“Relative humidity (RH) sensors with a cavity are specifically designed to measure the water vapor present in the air. Excessive humidity in the storage environment can directly increase the degree of decay, and humidity in the air can directly affect system reliability (corrosion) or product life, so relative humidity is an important measure.
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Relative humidity (RH) sensors with a cavity are specifically designed to measure the water vapor present in the air. Excessive humidity in the storage environment can directly increase the degree of decay, and humidity in the air can directly affect system reliability (corrosion) or product life, so relative humidity is an important measure.
One of the challenges with relative humidity sensors is that their exposed sensing elements degrade over time. Continued exposure to moist air and volatile organic compounds in the environment, such as benzene or ethylene glycol deposited on the sensor material, can cause the accuracy of the sensor to deviate from its specification. This requires decommissioning the system to replace or calibrate the sensor, increasing the total cost of ownership. In this article, I will discuss the accuracy and long-term drift of relative humidity sensors, and how these parameters affect the performance and longevity of end equipment.
Long-term drift and accuracy are some important specifications for humidity sensors. In today’s humidity sensors, especially capacitive sensors, typically have a relative humidity accuracy of 2% to 3% and a maximum relative humidity accuracy of 3% to 4%. This accuracy is the so-called initial accuracy of the sensor before it is put into actual use in the field, and does not include long-term drift.
The long-term drift of capacitive sensors is typically about 0.25% to 0.5% relative humidity per year. Although this drift is very small compared to resistive sensors (about 1% relative humidity per year), it can still reach 5% relative humidity drift within 10 years and become a significant source of error, affecting the intended operation of appliances or automobiles, etc. Equipment over 10 years old.
Figure 1 shows the accuracy and long-term drift to 8% relative humidity over 10 years. In the case of a refrigerator, if drift results in false high humidity readings, the compressor will turn on unnecessarily, resulting in less efficient operation and shortened compressor life.
Figure 1: Humidity sensor accuracy with long-term temperature drift
Humidity accuracy is also becoming increasingly important in cold chain applications such as storage and transportation of perishable food, life science and pharmaceutical products. With battery-powered wireless trackers and data loggers, sensors can continuously record temperature and humidity information for food shipments, as well as gardening and raw material powders. Loggers predict product life and therefore need to be highly reliable with accuracy traceable to National Institute of Standards and Technology (NIST) measurement standards, while complying with stringent International Organization for Standardization (ISO) and International Electrotechnical Commission (IEC) 17025 regulations .
Power consumption is another important specification; in battery-powered applications such as real-time data loggers and environmental sensors, power consumption can mean using a larger, more expensive battery, or using a battery that helps reduce costs Smaller battery. Low power consumption allows battery-powered applications to run longer, but accuracy becomes more important over time; if accuracy changes, calibration may be required.
The TI-designed HDC3020 relative humidity sensor extends system life while reducing power consumption. Its architecture enables accurate temperature and humidity measurements over the entire supply voltage range. Specifications: 1.5% RH typical and 2% RH maximum , the long-term drift is less than 0.21% relative humidity per year. These parameters help extend the life of appliances and other types of systems, reducing long-term error from 8% relative humidity to 4% relative humidity after 10 years, a reduction of up to 50%, as shown in Figure 1.
The HDC3020 features integrated drift correction technology that reduces drift caused by natural aging, environmental stress, or interaction with contaminants. Preliminary performance of this technology can be evaluated using the HDC3020 EVM. For more information on environmental impact, read the white paper: “Effect of 85°C/85% Accelerated Life Testing on Polymer-Based Relative Humidity Sensors.”
The HDC3020 also integrates high-accuracy 0.1°C (typ) and 0.4°C (max) temperature sensors with ultra-low power consumption. Both humidity and temperature are NIST traceable and tested to ISO 9001:2015 procedures, NIST, ISO and IEC 17025, among other standards. Removable tape and IP67 permanent filter cover options allow for washdown and conformal coating of printed circuit boards, and provide protection against dust and water, respectively, in harsh conditions.
Epilogue
When choosing a humidity sensor, you should not only consider accuracy, but also long-term drift and environmental factors, as exposed cavities on humidity sensors can accelerate sensor aging. NIST traceability for temperature and humidity is a great way to ensure that first-release systems provide excellent accuracy, but it doesn’t take into account other aspects, such as sensor aging or drift due to contamination. Ultra-small long-term drift and drift correction are necessary to maintain excellent sensor accuracy over the life of the sensor without recalibration. Choosing a highly reliable sensor leads directly to a higher performing, more reliable product.
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