WBGT: when simplifications undermine reliability

Heat stress by WBGT index

The ISO 7243 standard, updated in 2017, is the reference document for assessing heat stress using the calculation of the WBGT (Wet Bulb Globe Temperature) index. 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:

Tnw: wet bulb temperature with natural ventilation
Tg: black globe temperature
Ta: air temperature (only necessary in the presence of direct radiation “solar load”).

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.

Can a calculation effectively replace the direct measurement of the wet bulb temperature with natural ventilation?

Description of natural ventilation humid temperature sensor (Tnw) according to ISO 7243 standard

In Annex B.1, the standard provides a description of the sensor used to measure the wet bulb temperature with natural ventilation. This sensor consists of a temperature element covered by a cotton sock with high water absorbing power.In Annex D, the standard presents an alternative to direct measurement by sensor. The standard offers the possibility of obtaining this value indirectly through a calculation that uses the following parameters:

Ta: air temperature
RH: relative humidity
Tr: mean radiant temperature
Va: air speed

The following considerations are also reported in the standard:

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;
The calculation should only be used when direct measurement of Tnw is not possible;
It is preferable to measure the natural wet bulb temperature directly according to Annex B;
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;
The natural wet bulb as defined in Annex B should be used as the specification and most accurate method.

In the WBGT index formulas, it is important to note that the weight of the wet bulb temperature (Tnw) is significant as it is multiplied by a value of 0.7. 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 air temperature (Ta), used only in the WBGT formula with solar load, the multiplier is equal to 0.1. Instead, that of the mean radiant temperature (Tr), is equal to 0.3 or 0.2 depending on the formula used (with or without solar load).

Consideration on water-free systems

On the market, there are several systems to calculate the WBGT index defined as “water-free” that 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 “water-free” systems currently available on the market do not allow air velocity measurement. This introduces an error in the calculation of the WBGT index.

Direct and reliable LSI LASTEM solutions

For these reasons, LSI LASTEM has chosen not to adopt “water-free” systems. Instead, it opted for the use of systems that allow direct measurement of the Tnw using dedicated sensors, thus following the advice defined in the standard.LSI LASTEM provides different solutions for measuring the wet bulb temperature with natural ventilation, for indoor and outdoor acquisitions even over long periods (Figure).

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 DMA122 sensor (Figure 3), specifically designed for fixed applications, even outdoors, thanks to the large 1 liter tank which allows a more limited frequency of water topping up.

1. ESU121: small size sensor suitable for portable indoor measurements

2. Heat Shield: portable compact system for calculating WBGT with presence of Tnw sensor

3. DMA122: large size sensor suitable for outdoor applications with 1 liter water tank

Can the 5 cm globe thermometer replace the 15 cm one without introducing errors?

Description of the Globe thermometer sensor according to ISO 7243 standard

The ISO 7243 standard, in Annex B (section B-2), provides a description of the sensor used to measure the Black Globe Temperature (Tg). A sensor with a thermosensitive element positioned in the center of a hollow sphere is described with the following characteristics:

Diameter: 15 cm;
Average emission coefficient: 0.95 (emissivity: 95% of the radiation is absorbed).

In order for the emission coefficient to reach this value, the sphere must be painted with a matt black paint. To ensure the shortest possible response time, the sphere material must have a high thermal conductivity. The standard mentions “copper” as a material capable of providing a shorter response time.

Furthermore, the standard describes a sphere with a diameter of 15 cm, but also provides the possibility of using spheres with different diameters. Annex C of the standard, entitled “Alternative globes”, provides instructions on how to correct the measurements performed using diameters other than 15 cm. However, some important considerations are mentioned in the standard:

The globe temperature sensor as specified in Annex B is the only sensor specification that meets the requirement;
As an approximation, globes that vary from the specification may be used if a valid correction is made to provide estimation of the temperature;
The correction formula can be used to make the correction;
It is important to note that making a correction for globe size involves measurements of the environment (e.g. air temperature, air speed);
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;
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.

Consideration on systems with small globes

On the market there are systems for calculating the WBGT that use black globes with diameters of less than 15 cm. 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.

Standard and reliable LSI LASTEM solutions

LSI LASTEM recommends the use of globothermometric sensors with a diameter of 15 cm. 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 Heat Shield portable systems, there is a model with a 5 cm globothermometer, but in this case there is a specific correction formula. The correction is only active if the air speed is measured directly with a connected ESV125A 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.

1. Globothermometric
sensor DMA131A-EST131 with 15 cm diameter

2. Heat Shield models ELR605M-ELR615M with globothermometric sensor with 15 cm diameter

3. Heat Shield models ELR600M-ELR610M with globothermometric sensor with 5 cm diameter

4. ESV125A anemometer sensor for air speed measurement with Heat Shield. Useful for correcting the globothermometer dimension on ELR600M-ELR610M models

Conclusions

Although the ISO 7243 standard offers an alternative to the direct calculation of the wet bulb temperature with natural ventilation using a formula mentioned in its Annex D, it is important to note that the direct measurement remains the most accurate method especially in situations of low air speeds, when the formula described in Annex D involves major errors.
Although the ISO 7243 standard offers an alternative to the use of globe thermometers with a diameter of 15 cm, systems with different globe sizes, without air speed measurement, do not comply with the standard.

For this reason, LSI LASTEM has 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.

For the measurement of Wet Temperature, LSI LASTEM does not provide “water-free” systems which are too prone to errors, but only provides solutions that allow the direct measurement of the wet bulb temperature, offering reliability and precision in both indoor and outdoor portable and fixed measurements.