Black Bulb Sensors: Why Size Matters in WBGT & Heat Stress Readings


Black Bulb Sensors: Why Size Matters in WBGT & Heat Stress Readings

May 15, 2024
Written By
Saba Arif

What’s round, gets really hot, and saves lives? It’s a black bulb sensor, of course! 

Still don’t know what that is? Picture this: You’re pushing your limits halfway through afternoon practice under a relentless summer sun. But how do you know you’re pushing too hard? This is where the little black bulb plays a big role. Here’s all you need to know about how a black bulb sensor transforms your weather safety and training plans and why its size matters. 

What is a Black Bulb Sensor?

If you’re an athletic trainer, you’ve probably heard this term thrown around a LOT lately: Wet Bulb Globe Temperature (WBGT). To put it simply, WBGT is a measurement that assesses heat stress. The black bulb sensor, also known as the black globe, calculates the wet bulb globe temperature, a crucial tool for preventing heatstroke and heat exhaustion. So what is it, exactly?

A black bulb sensor is a type of thermometer housed inside a black ball that measures radiant heat in an area.

Black globe temperature indicates how hot it feels in direct sunlight.

Black bulb sensors provide a more accurate picture of how hot it feels outside, taking into account not just air temperature but also the sun’s intensity.

The Role of Black Bulb Sensors in Wet Bulb Globe Temperature Measurement

To calculate WBGT, we measure three crucial components:

Radiant Heat: This is measured by the black globe temperature. The black globe temperature represents the combined effects of radiation and wind.

Evaporative Heat: This measures the wet bulb temperature and mimics how sweat cools our bodies. Higher humidity reduces evaporation, leading to a higher wet bulb temperature.

Air Temperature: This is the classic “dry bulb” temperature we’re all familiar with.

How Do Black Bulb Sensors Work?

Black bulb temperature sensors calculate comfort temperature, which is the average of the conductive and radiant temperatures. This is the temperature that a person would feel comfortable at in the current environmental conditions. Black bulb sensors work by mimicking how surfaces heat up under the sun’s rays. Here’s a breakdown of the process:

Absorption: The key element is the black coating. Black is a fantastic absorber of radiant heat, including solar radiation, converting light energy into heat energy.

Heating Up: As the black bulb absorbs radiation, it heats up. A sensor inside the bulb then measures this temperature increase.

Temperature Measurement: Inside the black bulb is a temperature sensor, similar to what you might find in a regular thermometer.

WBGT Data Collection: The temperature sensor then calibrates and transmits this data to a WBGT meter or weather station.

The black coating on the bulb means that not only does a black globe sensor absorb heat faster, but that direct sunlight will make the globe hotter, and colder temperatures will cool off the globe. The wind carries heat away from the globe when the air is cooler, and increases heat transfer when the air is warmer than the globe, causing the globe to get closer to the ambient air temperature. Seems pretty obvious, doesn’t it? 

Size Matters: Why Sensor Size Impacts Accuracy

Accurate WBGT readings can help prevent heatstroke and heat exhaustion, keeping athletes safe. The size of the black bulb directly impacts the accuracy of the temperature reading in any weather monitoring device. Here’s how: 

  1. Surface Area: Larger black bulbs have a greater surface area, which means they can absorb more solar radiation, leading to more accurate readings of radiant heat. Conversely, smaller sensors absorb less radiation, potentially underestimating the actual heat stress risk, putting athletes at risk. 

  2. Stable Response Times: Larger bulbs have a higher heat absorption threshold, resulting in more stable globe temperature readings and avoiding inaccurate spikes.

  3. Cooling Effect: A larger size experiences less convective cooling from wind, further increasing the globe temperature reading.

In short, a bigger black bulb provides a clearer, more accurate picture of heat stress by better capturing the sun’s heat and minimizing misleading fluctuations. This accuracy is crucial, as underestimating or overestimating heat stress can lead to unnecessary disruptions in outdoor activities.

Perry Weather vs The Competition: Why Perry Weather’s Black Bulb is Better

When it comes to athlete safety, accurate heat stress monitoring is crucial. Here’s why Perry Weather stands out from competitors like Earth Networks, DTN WeatherSentry, and Kestrel in this critical area:

Real Sensor, Real Results:  Unlike competitors like Earth Networks and DTN WeatherSentry, whose weather stations are not equipped with black bulb sensors, Perry Weather’s weather station utilizes a large, 3.5-inch black bulb sensor to provide actual, measured WBGT data, and not just an estimation. This ensures a more accurate picture of heat stress risk, and precise WBGT readings.

The Bigger, The Better

The Kestrel 5400 uses a 1-inch black globe to monitor site-specific heat data. Perry Weather’s larger 3-inch black globe offers precise readings with fewer spikes, for a more accurate solution. This larger size reduces the risk of underestimating or overestimating the actual heat stress level.

No Waiting Necessary

Unlike the Kestrel 5400 which requires a 30-minute calibration period to adjust to true atmospheric conditions, Perry Weather’s black bulb sensors are ready to go – no waiting or additional calibration necessary. This allows for immediate monitoring and quicker action to prevent heat-related illnesses.

Placement Matters Too:  While sensor size is crucial, proper placement matters, too. Incorrect placement, such as in a shaded area or near a heat source, can lead to inaccurate readings and potentially dangerous situations. Perry Weather understands this and guides the placement of the sensor in a well-ventilated area representative of the work environment. This ensures the WBGT reading reflects the actual heat stress experienced.

Learn More About On-Site WBGT Sensors here.


Those little black bulb sensors are not just a design choice, they’re the key to a safer environment. By understanding their role and size, you can ensure accurate WBGT readings and create a safer space for everyone. Don’t settle for estimates when it comes to safety. Contact us today to learn more about our WBGT monitoring solutions and how they can help you create a safer training environment for your team.

More Like This