Athletics and schools
- A handheld unit that logs and alarms is the usual choice; the Kestrel 5400 is named explicitly in Oklahoma’s OSSAA policy.
- Check your state association’s wording first — some require an instrument, some accept apps.
A WBGT meter, also sold as a wet bulb globe thermometer or heat stress meter, measures three things at once and combines them into a single index. The three sensors are the whole point, and they are what separates a real instrument from a weather gadget that reports temperature and humidity.
The three sensors
Dry bulb
A shielded air temperature sensor. Weighted 0.1 outdoors, and the least important of the three.
Natural wet bulb
A wetted wick, deliberately left unaspirated and unshielded so it responds to the actual wind and sun. Weighted 0.7.
Black globe
A matte black sphere measuring radiant heat from sun, sky, turf and pavement. Weighted 0.2.
The combination
Outdoors, 0.7 natural wet bulb + 0.2 globe + 0.1 dry bulb. Indoors the dry bulb term drops and globe carries 0.3.
How accurate are WBGT meters?
This question has a published answer. Cooper et al. (2017), in the Journal of Athletic Training, tested six commercially available WBGT monitors against a custom reference unit built to ISO specification, and reported the error for each.
| Device | Mean absolute error | RMSE | Bias |
|---|---|---|---|
| QUEST°emp 34 | 0.24°C | 0.44°C | −0.64% |
| Extech HT30 | 0.61°C | 0.79°C | +0.44% |
| WBGT-103 Heat Stroke Checker | 0.78°C | 0.97°C | +2.21% |
| REED SD-2010 | 0.88°C | 1.00°C | +2.14% |
| Kestrel 4400 | 0.95°C | 1.23°C | −3.14% |
| General WBGT8778 | 1.18°C | 1.34°C | +4.25% |
Two things are worth drawing out. First, the spread is real but not disqualifying: every unit landed within about 1.2°C of the reference, which is roughly 2°F, and the authors judged the Extech, QUEST°emp and Kestrel units to give “conservative yet accurate” assessments suitable for field use.
Second, and more useful when choosing: the study found that instruments relying on psychrometric wet bulb temperature tended to underestimate WBGT in low wind. Underestimating in still air is the worst direction to be wrong in, because still air is when heat strain is highest.
Note the date. This was published in 2017 and tested the Kestrel 4400, the predecessor to the current 5400. Treat it as evidence about device classes and measurement methods rather than a current product ranking.
Cooper E, Grundstein A, Rosen A, Miles J, Ko J, Curry P. An Evaluation of Portable Wet Bulb Globe Temperature Monitor Accuracy. J Athl Train. 2017;52(12).
The two design choices that matter
Beneath the brand names, WBGT instruments differ in two ways that determine both price and behaviour.
How they get the wet bulb reading. Professional units such as the QUEST°emp 32/34/36 series use a genuine water-soaked wick, which means a reservoir to fill and a stabilisation wait before every session. Handheld units such as the Kestrel 5400 carry no water at all and derive natural wet bulb from temperature, humidity, wind speed and solar load. Kestrel markets this as an advantage, and for field use it plainly is — but it means the market-leading instrument is also computing part of its answer rather than measuring it.
Globe size. The ISO reference is a 150 mm black globe. Almost no portable unit carries one. The Kestrel 5400 uses a 25 mm copper globe and converts the reading to a 150 mm equivalent; the Extech HT30 uses a globe of around 40 mm. Smaller globes respond faster to changing conditions, which is genuinely useful, but the conversion is where error enters. If a device does not state that it corrects to a 150 mm equivalent, be sceptical of the number it reports.
Which class of device fits
Industrial hygiene and construction
- Area monitors on a tripod, such as the QUEST°emp series, are built to sit and log continuously at a fixed location.
- Wicked wet bulb and larger globes, at the cost of setup time and water.
Military and emergency services
- Ruggedised handhelds with logging, often the same Kestrel line.
- Clothing adjustment factors are applied on top of the reading, not by the device.
Occasional or budget use
- Entry-level meters exist and the study above suggests the better ones are usable.
- Be wary of the cheapest units: the widest error in that test was more than four times the best.
What to look for
- Calibration interval. South Carolina's athletic policy requires recalibration every two years. Check what the manufacturer specifies and what your governing body requires.
- Data logging. If your policy requires recorded readings every 30 minutes, a device that logs automatically saves an enormous amount of friction and produces better records.
- Indoor mode. A device that switches to the no-solar weighting matters if you assess gyms, warehouses or kitchens.
- Mounting. A tripod thread. Readings are taken at roughly chest height away from obstructions, not held in your hand or set on a truck bed.
Named instruments and what policies accept
Some governing bodies name acceptable equipment outright, which is the most reliable guide available. Oklahoma's OSSAA policy, for example, lists the Kestrel 5400 alongside on-site weather station services and several internet-based sources as acceptable methods.
