Comparison of Relative Humidity Sensors as a Proxy for Leaf Wetness

Relative humidity (RH) is an important measurement in agriculture as it helps with disease modeling, irrigation, and overall crop management. This study looks at three different RH sensors used in the North Carolina Environment and Climate Observing Network (ECONet): the Vaisala WXT-520, Vaisala HMP155, and the Rotronic HC2-S3. Currently, the WXT-520 is the operational sensor for the NC ECOnet. Starting in June 2013, a suite of Vaisala HMP155 and Rotronic HC2-S3 were installed at various ECONet sites across NC. Data analysis from June 2013 through December 2013 shows that while all three sensors are comparable at RH values < 90%, at above 90% RH values, a difference of between 5-7% is consistent between the WXT-520 and the other two sensors. During precipitation events, the Vaisala WXT-520 sensors average RH is 88.34%, while the Vaisala HMP155 and Rotronic HC2-S3 averaged 94.74% and 94.92%, respectively.

For disease management, relative humidity is used as a proxy for leaf wetness. Leaf wetness sensors were installed in the summer of 2013 at all ECONet stations to compare RH to leaf wetness. Using a proxy that 90% RH indicating a leaf is wet, the results showed a correct estimate of only 23% with the Vaisala WXT 520. However, the experimental sensors showed a slightly better correct estimate of 31%. False alarm rates are also higher with the WXT-520 (0.90) compared to the HMP 155 and HC2-S3 (0.71). Results here show that, for determining leaf wetness, the HMP 155 or HC2-S3 sensors are more reliable than the WXT 520.