The nemesis – condensation
Condensation or misting can be a rather complex and confusing issue because its formation depends on a number of (inter-related) factors including atmospheric pressure, altitude, temperature, time, surface ‘wettability’ and absorbency. Condensation can be thought of as the reverse of evaporation – it is the change of state from gas phase into liquid phase. For our purposes, it can be defined as the change in the state of water vapour to liquid water when in contact with a surface. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition – usually resulting in ice. For those of us of a certain age, the formation of ice on the inside of our bedroom windows will be a distant but familiar memory. Condensation is initiated by the formation of atomic or molecular clusters within its gaseous volume—like rain drop or snow flake formation within clouds—or at the contact between such gas and a surface. Condensation occurs when a vapour is cooled and/or compressed to its saturation limit when the molecular density in the gas phase reaches its maximal threshold. For outdoor equipment, the most common mechanism is when relatively warm air inside the equipment meets a chilled surface such as the aluminium or steel casework. In some cases this will lead to the formation of mist, then droplets and then condensate streams which, under gravity, will form pools of water at the lower points of the equipment. Condensation within outdoor equipment is a common, natural and usually unwanted phenomenon for design engineers. Other than the failure of optical or capacitive encoders, condensation may also cause or accelerate a number of effects such as corrosion, electrical shorting, mold, fungal growth and rotting of fabric or cellular materials.
What are the solutions?
Traditionally, there have been a number of solutions. The first is either lots of through ventilation and/or a heater inside the equipment so that the internal air doesn’t chill sufficiently to condense on cold surfaces. Forcibly ventilating electrical enclosures can, in practice, cause more problems than it solves in some moisture or particle laden environments (most notably with potentially explosive environments). A heater solution is more easily said than done and any heating system will tend to add complexity, extend start up times, increase energy consumption and in some cases exacerbate matters in unexpected ways. Next time you’ve got a misted up windscreen, see what happens when you blow warm rather than cold air on to it. Issues such as differential thermal expansion, thermal gradients/stress, mold and fungal growth must also be taken into account.
The second solution is to use a hermetically sealed unit and to fill the cavity with a gas which contains no water vapour, such as pure nitrogen. This is often the defacto solution for systems such as electro-optical assemblies, missile seekers, camera or infrared detectors. Important here, is the use of the word ‘hermetic’ – in other words, airtight. Importantly, O-rings and shaft seals are typically environmental rather than hermetic seals. In other words, they do not prevent the passage of air in and out of the equipment’s internal cavities. They may slow the passage of air, but even a modest pressure differential across a shaft seal will lead to air flow. This phenomenon is well known to aerospace engineers – especially with regards to ‘cavity pumping’ caused by changes in pressure from the ascent or descent of aircraft and this is why the use of hermetically sealed cavities is often a basic requirement for avionics. For most equipment designers, the position for encoders is typically where there are shafts and moving components and so by definition, they do not lend themselves to being hermetically sealed. This solution is seldom applicable to rotating shaft and encoder arrangements.
The third solution is to use a desiccant to remove water vapour from the internal environment. This can be a good method but requires periodic service for replacement or refill. Equipment users invariably don’t like this solution.
An alternative approach
Perhaps the most sensible solution is not to use equipment that is sensitive to condensation. Optical and capacitive encoders are not included in this category. Such an approach may seem obvious to some but it seems that the difficult lessons learned by an older generation of engineers has not necessarily been learned by a newer generation. Resolvers, synchros, InductosynsTM and inductive encoders (or ‘incoders’) are included in this category. Such devices use inductive or transformer principles to measure angular displacement or velocity and are generally unaffected by changes in temperature, humidity or even immersion in fluids. Inductive devices are the traditional choice of seasoned design engineers in the aerospace, defence, utility and petrochemical sectors where reliable outdoor operation over long periods is a key requirement.