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Calibration, Operation, Diagnostics, Load Cell Information
Basic Calibration
he Zero, Span, and Excitation potentiometers are used to cali-
T
brate the output. This calibration procedure does not account
for offsets or tare weights. If your system has an offset, tare
weight or deadweight, refer to the Offset Switch procedure.
Note: Perform the following calibration procedure any time
switch settings are changed.
Input and output ranges, if specified on your order, are factory
pre-configured (at 24°C ±1°C).
To achieve optimum results, the system should be calibrated
using an accurate bridge simulator, pressure calibrator, or
calibration weights depending on the application.
1. Apply power to the module and allow a minimum 20 minute
system warm up time.
2. Using an accurate voltmeter across terminals 10 and 12,
adjust the Excitation voltage potentiometer on front of the
APD 4058 for the exact voltage desired.
3. With the input set at zero or the minimum, adjust the front
Zero pot for a zero or low-end output (for example, 4 mA for
a 4-20 mA output or –10 V with a ±10V output).
4. The zero pot may also be adjusted for a zero reading on
the output display instrumentation, e.g. control system or
process indicator. Adjusting the zero pot this way eliminates
calibration errors in the display instrumentation.
5. Set the input at maximum, and then adjust the Span pot for
the exact maximum output desired. The Span control should
only be adjusted when the input signal is at its maximum.
Push-Pull Calibration
Use this calibration procedure for tension-compression appli-
cations and you will use a precision resistor for calibration.
Note: Perform the following calibration procedure any time
switch settings are changed.
1. Apply power to the module and allow a minimum 20 minute
system warm up time.
2. Using an accurate voltmeter across terminals 10 and 12,
adjust the Excitation voltage potentiometer to the exact volt-
age desired.
3. Connect the precision resistor between Exc+ and Sig+.
This will simulate the cell under tension and apply nega-
tive voltage to the input.
4. Adjust the Zero control to –80% output since the resistor
is scaled for 80% of defection.
5. Remove the precision resistor.
6. Connect the precision resistor between Exc– and Sig–.
This will simulate the cell under compression and apply a
positive voltage to the input.
7. Adjust the Span control for +80% output since the resistor
is scaled for 80% of defection.
8. Remove the precision resistor. The output should be near
0 V. It is possible for zero to be off a small amount due
to stacking of tolerances within both the load cell, wiring,
and the module.
Diagnostic Voltage Measurements
Using a meter with at least 10 megaohm input impedance,
measure the voltage coming from the strain gauge at the loca-
tions shown. Sensitivity is measured in mV/V.
A
A
P
P
BSOLUTE
BSOLUTE
ROCESS
ROCESS
Output Test Function
Models with the M01 or the M02 option do not have a Test
function. With either of these options the Test Cal. potentiom-
eter is non-functional.
When the Test button is depressed it will drive the output with a
known good signal that can be used as a diagnostic aid during
initial start-up or troubleshooting. When released, the output
will return to normal.
The Test Cal. potentiometer can be used to set the test output
to the desired level. It is factory set to approximately 50% out-
put. It is adjustable from 0 to 100% of the output span. Press
and hold the Test button and adjust the Test Cal. potentiometer
for the desired output level.
Calibration with Resistor Options M01 or M02
Use this calibration procedure if your APD 4058 was ordered
with a calibration resistor or if your sensor has its own internal
calibration resistor.
Note: Perform the following calibration procedure any time
switch settings are changed.
The M01 option uses a resistor installed internally in the APD
4058. The resistance is specified by the transducer manu-
facturer.
The M02 option is specified when the transducer incorporates
an internal calibration resistor. The transducer must be con-
nected per the manufacturer's specifications.
The sensor manufacturer should provide the percentage of
full-scale output for the transducer when using a calibration
resistor. This is often 80% of maximum output.
1. Apply power to the module and allow a minimum 20 minute
system warm up time.
2. Using an accurate voltmeter across terminals 10 and 12,
adjust the Excitation voltage potentiometer on front of the
APD 4058 for the exact voltage desired.
3. With the input set at zero or the minimum, adjust the Zero
potentiometer on front of the APD 4058 for a zero or low-
end output (for example, 4 mA for a 4-20 mA output).
4. The zero pot may also be adjusted for a zero reading on
the output display instrumentation, e.g. control system or
process indicator. Adjusting the zero pot this way eliminates
calibration errors in the display instrumentation.
5. Set the APD 4058 Test toggle switch to the Test position.
The calibration resistor is switched into the circuit to unbal-
ance the bridge.
6. Adjust the span pot to the for the % output specified by the
transducer manufacturer. This is often 80% of maximum
output.
7. Return the Test switch to the opposite position and readjust
the zero pot if necessary.
Positive
Negative
Meter Lead
Meter Lead
+ Exc.
– Exc.
+ Sig.
– Exc.
– Sig.
– Exc.
+ Sig.
– Sig.
I
I
NSTRUMENTS
NSTRUMENTS
Operation
Strain gauges and load cells are normally passive devices
that are commonly referred to as "bridges" due to their
four-resistor Wheatstone bridge configuration. These sensors
require a precise excitation source to produce an output that is
directly proportional to the load, pressure, etc. that is applied
to the sensor.
The exact output of the sensor (measured in millivolts) is deter-
mined by the sensitivity of the sensor (mV/V) and the excitation
voltage applied.
An additional input, the sense lead, monitors the voltage drop
in the sensor leads and automatically compensates the excita-
tion voltage at the module in order to maintain a constant
excitation voltage at the sensor.
The APD 4058 provides the excitation voltage to the sensors
and receives the resulting millivolt signal in return. This input
signal is filtered and amplified, then offset, if required, and
passed to the output stage. Depending on the output configu-
ration selected, a DC voltage or current output is generated.
GREEN LoopTracker
tion that a signal is being sensed by the input circuitry of the
module. It also indicates the input signal level by changing in
intensity as the process changes from minimum to maximum.
If the LED fails to illuminate, or fails to change in intensity as
the process changes, this may indicate a problem with module
power or signal input wiring.
RED LoopTracker Output LED – Provides a visual indication
that the output signal is functioning. It becomes brighter as the
input and the corresponding output change from minimum to
maximum. For current outputs, the RED LED will only light if
the output loop current path is complete. For either current or
voltage outputs, failure to illuminate or a failure to change in
intensity as the process changes may indicate a problem with
the module power or signal output wiring.
Meter Reading
No pressure/load
Excitation Voltage
Excitation Voltage
+ ½ Excitation Voltage
½ Excitation Voltage + (½ x Excitation Voltage x Sensitivity)
+ ½ Excitation Voltage
½ Excitation Voltage – (½ x Excitation Voltage x Sensitivity)
Zero Volts
Excitation Voltage x Sensitivity
1220 American Way Libertyville, IL 60048
Phone:
800-942-0315
Fax: 800-949-7502
APD 4058
®
Input LED – Provides a visual indica-
Meter Reading
Full pressure/load
api-usa.com
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