User's Manual
Heat Stress WBGT Meter
Model HT30
EXTECH INSTRUMENTS CORPORATION warrants this instrument to be free of defects in
parts and workmanship for one year from date of shipment (a six month limited warranty
applies to sensors and cables). If it should become necessary to return the instrument for
service during or beyond the warranty period, contact the Customer Service Department at
(781) 890-7440 ext. 210 for authorization or visit our website www.extech.com for contact
information. A Return Authorization (RA) number must be issued before any product is
returned to Extech. The sender is responsible for shipping charges, freight, insurance and
proper packaging to prevent damage in transit. This warranty does not apply to defects
resulting from action of the user such as misuse, improper wiring, operation outside of
specification, improper maintenance or repair, or unauthorized modification. Extech
specifically disclaims any implied warranties or merchantability or fitness for a specific
purpose and will not be liable for any direct, indirect, incidental or consequential damages.
Extech's total liability is limited to repair or replacement of the product. The warranty set
forth above is inclusive and no other warranty, whether written or oral, is expressed or
Congratulations on your purchase of Heat Stress WBGT (Wet Bulb Globe Temperature)
Meter. This meter measures and displays Heat Stress Index (WBGT), which is how hot it
feels when humidity is combined with temperature, air movement, and direct or radiant
sunlight. Black Globe Temperature (TG) monitors the effects of direct solar radiation on an
exposed surface. It also measures air temperature (TA) and Relative Humidity (RH).
Additional features include: selectable units of °F/°C, Auto power off with override, and a
built-in RS-232 interface with optional 407752 Windows® software. This meter is shipped
fully tested and calibrated and, with proper use, will provide years of reliable service.
HT30 V3.0 08/07
Meter Description
1. Black Globe temperature sensor
2. RH and Temperature sensors with protective cover
3. LCD display
7. RS-232 interface
8. Battery compartment (on rear)
Wet Bulb Globe Temperature
Black Globe Temperature
Air Temperature
Relative Humidity
Indoor (no sun)
Outdoor (in full sun)
Low Battery indicator
HT30 V3.0 08/07
Meter Operation
Press the
Slide down the protective sensor cover before taking measurements.
/SET button to power the instrument on/off.
Press the MODE/Sbutton to select the desired display mode: Wet Bulb Globe
Temperature Heat Index (WBGT), Air Temperature (TA), Black Globe Temperature
(TG), or Relative Humidity (RH). An icon will appear in the display indicating the
current selection.
To select the preferred temperature unit (°F or °C), simultaneously press and release
the MODE/Sand NEXT buttons. The F or C icon will appear in the display.
The meter measures WGBT index with (OUT) or without (IN) direct sun exposure.
Hold down the MODE/S for more than 1 second to alternate between settings. The
IN or OUT icon will appear in the display.
Alarm Setting
The HT30 has a user settable heat stress index alarm. The HT30 will emit an audible beep
when the heat stress index (WBGT) reaches the level previously set by the user. The
alarm will continue beep until the measured heat stress index temperature falls below the
alarm level set by the user, or the meter is turned off. The setting range is from 68.0 to
99.0°F (20.0 to 37.2°C).
/SET button for more than 2 seconds
To enter the alarm setup mode, hold down the
when turning the meter on.
The current alarm setting will be displayed with the most significant digit blinking.
Use the MODE/Sbutton to increment the digit.
To set the next digit, momentarily press the NEXT button.
After all of the digits are set, press and hold the NEXT button for 2 seconds to return
to the measuring mode.
NOTE: The OUT icon will appear in the display If the new setting is out of the setting range
of the alarm.
HT30 V3.0 08/07
Auto Power Off
The Auto Power Off feature automatically turns the meter off after approximately 20
minutes. To disable this feature, hold down the /SET and MODE/Sfor 2 seconds when
powering on the meter. The meter will momentarily display an “n” and enter measuring
mode. The meter will reset to auto power off mode when it is powered off.