Other associations are stricter about the instrument and looser about the brand. Georgia's By-Law 2.67 requires a “scientifically approved instrument” at each practice without naming one, and South Carolina requires a scientifically approved on-site thermometer while stating explicitly that phone apps are not approved.
Before buying, read your own governing body's wording. It will usually tell you whether you need an instrument at all, and occasionally it will tell you exactly which one.
When you need a meter, and when you do not
Being straight about this matters more than selling anything. These are different jobs.
| WBGT meter | App | |
|---|---|---|
| Measures your exact microclimate | Yes, by direct measurement | No, it models conditions rather than measuring them |
| Accounts for synthetic turf | Yes, if you stand it on the turf | Yes, Pro turf surface mode changes the surface modifier in the calculation |
| Accounts for real cloud cover | Yes, inherently | Yes, Pro lets you replace forecast cloud with what is overhead |
| Sees a trench, roof or hot process | Yes | No |
| Satisfies a measure-on-site policy | Yes | No |
| Forecasts ahead | No | Yes, hourly and 14 days |
| Works at a site you have not reached | No | Yes |
| Needs a wetted wick and calibration | Yes | No |
| Available to every supervisor at once | Only where the device is | Yes |
If your policy requires on-site measurement, buy the instrument. No model satisfies a rule that says measure here, now.
The gap between the two is narrower than it used to be. The usual argument against app readings is that they describe a weather station over grass rather than your synthetic turf, and the WBGT app’s Pro turf surface mode addresses that directly by changing the surface modifier in the calculation. What an instrument still does uniquely is measure a trench, a roof or the air beside a running compressor, and that is what you are buying it for.
Getting a reading worth recording
- Set the device up at the activity surface, not the parking lot or the building entrance.
- Put it in full sun if the activity is in full sun. Shading the instrument describes nobody.
- Chest height, on a tripod, clear of walls, vehicles and equipment exhaust.
- Wet the wick with distilled water. Tap water leaves deposits that bias the reading.
- Let it equilibrate, typically 10 to 20 minutes, before trusting the first number.
- Log every reading with the time and location. An unrecorded reading demonstrates nothing afterwards.
What the instrument cannot do
A meter tells you what is happening where it stands, right now. The app covers the other half: a reading for any location, a 14-day forecast for scheduling, and the same number on every supervisor's phone. Most programs that take heat seriously end up using both.
- Real-time WBGT for your location or any city
- Hourly and 14-day WBGT forecasts to plan ahead
- Color-coded flag levels with mode-specific guidance
- Pro: turf surface mode for synthetic fields
- Pro: overhead cloud adjustment from what you can see
- Humidity, wind, sun and cloud cover behind every reading
WBGT is estimated from weather data with the peer-reviewed Liljegren model. Where a policy requires on-site measurement, use the app for planning and as a cross-check alongside your instrument.
Sources
- Cooper E, Grundstein A, Rosen A, Miles J, Ko J, Curry P. An Evaluation of Portable Wet Bulb Globe Temperature Monitor Accuracy. J Athl Train. 2017;52(12):1161–1167.
- International Organization for Standardization. ISO 7243:2017, Ergonomics of the thermal environment — Assessment of heat stress using the WBGT (wet bulb globe temperature) index.
- Liljegren JC, Carhart RA, Lawday P, Tschopp S, Sharp R. Modeling the wet bulb globe temperature using standard meteorological measurements. J Occup Environ Hyg. 2008;5(10):645–655. doi:10.1080/15459620802310770
Common questions
What does a WBGT meter measure?
Three things simultaneously: dry bulb air temperature, natural wet bulb temperature from a wetted unaspirated wick, and black globe temperature from a matte black sphere measuring radiant heat. It combines them as 0.7 natural wet bulb plus 0.2 globe plus 0.1 dry bulb outdoors.
How much does a WBGT meter cost?
They range from inexpensive handheld units to professional logging instruments costing well into the hundreds or thousands. Price generally tracks globe size, whether the wet bulb is genuinely measured rather than modelled, data logging, and calibration traceability. Check what your governing body requires before choosing on price.
How often does a WBGT meter need calibration?
Follow the manufacturer's interval. Some governing bodies set their own: South Carolina's athletic heat guidelines require recalibration every two years or sooner if the manufacturer recommends it.
Can I use an app instead of a WBGT meter?
It depends on your policy. Oklahoma explicitly permits internet-based applications and names acceptable ones; Mississippi pointed schools to free apps. South Carolina states that phone apps are not scientifically approved, and Georgia and Florida specify an on-site instrument. See state WBGT rules.
What size should the black globe be?
The reference standard is a 150 mm globe. Smaller globes respond more quickly to change but need a correction applied to match the reference, so check that a device with a small globe reports a corrected value.
Where should a WBGT meter be placed?
At the activity surface, in the sun if the activity is in the sun, at roughly chest height on a tripod, clear of walls, vehicles and equipment exhaust, with the wick wetted using distilled water and 10 to 20 minutes to equilibrate.
Related WBGT guides
Know the real heat number
Real-time WBGT, flag levels and forecasts. Free on iPhone.