SERVICE TIP

Load Cell Weighing System

INTRODUCTION

A load cell is a device that outputs an electrical signal which is directly proportional to the force that is applied to it. Load cells are used extensively in electronic weighing applications.

TROUBLESHOOTING

Load cells fail in a variety of ways for a variety of reasons. These reasons may be mechanical, environmental, or electrical. We will discuss these reasons and make physical and electrical load cell inspections. Most load cells failures are caused by incorrect applications or abuse.

Mechanical Failure

The load cell may fail mechanically or physically. If the cell is too small for the application, the excessive weight will cause the cell to distort and not return to its "no-load" shape, thus keeping the strain gauges either in compression or tension. The total weight of the weigh structure (platform, hopper, vessel) plus the weight of the material being weighed must be considered. The number of structural support points also plays a role in load cell weight distribution. Normally the total weight of the structure is divided equally between all the load cells.

Shock loading also can cause mechanical failure. Shock loading occurs when the weight is dropped suddenly onto the scale, which can cause permanent distortion of the load cell. Observe the operators when they are loading the scale. If they are shock loading the scale, the operators require training on proper scale operation and/or larger capacity cells need to be used. Be careful as too large of a cell capacity can decrease load cell sensitivity or output below minimum indicator sensitivity requirements. Non-axial or side loading can also cause mechanical failure besides measurement inaccuracies. Side load can be minimized through proper use of various types of mounting hardware.

Most load cells are compensated to operate within a specified temperature range, usually 0º to 150ºF. The load cell may operate properly outside these limits. However, the calibration date supplied with the load cell is only valid when the cell is operated within its compensated range.

Moisture has a very negative effect on load cell operation. Moisture can cause no output, overload indications, or most commonly, continuous drift and erratic scale operation. Moisture enters a load cell through cut cables or through pressure. If a non-hermetically sealed load cell is used in a high pressure washdown application water will be forced into the load cell.

Chemicals can cause corrosion of the load cell. Corrosion can work its way into the strain gauges, especially if the material used to protect against the environment has worn away. A stainless steel load cell may be required to keep the cell from corroding, but may not prevent the penetration of moisture. Some chemicals such as chlorine can even corrode stainless steel.

The first step to take when troubleshooting a load cell is to check for distortion, cracks or rippling of the metal. All welds should be free of cracks or deep pox marks. Look for crimps, cuts and excessive abrasion on the load cell cable. Moisture can enter anywhere the cable is cut. The moisture will wick its way to the load cell and cause problems such as unstable readings.

Resistance to ground or electrical leakage is often caused by water contamination within the cell or cable. An unstable output is a good indication of water contamination.

Do not cut the load cell cable. The load cell is calibrated with a certain amount of cable attached.

Posted 30 July 2001