Load Cell Troubleshooting

Load Cell Troubleshooting

1. Introduction

Performance of a load cell force (or weigh) measurement system is dependent upon the integrity of the physical installation, correct interconnection of the components, proper performance of the basic components which make up the system, and calibration of the system. Load Cell Troubleshooting begins with checking if the installation was originally operating and was calibrated, load cell troubleshooting can begin by checking the components individually to determine if they have been damaged or have failed.

The basic components are:

Load cells
Mechanical supports and load connections
Interconnecting cables
Junction boxes
Signal conditioning electronics

1.1 Mechanical Installation

Load Cells which are not mounted in accordance with the manufacturer’s recommendations may not perform to manufacturer’s specifications. It is always worthwhile to check:

Mounting surfaces for cleanliness, flatness, and alignment.
Torque of all mounting hardware.
Load cell orientation: “Dead” end on mechanical reference or load forcing source, “live” end connected to the load to be measured. (Dead end is the end closest mechanically to the cable exit or connector.)

Proper hardware (thread sizes, jam nuts, swivels, etc) as required to connect the load to the load cell. A fundamental requirement is that there be one, and only one load path! This load path must be through the load axis of the load cell. This may sound elementary, however it is a commonly overlooked problem.

1.2 Electrical Installation

Proper load cell performance is also dependent upon the electrical “system.” The following items are common problem areas:

Loose or dirty electrical connections, or incorrect connection of color coded wires.
Failure to make use of remote sensing of excitation voltage on long cables.
Incorrect setting of excitation voltage. (The best setting is 10 VDC, because that voltage is used to calibrate the load cell in the factory. The maximum voltage allowed is 15 or 20 volts, depending on the model. Some battery-operated signal conditioners use smaller voltages, down to 1.25 volts, to conserve battery power.)
Loading of the bridge circuit. (Highly accurate load cell systems require highly accurate read-out instruments. Such instruments typically have very high input impedances to avoid circuit loading errors.)

2. Load Cell Evaluations

2.1 Check Bridge Circuitry and Zero Balance

(Numbers apply to standard 350 ohm bridges.)

Instrument required: Ohmmeter with 0.1 ohms resolution in the range of 250-400 ohms.
Bridge Input Resistance: RAD should be 350 ±3.5 ohms. (unless the cell has “standardized output,” in which case the resistance should be less than 390 ohms).
Bridge Output Resistance: RBC should be 350 ±3.5 ohms.
Bridge Leg Resistances: Comparing the leg resistances at no load permits evaluation of the cause of any permanent damage in the load cell flexure. The “computed unbalance” of the bridge shows the general condition of the cell.
The computed unbalance, in units of “mV/V,” is determined as follows: Unbalance = 1.4 • (RAC – RAB + RBD – RCD).
The Zero Offset, in units of “% of Rated Output”, is determined as follows: Zero Offset = 100 • Unbalance ÷ Rated Output.

If the ohmmeter resolution is 0.1 ohm or better, then a computed Zero Offset of greater than 20 percent is a clear indication of overload. A computed zero balance of 10-20% is an indication of probable overload. If the load cell has been overloaded, mechanical damage has been done that is not repairable, because overloading results in permanent deformation within the flexural element and gages, destroying the carefully balanced processing that results in performance to Interface specifications. While it is possible to electrically re-zero a load cell following overload, it is not recommended because this does nothing to restore the affected performance parameters or the degradation to structural integrity.

If the degree of overload is not severe the cell may in some cases be used at the user’s discretion, although some performance parameters may violate specifications and the cyclic life of the load cell may be reduced.

2.2 Insulation Resistance Tests

Insulation resistance, shield to conductors: Connect all the conductors together, and measure the resistance between all those wires and the shield in the cable.
Insulation resistance, load cell flexure to conductors: Connect all the conductors together, and measure the resistance between all those wires and the metal body of the load cell.

The tests described above can be performed using a standard ohm meter, although best results are obtained with a megohm meter. If resistance is beyond the standard ohmmeter range, about 10 megohms, the cell is probably OK. However, some kinds of electrical shorts show up only when using a megohm meter or with voltages higher than most ohmmeters can supply.

CAUTION:

Never use a voltage higher than 50 VDC or 35 VRMS AC to measure insulation resistance, or breakdown of the insulation between the gages and the flexure may result. Low resistance (below 5000 megohms) is often caused by moisture or pinched wires. The cause and extent of damage must be established at the factory to determine if the load cell may be salvaged.

3. Factory Evaluation

If the load cell is defective for reasons other than overload, return to factory for detailed evaluation. Factory evaluation may show that the cell is repairable or non-repairable and if repair or replacement will be under warranty. If non-warranty, the customer will be contacted with the cost of repairs and recalibration, and a delivery date after receipt of authorization to proceed.

