Low Humidity Testing
It’s well known that high humidity conditions can cause failures of equipment. MIL-STD-810G lists some of the mechanisms by which high humidity can cause equipment failure. These include causing electrical short circuits, by fogging optical surfaces, by causing oxidation and/or galvanic corrosion of metal surfaces, by causing the swelling of material due to sorption effects etc.
What is less well known is that low humidity can also be a stressor. A major reason is that low humidity greatly increases the production of static electricity.
As electronic components become smaller and smaller due to advances in technology, the problem of damage caused by electrostatic discharge becomes more and more a factor. This is because smaller components mean a further reduction in the spacing of insulators and circuits within the components. As a result, manufacturers and end users are demanding that their suppliers provide equipment or components that have undergone low humidity testing. Furthermore, certain medical and disinfection equipment needs to especially be functionally tested at low humidity since it is known that low humidity increases the spread and survivability of bacteria and viruses (indeed the reason that the flu spreads in winter is mainly not because of the cold per se but because of low humidity!).
To understand how low humidity environments occur, one needs to understand the difference between absolute humidity and relative humidity. Absolute humidity is simply the total mass of water vapour in a given volume of air. Relative humidity is the ratio of the partial pressure of water vapour to the equilibrium (i.e. saturated) water vapour pressure at the same temperature. Since the equilibrium water vapour pressure rapidly increases with increasing temperature, for the same absolute humidity, relative humidity falls with increasing temperature and vice versa. Thus when cold ambient air, which even if it has a moderate relative humidity still only has a low absolute humidity, is heated to a higher temperature, a low relative humidity occurs.
Low relative humidity environments are thus encountered when air is heated in winter – this could be by heaters or by heat generating equipment. Very low relative humidity is also found in the cabin of aircraft – including civilian airliners – cruising at high altitudes. This is because the air outside an aircraft cruising at high altitude is very cold and has a low absolute humidity. This air which is drawn into the cabin is then heated to comfortable room temperature which drives down its relative humidity. The whole process of temperature control of cabin air is more complicated than this but what you end up with is air inside an aircraft with a relative humidity around 10% and sometimes less.
The problem with low humidity testing is the challenge of producing very low humidity in a controlled laboratory environment. Since generating low relative humidity can be costly and difficult – especially at room and lower temperatures – most standard environmental test chambers struggle to get a relative humidity much lower than 40% at standard room temperature (23⁰C to 25⁰C depending on the standard). At lower temperatures, the minimum relative humidity that these chambers can achieve is even higher – since the absolute humidity required to achieve a given relative humidity must be lower.
For those requiring electrostatic discharge tests or needing very low humidity testing of electronic components, equipment intended to operate in aircraft cabins, medical/disinfection equipment or any other equipment that needs to survive very low humidity environments, Austest can now meet your needs.
Austest engineers have modified our climatic chambers, which can now achieve very low humidity. At the standard ambient temperature of 25⁰C, we can get relative humidity down to 7%. Even at 5⁰C, the modified chamber set up is able to achieve a relative humidity below 15%. Meanwhile at 40⁰C, we can drive relative humidity down to 5%. Feel free to contact our environmental engineers about your requirements.