Humidity management of outdoor electronic equipment: methods, pitfalls, and recommendations

Discusses the basics of the three approaches to mitigation of humidity related failures namely; 1) relative humidity control module (RHCM); 2) absolute humidity control (AHCM); 3) use of potting and coating, as well as conditions and limitations of their applicability. The RHCM approach (or Saturation Limit Control) uses heat dissipation from electronics to maintain an appropriate temperature difference between ambient and the critical surfaces to lower RH in their vicinity and at same time through the use of diffusion plugs ensures a relatively fast humidity equilibration between the enclosure air and ambient to minimize condensation caused by cold fronts and radiation cooling. The AHCM approach relies on minimization of water ingress through selection of appropriate materials and design and at same time uses desiccant to remove the water which made its way into the enclosure. The choice between these schemes depends mostly on thermal considerations. The more expensive and sensitive to errors AHCM scheme has to be used when the energy dissipation is either not high enough or it is highly dependent on the traffic.