Wrong
Mechanical
overload
Test #4
Shock
Resistance
Electrica
or in
shor
OK
Wrong
Wrong
The Zero
Balance is defined
as the load Wrong
cell output in a "no-lo
deadload)
has to beElectrical
removed
from the load
cell. Low capacity load c
Moisture
overload
Failed electrical
or
chemical
or
internal
connection
the ingress
load cell is designed
to measure force to prevent the weight of the
short circuit
The load cell should be connected to a stable power supply, preferab
R≤1
kΩ
R∞
R1kΣ
R4
of at least 10 volts. Disconnect any other load cell for multiple load c
Measure
across
Short circuit the voltage Broken
wire the load cell's output leads with a milli
to housing / screen
or component
excitation voltage to obtain the Zero Balance in mV/V. Compare the
certificate ( if available ) or to the data sheet.
Test #1: Zero Balance
TEST #1: ZERO BALANCE
A PPL I CAT I O N N OT E
The Zero Balance is defined as the load cell output in
Input
The Zero
BalanceTherefore,
is defined all
as weight
the load(including
cell output in a "no-load" situation.
Therefore, all weight (including
a "no-load"
situation.
deadload)
has has
to be
removed
from
Lowcapacity load cells should be measured in the position in which
deadload)
to be
removed
fromthe
theload
load cell. Low
~
capacity
cellsisshould
be to
measured
thetoposition
the load cell
designed
measure in
force
preventin
the weight of the element giving wrong results.
whichThe
the load
load cell
cell should
is designed
to measure
topower
prevent
be connected
to aforce
stable
supply, preferably a load cell indicator with an excitation voltage
the weight
of
the
element
giving
wrong
results.
of at least 10 volts. Disconnect any other load cell for multiple load cell systems.
Measure
the voltage
across the
cell's
output
leads with a millivoltmeter and divide this value by the input or
The load
cell should
be connected
to aload
stable
power
supply,
excitation
voltage
to obtainwith
the Zero
Balance involtage
mV/V. Compare the Zero balance to the original load cell calibration
preferably
a load
cell indicator
an excitation
( if available
) or toany
the other
data sheet.
of at certificate
least 10 volts.
Disconnect
load cell for
Output
multiple load cell systems.
V
Measure the voltage across the load
cell's output leads
Input
with a millivoltmeter and divide this value by the input ~
or excitation voltage to obtain the Zero Balance in mV/V.
Compare the Zero balance to the original load cell
calibration certificate ( if available ) orANALYSIS
to the data sheet.
zero output changes per time period are most likely
in the
strain gauge
resistance
Changes in Zero undergoing
Balancea change
usually
occur
if the
loadbecause
cell has been
Analysis
Output
of chemical or moisture intrusion. However, in this case
excessive shocks.theLoad
cells that experience progressive zero ou
insulation resistance and/or the bridge integrity will
Changes in Zero Balance usually occur if the load cell
also be
has been permanently deformed by overloading
and/or
undergoing
a change
incompromised.
the strain gauge resistance because of chem
excessive shocks. Load cells that experience progressive
the insulation resistance and/or the bridge integrity will also be comp
ANALYSIS
V
Changes in Zero Balance usually occur if the load cell has been permanently deformed by overloading and/or
For technical
support, contact
in Americas
www.vpgtransducers.com
Document
Number:
11867
excessive shocks. Load cells
that experience
progressive
zero [email protected],
output changes per time period
are most
likely
in Europe
[email protected],
in China
[email protected],
2
Revision
undergoing a change in the strain
gauge
resistance because
of chemical
or moisture intrusion. However,
in14-Dec-2011
this case
in Taiwan [email protected]
the insulation resistance and/or the bridge integrity will also be compromised.
body. Repeat the measurement between the same 4 or 6 leads and the
resistance between the load cell body and cable shield.
Never use a megohmmeter to measure the input or output resistance,
TEST #2: INSULATION RESISTANCE
exceeds the maximum excitation voltage by far!
VPG-08
The insulation resistance is measured between the load cell circuit a
VPG
Transducers
cell from the junction box or
indicator and
connect
all input, output a
Input
Measure the insulation resistance with a megohmmeter between the
the measurement between the same 4 or 6 leads and
Load body.
Cell Repeat
Troubleshooting
resistance between the load cell body and cable shield.
Never use a megohmmeter to measure the input or output resistan
Outputvoltage by far!
