In general, the main objective for water ingress applications is to reliably detect and notify of the presence of water inside an enclosure. Taking this into consideration, some end applications can require the ability to distinguish between water ingress and condensation. For the purposes of this investigation, the assumption is that any water in the enclosure can be deemed problematic, and is therefore treated as water ingress. Without an external disturbance to drive a specific factor, changes to temperature or RH typically occur slowly over longer periods of time. As such, testing windows when measuring temperature or RH are typically larger than for most electronics algorithms, with windows extending as high as 60 to 120 seconds in application.

Relative humidity is the most common humidity measure. This is defined as the percentage of water vapor present in air relative to the maximum capacity at that temperature. This essentially measures: How full of water is the air? At 100% RH, the air is fully saturated – any additional moisture can condense into liquid water. Mathematically, RH quantifies how far the existing partial pressure of water vapor is from the saturation vapor pressure at the same temperature and pressure, as shown in Equation 1.

RH is highly temperature-dependent – if temperature rises, RH falls (for the same absolute moisture content), and if temperature falls, RH rises. This is because as temperature increases, the saturation vapor pressure of the air increases as well while the actual vapor pressure remains constant (assuming no new moisture has been introduced). This behavior complicates a simple humidity threshold for leak detection. Given enough time, a vented enclosure's humidity can slowly climb to a high RH just from ambient conditions, potentially triggering a false alarm if only relying on an absolute RH level. However, such gradual changes happen very slowly, so focusing on the slew rate (or rate of change) of humidity can filter them out.

Absolute humidity (AH) alone is not a reliable water ingress indicator in a vented system. AH is not affected by temperature explicitly – this is just an amount per volume. However, because volume can change with temperature and pressure, AH of an air parcel can vary even if no water is added or removed. If air expands (for example, rising to high altitude where pressure is lower), the AH decreases (the same water molecules now occupy a larger volume). Vents can help with pressure equalization, but if an unvented closed container is cooled, the air contracts a bit and AH increases slightly; more significantly, some vapor can condense, which reduces the AH (because those molecules leave the gas phase).

Evaporation is the process of liquid water turning into water vapor through a state change. Typically, the liquid-to-vapor phase change is assumed to be the boiling point of water. However, evaporation occurs at any temperature with sufficient energy for molecules on the water’s surface. When water leaks occur, the liquid water can immediately begin evaporating. The evaporation can occur at different rates depending on the temperature of the system and volume of water. This new introduction of water vapor into the air is detected as a change in RH by the humidity sensor IC. Evaporation can differ based on the ambient humidity in the system. If the air is drier (low RH), then the rate of evaporation can increase. If the air is already humid (high RH), the rate of evaporation can decrease. This is relevant since water ingress can occur at different temperature and humidity conditions, so this can affect how the RH changes in the system over time.

Figure 1-1 Closed, Semi-Open, and Open Systems

A condition that was not evaluated during this experiment was an open system. The test enclosure described in the following was a semi-open system, and the results apply to a fully closed system as well. An open system means the electronics are completely exposed to the air and potentially water. For example, an open system with water ingress concerns can be a liquid cooled server. The server and the electronics have a free and uncontrolled air exchange with the environment, but the liquid is held within tubes and needs to be monitored for leaks. A closed system is one where there is no exchange of air inside and outside of the system. Hermetically sealed systems are closed systems. A semi-open system is one where there is some controlled and limited air exchange with the outside environment through a filtered vent or permeable membrane. By using the protective vent on the test setup, this application note covers how to detect water leaks in semi-open systems (findings can apply to closed systems as well). Open systems are more susceptible to large swings and RH levels in the ambient air, which can complicate using a rate of change threshold to detect water ingress.