Troubleshooting Transducers - A Guide

This article shares a few easy to follow steps to help one troubleshoot potential load cell issues. To begin with, a good quality digital multimeter – at least a 4 1/2-digit ohm meter – will be needed. The tests are: zero balance, physical inspection, bridge resistance, and resistance to ground.

STEP 2: Zero Balance

This test is effective in establishing if the load cell has undergone a physical distortion, possibly due to shock load, overload or metal fatigue. Before starting the test, the load cell must be in a “no load” condition. That is, the cell should be removed from the scale or the deadload should be counter balanced.

Now that the cell is not under any load, the signal leads can be disconnected, and the voltage across the +signal and-signal can be measured. The color code for establishing – and + signal leads is provided on the calibration data sheet. The output should be within the manufacturer’s specifications for zero balance, typically + 1% of full scale output. During the test, the excitation leads should stay connected with the excitation voltage supplied by the digital weight indicator. Be certain to use the exact same indicator that is employed in the cell’s regular operation to obtain a reading accurate to the application.

The typical value for 1% shift in zero balance is 0.3 mV, assuming 10 volts excitation on a 3 mV/V output load cell. To establish an application’s zero shift, users should multiply the excitation volts supplied by their indicator by the mV/V rating of a load cell. When performing the field test, remember that load cells can shift up to 10% of full scale and still function properly. If the test cell displays a shift under 10%, then there may be another problem with the suspect cell and additional testing is required. If the test cell displays a shift more than 10%, it has perhaps been physically distorted and should be changed.

Zero Balance

STEP 3: Bridge Resistance

Prior to testing bridge resistance, disconnect the load cell from the digital weight indicator. Find the + and – excitation leads and measure across them with a multimeter to locate the input resistance. Do not be startled if the reading surpasses the rated output for load cell. It is not uncommon for readings as high as 375 Ω for a 350 ohm load cell. The difference is caused by compensating resistors built into the input lines to balance out alterations caused by temperature or manufacturing imperfections. However, if the multimeter displays an input resistance greater than 110% of the stated output value (385 ohms for a 350 ohm cell or 770 Ω for a 700 ohm call), the cell may have been impaired and should be tested further.**

If the excitation resistance check is within specifications, test the output resistance across the + and -signal leads. This is a more dedicated reading and one should attain 350 ohms + 1% (350 ohm cell). Readings outside the 1% tolerance typically point to a damaged cell. Here is the tricky part. Even if the total output resistance test was within regular specifications, one could still have a damaged load cell. Frequently when a load cell is damaged by shock load or overload, opposite pairs of resistors will be deformed by the stress–equally, but in opposite directions. The only way to establish this is to test watch each leg of the bridge. The Wheatstone Bridge diagram illustrates a load cell resistance bridge and illustrates the test procedure and results of a sample cell damaged in such a manner. The legs which are in tension under load can be called T1 and T2, and the legs under compression C1 and C2.

With the multimeter, each leg was tested and the following readings were obtained:

  • T1 (-Sig, +Exc) = 282 Ω
  • C1 (-Sig, +Exc) = 278 Ω
  • T2 (-Sig, +Exc) = 282 Ω
  • C2 (-Sig, +Exc) = 278 Ω

Bridge Resistance

Note, when testing leg resistance, a reading of 0 W or ¥ means a broken wire or loose connection inside the cell. In a good load cell in a “no load” condition, all legs need not have precisely equal resistance, but the following relationships must be maintained:

  1. C1 = T2
  2. T1 = C2
  3. (C1 + T1) = (C2 + T2)

In this damaged load cell, both tension legs read 4 W higher than their corresponding compression legs. The equal damage imitates a balanced bridge in the output resistance test (3 above), but the individual leg test (1,2 above) indicate that the cell should be swapped.

** NOTE: On numerous cell applications for matched millivolt output, excitation resistance values may be greater than 110%.

STEP 4: Resistance to Ground

If the loaded cell has cleared all tests thus far, but is still not performing to specification, look for electrical leakage or shorts. Leakage is virtually always caused by water contamination within the load cell or cable. Electrical shorting caused by water is typically first detected in an indicator readout that is always unstable, as if the scale is continuously “in motion”.

The wrong cell in the wrong place is the main cause of water contamination. Almost always these leaking cells are “environmentally-protected” models built for regular non-washdown, not the “hermetically sealed” models which would have stood up to wash down and other difficult applications. Another cause is broken or loose solder connections. Loose or broken solder connections deliver an unstable readout only when the cell is bumped or moves enough so the loose wire contacts the load cell body. The reading is stable when the loaded scale is at rest.

To truly nail down electrical leakage issues though, users should test resistance to ground with a low voltage megohmmeter. Precaution should be taken because a high-voltage meter, which puts over 50 VDC into the cell, may damage the strain gauges. Twist all four leads together and test between them and load cell metal body if the shield is tied to the case. However, if the shield is not tied to the case, then twist all four leads and the shield wire together and test between them and the body.

If the result is not more than 5000 MW, it means current is leaking to the body at some place. If the cell fails this test, remove the shield wire and test with only the live leads to the metal body. If this tests correctly (more than 5000 MW), one can be reasonably sure that current is not leading through a break in the cable insulation or within the strain gage cavity.

Minor water infiltration issues can sometimes be solved outside the factor. If one is sure that water contamination has happened, and if one is sure that the cable entrance seal is the entry point, the following solutions can be tried:

  • Remove the cell to a warm, dry location for some days allowing the strain gage potting to dry
  • Prior to installing the cell back into service, seal with silicone around the cable entry point in the load cell body. This prevents the reentry of water vapor into the cell.

STEP 3: Bridge Resistance

Note, when testing leg resistance, a reading of 0 W or ¥ means a loose connection or broken wire inside the cell. In a good load cell in a “no load” condition, all legs do not have precisely equal resistance, but the following relationships should hold true:

  1. C1 = T2
  2. T1 = C2
  3. (C1 + T1) = (C2 + T2)

In this damaged load cell, both tension legs read 4 W higher than their corresponding compression legs. The equal damage imitates a balanced bridge in the output resistance test (3 above), but the individual leg test (1, 2 above) display that the cell must be swapped.

** NOTE: On numerous cell applications for matched millivolt output, excitation resistance values may be greater than 110%.

Users should attain 350 ohms + 1% (350 ohm cell). Readings outside the 1% tolerance typically indicate a damaged cell. Here is the tricky part. Even if the total output resistance test was within regular specifications, one could still have a damaged load cell. Usually, when a load cell is damaged by shock load or overload, opposite pairs of resistors will be deformed by the stress–equally, but in opposite directions. The only way to establish this is to test watch each leg of the bridge. The Wheatstone Bridge diagram illustrates a load cell resistance bridge and illustrates the test procedure and results of a sample cell damaged in such a manner. The legs which are in tension under load can be called T1 and T2, and the legs under compression C1 and C2.

With the multimeter, each leg was tested and the following readings were obtained:

  • T1 (-Sig, +Exc) = 282 Ω
  • C1 (-Sig, +Exc) = 278 Ω
  • T2 (-Sig, +Exc) = 282 Ω
  • C2 (-Sig, +Exc) = 278 Ω

HITEC Sensor Developments, Inc

This information has been sourced, reviewed and adapted from materials provided by HITEC Sensor Developments, Inc.

For more information on this source, please visit HITEC Sensor Developments, Inc.

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