Kestrel 4600 Heat Stress Meter - Manual (Page 8)

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THERMAL WORK LIMIT (TWL) ( 4400 ONLY)
Measuring
The unit will also display a measure of human heat stress known as “Thermal Work Limit,”
or “TWL.” TWL is based upon Globe Temperature, Relative Humidity, Ambient Temperature,
Barometric Pressure, Wind Speed, and parameters specific to the population using the
Thermal Work Limit measurement. These parameters are the Intrinsic Clothing Insulation
Factor (IClo), Vapor Permeation Factor (VPF), Position of the body (Pos), and surface area of
the person (Area). See Clothing Ensemble Level Setting Screen below. TWL is measured in
terms of the heat energy a person can dissipate from their surface area in Watts per square
meter (w/m2). TWL incorporates recommended acclimitization, buffer and withdrawal
zones as depicted in the following chart (page 16).
To Change TWL settings:
• Press while on the Thermal Work Limit screen.
• Use or to scroll to different ensembles.
• Press to select the desired ensemble.
• A bullet indicates the selected ensemble.
• Press to exit the WBGT settings screen.
• If Custom is selected, each factor specific to the user can be altered (shown above).
• To view the specifics of an ensemble, press after selecting it.
• If viewing the Custom ensemble specs, use or to adjust each value, and or
to scroll to different parameters.
• Press to exit the ensemble specs, and once more to exit TWL settings screen.
Note: “Acclim” will blink on the TWL screen when the meter detects that Acclimatization is the current zone.
In the Buffer or Withdrawal zones, the number will flash as shown in the screenshots below. For information on
TWL interventions, refer to Figure 2. For typical numeric values for each factor, refer to Figure 3.
TWL
245.7
Unrestricted
settings
w/m 2
ShSlv/Shorts
ShSlv/Pants
Coveralls
Business suit
ShSlv/Shorts
select exit
TWL
Withdrawl
settings
w/m 2
112.7
TWL
Withdrawl
settings
w/m 2
112.7
Figure 1
Figure 2
Figure 3: TWL values, working zones, and interventions.Source: Health Authority, Abu Dhabi.
URL: http://haad-safe.ae/index.php?option=com_content&view=article&id=27&Itemid=50
TWL (w/m ) 2
Working Zone
Interventions
> 220 140-220 115-140 < 115
Unrestricted Acclimatization Buffer Withdrawal
No limits on
self-paced
work for trained,
hydrated workers.
No restriction for
acclimatized workers
Workers with uncertain
acclimatization status
should not work alone
in this zone
• Be aware of increased
risk of heat illness
• Dehydration test for
first two shifts back
from leave
Buffer zone exists to identify situations
in which environmental conditions
may be limiting to work
• Any practicable intervention to reduce
heat stress should be implemented e.g.
provide shade, improve ventilation etc
• Working alone to be avoided if possible
• Unacclimatized* workers not to work in
this zone
• Use the technical information
sheets ‘Work-rest cycling – sample
schedules’ and ‘Fluid requirements for
working in heat’ to prescribe maximum
exposure time, work/rest cycling and
fluid intakes appropriate for type of
work and conditions
Work limited to essential maintenance
or rescue operations
• No person to work alone
• No unacclimatized* person to work
• Documentation required authorising
work in hostile thermal conditions for
specific purpose
• Specific induction required emphasizing
hydration and identifying signs of
heat strain
• Apply 20 minutes of work – 40 minutes
rest schedule
• Required fluid intake exceeds 600 ML
per 30 minutes
• Personal water bottle (2 liter capacity)
must be on the job at all times
• Mandatory dehydration testing at
end of shift
* Unacclimatized workers are defined as new workers who have been off work for more than 14 days due to
illness or leave (outside the tropics).
ENSEMBLE IClo VPF
Men’s business suit: Long sleeve shirt/
tweed suit jacket & long, loose trousers
Short sleeve shirt/denim shorts
Work Clothes: Short sleeve shirt/long
trousers (denim)
Work Clothes & Coveralls
1.13
0.41
0.50
0.96
0.37
0.43
0.40
0.39
Lying down
POSITION VALUE
0.00
1.00 Standing up
Figure 4: (left/above) Typical values for IClo,
VPF, and POS. Typical value for Area of a man
is 1.7. Sources: “Heat and Moisture Transfer
Through Clothing” (http://www.ibpsa.org/
proceedings/BS2009/BS09_1360_1366.pdf),
and “A Comprehensive Database for Estimating
Clothing Insulation,” Institute for Environmental
Research, Kansas State University; Elizabeth
McCullough and Byron James.