Sensor Accuracy over Time

Changes to temperature sensor accuracy over time

Typically 0.1°C/year

The drift increases with the measured temperature value. At 100°C, it may reach up to 0.3°C/year.

Changes to humidity sensor accuracy over time

Typically 1%RH/year

Causes of the drift

Unlike temperature sensors that can be protected from the environment with hermetic sealing, capacitive humidity sensors are exposed to the air or gas they are measuring, and thus to any potential contaminants. These contaminants can then increase the sensor response time or cause a temporary or permanent drift of its readings.

The resistance of RH sensors by different manufacturers to different contaminants varies considerably. It is therefore very important to consider the effects of various chemicals and their concentrations when choosing the most suitable sensor.

There are two common types of contaminants that can potentially affect the readings of a RH sensor.

1. Particulate contaminants

These are any particles suspended in the air that can be deposited on the RH sensor. Dust particles (depending on their type) may not have a significant effect on the sensor readings other than a potential increase in the sensor response time, depending on the buildup. Other particles, such as salts that can enter the environment from water sources, can have a significant effect on sensor readings in case of any significant buildup. In most cases, the RH sensor can be protected from such contaminants by selecting an appropriate filter to reduce the drift risk.

2. Vapor contaminants

Vapor contaminants are any volatile chemicals that have evaporated into the air. Some vapor contaminants are usually present in most environments in very small concentrations. The vapor concentration may increase in closed systems, such as environmental chambers. Once the vapor concentration reaches a certain level, the sensor readings may begin to drift. Unlike particulate contaminants, vapors cannot be easily filtered from the air. Other strategies, such as shorter calibration cycles, are often effective in reducing the risk of an out-of-tolerance condition.

While all capacitive RH sensors share a similar basic principle, the materials used and the general design will vary significantly. The long term stability and performance of a RH sensor is directly related to these differences in material and construction. Unfortunately, information regarding long term drift can be hard to find. Only a few datasheets list it. Sometimes it can be found in user manuals. But often it takes a call to the manufacturer to find out.

Recommendation

In a normal dust-free environment, HW group recommends to calibrate sensors once per year. In more demanding environments, the interval should be shortened to 0.5 years, or as needed.