Thermal Work Limit Weather Station

(Utilizing the TWL30 Firmware)

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


  • Accurate heat stress weather station
  • Protects staff in hot working conditions
  • Maintains productivity in hot working 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


  • OH & S heat stress management & alerts
  • Manage productivity & morale in thermally stressful working environments

TWL Summary

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.

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.

Environdata can now incorporate TWL into our weather stations, the TWL30 firmware option, to provide an accurate, reliable and robust Heat Stress Weather Station to determine TWL and prevent heat stress.

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.

Sensors Used

  • Air Temperature (Ambient)
  • Relative Humidity (Ambient)
  • Wind Speed (2m)
  • Radiant Heat (Black Globe Temperature)
  • Barometric Pressure (Optional)

Factory Programmable Factors

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, and 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.

Firmware Outputs

  • Wet Bulb Temperature (0C)
  • WBGT (0C)
  • TWL (Watts/m2)

Weather Stations

The TWL firmware (TWL30) is available in all Environdata’s new generation of heat stress weather stations:

  • Weather Maestro
  • WeatherMate 3000
  • WeatherMaster 3000

TWL Alerts

The Weather Maestro and WeatherMaster 3000 weather stations can provide alerts based on the TWL.

These TWL alerts can be sent via SMS where a modem and alarm firmware is fitted to the weather station.

TWL alerts from the weather stations can also control local devices or activate local alarm devices such as flashing beacons or sirens.

How to Use the TWL

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.