System Recalibrations

internally Amplified Load Cells

Dependent on option code. Standard internally amplified load cells such as a low profile load cell receive a standard calibration with applied load vs. voltage calibration points. ISO/IEC 17025:2017 accredited calibrations are available for voltage outputs. Current (mA) outputs are not within our ISO/IEC 17025:2017 accredited scope.

load Cell W/ External Amplifier

A load cell/amplifier system level calibration can be accomplished in one of two ways:

The first option is a system calibration using applied loads, this entails pairing the load cell and amplifier and performing the calibration under applied loads in the test machine. This method of system calibration (or ‘in-rig’ calibration) provides a single traceable calibration certificate for the load cell and amplifier combination. Calibration points are expressed as applied load points versus VDC/mA output. ISO/IEC 17025:2017 accredited calibrations are available for voltage outputs. Current (mA) outputs are not within our ISO17025 accredited scope.
The second option (referred to commonly as the standard Interface SYS-CAL1) is to calibrate/span the amplifier with a NIST traceable precision mV/V transfer standard using the load cells mV/V calibration data. As the amplifier is typically spanned to the capacity of the sensor, standard practice is to use the SEB output of the load cell to span the amplifier. If applicable, a shunt calibration is then be performed with the load cell and amplifier connected and the corresponding output recorded. This calibration method provides two separate calibration certificates, one for the load cell with mV/V calibration data and one for the load cell and amplifier with the calculated output and shunt calibration (RCAL) data. ISO/IEC 17025:2017 accredited calibrations are available for the load cell but are not available for the indicator/amplifier certificate.

load Cell W/ Indicator

A load cell and indicator system level calibration can be accomplished in one of two ways:

) System calibration using applied loads: This entails pairing the load cell and indicator and performing the calibration under applied loads in the test machine. This method of system calibration (or ‘inrig’ calibration) provides a single traceable calibration certificate for the load cell and indicator combination. Calibration points are expressed as applied load points versus indicated reading. ISO/IEC 17025:2017 accredited calibrations are available.
The second option (referred to commonly as the standard Interface SYS-CAL1) is to calibrate the indicator with a NIST traceable precision mV/V transfer standard using the load cell mV/V calibration data. As the indicator is typically calibrated to the capacity of the sensor, standard practice is to use the SEB output of the load cell to calibrate the indicator.

teds Load Cells W/ Indicator

Standard load cells with TEDS chips installed have the standard mV/V calibration certificate for its respective model as well as a TEDS data sheet to report what data is stored on the TEDS chip per IEEE 1451.4, template 33. To provide system level traceability, any indicator to be paired with a TEDS load cell should have a traceable internal mV/V calibration performed. ISO/IEC 17025:2017 accredited calibrations are available for the both the internal mV/V indicator calibration and the load cell calibration. An in-rig system calibration using applied loads can also be performed to verify and record system performance.

Internally Amplified Load Cells

internally Amplified Load Cell

Dependent on option code. Standard internally amplified load cells such as a low profile load cell receive a standard calibration with applied load vs. voltage calibration points. If the amplifier span or zero does not require adjustment, a combined “As-Found and Final” calibration certificate is provided. If the amplifier is adjusted, separate “As-Found” and “Final” certificates are provided. The “As-Found” certificate reports the calibration results before adjustment, the “Final” certificate reports the condition after final adjustment. ISO/IEC 17025:2017 accredited calibrations are available for voltage outputs. Current (mA) outputs are not within our ISO17025 accredited scope.

load Cell W/ External Amplifier

A load cell/amplifier system level recalibration is most commonly performed using method (A) described below. Method (B) can also be used, but would not document the as-found condition of the system.

The first option is a system recalibration using applied loads, this entails pairing the load cell and amplifier and performing the calibration under applied loads in the test machine. This method of system calibration (or ‘in-rig’ calibration) provides a single traceable calibration certificate for the load cell and amplifier combination. Calibration points are expressed as applied load points versus VDC/mA output. If the amplifier span or zero does not require adjustment, a combined “As-Found and Final” calibration certificate is provided. If the amplifier is adjusted, separate “As-Found” and “Final” certificates are provided. The “As-Found” certificate reports the calibration results before adjustment, the “Final” certificate reports the condition after final adjustment. ISO/IEC 17025:2017 accredited calibrations are available for voltage outputs. Current (mA) outputs are not within our ISO/IEC 17025:2017 accredited scope.
The second option (referred to commonly as the standard Interface SYS-CAL1) is be to calibrate/span the amplifier with a traceable precision mV/V transfer standard. This option requires the load cell to first be calibrated independently of the amplifier in mV/V. This calibration data is then used to calculate the correct scaling and the mV/V transfer standard is used to calibrate/span the amplifier. As the amplifier is typically spanned to the capacity of the sensor, standard practice is to use the SEB output of the load cell to span the amplifier. If applicable, the shunt calibration is then be performed with the load cell and amplifier connected. This calibration method provides two separate calibration certificates, one for the load cell with mV/V calibration data and one for the amplifier with the calculated output and shunt calibration (RCAL) data. ISO/IEC 17025:2017 accredited calibrations are available for the load cell but are not available for the indicator/amplifier certificate.

load Cell W/ Indicator

A load cell/Indicator system level recalibration is most commonly performed using method (A) described below. Method (B) can also be used, but the as-found condition of the system would not be documented. Load cells used with the Interface Gold Standard System or with the Interface 9840 are addressed in method (C).

