Measuring WBGT for indoor heat limits
What wet bulb globe temperature is, where to stand, and why no weather feed can give it to you.
Wet bulb globe temperature combines air temperature, humidity, air movement, and radiant heat into one number. Minnesota indoor limits are written in WBGT, and heat stress professionals prefer it because it captures the radiant load a heat index ignores. There is no weather service feed for WBGT, because radiant heat is local to the spot, so somebody has to walk the space with an instrument.
Takeaways
- Explain what WBGT captures that temperature and heat index do not.
- Take a reading in the right place, at the right time.
- Pair every reading with a workload so it can be evaluated.
- Choose between a full WBGT meter and the alternatives.
The four inputs WBGT combines
Where and when to take the reading
A WBGT with no workload is unreadable
What to buy, and the alternatives
Frequently asked questions
- Can I use heat index instead of WBGT for a Minnesota indoor site? Not for the Minnesota limits, which are written in WBGT. Heat index ignores radiant heat and air movement, so it can read comfortable in a space that exceeds the WBGT ceiling.
- How often should I measure? Often enough to describe the conditions employees actually worked in. Once at the hottest part of the shift is a practical minimum, and more often when a process cycles or a bay door changes the airflow.
- Does clothing change the WBGT limit? Clothing does not change the measured WBGT, but heat stress practice applies a clothing adjustment factor to the limit. Vapor-barrier and multi-layer clothing effectively lower the temperature at which the limit is reached.
- Is outdoor WBGT worth measuring? It is the most accurate outdoor heat stress metric and it underpins the NIOSH approach that Oregon accepts as an alternative to its rest table. It takes more effort than reading a heat index, which is why most rules do not require it outdoors.