Kestrel 4400 Heat Stress - DISCONTINUED

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Vendor Kestrel
Kestrel Meter 4400 Heat Stress Meter

The Kestrel 4400 was DISCONTINUED in December 2015.  It was replaced by the Kestrel 5400.


For Historical Reference, here is information on the discontinued Kestrel 4250


The Kestrel 4400 was revolutionary in size, cost and capability for those in need of measuring Heat Stress.  You may have seen these in Tan or Olive and/or with Bluetooth.  Going forward as of summer 2015, the 4400 is only offered at the new lower price of $469, but has no case, and no vane mount, with no bluetooth option.  If you need Bluetooth, check the Kestrel 4600, it's the same heat stress meter with a compass and bluetooth.  

Along with having a Wet Bulb Global Temperature reading, the Kestrel 4400 displays thermal work limit (“TWL”), another recognized composite heat stress prediction tool. For both WBGT and TWL, the Kestrel 4400 provides on-screen alarms when conditions enter the caution and danger zones, providing clear and immediate guidance that heat illness prevention steps must be taken.  Clothing levels can also be customized, making the Kestrel 4400 particularly useful in activities requiring heavy protective gear which worsens heat stress.  The Kestrel 4400 also displays natural wet bulb temperature, and black globe temperature.

Athlete deaths from heat exposure are on the rise.   Warming summers have been accompanied by weekly reports of athletes young and old collapsing from severe heat-induced illness.  Our military training units struggle to keep soldiers healthy while preparing them for the grueling heat of Iraq and Afghanistan.  OSHA is campaigning to prevent heat illness in outdoor workers and reports that every year thousands of workers become sick working in the heat, and some even die.  In every instance, accurate and localized measurement of heat stress conditions is a key component of a heat illness prevention plan.  The Kestrel 4400 Heat Stress Tracker, new from NK, answers this need. 

Human heat stress results from a combination of many environmental factors – air temperature and humidity along with radiant heat from the sun and surfaces, balanced by the cooling effect of breezes or air flow. Anyone who has stood under welcome shade adjacent to a sizzling parking lot understands that these conditions must be evaluated exactly where peoples’ activities are taking place.  The most common composite measurement used to determine appropriate exposure to heat stress conditions is Wet Bulb Globe Temperature or “WBGT.”  The American College of Sports Medicine, the American Conference of Governmental Industrial Hygienists, and the US Military all have published threshold limit values (TLV’s) or reference guidelines for conditions, activity levels and hydration based on WBGT, clothing levels and acclimatization of participants.

How Does the Kestrel 4400 Work?

The Kestrel 4400 uses a 1-inch black globe to gather information about the environment, which isn't possible with current Kestrels. As is true with the original Wet Bulb Globe Temperature apparatus, a temperature sensor is installed inside the globe that will measure the temperature inside. The temperature inside the globe is a function of the ambient air temperature, solar insolation and wind speed.

As one can imagine, a sunny day will make the globe hotter because it's painted black, and colder ambient temperatures will have a tendency to cool off the globe. Lastly, the wind will cause the globe to get closer to the ambient air temperature because it will carry heat away from the globe when the air is cooler, or increase heat transfer to the globe if the air is warmer than the globe.

The Kestrel must sit out for a period of time to allow the globe to adjust to true atmospheric conditions; we recommend a minimum of 7-10 minutes. The globe temperature is then taken into account along with other measurements, such as ambient temperature, humidity and pressure, to calculate more meaningful measurements, such as Wet Bulb Globe Temperature and Thermal Work Limit.

Why Use WBGT as a Heat Stress Indicator vs. just using the Heat Index?

Wet bulb globe temperature (WBGT) is based on an equation that uses a combination of environmental elements to calculate the reading. The calculation is a measure of the heat stress in direct sunlight, which takes into account: temperature, humidity, wind speed, sun angle and cloud cover (solar radiation).

Heat index takes into consideration temperature and humidity, and is calculated for shady areas.

There is a difference.

Monitoring the WBGT of the environment is quickly becoming the accepted and preferred method of measuring heat stress. Several sports organizations use WBGT to assess the potential risk of environmental stress on athletes. These organizations include the National Athletic Trainers Association (NATA), National Collegiate Athletic Association (NCAA) and the Women’s Tennis Association (WTA).

Refer to the chart below for a comparison of WBGT and heat stress index.

Roberts WO. Medical management and administration manual for long distance road racing.

In: Brown CH, Gudjonsson B, eds. IAAF Medical Manual for Athletics and Road Racing Competitions:

A Practical Guide. Monaco: International Amateur Athletic Federation Publications; 1998: 39-75.

