Thermal Work Limit

Thermal Work Limit (TWL), now included in the AIOH Heat Stress Standard, is a rational heat stress index that calculates the "limiting" metabolic (work) rate that moderately fit and healthy, euhydrated, acclimatised persons can sustain in any given environment and takes into account all the relevant environmental factors (air temperature, wind speed, radiant heat, relative humidity and barometric pressure), clothing insulation and vapour permeation and user-defined limits on allowable sweat rates and deep body core temperatures. TWL has become the de-facto standard in underground mines in Australia, probably the most adverse occupational setting for heat stress in Australia. However, it can easily be applied to sporting or other environments.

D.J. Brake PhD Thesis

Abstract from The Deep Body Core Temperatures, Physical Fatigue and Fluid Status of Thermally stressed Workers and the Development of Thermal Work Limit as an Index of Heat Stress, PhD thesis, Curtin University by D. J. Brake, 2002
Download Abstract as .pdf
Full Thesis
You can download the full thesis by clicking on the link below.
http://adt.curtin.edu.au/theses/available/adt-WCU20030214.133140/

More information on Thermal Work Limit can be obtained from this link to Pointhealth Pty Ltd. this link includes additional technical papers on TWL as well as an online calculator. Note that all the formulas for the original TWL formulation are avaialble in the paper "Limiting work rate (Thermal Work Limit) as an index of thermal stress" avaialble from the

"Papers" page on this site. There have been some changes to these formulas that mostly affect very hot dry environments but these are as yet unpublished. http://www.pointhealth.com.au/

TWL can now be read directly by two commercial available instruments. Note that MVA has no commercial or any other interest in these instruments, direct or other:

* Kestrel 4400 Heat stress tracker, see http://pacdatasys.com.au/our-products/weather/kestrel-handheld-meters
* Scarlet TWL-1S meter, see http://www.scarlet.com.tw/twl-1s.html

For a discussion on the differences between Thermal Work Limit (TWL) and Air Cooling Power (ACP) read this Technical Guidance Note.

Papers Written By Rick Bake Available For Download

Click on the download icon to view the papers.

Heat Stress

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Brake D J, 2002. The Deep Body Core Temperatures, Physical Fatigue and Fluid Status of Thermally Stressed Workers and the Development of Thermal Work Limit as an Index of Heat Stress, PhD thesis, School of Public Health, Curtin University, Perth
3 MB
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Brake DJ and Bates GP, 2002. Limiting work rate (Thermal Work Limit) as an index of thermal stress. Journal of Applied Occupational and Environmental Health (ACGIH). Vol 13(3):176-186, March. [The journal of the ACGIH, the American College of Government Industrial Hygienists]
185 kB
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Brake D J and Bates G P, 2002. Deep body core temperatures in industrial workers under thermal stress. J Occup Environ Med. Vol 44, No 2, pp 125-135. [The journal of the American College of Occupational and Environmental Medicine]
313 kB
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Brake D J and Bates G P, 2002. A valid method for comparing rational and empirical heat stress indices. Annals of Occupational Hygiene. Vol 46, No 2 pp 165-174 [Journal of the British Occupational Hygiene Society]
257 kB
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Brake DJ and Bates GP, 2001. Fatigue in industrial workers under thermal stress on extended shift lengths. Occ Med (Society of Occ Med, UK). Vol 51, No 7, pp 456-463 [The journal of the British Society of Occupational Medicine]
475 kB
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Brake D J and Bates G P, 2003. Fluid Losses and Hydration Status of Industrial Workers under Thermal Stress Working Extended Shifts. British Journal of Occupational and Environmental Medicine. Vol 60, No 2 pp 90-96.
173 kB
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Brake D. 2001. Fluid consumption, sweat rates and hydration status of thermally-stressed underground miners and the implications for heat illness and shortened shifts. Proc 2001 Qld Mining Ind Occ Health and Safety Conf. Townsville, 2001, pp 275-287. Qld Mining Council, Brisbane (2001)
208 kB
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Brake, D J, Donoghue M and Bates G, 1998. Management Of Heat Stress In A Hot, Humid, Underground Environment. Paper presented to the 1998 International Safety Conference and Exposition. Melbourne. 25-28 February
89 kB
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Brake, D J, Donoghue, M D and Bates G P. A New Generation of Health and Safety Protocols for Working in Heat. Proc 1998 Qld Mining Ind Occ Health and Safety Conf. Yeppoon, 1998, pp. 91-100. Qld Mining Council, Brisbane (1998)
76 kB
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Brake, D.J.; Bates, G.P.: Occupational Heat Illness: An Interventional Study. In: Proceedings of the International Conference on Physiological and Cognitive Performance in Extreme Environments, 2000, pp. 170-172. W.M.Lau, Ed. Defence Scientific and Technology Organisation, Australian Department of Defence, Canberra (2000)
128 kB
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Brake, D J and Bates G P, 2000. Assessment of Underground Thermal Environments and the Prevention of Heat Illness. Intl Mine Safety Conf, Perth
886 kB
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Brake D J, 2004. The Application of a Rational Heat Stress Index (Thermal Work Limit) to Sports Medicine. Sports Med Aust annual conf, Alice Springs, Oct (presentation)
1.4 MB
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Brake, D J, 2002. A quantitative method for assessing the impact of acclimatisation in the workplace. Proc Queensland Mining Industry Safety and Health Conference, July, pp 33 to 41
198 kB
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Technical Guidance Note: Air Cooling Power (ACP) and Thermal Work Limit (TWL)—What are the differences?
37 kB

