A psychrometric interpretation for environmental conditions and its impact on lifespan of biomedical electronic devices

Abstract

Traditionally, environmental factors have been utilised in assessments of electronic systems reliability (ESR) of electronic components, systems or devices. Most of these available interpretations are, however, confined to descriptions of the environment being ‘Ground’, ‘Airborne’ ‘Naval’, ‘Commercial’ and so on, which only describe the ‘condition’ of the environment and not the ‘physical’ or ‘geographic’ location that the electronic system is deployed for use in. Environments that combine different types of ambiance (dust, humidity, etc.) are known to be among the most detrimental to ESR. For example, moisture, considered as a chemical agent, is responsible for the deterioration of many types of materials including electronic components, systems and devices. In this work, arguments regarding the use of psychrometry, the study of the composition of air, its properties and their effects on different materials from place to place, in order to reinterpret the impact of physical nature of the environment on a device's reliability are advanced. Humidity connotes the amount of an invisible water vapour in the air and is at the centre of all psychrometric analysis. Generally, humidity could be considered inimical to ESR because low values are known to cause condensation and the build-up of static electricity in electronic equipment, which results in brittleness, dielectric breakdown and ultimately permanent damage to devices. At the other extreme, high values of humidity lead to increased conductivity, which leads to numerous hazards to both the device and surrounding environment. Aridity is another physical attribute used to characterise the extent of dryness or availability of water in an area, using which environmental conditions peculiar to any location can be deduced. Therefore, arid areas are characterised by high temperatures, low humidity, medium to large amounts of dust, solar and atmospheric pressure, etc. Geographically, the values of these psychrometric parameters anywhere on the global can be easily discerned. For example, it is well-known that areas surrounding the equator are the most arid, while humidity increases as we leave the equator towards the poles; thus, providing a model to estimate lifespan of electronic systems based on the location the device is deployed for use. When the physical attributes of the environment are efficaciously cogitated they can help to better interpret contributions of (physical or geographic) environmental conditions to the smooth, reliable and prolonged operation of electronic equipment. Building on an earlier work that formulated contributions of aridity in assessing environmental impacts on ESR and broadening this assessment from the established environmental ‘conditional’ interpretations to more ‘physical’ (or geographic) descriptions vis-a-vis its influence in terms of a device's failure rate, a property loosely referred to as the lifespan of an electronic system; in this study an attempt is made to model the impact of relative humidity on the lifespan of a biomedical electronic device, specifically the electrocardiogram, a device widely used to monitor and manage cardiovascular ailments. To validate the proposal, a conscientious and veridical analysis of psychrometric data gathered (in Riyadh, the capital city of the Kingdom of Saudi Arabia) over a 12-month period was undertaken in order to ascertain the relationship between some psychrometric (environmental) variables. Based on these relationships, an apposite formulation for electronic system lifespan was attained. Comparisons between the expected lifespan of the aridity based assessment of the lifespan of an ECG device with that based on the reformulation of lifespan based on the psychrometric conditions prevalent in Riyadh show an 80% correlation with the overall device lifespan reducing to 11 years. The results provide a more detailed and accurate assessment of the ECG lifespan confirming that that ESR is influenced by the geographical location (physical environment) that it is deployed for use.