Heat stress is about the work.
Not just the weather.
Heat stress monitoring measures the combined effects of air temperature, humidity, radiant heat and air movement on people working in hot conditions. Wet Bulb Globe Temperature (WBGT) indicates the level of thermal stress. Thermal Work Limit (TWL) goes further, estimating the sustainable work rate for an acclimatised person under the measured conditions.
Gill developed heat stress measurement within the MaxiMet range to bring these measurements together in a compact, fixed-site weather station. The MaxiMet GMX552 measures the environmental parameters needed to calculate WBGT and TWL onboard, giving site operators continuous data to support decisions about work, rest and exposure.
How do you measure heat stress at a work site?
Heat stress is measured where people work, not where the forecast is made. An index such as WBGT combines air temperature, humidity, radiant heat and air movement into one figure that a workplace policy can act on. A fixed station such as MaxiMet GMX552 measures those conditions continuously and calculates WBGT onboard, with the WBGT calculation in accordance with ISO 7243:2017.
The temperature in a weather forecast is not necessarily the temperature experienced by someone working outdoors. A person on an exposed haul road, beside hot machinery or on a sunlit construction site may face very different conditions from those recorded at a nearby weather station.
Radiant heat from direct sunlight and hot surfaces adds to the body’s thermal load. High humidity limits the effectiveness of sweating, while air movement can help the body lose heat. These factors vary across a site and throughout the day, particularly during periods of intense physical work.
Air temperature alone cannot describe that combined effect. Nor can an occasional reading reliably represent conditions throughout a shift.
For organisations managing outdoor work, the challenge is to measure the conditions where people are exposed, maintain a record as those conditions change, and make the resulting information available to the people responsible for operational decisions.
That is particularly important on large or exposed sites, where conditions can differ between work areas and where stopping, rescheduling or slowing work has operational consequences.
What needs to be measured
Heat stress indices combine several environmental measurements into a figure that can be interpreted within an established assessment or workplace policy. The quality of the result depends on measuring the relevant conditions appropriately.
Air temperature
Air temperature provides an important part of the picture, but it does not account for radiant heat or the effect of humidity and air movement on the body.
Humidity and wet-bulb temperature
Humidity affects the body’s ability to cool itself through evaporation. WBGT incorporates natural wet-bulb temperature, which reflects the combined influence of humidity, air movement and other environmental factors on evaporative cooling.
Radiant heat
A black globe absorbs radiant heat from sunlight and surrounding surfaces. This helps distinguish conditions in direct sun from those in shade, even when the measured air temperature is similar. The MaxiMet GMX552 uses a 150 mm black globe, and its WBGT calculation is in accordance with ISO 7243:2017.
WBGT calculation in accordance with ISO 7243:2017
Wind speed
Air movement affects heat exchange between the body and its surroundings. Wind speed is also an input to the Thermal Work Limit calculation, making it relevant to the assessment of sustainable work rates.
Atmospheric pressure
Pressure is another input used in the onboard TWL calculation. Measuring the relevant environmental parameters together allows the station to calculate the index without requiring a separate downstream calculation.
WBGT and TWL onboard
Choosing a heat stress measurement approach
The appropriate approach depends on the size and nature of the site, how frequently conditions need to be assessed, how decisions are made and whether data needs to be integrated into another system.
| Consideration | Handheld WBGT meter |  Fixed station: MaxiMet GMX552 | Forecast or general heat index |
|---|---|---|---|
| Measurement location | Wherever the user takes the reading | Continuously at the installed location | Typically a regional or forecast location |
| Measurement frequency | When a person takes a reading | Continuous measurement and logging through the connected system | Depends on the forecast or service |
| Radiant heat | Depends on the meter and configuration | 150 mm black globe | Not measured at the work site |
| WBGT | Depends on the meter | Calculated onboard, in accordance with ISO 7243:2017 | Not a substitute for site-specific WBGT measurement |
| TWL | Depends on the instrument and calculation method | Calculated onboard | Not provided as a site-specific TWL measurement |
| Data integration | Often standalone, depending on the instrument | Digital output for connection to site systems | Depends on the service |
| Most appropriate use | Spot checks, surveys and mobile teams | Work areas requiring continuous, fixed-site measurement | Planning and general awareness of forecast conditions |
A handheld meter can be the appropriate choice when teams move between locations or need to assess several work areas. A fixed station is more appropriate where a particular work area requires continuous measurement and a consistent record. On a large site, fixed stations and handheld checks may be complementary rather than competing approaches.
