Environmental protection and IP classification of load cell

In the last article in this series, we looked at how to go about choosing load cells of the appropriate mass capacity, applying a de-rating to allow for shock loading, etc. Part II ended with a discussion on accuracy considerations. This month examines environmental protection and considerations that significantly effect load cell installations.

Environmental protection

No area of load cell operation causes more confusion and contention than that of environmental protection and sealing standards. Although our industries have in-depth standards and test procedures to define load cell and weighing system performance, no standards have been developed to cover product suitability for specific environmental conditions. In the absence of such standards, most manufacturers have adopted the International Protection system (IP/IEC 529 or EN 40.050) or National Electrical Manufacturers Association Standards (NEMA publication 250). Define the level of sealing for their products. Both standards are good test procedures for environmental sealing when applied to the products for which they were intended - those being electrical enclosures, but they are not very well suited to load cells.

IP classification

The IP standard describes a system for classifying the degree of protection provided by the enclosures of electrical equipment.

* Protection of persons against access to hazardous parts inside the enclosure.

* Protection of the weighing equipment inside the enclosure against the ingress of solid foreign objects.

* Protection of equipment inside the enclosure against harmful effects due to the ingress of water.

Unfortunately, no definition is given for the term 'harmful effects'. Presumably, for enclosures, the main problem with water could be one of electrical shock to persons in contact with the enclosure, rather than malfunctioning of the unit. Furthermore, the standard only relates to water ingress and ignores moisture, chemicals, corrosion, etc.

The commonly used categories to describe load cell sealing are:

* IP65 protected against low pressure jets of water from all directions, limited entrance allowed.

* IP66 protected against strong jets of water, eg for use on ship decks, limited entrance allowed.

* IP67 protected against the effects of immersion between 15 cm and 1 m.

* IP68 Protected against long periods of immersion under pressure.

NEMA Classification Classifications in the NEMA system run from NEMA 1 to NEMA 12, but load cell manufacturers concern themselves with NEMA 4 and NEMA 6. Unlike the IP system, NEMA does concern itself with environmental conditions such as corrosion, rust, freezing, oil and coolants. NEMA 4 enclosures are intended for indoor and outdoor use, providing a degree of protection against windblown dust, rain, splashing water, and hose directed water. However, no consideration is given for the effects of internal condensation. Nema 4X enclosures meet the same standards as NEMA 4 and are constructed of 304 stainless steel or other material offering equal corrosion resistance.

NEMA 6 enclosures are used where there is a chance of temporary immersion. This standard calls for the highest part of the enclosure to remain submerged in water, with its highest point 1,83 m below the surface for 30 minutes. NEMA 6P enclosures are used where prolonged immersion may occur and resistance to corrosion is needed.

While it may seem that NEMA standards offer some advantages over the IP system for corrosion resistance, they only relate to external corrosion of enclosures. This is very limited when applied to the more complex load cell construction and the different effects of corrosion or water ingress. 1.4.3