In the weighing industry the most common technology used is strain gauge load cells. Strain gauges are very sensitive and need to be protected against moisture, chemicals, and mechanical damage. Many people believe that all load cells made from stainless steel are protected, but this is not true. To obtain the ultimate degree of moisture protection, the load cell must be stainless steel and hermetically sealed. Hermetic sealing will ensure the integrity of the strain gauge section of the cell in corrosive or washdown applications.
Right Equipment for the Job
The success or failure of any electronic weighing system ultimately lies with selecting the correct equipment. An important aspect of choosing the correct load cell is to make sure that the load cell is properly sealed for the environment where it will be used. If the environment is wet, has high humidity, or the load cells will be washed down, the only choice that should be considered is a hermetically sealed stainless steel load cell.
is in the Seal
The only difference between a tool steel and a stainless steel load cell is the element material used. The method of sealing these load cells is the same. Only the sealing process of a true hermetic seal will prevent moisture from entering the load cell. An environmentally sealed load cell entails potting the cavity where the strain gauges are installed, covering that potting with a plate that is glued in place, and using washers and o-rings where the load cell cable enters the body of the load cell. A true hermetically sealed load cell incorporates a cover over the gauge cavity that is welded in place and a welded header where the cable is attached. This welded header is the key. Some stainless steel load cells offer a welded cover over the gauge area but do not incorporate the welded cable header. Although the welded cover provides some protection of the strain gauges, moisture can still enter the load cell through the cable entry area and damage the load cell.
Worth the Investment
Hermetically sealed, stainless steel load cells are made from 17-4PH, which offers the best performance qualities of any stainless alloy. Stainless steel has a higher resistance to moisture and chemical corrosion than tool steel or aluminum.
Hermetically sealed, stainless steel load cells are more expensive than other load cells due to the additional costs of sealing the load cell.
If a non-hermetic cell is used in a washdown application and the cell is replaced several times due to failure from moisture, the cost is much greater than using a hermetically sealed load cell initially.