Test #2: Insulation Resistance exceeds the maximum excitation
Σ
The insulation resistance is measured between the load cell
circuit and element or cable shield. Disconnect the load
cell from the junction box or indicator and connect all
input, output and sense (if applicable) leads together.
Input
ANALYSIS
Measure the insulation resistance with a megohmmeter
between these four or six connected leads and the
load
cell
The
insulation
resistance of all load cells should be 5000 MΩΩ or more fo
body. Repeat the measurement between the same
4
or
6
cable screen
and housing to cable screen.
leads and the cable shield. Finally measure the insulation
Output
A lower value indicates electrical
leakage, which is usually caused by mo
resistance between the load cell body and cable shield.
Σ
the load cell or cable. Extremely low values (≤ 1k ΩΩ ) indicate a short cir
results usually in unstable load cell or scale reading ou
Never use a megohmmeter to measure the input or output
Electrical leakage
resistance, as it normally operates at a voltage which exceeds
temperature.
the maximum excitation voltage by far!
Analysis
ANALYSIS
load
cell or cable.
low values
(≤1kbeΩ 5000
) indicate
The insulation resistance of all load cells should
be
The insulation
resistance
of Extremely
all load cells
should
MΩΩa or mor
short circuit rather than moisture ingress.
5000 MΩ or more for bridge circuit to housing, bridge
cable screen and housing to cable screen.
circuit to cable screen and housing to cable screen.
TEST
BRIDGE
INTEGRITY
Electrical
leakage
results
usually which
in unstable
load cell
A lower#3:
value
indicates
electrical
leakage,
is usually
caused b
or scale reading output. The stability might vary with
A lower value indicates electrical leakage, which is usually
the
load
cell
or
cable.
Extremely
low
values
(≤
1k
ΩΩ
)
indicate
a shor
bridge
is verified by measuring the input and output resistan
temperature.
caused by moisture or chemical contaminations The
within
the integrity
Test #3: Bridge Integrity
Electrical leakage
usually
in unstable
cell or scale
readin
Disconnect
the load results
cell from
the junction
box orload
measuring
device.
The
input
and
output
resistance
is
measured
with
an
ohmmeter
across
eac
temperature.
the input and output resistance to the original calibration certificate (if av
The bridge balance is obtained by comparing the resistance from -output
difference between both values should be smaller than, or equal to 1 ΩΩ.
The bridge integrity is verified by measuring the input and
TEST #3:
output resistance as well as the bridge balance. Disconnect
the load cell from the junction box or measuring device.
BRIDGE INTEGRITY
Input
Σ
resistance from -output to -input, and -output to +input.
The difference between both values should be smaller
than, or equal to 1Ω.
Analysis
Input
Σ
A PPL I CAT I O N N OT E
The bridge integrity is verified by measuring the input and output res
The input and output resistance is measured with an
Disconnect
the load cell from the junction box or measuring device.
ohmmeter across each pair of input and output
leads.
Σ
input and output resistance is measured with an ohmmeter
acros
Compare the input and output resistance to theThe
original
calibration certificate (if available) or to the data
sheet and output resistance to the original calibration certificate (
the input
specifications.
The bridge balance is obtained by comparing the resistance from -ou
Output
Σ
The bridge balance is obtained by comparing
the between both values
difference
should be smaller
than, or equal to 1
ANALYSIS result from over-voltage (lightning or welding), physical
Changes in bridge resistance or bridge balance are most
resistance
or bridge
balance
most excessive
often caused by
damage
from shock,
vibration
or are
fatigue,
often caused by a broken or burned wire, anChanges
electrical in bridge
temperature,
or from
production
inconsistencies.
component failure or internal short circuit. This
might failure
component
or internal
short
circuit. This
might result fromΣover-v
damage from shock, vibration or fatigue, excessive temperature, or from
Output
Document Number: 11867
Revision 14-Dec-2011
For technical support, contact in Americas [email protected],
in Europe [email protected], in China [email protected],
inANALYSIS
Taiwan [email protected]
Σ
www.vpgtransducers.com
3
TEST #4: SHOCK RESISTANCE
VPG-08
VPG Transducers
The load cell should be connected to a stable power supply, preferabl
of at least 10 volts. Disconnect all other load cells for multiple load ce
With a voltmeter connected to the output leads, lightly rap on the load
Load Cell
Troubleshooting
Exercise
extreme care not to overload low capacity load cells while te
Watch the readings during the test. The readings should not become e
return to original zero readings.