Forecasts remain useful for planning, but they cannot establish the actual heat-stress conditions at a particular work location.
Siting, data and operational decisions
Siting changes the reading
A technically suitable station can still produce unrepresentative readings if it is installed in the wrong place. The black globe needs to reflect the radiant heat exposure relevant to the work being assessed. Shelter, nearby structures, reflective surfaces and the position of the measurement point can all affect the reading.
One point, not the whole site
A fixed station also measures the conditions at its own location, not across an entire site. Large operations or sites with materially different exposures may require several measurement points, supported by handheld assessments where appropriate.
The data has to reach the people who use it
Before installation, establish whether readings will be displayed locally, logged for later review or integrated into a site monitoring system. Any alert thresholds and resulting work-rest actions should be defined through the organisation’s heat-management procedures and appropriate occupational health advice.
Measurement supports the policy, not replaces it
Continuous measurement supports a more complete record of changing conditions, but it does not replace a risk assessment, a heat-stress policy or decisions about individual workers.
The engineering behind MaxiMet GMX552
The GMX552 brings together the environmental measurements required for heat stress assessment in a compact weather station. Rather than assembling separate instruments for wind, temperature, humidity, pressure and radiant heat, users can obtain the relevant measurements from one integrated unit.
This approach is particularly useful where installation space, cabling and access to the measurement point matter. The station combines a WindSonic-based ultrasonic wind sensor with temperature, humidity, pressure and a black globe, using one cable.
The integrated design also allows WBGT and TWL to be calculated onboard. That reduces the need to build separate index calculations into the receiving system, although users still need to establish how the data will be displayed, recorded and used within their own operational procedures.
A further benefit is the ability to update compatible GMX552 units already in service to calculate TWL through firmware. This can extend functionality without requiring new hardware, subject to compatibility and the applicable update process.
The measurement station is one part of the solution. Siting, data access, alarm configuration where required, record retention and the organisation’s response procedures must also be considered.
MaxiMet GMX552
Heat stress weather station
The GMX552 combines environmental measurement and onboard heat-stress calculations in a compact station for fixed-site monitoring.
TWL calculation is available by firmware update on compatible units already in service. Confirm compatibility and current specifications before planning an upgrade.
MaxiMet range
When a wider weather station is more appropriate
Not every site needs onboard WBGT and TWL calculations. Some applications need a broader picture of local weather, with measurements selected for the environment and operational purpose.
The wider MaxiMet range combines parameters such as wind, temperature, humidity, pressure, rainfall and solar radiation in different configurations. The appropriate model depends on the measurements required and the system into which the data will be integrated.
For heat-stress applications, the GMX552 is the relevant MaxiMet model. For general site weather monitoring, the wider range provides alternative combinations of environmental measurements.
Calibration and measurement confidence
UKAS-accredited, through Labcal
Heat-stress assessments depend on the quality of the underlying measurements. Temperature and humidity calibration can form part of a measurement assurance programme, helping organisations understand the status of their instruments and maintain appropriate records.
Labcal, part of the Gill Group, is a UKAS-accredited calibration laboratory (UKAS Lab 0625) operating to ISO/IEC 17025:2017. It provides temperature and humidity calibration for instruments from Gill and other manufacturers.
Calibration requirements depend on the instrument, the parameters being measured and the assurance requirements of the application. Confirm the relevant scope and service before specifying calibration.
insight
WBGT and TWL answer different questions
WBGT and TWL are related, but they are not interchangeable. Understanding the distinction helps determine which measurements are required and how they should inform workplace decisions.
WBGT: how stressful are the conditions? Wet Bulb Globe Temperature combines natural wet-bulb temperature, black globe temperature and air temperature into an index defined by ISO 7243. It is widely used in occupational heat-stress assessment and workplace heat policies. The resulting value can be considered alongside workload, clothing, acclimatisation and the organisation’s chosen assessment method.
TWL: what work rate can be sustained? Thermal Work Limit estimates the maximum sustainable metabolic work rate for an acclimatised person under the measured environmental conditions. It incorporates air temperature, wet-bulb temperature, radiant heat, wind speed and atmospheric pressure, returning a value in watts per square metre.
