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Environmental Conditions

Time:2015.12.03    Click:

Scope
Most of the process industries rely heavily upon strain gage based load cells for accurate and consistent weight data and expect them to perform under a wide range of adverse conditions, including mechanical and chemical attack.
The problem is that premature failure of load cells can have far reaching effects on the overall processes, and have a consequent impact on cost, safety and product reputation. Replacing a load cell in the field will involve not only the cost of the component itself, but also the expenses associated with labor, downtime and re-calibration.
There are several factors that can cause load cell failure; one of the most important is the environment. This Technical Note takes a look at the effects the environment can have on load cells and offers guidelines on how to minimize these effects through proper selection and application. In addition, existing load cell classification standards are overviewed.
Classification Standards
No area of load cell operation causes more confusion and contention than that of environmental protection and sealing standards. Although the weighing and load cell 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 IP classification (Ingress Protection by IEC/EN60.529 or DIN 40.050) or National Electrical Manufacturers Association Standards (NEMA) Publication 250 classifications to 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
• Protection of persons against access to hazardous parts inside the enclosure.
• Protection of the 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.
The IP code consists of five categories or brackets identified by a number or letter that indicate the degree of some element to the standard. The first characteristic number relates to access to the hazardous part by persons or solid foreign objects. A number from 0 to 6 defines the physical size of the accessing object.
Numbers 1 and 2 relate to solid objects and parts of the human anatomy, while 3 to 6 relate to solid objects such as tools, wire and dust particles. As shown in the accompanying table on the next page, the higher the number, the smaller the accessing object.
Most load cell manufacturers use the number 6 for this category to signify that their products are dust tight. However the effectiveness of this classification depends on what constitutes an enclosure. Of particular significance here are load cells of a more open nature, such as single point cells, where the introduction of a tool, such as a screwdriver, could have catastrophic results even though the load cells are dust tight with regard to the critical components.
The second characteristic number relates to the entrance of water with what is described as harmful effects. Unfortunately, the standard does not define harmful. Presumably, for electrical enclosures, the main problem with water could be one of electrical shock to persons in contact with the enclosures, rather than the malfunctioning of the unit. The characteristic describes conditions ranging from vertically dripping water, through spraying and jetting, to continuous immersion.

Load cell manufacturers usually adopt either the 7 or the 8 designation for their products. However the standard clearly states that “An enclosure designated with a second characteristic number 7 or 8 is considered unsuitable for exposure to water jets (designated by the second characteristic 5 or 6) and need not comply with requirements for number 5 or 6 unless it is dual coded, e.g., IP66/IP68”. In other words, under certain conditions and for certain product designs, a product that passed a half-hour immersion test may not necessarily pass one that involves high pressure water jets from all angles.
As with IP66 and IP67, IP68 conditions are set by the manufacturer of the product, but must be at minimum more severe than for IP67 (i.e., longer duration or greater depth of immersion). The requirements for IP67 are that the enclosure can withstand immersion to a maximum depth of 1 meter for 30 minutes.
While the IP standard is an acceptable starting point there are shortcomings:
• The IP definition of enclosure is too loose to be meaningful for load cells.
• The IP system only relates to water entrance and ignores moisture, chemicals etc.
• The IP system can not differentiate between load cells with different constructions with the same IP rating.
• No definition is given for the term “harmful effects”, so the effect on load cell performance is open to interpretation.

NEMA Classification
Classifications in the NEMA system run from NEMA1 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, icing, oil and coolants.
NEMA 4 enclosures are intended for indoor and outdoor use, and provide a degree of protection against windblown dust and 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. The standard calls for the highest part of the enclosure to remain submerged in water, with its highest point 1.83 metres 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. Also, neither system concerns itself with internal condensation or the subject of cable entry into the enclosures.

Damp Heat Cycling
The IP standard clearly states that it does not deal with internal condensation or moisture within the enclosure, saying that this is the responsibility of the relevant product standard. However, moisture or condensation is of vital importance in load cell operation.
Moisture may enter the inside of the load cell over a long period and have a catastrophic effect, especially when acids or alkalis are present. One test used to determine a load cells ability to withstand moisture or condensation is the Damp Heat Cycling Test. Although there are several versions of the test, the one most universally accepted is (IEC) 68-2-30. The object of the IEC standard is “To determine the suitability of components, equipment, or other articles for use and storage under conditions of high humidity when combined with cyclical temperature changes”.
It is obvious that this standard is a much more useful classification than the IP rating when it comes to defining load cell environmental suitability.
Load cells certified to OIML R-60 are tested to withstand 12 damp heat cycles of 24 hours each. Load cells which are not suited to withstand this test should be marked with “NH” (non-humidity) behind the appropriate accuracy grade.
Load Cell Construction
Besides a given IP-rating or NEMA-classification load cells should also be classified according to their design in terms of cable entry, material of construction and gages sealing method. Load cells can be divided into six main groups in terms of sealing:

Whether or not the products 4 to 5 in the above listing meet the IP66 classification may depend upon how the load cells are used and whether additional mechanical protection is provided. In parallel with these six classifications, different load cells are constructed from different materials. The main ones are aluminium; copper beryllium; tool steel-painted; tool steel-nickel plated or stainless steel. Certain products may be a combination of these e.g., tool steel-nickel plated body with stainless steel bellows or cups.

Conclusion
Selecting the wrong load cell for a par ticular application in terms of environmental compatibility can have far reaching consequences in terms of costs, safety and product reputation. Current classifications fall well short of defining adequate environmental standards for load cells. As a result, this subject needs careful review by both load cell users and manufacturers to ensure that clear guidelines are available.
Users should be able to compare like-for- like features when selecting products from different manufacturers. If in doubt, they should ask pertinent questions relating to:
• Construction of the load cell
• Cable entry method
• Past experiences (long-term environmental success stories)
For applications in harsh environments, additional protection for the load cells may be needed to assure their reasonable working life. This can be achieved with enhanced scale designs and the use of additional coatings on the load cell, such as paints, greases and plating. The scale or system design should minimize the possibility of material build-up around the cells. If appropriate, the design should also provide mechanical protection from the effects of direct water and solvents whilst cleaning. Sealing compounds and rubbers used on some load cells can deteriorate when exposed to chemicals or direct sunlight. Because they embrittle rubber, chlorine-based compounds are a particular problem.
Load cells correctly selected and regularly maintained should be capable of a working life in excess of ten years. There are always exceptions, but the engineer needs to be able to obtain the optimum per formance out of his or her selected load cells.





 

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