{"id":108652,"date":"2026-03-18T12:03:29","date_gmt":"2026-03-18T11:03:29","guid":{"rendered":"https:\/\/34.38.16.0\/?p=108652"},"modified":"2026-04-13T15:09:42","modified_gmt":"2026-04-13T13:09:42","slug":"wbgt-when-simplifications-undermine-reliability","status":"publish","type":"post","link":"https:\/\/lsi-lastem.com\/es\/wbgt-when-simplifications-undermine-reliability\/","title":{"rendered":"WBGT: when simplifications undermine reliability"},"content":{"rendered":"\t\t
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WBGT: when simplifications undermine reliability<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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Heat stress by WBGT index\n<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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The\u00a0ISO 7243 standard<\/b>, updated in 2017, is the\u00a0reference document<\/b>\u00a0for assessing heat stress using the calculation of the\u00a0WBGT<\/b>\u00a0(Wet Bulb Globe Temperature)\u00a0index<\/b>. This standard includes, not only the procedures necessary to calculate the index and the tables that define the limits, but also details on the characteristics of the sensors necessary to acquire the environmental parameters necessary for the execution of the formulas:<\/p>

  • Tnw: wet bulb temperature with natural ventilation<\/li>
  • Tg: black globe temperature<\/li>
  • Ta: air temperature (only necessary in the presence of direct radiation \u201csolar load\u201d).<\/li><\/ul>

    The instrumentation used to measure these parameters and to calculate the WBGT index should comply with the requirements of ISO7243 standard. In this article we will evaluate two aspects that are often overlooked during the choice of instrumentation, which, being used in legal fields, must instead be strictly compliant with the standard.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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    Can a calculation effectively replace the direct measurement of the wet bulb temperature with natural ventilation?<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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    Description of natural ventilation humid temperature sensor (Tnw) according to ISO 7243 standard<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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    In\u00a0Annex B.1<\/b>, the standard provides a\u00a0description of the sensor<\/b>\u00a0used\u00a0to measure<\/b>\u00a0the wet bulb temperature with natural ventilation<\/b>. This sensor consists of a temperature element covered by a cotton sock with high water absorbing power.In\u00a0Annex D<\/b>, the standard presents an alternative to direct measurement by sensor. The standard offers the possibility of obtaining this value\u00a0indirectly<\/b>\u00a0through a calculation that uses the following parameters:<\/p>

    • Ta: air temperature<\/li>
    • RH: relative humidity<\/li>
    • Tr: mean radiant temperature<\/li>
    • Va: air speed<\/li><\/ul>

      The following considerations are also reported in the standard:<\/p>

      • The indirect evaluation of Tnw by calculations is neither simple nor reliable, especially when air velocity is low in conditions of natural convections. It is not recommended;<\/li>
      • The calculation should only be used when direct measurement of Tnw is not possible;<\/li>
      • It is preferable to measure the natural wet bulb temperature directly according to Annex B;<\/li>
      • It is important to remember that when making calculation of natural wet temperature, the environmental measures used will have associated measurement errors. These can accumulate in any prediction and hence any calculation should be viewed with caution;<\/li>
      • The natural wet bulb as defined in Annex B should be used as the specification and most accurate method.<\/li><\/ul>

        In the WBGT index formulas, it is important to note that the weight of the\u00a0wet bulb temperature<\/b>\u00a0(Tnw) is significant as it is multiplied by a value of\u00a00.7<\/b>. Therefore, using a Tnw value with a high uncertainty can cause a considerable error in the final calculation compared to the other quantities. Regarding the\u00a0air temperature<\/b>\u00a0(Ta), used only in the WBGT formula with solar load, the multiplier is equal to\u00a00.1<\/b>. Instead, that of the\u00a0mean radiant temperature<\/b>\u00a0(Tr), is equal to\u00a00.3<\/b>\u00a0or\u00a00.2\u00a0<\/b>depending on the formula used (with or without solar load).<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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        Consideration on water-free systems<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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        On the market, there are several\u00a0systems to calculate<\/b>\u00a0the\u00a0WBGT<\/b>\u00a0index defined as\u00a0\u201cwater-free\u201d<\/b>\u00a0that do not have the direct measurement of the Tnw but are based on an indirect calculation by means of the formula specified in Annex D of the standard, which however requires measurement of air speed. Most of the \u201cwater-free\u201d systems currently available on the market do not allow air velocity measurement. This introduces an error in the calculation of the WBGT index.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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        Direct and reliable LSI LASTEM solutions\n<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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        For these reasons, LSI LASTEM has chosen not to adopt \u201cwater-free\u201d systems. Instead, it opted for the use of\u00a0systems<\/b>\u00a0that allow\u00a0direct measurement<\/b>\u00a0of the Tnw using dedicated sensors, thus following the advice defined in the standard.LSI LASTEM provides different\u00a0solutions<\/b>\u00a0for\u00a0measuring<\/b>\u00a0the wet bulb temperature with natural ventilation, for\u00a0indoor\u00a0<\/b>and\u00a0outdoor<\/b>\u00a0acquisitions even over\u00a0long periods<\/b>\u00a0(Figure).<\/p>