Battery Replacement
When it is time to replace the batteries, the low battery indicator
left-hand corner of the LCD display. To replace the battery:
1. Open the battery compartment on the back of the meter.
appears in the bottom
2. Replace the two AAA batteries and close the compartment.
RS-232 PC Interface
The RS-232 serial data port (3.5mm phono jack) is located on the right side of the meter.
The PC interface hardware is intended for use with the Extech software package, Part
Number 407752 which includes Windows 95 / 98 / ME / NT / 2000/ XP compatible
software and PC interface cable. For more information, contact Extech or refer to the
407752 User's Manual.
Error Messages
An error message will appear on the display if the meter fails an internal diagnostic test.
1. E2: The value is under range.
2. E3: The value is over range.
3. E4: The value is in error.
4. E11: RH Calibration error.
5. E33: Circuit error. Repair/replacement necessary.
HT30 V3.0 08/07
Wet Bulb Globe Temperature (WBGT)
32°F to 122°F (0°C to 50°C)
WBGT Accuracy
Calculated from measured parameters
TG Black Globe Temperature range
32°F to 176°F (0 to 80°C)
TG Accuracy
±4°F (2°C)
±5.5°F (3°C)
TA Air Temperature range
32°F to 122°F (0°C to 50°C)
TA Accuracy
±1.8°F (1°C)
Relative Humidity (RH)
0 to 100%RH
RH Accuracy
±3% (at 25°C, 10 to 95%RH)
0.1°F/°C; 0.1%RH
Operating Temperature
32°F to 122°F (0°C to 50°C)
Operating Humidity
Max. 80% RH
Power Supply
Two AAA batteries
Battery Life
Approx 1000 hours
Meter: 10x1.9x1.1" (254 x 48.7 x 29.4mm)
Black Ball: 1.57 Dia., 1.37H (40mm, 35mm)
4.8oz (136g)
Optional Accessories
PC software and cable (407752)
HT30 V3.0 08/07
The following is reprinted with permission of the National Oceanic and Atmospheric Administration.
A National Problem
Heat kills by taxing the human body beyond its abilities. In a normal year, about 175 Americans succumb
to the demands of summer heat. Among the large continental family of natural hazards, only the cold of
winter -- not lightning, hurricanes, tornadoes, floods, or earthquakes -- takes a greater toll. In the 40-year
period from 1936 through 1975, nearly 20,000 people were killed in the United States by the effects of heat
and solar radiation. In the disastrous heat wave of 1980, more than 1,250 people died. And those are the
direct causalities. No one can know how many more deaths are advanced by heat wave weather -- how
many diseased or aging hearts surrender, that under better conditions would have continued functioning.
North American summers are hot; most summers see heat waves in one section or another of the United
States. East of the Rockies, they tend to combine both high temperatures and high humidity although
some of the worst have been catastrophically dry.
Considering this tragic death toll, the National Weather Service has stepped up its efforts to alert more
effectively the general public and appropriate authorities to the hazards of heat waves -- those prolonged
excessive heat/humidity episodes. Based on the latest research findings, the NWS has devised the "Heat
Index" (HI), (sometimes referred to as the "apparent temperature"). The HI, given in degrees Fahrenheit, is
an accurate measure of how hot it really feels when the relative humidity (RH) is added to the actual air
temperature. To find the Heat Index, look at the Heat Index Chart. As an example, if the air temperature is
95°F (found on the left side of the table), and the relative humidity is 55% (found at the top of the table),
the HI -- or how hot it really feels -- is 110°F. This is at the intersection of the 95° row and the 55%
column. Important: Since HI values were devised for shady, light wind conditions, exposure to full
sunshine can increase HI values by up to 15°F. Also, strong winds, particularly with very hot, dry
air, can be extremely hazardous. Note on the HI chart the shaded zone above 105°F. This corresponds
to a level of HI that may cause increasingly severe heat disorders with continued exposure and/or physical
HT30 V3.0 08/07
Heat Index / Heat Disorders
Heat Index / Heat Disorders
Possible heat disorders for
people in higher risk groups
130°F or
Heatstroke/sunstroke highly likely with continued exposure.
