Guide to Determining a Faulty Load Cell - Weigh Scales

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For Scales and Industrial Weighing Equipment.

What you will need:

A toolkit (Screwdrivers, wire cutters etc) a multimeter that provides millivolt readings, a 9 volt battery (rectangular shape), a pen and paper to record the readings.

Although there are further tests that be carried out to determine that a fault exists with a load cell by taking readings in ohms (resistance) or by using a load cell tester, these can be quite in-depth and the cost of the equipment, quite expensive. Therefore by using this simple test, you should be able to determine if a load cell has a fault.

Scenario:

On most scale equipment, your weighing system will be made up of 3 - 4 load cells, the cabling, a junction box and a weight indicator. Although any of these could be the cause of the issues with your equipment, by trial and elimination and for this guide, we shall say that all the terminal connections are satisfactory and 1 of the load cells is thought to be the cause, the load cell  cables have been visually checked for cuts or crushing and no damage was found.

How the load cells work:

Load cells will be connected and powered from the weight indicator either by being directly wired or if multiple load cells are used, then more commonly connected into a junction box. The load cell then receives it’s voltage through the Excitation + (Positive) and Excitation - (Negative) wires.

When weight is placed onto the loading point of the load cell, it will send a signal (Millivoltage) back to the indicator through the Signal + (Positive) and Signal - (Negative) wires. The more pressure applied, the more millivoltage will be shown.

Because the load cells are interconnected and we are unsure as to which is at fault, we need to ensure that the load cell being tested cannot pick up any interference from the other load cells and with this in mind, we shall begin the test.

Tip:

Before disconnecting the wires of the load cells, always make a diagram of the wiring and connections or using a camera phone or camera, take a photo.

Although not always possible due to large scale weighing systems, try to ensure that all weight is removed from the load cells. For example, with a platform scale, you could turn the platform scale upside down so the load cells are off the floor and therefore, no weight is being applied.

Carrying out the test:

The first thing to do is to identify the load cells colour coding. This can be done by looking on the load cell itself where there is sometimes a label with the markings, S+ (Signal Positive),- S- (Signal Negative), EX+ (Excitation Positive), EX- (Excitation Negative). There may be sense wires also marked SE+ (Sense Positive) for example, however for this test, we only need the excitation and signal wires.

If no markings can be found then look for markings on the load cell cable or failing that then use the markings at the junction box / weight indicator terminal block.

With this done, make a note of the coloured wire and it’s reference,. Example: Green = Excitation + (Positive power supply)

With the first load cell chosen to be tested, disconnect all the load cell wires from the terminal block.
Connect the EX+ (Excitation Positive)  wire of the load cell to the positive terminal on the battery and the EX- (Excitation Negative) wire of the load cell to the negative terminal on the battery. With the load cell now powered, take your multimeter (Set to Millivolts) and connect the red (Positive Lead) of your meter to the S+ (Signal Positive) load cell wire and the black (Negative Lead) of your multimeter to the S- (Signal Negative) wire of the load cell.

With this done, you are looking for a reading of zero millivolts with no weight applied or if the load cell has a large weight applied (For example is under load from carrying a large vessel) then you will be looking for maybe a very slight reading, maybe around 1 millivolt at the most.

A faulty load cell will usually give a reading of much higher and if this is the case, then you will have identified the problem.

If you are dealing with a weighing system of more than one load cell installed, then make a note of the readings and leaving the tested load cell disconnected, proceed to test the others using the same procedure, making notes of the readings as you carry the tests out.

With all the load cells tested and from your readings, you will find that one load cell gives substantially higher readings from the others. This load cell with be the cause of your problems and you are now on the way to getting your weighing equipment back into a working condition.

Closing note:

Although this is a guide into load cell testing,  if you are uncertain of the results or before replacing parts, always seek advice or have the tests carried out by an experienced scale technician and if we can be of help in any way, then please get in contact.

We hope that this guide has given you some insight and that you have enjoyed reading it. We also welcome any feedback.

If you would like any other guides to be written on scales and weighing equipment then please send us a message via our seller contact information.



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