        When sensors of this kind are used in fixed outdoor applications, one of the most significant limitations is related to the limited capacity of the water tank, which requires it to be topped up with a certain frequency. To overcome this limitation, LSI LASTEM proposes the\u00a0DMA122<\/b>\u00a0sensor (Figure 3), specifically designed for fixed applications, even outdoors, thanks to the\u00a0large<\/b>\u00a01 liter\u00a0tank<\/b>\u00a0which allows a more limited frequency of water topping up.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t

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        1.\u00a0<\/strong>ESU121:<\/strong>\u00a0small size sensor suitable for portable indoor measurements<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t

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        2. Heat Shield:<\/b>\u00a0portable compact system for calculating WBGT with presence of Tnw sensor<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t

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        3. DMA122:<\/b>\u00a0large size sensor suitable for outdoor applications with 1 liter water tank<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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        Can the 5 cm globe thermometer replace the 15 cm one without introducing errors?<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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        Description of the Globe thermometer sensor according to ISO 7243 standard\n<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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        The ISO 7243 standard, in Annex B (section\u00a0B-2<\/b>), provides a\u00a0description of the sensor<\/b>\u00a0used\u00a0to measure the Black Globe Temperature<\/b>\u00a0(Tg). A sensor with a thermosensitive element positioned in the center of a hollow sphere is described with the following characteristics:<\/p>

        • Diameter: 15 cm;<\/li>
        • Average emission coefficient: 0.97 (emissivity: 97% of the radiation is absorbed).<\/li><\/ul>

          In order for the emission coefficient to reach this value, the sphere must be painted with a\u00a0matt black paint<\/b>. To ensure the shortest possible response time, the sphere material must have a\u00a0high thermal conductivity<\/b>. The standard mentions \u201ccopper\u201d as a material capable of providing a shorter response time.<\/p>

          Furthermore, the standard describes a sphere with a diameter of\u00a015 cm<\/b>, but also provides the possibility of using spheres with different diameters. Annex C of the standard, entitled \u201cAlternative globes\u201d, provides instructions on how to correct the measurements performed using diameters other than 15 cm. However, some important considerations are mentioned in the standard:<\/p>

          • The globe temperature sensor as specified in Annex B is the only sensor specification that meets the requirement;<\/li>
          • As an approximation, globes that vary from the specification may be used if a valid correction is made to provide estimation of the temperature;<\/li>
          • The correction formula can be used to make the correction;<\/li>
          • It is important to note that making a correction for globe size involves measurements of the environment (e.g. air temperature, air speed);<\/li>
          • The accuracy of any prediction will therefore be dependent on the accuracy of the environmental measures. Errors in measurement can be significant so any correction will include these inaccuracies;<\/li>
          • It can be seen that, to make a correction for globe size, air speed is required. If air velocity is not known, then it will not be possible to make a correction.<\/li><\/ul><\/div><\/div><\/div><\/div><\/div><\/div>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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            Consideration on systems with small globes<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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            On the market there are\u00a0systems for calculating the WBGT<\/b>\u00a0that use\u00a0black globes<\/b>\u00a0with\u00a0diameters of less<\/b>\u00a0than\u00a015 cm<\/b>. Those with a diameter of 5 cm are very common, chosen for their portability, for a faster response time, for the presumed greater resistance and for adapting to the preferences of some markets.However, many of these systems do not include air speed measurement, and therefore do not correct globe diameter as required by the standard. Consequently, these systems do not comply with the ISO 7243 standard.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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            Standard and reliable LSI LASTEM solutions<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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            LSI LASTEM recommends the use of\u00a0globothermometric sensors<\/b>\u00a0with a\u00a0diameter<\/b>\u00a0of\u00a015 cm<\/b>. LSI LASTEM sensors are made of copper, covered with a highly absorbent paint, better than required by the standard (reflectivity <2% therefore absorption >= 0.98). In\u00a0Heat Shield<\/b>\u00a0portable\u00a0systems<\/b>, there is a model with a\u00a05 cm globothermometer<\/b>, but in this case there is a\u00a0specific correction formula<\/b>. The correction is only active if the air speed is measured directly with a connected ESV126 <\/span>anemometric sensor. Alternatively, it is possible to enter the air speed value manually via the keyboard; this possibility is recommended for indoor measurements when the air speed is definitely absent.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t

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            \n\t\t\t\t\t\t\t\t\t1. Globothermometric<\/strong>\u00a0
            sensor DMA131A-EST131 with\u00a015 cm\u00a0diameter\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t
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            2. Heat Shield\u00a0<\/b>models ELR605M-ELR615M with globothermometric sensor with\u00a015 cm\u00a0diameter<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t

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            3. Heat Shield<\/b>\u00a0models ELR600M-ELR610M with globothermometric sensor with\u00a05 cm\u00a0diameter<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t