105° 130°F
Sunstroke, heat cramps or heat exhaustion likely, and heatstroke
possible with prolonged exposure and/or physical activity.
90° 105°F
Sunstroke, heat cramps and heat exhaustion possible with
prolonged exposure and/or physical activity.
80° - 90°F Fatigue possible with prolonged exposure and/or physical activity.
The "Heat Index/Heat Disorders" table relates ranges of HI with specific disorders, particularly for people in
the higher risk groups.
WARNING: Sudden temperature and humidity changes (e.g. going indoors to
outdoors) may cause inaccurate Temperature, Humidity and Heat Index
readings for up to 45 minutes. Until readings stabilize (e.g. stop steadily
increasing or decreasing), users should NOT rely on the Temperature,
Humidity or Heat Index readings of this meter, or on the meter’s alarm
function in regards to such readings. This meter is NOT a medical device
and users should NEVER rely on readings for medical purposes.
HT30 V3.0 08/07
How Heat Affects the Body
Human bodies dissipate heat by varying the rate and depth of blood circulation, by losing water through the
skin and sweat glands, and -- as the last extremity is reached -- by panting, when blood is heated above
98.6 degrees. The heart begins to pump more blood, blood vessels dilate to accommodate the increased
flow, and the bundles of tiny capillaries threading through the upper layers of skin are put into operation.
The body's blood is circulated closer to the skin's surface, and excess heat drains off into the cooler
atmosphere. At the same time, water diffuses through the skin as perspiration. The skin handles about 90
percent of the body's heat dissipating function.
Sweating, by itself, does nothing to cool the body, unless the water is removed by evaporation -- and high
relative humidity retards evaporation. The evaporation process itself works this way: the heat energy
required to evaporate the sweat is extracted from the body, thereby cooling it. Under conditions of high
temperature (above 90 degrees) and high relative humidity, the body is doing everything it can to maintain
98.6 degrees inside.The heart is pumping a torrent of blood through dilated circulatory vessels; the sweat
glands are pouring liquid -- including essential dissolved chemicals, like sodium and chloride -- onto the
surface of the skin.
Too Much Heat
Heat disorders generally have to do with a reduction or collapse of the body's ability to shed heat by
circulatory changes and sweating, or a chemical (salt) imbalance caused by too much sweating. When
heat gain exceeds the level the body can remove, or when the body cannot compensate for fluids and salt
lost through perspiration, the temperature of the body's inner core begins to rise and heat-related illness
may develop.
Ranging in severity, heat disorders share one common feature: the individual has overexposed or
overexercised for his/her age and physical condition in the existing thermal environment.
Sunburn, with its ultraviolet radiation burns, can significantly retard the skin's ability to shed excess heat.
Studies indicate that, other things being equal, the severity of heat disorders tend to increase with age -heat cramps in a 17-year-old may be heat exhaustion in someone 40, and heat stroke in a person over 60.
Acclimatization has to do with adjusting sweat-salt concentration, among other things. The idea is to lose
enough water to regulate body temperature, with the least possible chemical disturbance.
Cities Pose Special Hazards
The stagnant atmospheric conditions of the heat wave trap pollutants in urban areas and add the stresses
of severe pollution to the already dangerous stresses of hot weather, creating a health problem of
undiscovered dimensions. A map of heat-related deaths in St. Louis during 1966, for example, shows a
heavier concentration in the crowded alleys and towers of the inner city, where air quality would also be
poor during a heat wave.
The high inner-city death rates also can be read as poor access to air-conditioned rooms. While airconditioning may be a luxury in normal times, it can be a lifesaver during heat wave conditions.
The cost of cool air moves steadily higher, adding what appears to be a cruel economic side to heat wave
fatalities. Indications from the 1978 Texas heat wave suggest that some elderly people on fixed incomes,
many of them in buildings that could not be ventilated without air conditioning, found the cost too high,
turned off their units, and ultimately succumbed to the stresses of heat.
