The difference between stress testing and overstress testing

Stress tests are vital to RH sensor reliability, as the results of a stress test can predict the longevity of a RH sensor under harsh environmental conditions; however, developers using humidity sensors in an application should consider the special storage and handling guidelines.

As shown in Figure 2, RH sensors have an open cavity that exposes a polymer to the air, enabling a chemical reaction with which it is possible to calculate the RH of the environment. Exposed polymers can be affected by extreme conditions (those exceeding data sheet specifications) including 85°C/85%, leading to shifts in RH measurements.

If the goal is to ensure that the system is still functionally operational, this testing may be OK – in fact, this is the expectation when running BHAST at the chip level. But, if data-sheet accuracy parameters need to remain within specifications after stress testing beyond data-sheet conditions, system developers may have a problem. A stress test of an RH sensor by definition involves selecting RH percentages and temperatures that can represent expected sensor performance in the field, even under harsh but realistic environmental conditions. Selecting RH percentages and temperatures that are beyond data-sheet specifications cannot serve as a reliable predictor of sensor performance in the field.

Figure 3 illustrates a meaningful approach to stressing a humidity sensor. The graph shows the world record temperature for dew point (35°C/95°F), which represents the largest known amount of moisture (100% RH) to have been held in the earth’s air (42.0711 mmHg). 85°C and 85% RH would translate to a dew point of 81°C, which is far beyond what is possible in the Earth’s atmosphere. Assuming constant air moisture, increasing the temperature enables a calculation of theoretical RH. For example, in Figure 3, at 85°C the RH is only 9.7%. Test points of temperature and RH exceeding those in Figure 3 are overstressing the sensor and do not represent expected sensor performance in possible field stress scenarios, raising false alarms about sensor quality and performance.

As the surface temperature increases, so does atmospheric humidity. Those wanting to account for global warming over time could use a guard band, as shown in Figure 4. The saturated vapor pressure increases approximately 7% per 1°C of warming.

Figure 4 shows a hypothetical dew-point extrapolation at 40°C and 50°C, in addition to the measured dew-point record of 35°C. Note that at 85°C, as well as other high temperatures, the RH is still very low.