The first option is a system calibration using applied loads, this entails pairing the load cell and indicator and performing the calibration under applied loads in the test machine. This method of system calibration (or ‘in-rig’ calibration) provides a single traceable calibration certificate for the load cell and indicator combination. Calibration points are expressed as applied load points versus indicated reading. If the indicator does not require adjustment, a combined “As-Found and Final” calibration certificate is provided. If the indicator is adjusted, separate “As-Found” and “Final” certificates are provided. The “As-Found” certificate reports the calibration results before adjustment, the “Final” certificate reports the condition after final adjustment. ISO/IEC 17025:2017 accredited calibration is available
The second option (referred to commonly as the standard Interface SYS-CAL1) is to calibrate the indicator with a traceable precision mV/V transfer standard. This option requires the load cell to first be calibrated independently of the indicator in mV/V. This calibration data is then used to calculate the correct scaling and the mV/V transfer standard is used to calibrate/span the indicator. As the indicator is typically calibrated to the capacity of the sensor, standard practice is to use the SEB output of the load cell to calibrate the indicator. If applicable, the shunt calibration is then performed with the load cell and amplifier connected. This calibration method provides two separate calibration certificates, one for the load cell with mV/V calibration data and one for the indicator with the calculated indicated reading and shunt calibration (RCAL) data. ISO/IEC 17025:2017 accredited calibration is available for the load cell but is not be available for the indicator/amplifier certificate.
Gold Standard and 9840- multi load cell indicators

teds Load Cells

Standard load cells with TEDS chips installed have the standard mV/V “As-Found and Final” calibration certificate for its respective model as well as a TEDS data sheet to report what data was stored on the TEDS chip per IEEE 1451.4, template 33. To provide system level traceability, any indicator to be paired with a TEDS load cell(s) should have a NIST traceable internal mV/V calibration performed. ISO/IEC 17025:2017 accredited calibrations are available for the internal mV/V Interface indicator calibrations.

teds Load Cells W/ Indicator

System level recalibration of TEDS enabled load cell with a compatible indicator has the below steps performed:

System level “As-Found” calibration performed with the load cell paired to the indicator, calibration performed using applied loads and reported as applied load vs. indicated reading. Dependent on customer requirements, if no adjustment is required or performed an “As-Found and Final” may be issued with no further steps (not Interface recommended).
Standard mV/V calibration performed per the load cell model, TEDS chip reprogrammed with updated calibration data. “As-Found and Final” mV/V calibration certificate issued for the load cell as well as a TEDS data sheet to report what data is stored on the TEDS chip per IEEE 1451.4, template 33. ISO/IEC 17025:2017 accredited calibrations are available.
NIST traceable internal mV/V calibration of indicator performed. If no adjustment is required or performed an “As-Found and Final” certificate is issued. If adjustments are performed separate “As-Found” and “Final” certificates are issued to report the as-received condition and the condition after adjustment. ISO/IEC 17025:2017 accredited calibrations are available for the internal mV/V indicator calibration.
System level “Final” calibration performed with the load cell paired to the indicator with the updated TEDS programming and indicator internal mV/V calibration. System calibration performed using applied loads and reported as applied load vs. indicated reading. ISO/IEC 17025:2017 accredited calibration is available.

shunt Calibration Data

Where applicable, shunt calibration readings or outputs are recorded as part of the load cell or load cell and amplifier system calibration. The shunt calibration data provides a means for the user in the field to verify system scaling, and as many instruments can be scaled using this data it provides a means to temporarily restore a calibration in the field.

System level shunt calibration: Readings or outputs for a load cell and indicator or amplifier system are most commonly taken at the conclusion of the calibration. The load cell is placed into an unloaded condition and the RCAL (alternatively referred to as a shunt cal) is activated, the indicated reading (indicator) or output voltage/current indicator is then recorded.
Load cell shunt calibration: Shunt outputs for a load cell are taken at the conclusion of the calibration with the load cell in an unloaded condition, if applicable readings are taken for both tension and compression. As seen in the below example, the applicable connections are also noted. A straight line conversion is also provided, this is the calculated force equivalent to the shunt output and is calculated using the SEB or best fit straight line output.

shunt Calibration

	Shunt (+/- 0.01%)	Output	Straight Line Conversion	Connections*
Tension Compression	60.0 KOhm 60.0 KOhm	1.45393 mV/V – 1.45464 mV/V	1817.7 N 1818.1 N	– Out to – Exc – Out to + Exc
*For models wired with + Sense, – Sense or – SCal leads, resistor connections are actually to these leads in place of + Exc, – Exc, or – Out respectively.

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