TWL was developed in Australian mining and is used in some industries and regions, including mining operations in Australia and the Middle East. Its applicability depends on the workforce, work conditions and relevant procedures.
The practical distinction is important. WBGT characterises the level of thermal stress. TWL provides an estimate of sustainable work rate under defined assumptions. Neither index, on its own, determines whether a person can safely continue working. Organisations must apply appropriate occupational health guidance, acclimatisation provisions, workload considerations and site-specific policies.
On the MaxiMet GMX552, both indices are calculated onboard, giving users access to WBGT and TWL from one station.
The full explanation is in What should I consider when choosing a weather station?
Where heat-stress monitoring is used
IndustryMining, quarrying and cement
Open pits, processing areas and other exposed work locations can combine radiant heat, high air temperatures and physically demanding work. Site-specific measurement can support the assessment of conditions and the application of workplace procedures.
IndustryDefence and security
Training areas and deployed sites can expose personnel to demanding outdoor conditions. Monitoring needs to reflect the actual working environment and the operational procedures in place.
ApplicationTemporary events and crowd safety
Outdoor events can involve changing exposure to sun, crowd density and local weather. Heat-stress measurement may form part of wider environmental and event-safety planning.
NewsProduct development
Gill’s heat-stress measurement capability brings together environmental sensing and onboard index calculations in the MaxiMet platform. Read about the launch of Gill heat-stress measurement.
Speak to our team
Tell us what you need to measure, where and under what conditions. We’ll help you work out the appropriate approach.
Heat stress questions
Wet Bulb Globe Temperature (WBGT) is an index used to assess environmental heat stress. It combines natural wet-bulb temperature, black globe temperature and air temperature according to ISO 7243. It is used alongside factors such as workload, clothing and acclimatisation when assessing heat exposure.
WBGT is calculated from measurements of natural wet-bulb temperature, black globe temperature and air temperature. The black globe captures the influence of radiant heat, while natural wet-bulb temperature reflects the combined effects of humidity and air movement. The measurement method and equipment should be appropriate to the applicable standard and assessment.
No. WBGT and the heat index are different measures. WBGT incorporates natural wet-bulb temperature and black globe temperature as well as air temperature, making it suitable for particular occupational heat-stress assessments. The heat index generally combines air temperature and humidity and does not account for radiant heat in the same way.
Thermal Work Limit (TWL) estimates the maximum sustainable metabolic work rate for an acclimatised person under specified environmental conditions. It uses environmental inputs including air temperature, wet-bulb temperature, radiant heat, wind speed and atmospheric pressure, and expresses the result in watts per square metre. Its use must reflect the assumptions and procedures appropriate to the workforce and application.
The choice depends on the applicable occupational health guidance, workplace policy, work conditions and the decisions the organisation needs to make. WBGT is widely used in occupational heat-stress assessment. TWL estimates sustainable work rate for an acclimatised person under its defined assumptions and is used in some industries and regions. The indices are not interchangeable, and neither should be treated as a complete safety decision on its own.
A suitably configured weather station can measure the environmental parameters needed to calculate a heat-stress index. Gill’s MaxiMet GMX552 calculates WBGT and TWL onboard, using its integrated wind, temperature, humidity, pressure and black-globe measurements. Correct siting and appropriate use of the resulting data remain essential.
A handheld meter provides a reading at the location and time of the assessment. A fixed station can measure continuously at an installed work area, supporting a record of changing conditions. Handheld equipment is useful for mobile teams and spot checks, while a fixed station is useful where ongoing monitoring is required. The two approaches can be complementary.
TWL calculation can be added to compatible GMX552 units through a firmware update, without new hardware. Confirm the compatibility of the specific unit and the current upgrade procedure with Gill.
No. A measurement station provides environmental data and calculated indices. The thresholds, work-rest arrangements and actions taken in response must be established through the organisation’s risk assessment, heat-management policy and appropriate occupational health advice. Decisions may also need to account for workload, clothing, acclimatisation and individual circumstances.
The station should be positioned to represent the conditions relevant to the work being assessed, including exposure to radiant heat and local air movement. Nearby structures, shade and reflective surfaces can influence readings. Because a fixed station measures conditions at its installation point, large or varied sites may need more than one monitoring location.