Scale Trouble Shooting (3)

B. Scale not Weighing

This section describes what to do if the scale is not weighing at all, or failed the cornering or Gross Calibration test. In this case there is something wrong with the scale either mechanically or electrically, and the objective is to identify it and correct it.

A. Inspect the platform/hopper for mechanical interference. The scale cannot touch any mechanical structure other than through the load cells. If the scale is of a nature that control cables or hoses attach to the scale, make sure that they are flexible and have loops in them to minimize the forces they can apply to a scale. Note that an air hose can exert several pounds of force on a scale, and fixed conduit can exert hundreds of pounds if run vertically to the scale.
B. Check all load cells to make sure that their mounting arrangement is correct, and that the weight is transmitted through the load cells without interference. Common problems are interference from self jacking screws, load pins, or suspending rod.
C. On the back of the indicator, locate the connector for the load cell input, and have ready a Volt-Ohm meter for the tests to follow.

1. Combined Load Cell Resistance Test
The following test will check the overall wiring and load cell configuration to determine if the cabling or individual load cells need to be tested. This is a passive resistance test of the parallel combination of the load cells.
A. Unplug the load cell connector from the back of the indicator, and perform the following measurements on this connector while it is disconnected.
B. Measure the resistance between the +E and –E terminals of the connector. This should be the
specified excitation resistance divided by the number of load cells. For a four load cell system with 400 ohm cells, you should see 100 ohms +/- 3 Ohms. If it is not, then proceed to the individual load cell resistance test in section 4.2.3. Note that if you don’t measure any resistance at all (open), then you probably have a problem with the cable between the indicator and the Junction box.
C. Measure the resistance between the + SIG and –SIG terminals of the load cell connector. This
should be the specified signal resistance divided by the number of load cells. For a four load cell system with 350 ohm cells you should see 88 Ohms +/- 2 Ohms. If it is not then proceed to the individual load cell resistance test in section 4.2.3. Note that if you don’t measure any resistance at all (open), then you probably have a problem with the cable between the Indicator and the Junction box.
D. Measure the resistance between the shield and each of the individual wires on the load cell connector. You should measure over 1 Meg Ohm of resistance. If there is any substantial continuity between the shield and a load cell wire then you have a problem either with the cable between the indicator and the Junction Box, or with an individual load cell. You can test each individual load cell at the junction box (section 4.2.3) to determine which. This must be corrected or the scale will not weigh properly.

B. Combined Load Cell Signal Test
This test will measure the power to the load cells as well as their output to determine if there is problem that may be attributable to an individual cell.
A. Plug the load cell connector back into the indicator, and have a volt meter that can measure in
both Volts and Millivolts.
B. Set the meter to a DC range that can handle 20volts.
C. Measure between the +E and –E terminals on the load cell connector while it is plugged into the indicator. You should read the excitation voltage specified in the indicator manual within 1VDC.

D. If the Excitation does not read as specified, then unplug the load cell connector from the indicator and measure it again on the indicator. If it still is out of spec then there is a problem with the indicator. If it reads correctly with out the load cells plugged in, but incorrectly with them plugged in, then you may have a cable or load cell problem (however this should have shown up in section 4.2.1). Inspect all cables for shorts, and try plugging the load cell connector into another indicator to see if you get similar results. This will determine if it is the Indicator or a Load Cell problem
E. If you believe it is a load cell problem, you can disconnect the load cells one at a time in the junction box to isolate the problem (make sure you disconnect both the signal and excitation connectors).
F. Calculate what the full load output of the load cells should be. You can do this by multiplying the excitation voltage output of the indicator times the specified mv/V output of the load cells. As an example, if the indicator had a 15vdc excitation, and the load cells were 2mv/V, then the full scale output would be 30mv.
G. With the load cell connector plugged into the indicator, and the voltmeter set to a scale that can read millivolts, measure between the +SIG and –SIG of the load cell connector.
H. As a rule of thumb This should be approximately equal to the dead load of the scale, divided by the total load cell capacity, times the full scale output of the cells (as calculated in Step F).
If it is outside of this range then there is a possibility of either a bad load cell or a mechanical problem with the scale. Note that it is possible to have a load cell with a large zero offset that can cause this value to be outside this range, but still weigh correctly (see next step), but it is a symptom of something that needs to be looked at. If the signal is significantly outside of this range (either negative, or over 20mv then there is most likely a problem than needs to be corrected. The actual change in this signal in proportion to weight is the most important measurement. The steps below will help determine this.
I. Empty the scale and measure between +SIG and –SIG and record the millivolt reading.
J. Arrange to put the test weights on the scale and record the millivolt reading again between the +SIG and –SIG pins on the back of the indicator.
K. The signal reading should change ( subtract the reading in I from the reading in J) by the amount calculated as follows: Divide the amount of test weights applied by the total load cell capacity in the scale, and multiply by the full scale output calculated in F. Note that voltmeters are not nearly as accurate as the indicator, and this is only a rough reading to see if the scale is basically doing what it should. If this reading is within 10% of what it should be it is ok. If it is not then either there is a bad load cell or a mechanical problem with the scale.
See section D for individual load cell tests.