HT30 V3.0 08/07
Know These Heat Disorder Symptoms
First Aid
Redness and pain. In severe cases,
swelling of skin, blisters, fever,
Ointment for mild cases if blisters appear. If breaking
occurs, apply dry sterile dressing. Serious, extensive
cases should be seen by a physician.
Painful spasms usually in muscles of
legs and abdomen possible. Heavy
Firm pressure on cramping muscles, or gentle
massage to relieve spasm. Give sips of water. If
nausea occurs, discontinue use.
Heavy sweating, weakness, skin
Exhaustion cold, pale and clammy. Pulse
thready. Normal temperature
possible. Fainting and vomiting.
Get victim out of sun. Lay down and loosen clothing.
Apply cool wet cloths. Fan or move victim to air
conditioned room. Sips of water. If nausea occurs,
discontinue use. If vomiting continues, seek
immediate medical attention.
Heat stroke is a severe medical emergency.
Summon medical assistance or get the victim to
a hospital immediately. Delay can be fatal.
High body temperature (106°F, or
higher). Hot dry skin. Rapid and
strong pulse. Possible
Move the victim to a cooler environment. Reduce
body temperature with cold bath or sponging. Use
extreme caution. Remove clothing, use fans and air
conditioners. If temperature rises again, repeat
process. Do not give fluids.
WARNING: Sudden temperature and humidity changes (e.g. going indoors to
outdoors) may cause inaccurate Temperature, Humidity and Heat Index
readings for up to 45 minutes. Until readings stabilize (e.g. stop steadily
increasing or decreasing), users should NOT rely on the Temperature,
Humidity or Heat Index readings of this meter, or on the meter’s alarm
function in regards to such readings. This meter is NOT a medical device
and users should NEVER rely on readings for medical purposes.
HT30 V3.0 08/07
Preventing Heat-Related Illness
Elderly persons, small children, chronic invalids, those on certain medications or drugs (especially
tranquilizers and anticholinergics), and persons with weight and alcohol problems are particularly
susceptible to heat reactions, especially during heat waves in areas where moderate climate usually
Heat Wave Safety Tips
Slow down. Strenuous activities should be reduced, eliminated, or rescheduled to the coolest time of the
day. Individuals at risk should stay in the coolest available place, not necessarily indoors.
Dress for summer. Lightweight, light-colored clothing reflects heat and sunlight, and helps your body
maintain normal temperatures.
Put less fuel on your inner fires. Foods (like proteins) that increase metabolic heat production also
increase water loss.
Drink plenty of water or other nonalcoholic fluids. Your body needs water to keep cool. Drink plenty of
fluids even if you don't feel thirsty. Persons who (1) have epilepsy or heart, kidney, or liver disease, (2) are
on fluid restrictive diets, or (3) have a problem with fluid retention should consult a physician before
increasing their consumption of fluids.
Do not drink alcoholic beverages.
Do not take salt tablets unless specified by a physician. Persons on salt restrictive diets should consult
a physician before increasing their salt intake.
Spend more time in air-conditioned places. Air conditioning in homes and other buildings markedly
reduces danger from the heat. If you cannot afford an air conditioner, spending some time each day
(during hot weather) in an air conditioned environment affords some protection.
Don't get too much sun. Sunburn makes the job of heat dissipation that much more difficult.
HT30 V3.0 08/07
Calibration and Repair Services
Extech offers repair and calibration services for the products we sell. Extech also
provides NIST certification for most products. Call the Customer Service Department for
information on calibration services available for this product. Extech recommends that
annual calibrations be performed to verify meter performance and accuracy.
Support line (781) 890-7440
Technical support: Extension 200; E-mail: [email protected]
Repair & Returns: Extension 210; E-mail: [email protected]
Product specifications subject to change without notice
For the latest version of this User Guide, Software updates, and other
Up-to-the-minute product information, visit our website: www.extech.com
Extech Instruments Corporation, 285 Bear Hill Road, Waltham, MA 02451
Copyright © 2007 Extech Instruments Corporation.
All rights reserved including the right of reproduction in whole or in part in any form.
HT30 V3.0 08/07