Thermal Work Limit Weather Stations


Environdata’s Thermal Work Limit (TWL) weather stations can provide accurate, reliable on-site monitoring to manage the risk of human heat stress in the workplace.

Our heat stress weather stations have alert functions and robust Australian Made reliability, built to survive Australia’s harsh conditions and to keep your workers safe and productive.

The TWL firmware (TWL30) is available in all Environdata’s weather stations, with the correct sensors added.  We also calculate the WBGT index for you as part of the package, so you have the full suite of heat stress indices at your fingertips.

Environdata weather station for monitoring thermal work limit located in the Sahara Desert

Key Features

  • Accurate heat stress weather station

  • Protects staff in hot working conditions

  • Maintains productivity in hot conditions

  • Tested throughout Australia & Middle East

  • Accounts for the accumulative effects of sun, wind, temperature & humidity

  • Default values for clothing, posture & sweat rate, or factory set to your specific needs

TWL Applications

  • OH&S heat stress management & alerts

  • Manage productivity & morale in thermally working environments

Environdata weather station for monitoring thermal work limit located at a remote cattle station

Thermal Work Limit Calculations

Some key factors used in calculating TWL can be adjusted during our factory set up of your TWL weather station:

  • Clothing Insulation Factor
  • Clothing Permeability
  • Posture Factor
  • Maximum Sweat Rate

The Clothing Insulation Factor is the level of insulation a body feels from the clothing, and the clothing permeation factor represents the amount of airflow and evaporative cooling that can be achieved in those clothes. Both of these factors in Environdata’s TWL weather stations have been based on the Curtin University research, assuming cotton trousers and long sleeved shirt, with sweat impact on the material also taken into account.

The Posture Factor is based on the posture of normal manual workers (upright).

The Maximum Sweat Rate is the normal maximum sweat rate achieved in steady self paced work by acclimatized workers.

If you wish to adjust these settings in our firmware, we can program them to tune the TWL to your specific requirements, either in the factory programming of your weather station, or in a later revision from us. Changing these parameters should only be done by an appropriately qualified TWL & Heat Stress professional.

About Thermal Work Limit (TWL)

Black Globe Temperature Sensor
Weather Maestro for Monitoring Heat Stress in Cattle - Feedlot Weather Station



Weather Wombat Weather Station with Lockable Housing and Black Globe Temperature Sensor

What is Heat Stress and Thermal Work Limit?

Heat stress occurs when the body’s metabolic heat production exceeds the individual’s ability to dissipate the heat produced.

Heat is dissipated by evaporative cooling from the skin, enhanced by sweating. In humid conditions the efficiency of sweat evaporating and providing cooling from the skin decreases. In windy conditions the evaporative cooling efficiency increases. In sunny conditions, the solar radiation absorbed by the skin means a human absorbs more heat, which then needs to be dissipated as well as the metabolic heat generated by work.

As the principal factor driving metabolic heat production is muscular activity, those working in hot conditions are at greatest risk. For any set of weather conditions, there is a maximum rate at which an individual can dissipate heat i.e. a limiting metabolic rate, and therefore a maximum rate at which they can safely work.

Environments where Heat Stress Protection is Vital

Many industries, particularly in the military, resource or construction sectors, have workers exposed to thermally stressful work environments.

These workers need to be protected from heat stress, but without unnecessarily compromising productivity. Many current measures of heat stress are very conservative in many sets of conditions, as they are too simplistic. This leads to standing workers down when they are safe to continue working with no risk of heat stress.

In research conducted throughout Australia and the Middle East by a research team from Curtin University (Western Australia), Thermal Work Limit (TWL) has been developed to indicate the limiting metabolic rate a worker can sustain given the environmental factors to which they are exposed.

Weather Stations for Thermal Work Limit (TWL)

To be able to calculate TWL on site, you will need a weather station to provide accurate and reliable weather data.  Preferably, the weather station will include built in TWL firmware to determine TWL and help prevent heat stress incidents.  Environdata has incorporated this firmware and it is available for you to take advantage of now.