Customized Warning Zones

Users from different work environments or different standards around the world have the ability to create their own warnings on the Kestrel 4400.  With the user friendly functionality, the user may  go into the Kestrel 4400's WBGT function and turn different warning zones on or off and set these warning zones to be triggered at different degrees as the temperature rises. By customizing the Kestrel 4400's WBGT warning zones, monitoring a team or crew becomes much more personal. A chart with suggested warning zones will be included with every Kestrel 4400 ordered. 

Kestrel Meter Heat Index Reference Guide


Download Heat Index Reference Guide - Download Now (PDF)

The Kestrel Meter 4400 Heat Stress Meter measures/features

  • Wet Bulb Globe Temperature ("WBGT")
  • Thermal Work Limit ("TWL")
  • Globe Temperature
  • Naturally Aspirated Wet Bulb Temperature
  • Wind Speed (Current, Average, & Maximum)
  • Altitude
  • Pressure trend
  • Barometric pressure
  • Relative humidity in %
  • Heat stress index 
  • Dewpoint
  • Wet bulb temperature
  • Density altitude
  • Wind chill
  • Air, water, and snow temperature °F or °C
  • Waterproof
  • Time and date
  • Easy- to-read backlit display
  • Data Logger (automatic and manual)
  • Customizable data storage - 2900 data points
  • Humidity sensor can be calibrated in the field with our Relative Humidity Calibration Kit.
  • Customize screens to display user-selected measurements
  • Graph and recall trends
  • User-replaceable impeller
  • Data charting
  • Upload to a computer (with optional interface)
  • Meets MIL-STD-810F 516.5 Procedure IV Transit Shock test - 20 drops from 5 ft. IP-67 sealed to exclude dust and water at 3 ft. submersion
  • Five languages (English, French, Spanish, German, and Italian)
  • US Patent Nos. 5,783,753 and 5,939,645 and 6,257,074
  • Made in the USA

    Here are just a few of the markets that need to reference WBGT for work or play:  

    • Athletes (high school, college, professional)
      • Athletic trainers 
    • Military training units & medics
    • Law enforcement medics 
    • Industrial hygienists
    • Construction supervisors 
    • Others working in extreme heat conditions

    Advantages of the Kestrel 4400 vs. Competition

    • Fraction of the size
    • Less than half of the cost
    • Equal or better accuracy
    • Fast response
    • Simple user interface 

    Kestrel Meter 4400 Includes

    • Batteries - 2 AAA*
    • Kestrel Certificate of Conformity 
    • A chart with suggested Heat Stress warning zones.
    • NOTE: Closeout models include the above, plus a neck lanyard, a sturdy zippered case, and a portable vane mount


    Historical Facts:


    • Was available in Yellow, Olive Drab, or Tan
    • Discontinued in December 2015


      *We ship every Kestrel 4000 Series Meter exclusively with Made in the USA Energizer® Brand Ultimate Lithium AAA batteries for improved reliability, capacity, cold-weather performance and weight. We strongly recommend that you use only Energizer Ultimate Lithium batteries in your Kestrel Meter. If you choose to install alkaline batteries, we recommend that you do NOT store your Kestrel with these batteries installed, and that you check your batteries for leakage and corrosion a minimum of every 30 days. Damage due to leaking alkaline batteries is not covered under warranty. 

      The History of Adopting Heat Acclimatization Guidelines

      In 2003 the NCAA began implementing heat acclimatization guidelines. Since the NCAA started these guidelines there has only been one death in collegiate football during the pre-season period, where prior to 2003 there had been an average of 1-2 every pre-season (saving an projected 15 lives). In 2009 the NATA released guidelines very similar to those of the NCAA.  Since these have been released, KSI has been working with many state athletic associations (as well as others) to assist these states in the adoption of the guidelines for high schools as well as other sport levels.

      Kestrel Meter 4400 Additional Information 

      • Kestrel Meter 4400 Heat Stress Tracker  (PDF) Instructions English
      • WBGT Guidelines Insert | Download PDF
      • Athletics Booklet | Download PDF
      • Kestrel Meter 4400 Heat Stress Meter Detailed Specs.
      • Kestrel Meters Catalog (PDF) 
      • Kestrel Communicator Download for Bluetooth Model
      • Dimensions: 6.5 x 2.3 x 1.1 in. / 16.5 x 5.9 x 2.8 cm
      • Weight: 4.4 Ounces / 125 Grams
      • Model: Kestrel 4400, Part # 0844
      • Model: Kestrel 4400 Bluetooth, Part # 0844B - CLOSEOUT!
      • Model: Kestrel 4400 Night Vision, Part # 0844NVOLV / 0844NVTAN - CLOSEOUT!
      • Model: Kestrel 4400 Bluetooth Night Vision, Part # 0844BNVOLV / 0844BNVTAN - CLOSEOUT!
      • Place of Origin: USA 

      Kestrel 4400 Heat Stress Features