Protecting Data Acquisition Equipment from the Environment

The performance of any measuring system can be compromised by the environment in which it is operating. Naturally, the harsher the environment, the greater the likely impact. Moisture is probably the greatest single cause of system errors and failure.

Avoiding Condensation

Condensation is an insidious problem that can cause a wide range of problems with measuring equipment. Water and electricity simply don't mix!

Condensation occurs when the temperature of moisture laden air is taken below its dew point. Typically this occurs on the surface of objects that become cooler than the air surrounding them. The condensation will form water droplets on the surface. In principal this water is very pure as it has gone through a distillation like process, however most surfaces are contaminated by dust, salts, fluxes and other deposits. It is these deposits combined with the water that tend to cause problems. The resulting symptoms include corrosion, electro-erosion, dendritic growths, electroplating and electrical leakage.

Sea Spray

Sea spray and salt laden air is toxic to electronic equipment. The salt when moist is an excellent electrical conductor and will induce malfunction. Sea salt is slightly hygroscopic and will tend to absorb moisture from the air.

For trouble free operation electronic equipment must be sealed against sea spray. Circuit boards can be coated with appropriated varnish or seal within air tight enclosure.

Corrosive Vapors

Acid vapors associated with battery charging, electroplating, welding and other industrial processes can significantly impact the performance of electronic equipment. Seal the equipment and provide a vent to outside clean air.

Solvent Vapors

Solvents are widely used in industry. Vapors from these materials can create problems with some plastic and epoxy component packages. They can also be an exposition risk and a health risk.

Sealing Enclosures

The presence of sea spray, corrosive vapors and solvents demands that electronic systems be mounted in sealed enclosures. The operative word is "sealed" and by that we mean air tight. An air tight seal is required to prevent the regular atmospheric pressure changes from effectively causing an air change over, accompanied by the ingress of the unwanted vapors

Lightning Protection

Lightning is a major cause of monitoring system failure. While direct lightning strikes can devastate equipment, it is the voltages that are induced by indirect strikes that cause the most frequent problems - simply because indirect strikes are far more common.

A lightning strike can be 100's of meters away yet still induce destructive voltages. In striking the ground or a structure, lightning cause massive currents to flow for perhaps 50µs. These currents are typically 5,000A to 15,000A but can be as high as 50,000A. Large voltages are induced in two ways - by simple resistive voltage drops in the earth and by magnetic induction.

Vermin and Other Biological Risks

Parrots can chew cables, rats defecate on circuit boards, insects can build nests in equipment, mould can grow on sensitive electronics, cattle can wreck automatic meteorological stations, human vandals use field equipment for shooting target practice. Tack precautions by sealing and concealing equipment.

IP Codes

   The widely accepted standard IEC 60529 (2001-02) (published by the International Electrotechnical Commission) defines a classification system for the effectiveness of enclosures in preventing the ingress of solids and liquids. This system uses two letters IP (for Ingress Protection) followed by two digits - the first for solids the second for liquids according to the following table:

 For example IP 65 implies a dust tight case able to with stand a water jet and is a common level of protection for outdoor equipment.

NEMA Codes

In the USA and several other countries, it is more common to apply a NEMA (National Electrical Manufacturers Association) standard NEMA 250-1997 to the environment protection of equipment. The standard is less systematic than IEC60529 but is neither the less useful as it deals with additional issues such as ice formation.

Type 1 - Indoor use to provide protection against falling dirt.

Type 2 - Indoor use to provide protection against falling dirt, and against dripping and light splashing of liquids.

Type 3 - Indoor or outdoor use to provide protection against falling dirt, rain, sleet, snow, and windblown dust; and that will be undamaged by the external formation of ice on the enclosure.

Type 3R - Indoor or outdoor use to provide protection against falling dirt, rain, sleet, and snow; and that will be undamaged by the external formation of ice on the enclosure.

Type 3S - Outdoor use to provide protection against falling dirt, rain, sleet, snow, and windblown dust; and in which the external mechanism(s) remain operable when ice laden.

Type 4 - Outdoor use to provide protection to personnel against falling dirt, rain, sleet, snow, windblown dust, splashing water, and hose-directed water; and that will be undamaged by the external formation of ice on the enclosure.

Type 4X - Outdoor use to provide protection against falling dirt, rain, sleet, snow, windblown dust, splashing water, hose-directed water, and corrosion; and that will be undamaged by the external formation of ice on the enclosure.

Type 5 - Indoor use to provide protection against falling dirt; against settling airborne dust, lint, fibers, and flyings; and to provide protection against dripping and light splashing of liquids.

Type 6 - Outdoor use to provide a degree of protection against falling dirt; against hose-directed water and the entry of water during occasional temporary submersion at a limited depth; and that will be undamaged by the external formation of ice on the enclosure.

Type 6P - Outdoor use to provide protection against falling dirt; against hose-directed water and the entry of water during prolonged submersion at a limited depth; and that will be undamaged by the external formation of ice on the enclosure.

Type 12 - Indoor use to provide protection against falling dirt; against circulating dust, lint, fibers, and flyings; and against dripping and light splashing of liquids.

Type 12K - Same as Type 12 with knockouts.

Type 13 - Indoor use to provide protection against falling dirt; against circulating dust, lint, fibers, and flyings; and against the spraying, splashing, and seepage of water, oil, and non-corrosive coolants.

The above Type classifications also imply a protection to personnel from dangerous voltages (up to 1000V), and that outdoor rated enclosures can also be used indoors.

The following table provides a conversion from NEMA to IP although not in the reverse direction as the NEMA protection level is matched against an equal or higher IP level.

The IP or NEMA rating of an enclosure or case for electronic equipment is not all that should be considered. For example, condensation of moisture is a common cause of equipment failure and more subtly, errors in measuring systems.