Dear Customer,
We are writing to notify you of an accuracy issue we have discovered in certain Kestrel 4000 series weather meters. For the majority of Kestrel customers, this issue will not impact the ability of the Kestrel to provide the accurate environmental readings they require for their applications. In fact, only two customers to date have actually reported seeing the impact of this accuracy issue in their application. However, because we are committed to ensuring the Kestrel line’s absolute accuracy and adherence to specifications, we are providing notice to you and providing test, repair and recalibration of any Kestrel meter displaying this flaw at our expense.
All Kestrel 4000 series meters manufactured after 3 February 2006 make use of a temperature sensor inside the relative humidity housing (the white cylinder) to provide accurate humidity readings more quickly in changing conditions. Due to a supplier error, certain units were manufactured with an internal thermistor with response characteristics that did not match the external thermistor. The result of this is that at very low temperatures the Kestrel displays higher values for relative humidity than the true value, and at very high temperatures the Kestrel displays values lower than the true relative humidity.
Please note that at room temperature, the Kestrel is within specifications, and deviates only at the extremes of the temperature range. Note also that only humidity and humidity related measurements (heat stress index, evaporation rate, dewpoint, humidity ratio, relative air density, wet bulb, and density altitude) are affected. Wind speed, temperature, wind chill, pressure and altitude are unaffected and perform within specifications throughout the specification range. This problem impacts only Kestrel 4000 to 4500 models and does not affect the Kestrel 3000 or 3500. Please refer to our list of possible scenarios to get a better idea of how this problem may affect the measurements you use in your application.
Fahrenheit
Celsius
Affected units will have serial numbers between 571631 and 608738 (manufactured between April 2008 and April 2010). We would like to request that you check your unit(s) for this serial number range. If you have a unit(s) in this range, please visit our online return authorization page to obtain a RA number. You can request a prepaid UPS return label at the bottom of the page. Please contact us if you have any questions or problems with the return authorization page. Once received, we will repair and return your unit(s) as soon as possible. If you have any questions, please do not hesitate to ask.
Thank you for your time and we apologize once again for any inconvenience.
Please see below for specific examples of how the Kestrel is used and how it may affect you:
Firefighting: I am a firefighter and use my Kestrel to aid my decisions when planning prescribed burns. How would the thermistor mismatch affect my readings?
The error in the thermistor mismatch causes the Kestrel to display lower relative humidity than actual at temperatures higher than room temperature. As a result, in very hot/dry climates, the Kestrel will err on the safe side because it will indicate that the conditions are drier than they are. Due to the extreme importance of relative humidity accuracy in firefighting, however, we recommend that all firefighters return their unit for a free repair and recalibration.
Long-range Shooting: I use my Kestrel 4000 to take measurements relating to ballistics performance – specifically wind, temperature, barometric pressure and density altitude. How will this issue affect the accuracy of my firing solution?
Wind, temperature and pressure are all unaffected, so these readings will be completely within specifications. Relative humidity has an extremely small effect on air density (whether input as density altitude or humidity into your ballistics calculator), so the humidity inaccuracy is unlikely to impact the firing solution result at all. If this is the only context in which you use your Kestrel 4000 (or 4500), you can safely continue to use it for this purpose.
HVAC: I rely on my Kestrel to give me the necessary information to aid in appropriately sizing AC units and humidification/dehumidification requirements. Could I have sized a unit incorrectly as a result of this thermistor issue?
For residential/commercial AC, an error is very is unlikely because you almost certainly took your measurements at or around room temperature (70-80° F /21-27° C). In this temperature range, the relative humidity measurements are within the +/- 3% accuracy range even with the thermistor mismatch.
Racing: I use my Kestrel to determine air density impact on my ET in bracket races, or to tune my car to the perfect fuel/air ratio. Could the thermistor mismatch have impacted my results?
As discussed above, relative humidity has a very small impact on air density compared to temperature and pressure. So, whether you were referring to density altitude or inputting absolute pressure, temperature and humidity into an ET calculator, your results are unlikely to have been impacted. Furthermore, the errors are only seen at the extremes of the temperature range. At 100° F and 40% relative humidity (true), the Kestrel would measure 36.1%. Less than 1% more than the specified accuracy.
Concrete Pouring: I’ve used my Kestrel to determine if my concrete will dry too fast due to evaporation. Did the Kestrel cause me to compromise the integrity of the concrete?
Highly unlikely. At the temperatures suitable for pouring concrete, the Kestrel would either be within specifications or would under-report humidity, thereby erring on the side of caution. If it was extremely hot at the pour location, the Kestrel would read a lower relative humidity (and a higher evaporation rate in a 4300) than actual. At worst, you may have taken steps to reduce evaporation (covering, misting, add-ins) that were not necessary because the Kestrel showed a faster drying rate than actual.