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            4. ESV126<\/strong> hot-wire <\/span><\/b>anemometer sensor with interchangeable head, for air speed measurement with Heat Shield. Useful for correcting the globothermometer dimension on ELR600M-ELR610M models<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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            Conclusions<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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            • Although the ISO 7243 standard offers an alternative to the direct calculation of the\u00a0wet bulb temperature with natural ventilation<\/b>\u00a0using a formula mentioned in its Annex D, it is important to note that the\u00a0direct measurement remains the most accurate method\u00a0<\/b>especially in situations of low air speeds, when the formula described in Annex D involves major errors.<\/li>
            • Although the ISO 7243 standard offers an alternative to the use of globe thermometers with a diameter of 15 cm,\u00a0systems with different globe sizes<\/b>,\u00a0without air speed measurement, do not comply with the standard<\/b>.<\/li><\/ul>

              For this reason,\u00a0LSI LASTEM<\/b>\u00a0has chosen to follow the recommendations of the standard by recommending the use of globe thermometric sensors with a diameter of 15 cm, or with a diameter of 5 cm but, in this case, with a correction formula that uses the measurement of the air speed coming from a connected anemometer.<\/p><\/div>

              For the measurement of Wet Temperature, LSI LASTEM does not provide \u201cwater-free\u201d systems which are too prone to errors, but only provides\u00a0solutions\u00a0<\/b>that allow the direct measurement of the wet bulb temperature, offering\u00a0reliability<\/b>\u00a0and\u00a0precision\u00a0<\/b>in both indoor and outdoor portable and fixed measurements.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t

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              FAQ: Frequently Asked Questions<\/h3>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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              What is WBGT? <\/div><\/span>\n\t\t\t\t\t\t\t\n\t\t\t<\/path><\/svg><\/span>\n\t\t\t<\/path><\/svg><\/span>\n\t\t<\/span>\n\n\t\t\t\t\t\t<\/summary>\n\t\t\t\t
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              WBGT is an index used to assess heat stress because it takes into account air temperature, humidity, radiant heat, and ventilation together. For this reason, it is more reliable than air temperature alone when estimating the actual risk for workers.<\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/details>\n\t\t\t\t\t\t

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              Is a 5 cm black globe always equivalent to a 15 cm one? <\/div><\/span>\n\t\t\t\t\t\t\t\n\t\t\t<\/path><\/svg><\/span>\n\t\t\t<\/path><\/svg><\/span>\n\t\t<\/span>\n\n\t\t\t\t\t\t<\/summary>\n\t\t\t\t
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              No. ISO 7243 uses a 15 cm black globe as the reference; if globes with different diameters are used, specific corrections are required. Without measuring air velocity, these corrections cannot be applied properly.<\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/details>\n\t\t\t\t\t\t

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              Why use a portable wireless system for WBGT? <\/div><\/span>\n\t\t\t\t\t\t\t\n\t\t\t<\/path><\/svg><\/span>\n\t\t\t<\/path><\/svg><\/span>\n\t\t<\/span>\n\n\t\t\t\t\t\t<\/summary>\n\t\t\t\t
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              Because it allows measurements to be taken directly in the field, close to the workstation, where the risk must actually be assessed. The Heat Shield system, for example, integrates the parameters needed to calculate WBGT and can also manage WBGT Eff, WBGT Ref, and the average value across the three levels: head, abdomen, and ankles.<\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/details>\n\t\t\t\t\t\t

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              Is WBGT alone enough to manage heat stress risk? <\/div><\/span>\n\t\t\t\t\t\t\t\n\t\t\t<\/path><\/svg><\/span>\n\t\t\t<\/path><\/svg><\/span>\n\t\t<\/span>\n\n\t\t\t\t\t\t<\/summary>\n\t\t\t\t
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              No, WBGT is essential, but it does not cover the entire assessment. Heat risk management must also take into account acclimatization, hydration, work intensity, clothing, and work organization, all of which can significantly increase the thermal load on the worker.<\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/details>\n\t\t\t\t\t\t

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              Is LSI LASTEM instrumentation suitable for accurate and compliant WBGT measurements? <\/div><\/span>\n\t\t\t\t\t\t\t\n\t\t\t<\/path><\/svg><\/span>\n\t\t\t<\/path><\/svg><\/span>\n\t\t<\/span>\n\n\t\t\t\t\t\t<\/summary>\n\t\t\t\t
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              Yes. The Heat Shield solution directly measures the key parameters for WBGT \u2014 Tg, Tnw, Ta, RH and, when needed, air velocity \u2014 following an approach consistent with ISO 7243 and with the article\u2019s emphasis on the importance of direct Tnw measurement. In addition, it can apply corrections in 5 cm globe configurations using air velocity measurement and also includes a high-precision calibration verification probe, all of which help ensure reliable and technically robust data.<\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/details>\n\t\t\t\t\t<\/div>\n\t\t\t\t\t