Some firefighters have questioned the accuracy of sling psychromters to measure relative humidity, especially when comparing them to data from digital weather meters. Of two studies, both preliminary or draft, that analyzed the accuracy of these devices, one found that they were inaccurate at low RH levels, and the other concluded the same thing at temperatures higher than 85°F.
The second study mentioned above is titled Accuracy and Linearity of Sling and Digital Psychrometers, by Phillip K. Parson and Tom Swan.
The Parson-Swan study above, which was marked “draft”, concluded:
Relative humidity taken using a sling psychrometer appears to be much greater than electronic sensors in the 85°F range and greater. Both electronic meters appear to track well together to slightly over 100°F. Under field conditions, it is much more likely inspectors/operators would get much more reliable data using electronic psychrometers.
The other study, which appears to be preliminary, is titled Comparison of Sling Psychrometer to Digital Weather Meters, by Chuck McHugh and Larry Bradshaw of the USFS Fire Sciences Lab in Missoula, Montana. A 47-minute video of their presentation is available, but the audio is terrible. Below are some screen shots.
The image below shows RHs measured by a sling psychrometer on a prescribed fire that are 3 to 14 percent higher than data from a Kestrel digital weather meter.
The weather instruments were tested in an environmental chamber.
In the graphs above, “Log Tag”, the solid RED line, appears to be data from a highly accurate calibrated meter.
Chuck McHugh here. Regarding the current work that Larry Bradshaw and I are doing I want to stress that the referenced video recording and the materials currently being shown on this website are PRELIMINARY only from Phase 1 of our study. We are just beginning Phase 2 of measurements this week. So please use or interpret this information with caution understanding that the study is not complete at this time. Our current plan is to have the measurements completed by next week and then finish the analysis and work on having the results formally published and release the findings via the Lessons Learned Center via webinar down the road.
Thanks Chuck. And, good luck with the audio during the next webinar.
“the dry bulb temperature may vary by ± 10 oF or more from the reading observed prior to slinging it”
I wonder how well versed in using the psychrometer the user/testers are. I know that almost everyone I observed using them swings them much too fast. If you swing them at the correct speed, the dry bulb temp shouldn’t go down, as it was explained to me.
Probably should read the whole report before I comment, but just reading the introduction that quote struck me.
Parson/Swan conclude that determining the accuracy of the test instruments was “beyond the scope” of their study.
In spite of that, the quote that begins “Relative Humidity taken using a sling psychrometer…” was taken from the “CONCLUSIONS” section of the report.
Regarding “Why Study?”, some firefighters can’t HANDLE the truth.