Testing a Load Cell with a Multimeter
Troubleshooting a Load Cell
There are a number of mechanical and/or electrical failures that can cause errors in load cell weighing systems. Fortunately, most issues can be found with a few basic tests which are detailed below on how to troubleshoot a load cell. These tests will help you to determine if a load cell repair is needed.
At the bottom of the page, you will find a printable form to record data during testing. You can also submit your data for free analysis. Hit the "continue" button at the bottom of the form, and a Load Cell Central technician will review the data and contact you regarding the results.
Before you begin, make sure you establish the following:
- A stable power source is required. If the power source fluctuates then so will the load cell reading. Preferably 10V but not to exceed 15V.
Some smaller load cells are limited to 6V. Be sure to check the load cell datasheet for the required excitation voltage. - Be sure to use a highly accurate multimeter, having a minimum of 2 decimal places in the millivolt (mV) range. Many shop multimeters do not have a high enough sensitivity to provide viable feedback.
- Remove the load cell from any housing or mounting assembly. This includes eye bolts and shackles. If the hardware cannot be removed, the additional weight must be accounted for in order to accurately assess the zero balance.
Physical Inspection
When evaluating a load cell, the first step is always to perform a physical inspection. Often the problem with the load cell can be detected by a trained eye before any other tests are performed.
What to look for:
- Irregularities in the shape or structural integrity of any part of the cell: Buckling, bending, rippling, or other deformation of the metal body of the cell indicates that the load cell has experienced overloading, shock loading, or side (non-axial) loading. When physical damage prevents a load cell from returning to its zero-load shape, the integrity of the cell has been compromised and the load cell must be replaced.
- Damaged welding: Welds should be free of cracks, deep gouges, or other anomalies. Breaches in the integrity of the weld can allow moisture and dust to enter the load cell.
- Excessive rust: While a small amount of external rust is usually benign, excessive rust on the exterior indicates there could be moisture damage on the interior of the load cell. Exposure to moisture causes harm to a load cell both internally and externally. This problem can usually be avoided by choosing a stainless steel, hermetically sealed load cell for use in wet, moist, or humid environments. Be sure to inspect seal areas closely to see if there is evidence of the seal being breached by rust.
- Excessive corrosion: Exposure to chemicals, whether powder, gas, or liquid, can cause corrosion of the load cell body and electrical components. Chemical corrosion can occur even in stainless steel load cells. If corrosion appears to be more than just superficial, there could be internal damage to the load cell. As with rust, be sure to inspect seal areas to see if there is evidence of the seal being breached by corrosion.
- Cable damage. The load cell cable must be checked along its full length, starting with where it connects to the cell. A damaged or broken cable can cause a multitude of problems. Broken or damaged wires are the obvious problem, but even small cuts or abrasions in the casing can allow moisture to wick up into the load cell causing moisture damage to the internal workings of the cell.
Tap Test
The tap test can be done simply by monitoring the indicator while lightly tapping it with the handle of a screwdriver or a rubber mallet (never use a hammer or other metal object). Drastic jumps in the display would indicate internal damage to the load cell. Make sure all taps are light enough so they do not damage the housing of the load cell.
If the load cell has already been disconnected from the indicator, this test can be done using a multimeter during the zero balance test.
Zero Balance
Zero balance (expressed in percent of rated output) is the measurement of the output signal from the load cell with rated excitation and no load applied.
- If not already done, disconnect the load cell from the indicator.
- Make sure the cell has no load applied and that the excitation leads are connected to a voltage source set at the recommended excitation voltage given on the datasheet. (This excitation voltage can often be supplied by the indicator by leaving only the excitation leads attached)
- Using a multimeter, measure the voltage across the signal leads.
- Note: Load cells are rated in mV/V; excitation and output are in V. Using 10V excitation, a load cell rated at 2mV/V would output 20mV at full scale.
- A zero balance variation greater than the tolerance given on the load cell data sheet (USUALLY 1%) is evidence that the cell has likely been damaged by overloading.
Example: Given the data from above (c) you should see no more than 0.2mV on your meter at zero load.
If an excitation voltage other than 10V is applied, use the following equation:
F x E = O
Where “F” is the rated full-scale output of the load cell in mV/V, “E” is the excitation voltage in Volts, and “O” is the full-scale output in mV. Your zero balance should be no greater than 1% (or other % listed on the datasheet) of “O” in either positive or negative.
- Balance check: Using the ohm (Ω) setting on your multimeter, compare the resistance on each side of the bridge circuit.
- Make sure the load cell is disconnected from power and has no load applied.
- Short the signal leads together by twisting or clipping them.
- Measure and record the resistance between the signal leads and the -Exc lead.
- Measure and record the resistance between the signal leads and the +Exc lead.
- The two readings should usually be within 1Ω of each other. (Results varying by more than 1 Ω almost always indicate a cell that has been improperly loaded)
- Excitation/signal resistance: Using your multimeter (ohm, Ω setting), compare the bridge input resistance and output resistance.
- To obtain bridge input resistance, place your meter between the +Exc and -Exc leads.
- To obtain bridge output resistance, place your meter between the +Sig and -Sig leads.
- Both readings should be within the specs listed on the load cell datasheet.
- Bridge resistance: In the same fashion as above, obtain the following measurements:
This test checks each gauge individually.
- +Exc to +Sig
- –Exc to +Sig
- +Exc to –Sig
- –Exc to –Sig
- In most cases, these readings should be within 1 Ω of each other but older load cells can be an exception.
In order to make an accurate diagnosis, please input your readings into the form below and a Load Cell Central technician will review the data and contact you regarding the results.
What is excitation voltage and does it matter?
The amount of voltage used to excite the Wheatstone Bridge circuit inside a load cell. This voltage is required to produce a linear output signal, which can then be converted into a force measurement.
A higher excitation voltage produces a higher output signal, which is easier to measure and digitize. If the maximum excitation voltage is exceeded, however, the increased current flow produces excessive heat leading to signal disturbance and strain gauge damage.
What is the difference between full-scale output, rated output, and sensitivity?
Rated output is the load cell’s electrical output when it is loaded to its rated capacity load, and full-scale output is generally the range of output values (also called the span) throughout the measuring range of the load cell (that is its output at maximum load minus the output at minimum load).