These TWL weather stations give OH&S professionals and line management the means to implement safe working limits based on weather conditions at any given moment. With the added knowledge that the workers on site are protected by a reliable weather station to alert them based on weather conditions relevant to them, worker morale and productivity can improve.

TWL has been shown to be more appropriate than the traditional Wet Bulb Globe Temperature (WBGT) as a predictor of heat stress. WBGT can also be provided as a standard output from our weather stations, so both measures can be used and compared in heat stress management.

Thermal Work Limit (TWL) Alerts

  • The Weather Maestro weather stations can provide alerts based on the Thermal Work Limit.
  • These TWL alerts can be sent via SMS and email when your system is connected to our WeatherMation LIVE Service. 
  • TWL alerts from the weather stations themselves can also control local devices or activate local alarm devices such as flashing beacons or sirens.

How to use the Thermal Work Limit

  • The TWL output represents the maximum metabolic rate a worker can sustain in terms of the prevailing environmental variables.
  • It presumes the worker is acclimatized and has access to cool drinking water for hydration.
  • As the TWL decreases, this indicates a lower metabolic rate and a corresponding lower level of work that is sustainable.
  • Essentially, for self paced work by environmentally acclimatized workers with access to cool drinking water and shade for breaks, there is no restriction on work if the TWL is above 140w/m2. Between 115w/m2 and 140w/m2 there are strict guidelines to follow to ensure heat stress risk is minimized by those working in these conditions, and below 115w/m2 only essential work should be done with very strict rest-work periods, shade and hydration.

Please refer to your relevant OH&S professional for management guidelines based on TWL. However a quick guide is included.

Thermal Work Limit Chart

* Table reproduced with kind permission of Dr Graham Bates, the lead Researcher into TWL.

Typical Weather Stations

Weather Maestro Weather Station for calculating TWL on remote Mine site


Weather Maestro

The Weather Maestro weather stations include a bigger data logger for more sensors, and the ability to incorporate Cellular, UHF or satellite telemetry.

Most suited when expansion is important and remote communications are required.  TWL can be retrofitted to your current system with the correct suite of sensors and a new program.  Alternatively take advantage of your new system by adding it now.

More infoContact Us

WeatherMate 3000 Weather Station able to calculate TWL once Black Globe is added


WeatherMate 3000

The basic WeatherMate weather station for TWL includes wind speed, relative humidity, air temperature and black globe temperature sensors. Also includes the mounting hardware, solar power, weatherproof housing, pre-wiring, programming.

Most suited to a direct cable connection to a nearby office, this system provides an entry level high quality robust system.

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Some Typical Sensors

Black Globe Temperature Sensor


Radiant Heat Sensor - also known as a Black Globe Temperature Sensor.  Designed to exacting specifications to represent the heat load on a body, the sensor consists of a 6" Copper Globe powder coated matt Black, with a high accuracy and high stability temperature probe inserted in the centre.  Sensors with smaller diameter globes can often produce much different "Globe" temperatures, putting both the data and human health at risk.

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3-cup Anemometer wind speed sensor


Wind Speed Sensor - 3 Cup Anemometer

Accurate and reliable wind speed measurements are crucial to an accurate TWL calculation, as the wind's ability to remove heat from the body's surface through enhanced evaporative cooling is a critical component of managing thermal stress and removing excess metabolic heat.


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Ambient Air Temperature Sensor


Ambient Temperature is another key part of the TWL equation.  As the ability for the body to remove heat hinges on the heat within the immediate environment, so the TWL equation relies on an accurate and reliable temperature sensor.  Ideally you want a sensor accurate to +/- 0.1 Degrees Celcius and able to be regularly calibrated.

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Environdata are Australia’s weather station specialists since 1982, so please contact us for advice on your weather monitoring requirements, positioning on your site, and general advice on all things weather station related.

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