C. Individual Load Cell Resistance Tests
This section describes how to test the resistance of each individual load cell, which can help identify a bad load cell or wiring problem. This test should be done if the combined resistance test in section A was out of spec, or if there are other reasons to suspect a bad load cell.
A. Locate the load cell junction box. All load cells terminate here, and a single cable leaves this box and goes to the weight indicator.
B. Inside this junction box is normally a circuit board with terminal strips that should be clearly labeled as to which load cell connects to which, and which terminals are signal and excitation.
C. Have read a voltmeter set to read ohms.
D. Disconnect the 4 wires coming from load cell 1 from the terminal strip in the junction box, and identify the signal and excitation leads (refer to your load cell specification sheet for the color codes as they can be different depending on the manufacture)r.
E. On the removed excitation wires, measure the resistance between them. It should be 390 Ohms +/- 10 ohms for a typical load cell (but check your load cell specification sheet to verify).
F. Measure the resistance on the removed signal wires. It should be 352 Ohms +/- 2 ohms for a
typical load cell, however verify as above.

G. Measure the resistance between each of the four wires, and the shield. It should be greater than 1 meg ohm.
H. Repeat steps D through H for all load cells.
I. If any of the resistances are outside the specified range, then the cabling or load cell may be bad.

D. Individual Load Cell Signal Test
This test will help determine if one of the load cells is outputting an incorrect signal. Because the loading of the load cells is rarely symmetrical (one cell may support more of the load than the others), the test may not provide conclusive results. This test should be performed if the combined load cell signal test in section B was out of range, or a bad cell is suspected for other reasons. This test will also help point out mechanical problems with the scale.
A. Locate the load cell junction box. Make sure all wires for the load cells and the cable to the indicator are connected to the terminal strips with the EXCEPTION of the signal wires from each load cell (those need to be disconnected from the junction box terminals).
B. Have a Volt meter ready that can read millivolts, and the test weights used in section B (C). With the scale empty, record the millivolt reading on each of the load cells signal leads. Each load cell should read about the same as what was measured in step B (G) (with all load cells hooked together). A variation of more than 25% between the reading in B (G) and any of the load cell readings is cause for suspicion, and may warrant a check of the load cell or mechanical structure.
D. Apply the test weights to the scale in a fashion that they are either in the center of the scale, or evenly spaced around the scale so that the load on each corner is similar.
E. Record the millivolt reading on each of the load cell signal leads
F. Calculate the change in millivolts on each cell from step C to step E. For each load cell this reading should be about the same as the change calculated in B (K) (with all load cells hooked together). A variation of more than 25% between each load cell’s change and that measured in B(K) may indicate a problem with that load cell or the structure.
G. When the change in output voltage from the cells was measured between empty and full you will see some cells change differently than others. If there is a wide variation between the cells, and the average is incorrect, then it is possible that there is either a mechanical bind or a bad cell. Usually the cell with the low output change is the one with a problem.
H. You can also do a quick bench test of a load cell that is not in service by temporarily hooking it directly to an indicator, or alternately hooking the excitation leads to a 15vdc supply. With no load on the cell it’s signal output should be between +/- 1mv. Any reading substantially outside this range would indicate that the cell has been damaged. Note that you can also do the resistance tests on the cell as described above. Note that these tests are not conclusive, as the cell can pass these and still be bad. The only sure test is put the cell under a range of known loads and check it’s output.

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