Kestrel Meter Found More Accurate Than Sling Psychrometer in Lab Test

Accurate weather readings are essential for the safe and effective management of wildland fires. The ideal weather monitoring tool must be easy to use, extremely accurate, and quick to deploy. Both the sling psychrometer and Kestrel Weather Meter have been used for decades by firefighters to gather environmental conditions. Which device performs at the highest level of accuracy? This question was put to the test in a lab setting by PhysioZing, an affiliate company of The Montana Center for Work Physiology and Exercise Metabolism and frequent research collaborators with the Montana Technology and Development Center (MTDC) at The University of Montana.

Lab Testing Finds Kestrel Meter RH Measurement To Be More Accurate Than Traditional Sling Psychrometer

The testing compared the accuracy and reliability of the sling versus the Kestrel 5500FW Fire Weather Meter Pro during controlled trials in varied weather conditions. The study involved Five veteran Type 1 wildland firefighters with a combined 52 years of experience using the sling and the Kestrel 5500FW in an environmental chamber. The chamber was set to specific temperatures and relative humidities as reported by a recently calibrated United States Forest Service remote access weather station (RAWS) and verified by an independently calibrated chilled mirror hygrometer. The data collected over two days show that the Kestrel 5500FW measured RH more accurately and with higher consistency than the sling psychrometer.

These data findings disagree with the interpretation that the sling system should be used as the gold standard. The current data rather demonstrates that the sling consistently measures inflated values compared to the Kestrel system when its compared to a calibrated RAWS and a chilled mirror hygrometer.
- Brent Ruby, Ph.D


  1. The Kestrel 5500FW measured RH more consistently and more accurately than the slings at 70°F and 100°F compared against the RAWS unit.
  2. When user technique was controlled for, the sling psychrometers still showed higher RH measurements than both the Kestrel 5500FW and the RAWS.

“These data show it’s really hard to get reliable repeatability with the sling. That means you can’t trust the sling for trend data. Our data show that even trained firefighters demonstrate tremendous variability both in technique and ultimately the end result. Even when we standardized the sling protocol, we still saw inaccuracies and inconsistencies as compared to the Kestrel [meters].”

Watch this video to see how the study was done and more details on the results.

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