Mine Ventilation

Brake D J, 2001. Calculation of the natural (unventilated) wet bulb temperature, psychrometric dry bulb temperature and Wet Bulb Globe Temperature from standard psychrometric measurements. J Mine Vent Soc of Sth Africa. Vol 54, No 4, Oct-Dec 2001, pp 108-112
179 kB
Brake D J, 2002. Fan total pressure or fan static pressure: which is correct when solving ventilation problems? J. Mine Vent Soc of Sth Africa. Vol 55, No 1, Jan-Mar 2002
118 kB
Brake, D and Nixon, T. 2005. Current trends and future opportunities in underground hardrock ventilation practices. AusIMM Underground operator’s conference, Perth, pp403-408
65 kB
Brake, D and Nixon T, 2006. Design and operational aspects in the use of booster, circuit and auxiliary fan systems. Proc 11th US Mine Vent sym, Penn State, pp 543-553
1.1 MB
Brake D J and Nixon C A, 2008. Correctly estimating primary airflow requirements for underground metalliferous mines. Proc 10th AusIMM Underground op conf, Launceston, Apr. pp 199-204
95 kB
Brake D J, 2009. The growing use of hazardous primary ventilation systems in hardrock mines, 9th Int Mine Vent Cong (Panigrahi D, ed), Nov, New Delhi. pp 43-60
326 kB
Brake D J, 2010. The design of push-pull primary and secondary ventilation systems and a vertically-split intake-exhaust ventilation shaft. Proc 12th US-Nth Am Mine Vent sym (Hardcastle S & McKinnon D, eds), Sudbury. pp 181-191
905 kB
Brake D J and Jones C, 2011. The use of the main ramp as a mine exhaust at the Kencana mine, Indonesia, 11th AusIMM Underground Operators’ Conference 2011, Mar, Canberra. pp 13-18
222 kB
Brake D J, 2012. Developments in underground hardrock mine ventilation, in Australasian Mining and Metallurgical Operating Practices (AMMOP), 3rd ed. AusIMM.
117 kB
Brake D J, 2012. Series ventilation circuits in hardrock mines - Can they be designed and operated safely? Proc 13th US-Nth Am Mine Vent sym (Calizaya F, ed), Salt Lake City. pp 175-182
235 kB
Brake D J, 2013. Performance and Acceptance Testing of Main Mine Ventilation Fans. Proc 2nd Australian Mine Vent conf (Chalmers D, ed). Adelaide. The AusIMM. pp 141-148
550 kB
Brake D J, 2013. Fire Modelling in Underground Mines using Ventsim Visual VentFIRE Software. Proc 2nd Australian Mine Vent conf (Chalmers D, ed). Adelaide. The AusIMM. pp 265-276.
795 kB
Brake D J, 2013. Ventilation Challenges Facing the Metalliferous Sector. Proc 2nd Australian Mine Vent conf (Chalmers D, ed). Adelaide. The AusIMM. pp 3-12
688 kB
Brake D J, 2014. Pressure piling and the impacts of blast relief to protect primary fans in a highwall longwall operation. 10th International Mine Ventilation Congress, IMVC2014 pp55-65
1.33 MB
Brake D J, 2015. A Review of Good Practice Standards and re-Entry Procedures after Blasting and Gas Detection Generally in Underground Hardrock Mines, Proc 15th North American Mine Ventilation Symposium, 2015 — Sarver, E., Schafrik, S., Jong, E., Luxbacher, K. Virginia Tech Department of Mining and Minerals Engineering pp.
539 kB
Brake D J, 2015. Modeling and Simulation of Multiple Underground Mine Fire Scenarios at Freeport Indonesia, Proc 15th North American Mine Ventilation Symposium, 2015 — Sarver, E., Schafrik, S., Jong, E., Luxbacher, K. Virginia Tech Department of Mining and Minerals Engineering pp.
793 kB
Brake D J, 2015. Specification and Selection of handheld gas monitors (detectors) for underground mines, Proc 3rd Aust Mine Vent conf (Belle B (ed)). AusIMM. pp 143-153.
536 kB
Brake D J, 2015. Quality Assurance Standards for Mine Ventilation Models and Ventilation Planning, Proc 3rd Aust Mine Vent conf (Belle B (ed)). AusIMM. pp 221-228.
294 kB
Brake D J, 2017. Headframe, intake shaft and intake portal design issues for heater design. Proc 16th North American Mine Ventilation Symposium, ed J Brune. Colorado. pp 4-17 to 4-22.
347 kB
Brake D J, 2017. Fan specification and tender adjudication for mine ventilation engineers. AusIMM Bulletin. Aug 2017, pp 76-78 [this is an abridged version of paper V 24]
549 kB
Brake D J, 2017. Auxiliary ventilation design: why and how mines waste so much power on inferior systems. Paper accepted for 4th Australian Mine Ventilation Conference, 2017. Brisbane.
585 kB
Brake D J, 2017. Fan specification and tender adjudication for mine ventilation engineers with particular reference to turnkey projects. Paper accepted for 4th Australian Mine Ventilation Conference, 2017. Brisbane
216 kB
Brake D J, 2018. Management of occupational exposure limits: a guide for mine ventilation engineers, 11th Int mine vent congress, Xi’an, Sep. pp 799-812
1.92 MB
Brake D J and Fulker R, 2000. The ventilation and refrigeration design for Australia’s deepest and hottest underground operation: the Enterprise mine. Proc MassMin 2000 611-621. Aust Inst Mining and Met. 2000
243 kB