Test #4: Shock Resistance
The load cell should be connected to a stable power supply,
preferably a load cell indicator with an excitation voltage
of at least 10 volts. Disconnect all other load cells for
multiple load cell systems.
With a voltmeter connected to the output leads, lightly
rap on the load cell with a small mallet to mildly shock it.
Exercise extreme care not to overload low capacity load cells
while testing their shock resistance.
Input
~
Output
Watch the readings during the test. The readings should
not become erratic, should remain reasonably stable and
return to original zero readings.
Analysis
V
ANALYSIS
Erratic readings may indicate a failed electrical connection
Erratic readings may indicate a failed electrical
or a damaged glue layer between strain gauge and element
element as a result of an electrical transient.
as a result of an electrical transient.
connection or a dama
LOAD CELL EVALUATION FORM
Load Cell Evaluation Form
The following load cell evaluation form should be used as a guide for the testing and evaluation of load cells. We recommend
that this form be included in the customer dossier and used as the basis to discuss the test results and diagnostics with third
parties.
A PPL I CAT I O N N OT E
A load cell evaluation form is included in this application
testing
and
cells.
We recommend
this
If a load cell is returned to VPG Transducers, thefor
Evaluation
Form
will evaluating
assist our repair load
department
in further
diagnostics
and repair of the cell.
and its use as a basis to discuss the test results and diagnost
If a load cell is returned to Vishay Revere Transducers, th
department in further diagnostics and repairing the cell.
Customer support:
The Vishay Revere Transducers combines sixty years of lo
application know how. For further information, please con
one of our regional sales offices.
www.vpgtransducers.com
4
For technical support, contact in Americas [email protected],
in Europe [email protected], in China [email protected],
in Taiwan [email protected]
Vishay Revere Transducers B.V.
Document Number: 11867
Revision 14-Dec-2011
VPG-08
VPG Transducers
Load Cell Troubleshooting
Load Cell Evaluation Form
Company:________________________________________________ Contact person:___________________________________
Address:___________________________________________________ City / Country:___________________________________
Tel. / Fax.:__________________________________________________ Repair order:__________ Date:____________________
Load cell type:______________________________________________ Serial number:___________________________________
Capacity:_________________________________________________ Accuracy grade:___________________________________
Short description of system failure and application:______________________________________________________________
__________________________________________________________________________________________________________
__________________________________________________________________________________________________________
__________________________________________________________________________________________________________
Visual Inspection
Label
❒OK
❒Unreadable
❒ Missing
Condition
❒ Like new
❒ Broken welds
❒ Cable cut
❒ Visual mechanical overload
❒ J-box damage
❒ Dents/cracks in parts
Corroded parts
❒Weld(s)
❒Housing/element
❒ J-box/cable entry
❒ Top/bottom plate
❒Diaphragm
❒Bellow/tube/cubs
Affected by chemicals
❒No
❒Unknown
❒ Yes: _______________
Electrical Inspection
Bridge Measurements
Actual
Specification
Zero balance
mV/V
≤±1% of rated span
❒OK
Conclusion
❒Wrong
Input resistance
Ω
Ω±1%
❒OK
❒Wrong
Output resistance
Ω
Ω±1%
❒OK
❒Wrong
Output – to input –
Ω
❒OK
❒Wrong
Out put – to input +
Ω
difference ≤±1%
❒OK
❒Wrong
Insulation Resistance
Actual
Specification
Bridge to housing
MΩ
≥5000 MΩ
❒OK
❒Wrong
Bridge to shield
MΩ
≥5000 MΩ
❒OK
❒Wrong
Shield to housing
MΩ
≥5000 MΩ
❒OK
❒Wrong
Conclusion
❒Moisture ingress
❒ Short circuit
❒ Broken wire/component
❒ Excessive heat
❒Electrical transients
❒ Mechanical overload
❒ Excessive corrosion
❒ Broken cable
❒ Other: ____________________________________________
Recommendation
❒
❒
❒
❒
Return load cell to supplier for further evaluation and repair (if possible)
Return load cell to supplier for warranty
Load cell beyond (economic) repair
________________________________________________________________________________
Document Number: 11867
Revision 14-Dec-2011
For technical support, contact in Americas [email protected],
in Europe [email protected], in China [email protected],
in Taiwan [email protected]
www.vpgtransducers.com
5
A PPL I CAT I O N N OT E
Expected Reason for Failure