Why the Kestrel 5400 Occasionally Gives Higher Readings Than the Zelus WBGT System

Wet Bulb Globe Temperature (WBGT) is a crucial metric in environmental and occupational health, but it’s not directly measured. Instead, WBGT is a calculated value derived from multiple measurements. With no standardized method for this calculation, variations can occur when different systems are used.

The Kestrel 5400, a widely used device, has a unique approach to measuring and calculating WBGT. This uniqueness, however, has been linked to higher than expected readings. Multiple independent studies have demonstrated that the Kestrel 5400 tends to provide WBGT readings that are higher than those from a gold-standard piece of equipment. This variance becomes more pronounced as WBGT values climb. For instance, at a WBGT of 86° F, about 75% of Kestrel 5400 readings are higher than the actual WBGT. This disparity escalates dramatically, reaching nearly 100% at a WBGT of 92° F.

Graph showing warm bias of Kestrel 5400

There are a couple of probable reasons for this trend:

Insolation Effect or Hot-Car Error

The Kestrel 5400 is subject to this effect, much like a car heating up in the sun. This results in elevated temperature readings, especially on sunny days. Kestrel’s literature acknowledges this issue, noting it cannot be corrected.

Kestrel handout with insolation error highlighted

Equations for Wet Bulb Temperature

The Kestrel 5400 uses equations developed for an aluminum smelting plant to calculate Natural Wet Bulb Temperature. On days with high sun and low wind, these equations assume additional radiant heat sources, like an aluminum smelter, are near, which may contribute to the device’s progressively inaccurate readings as WBGT increases.  More research needs to be performed to determine if this contributes to the progressively incorrect readings the Kestrel 5400 reports as the WBGT increases.

In contrast, the Zelus WBGT system is not subject to these errors. It does not experience insolation error and uses different equations that are not based on the conditions of an aluminum smelting plant. Consequently, the Zelus system occasionally reports lower WBGT readings than the Kestrel 5400, especially under conditions where the Kestrel is most prone to inaccuracies.

Understanding these differences is vital for accurate environmental monitoring and ensuring safety in heat-stressed environments.

Until next time, stay cool!😎

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