Mine Cooling and Refrigeration

Brake D, 2001. The application of refrigeration in mechanised mines. Proc AusIMM. 2001(1):1-10
135 kB
Brake D, 2001. Key engineering considerations in the specification and selection of mine refrigeration plants. Proc AusIMM. 2001(2):1-16
200 kB
Brake, D, 2002. Design of the world’s largest bulk air cooler for the Enterprise mine in northern Australia. 9th US mine ventilation symposium, Kingston, Canada, June. pp 381-390
339 kB
Brake D, 2005. Performance and acceptance testing of mine refrigeration plants. Proc 8th Intl Mine Vent Cong, Brisbane, pp 3-12
2.23 MB

Mine Ventilation Technical Training, Quality Standards and Professional Development

Brake D J and Nixon T, 2004. The impact of changing employment and mining practices on the mine ventilation profession in Australia, in Mine Ventilation (Proc Tenth US Mine Vent Sym, Anchorage), pp 409-416
61 kB
Brake D J and Greenhill G, 2005. New integrated pathways for mine ventilation technical training in Australia. (Qld Mining Safety and Health Conf, Tvl, 15-17 Aug, pp 29-33
56 kB
Brake, D J, 2006. Developments in mine ventilation legislation, statutory qualifications and ventilation training in Australia. Proc 11th US Mine Vent sym, Penn State, pp 609-617
57 kB
Brake, D J, 2008. A protocol and standard for mine ventilation studies. Proc 12th US/North American Mine Vent Sym (Wallace K, ed), Jun. Reno. pp 3-11
215 kB

Egress and Entrapment

Brake, D J, 1999. An integrated strategy for emergency egress from an underground metal mine. Proc 8th US Mine Ventilation Congress. Rolla, pp 649-657. (University of Missouri-Rolla)
54 kB
Brake, D. J. and Bates, G. P, 1999. Criteria for the design of emergency refuge stations for an underground metal mine. Proc AusIMM 304(2):1-8
62 kB
Brake D J, 1999. Entrapment and escape from metal mines – a case study. Proc 1999 Qld Mining Ind Occ Health and Safety Conf. Yeppoon, 1999. Qld Mining Council, Brisbane (1999)
50 kB
Brake D J, 2017. Egress and Entrapment in hardrock mines (revisited). Proc 16th North American Mine Ventilation Symposium, ed J Brune. Colorado. pp 2-9 to 2-16.
392 kB
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