Heat Stress Meter / WBGT Meter

High temperatures and humidity put a strain on humans and many animals, whose organisms are geared towards an optimum temperature. Exposure to heat that cannot be compensated for by the body leads to health problems and even heat-related death due to reduced performance. Not only people in traditional hot workplaces such as foundries are at risk from heat stress, but also people who are exposed to unaccustomed heat outdoors or indoors for long periods of time. This can apply to office workers as well as employees and residents of nursing homes, workers in agriculture and forestry or in road and building construction.

Employees traveling to warmer regions for assembly or maintenance work and soldiers on deployment abroad are also at risk from the unfamiliar climatic conditions. However, individual sensitivity to heat varies greatly and direct sunlight, humidity and air flow conditions also play an important role in the effects on the body. For this reason, the definition of a special parameter that takes these factors into account has created a basis for a more precise assessment of the health risks posed by heat. A WBGT heat stress measuring device that can determine and directly display this parameter makes it easier to plan and implement protective measures.

The Wet Bulb Globe Temperature Index (WBGT Index) is a temperature value that takes into account the effect of humidity, solar radiation and air velocity on physical exposure to high temperatures. For the WBGT heat stress index, the natural wet bulb temperature, the globe temperature (black globe temperature) and the air temperature are added together with different weightings:

The term Wet Bulb Globe Temperature Index (WBGT) therefore refers to two of the temperature values it contains. Because of the addition of the different temperature values, the WBGT index is also referred to as a climate sum measure. These temperatures can be measured separately and used in the formula. A special heat stress meter with several sensors makes it much easier to assess the heat load, as it can record all three measured variables and display the WBGT index directly. DIN EN ISO 7243 specifies the maximum WBGT index for workplaces for a continuous exposure of 8 hours, depending on how energy-intensive the activity is. The limit values range from WBGT = 33 °C for heat-acclimatized persons with resting activities and fall to WBGT = 20 °C for non-heat-acclimatized persons with heavy physical work.

The wet bulb temperature tnw is also referred to as the natural humidity temperature or wet bulb temperature. Air that is not yet saturated with water vapor can absorb moisture from the environment. The energy required for the transition from the liquid to the gaseous state of water is extracted from the environment, causing it to cool down. This evaporative cooling can be felt, for example, when damp clothing dries on the body. The effect is intensified at higher air speeds, as dry air that can be absorbed more quickly is then drawn in. The wet bulb temperature describes the temperature to which air can be cooled by the evaporation of water under the existing temperature and humidity conditions. Mercury or resistance thermometers whose temperature sensor is encased in a material that is easy to moisten can be used for measurement.

Resistance thermometers are also suitable for continuous measurements. If the lowest temperature achievable through water evaporation - the cooling limit temperature - is to be determined for technical applications, forced-air humid thermometers are used. To determine the heat load, however, the natural wet bulb temperature is measured, which is set at the air flow present at the measuring point. If a Wet Bulb Globe Heat Stress Meter has a humidity sensor, the wet bulb temperature can also be calculated from the air temperature and humidity in the measuring device without measuring on a wet bulb thermometer.

The globe temperature tg allows additional heat radiation to be taken into account. It is measured inside a matt black sphere. The temperature there is higher than outside the sphere when the radiation hits it. For a correct measurement, the heat stress measuring device must be exposed to the radiation for a sufficiently long time.

The air temperature ta as the dry temperature of the air should not be increased by additional heat radiation hitting the sensor. Radiation-protected temperature sensors are therefore used in the heat stress measuring device for this measurement.

The German Social Accident Insurance also refers to the use of the WBGT index for assessing heat stress at workplaces. DGUV Information 213-002 (BGI 579) entitled "Heat work; recognize - assess - protect" from August 2013 is available to download free of charge. It quotes the guide values for the WBGT index from DIN EN ISO 2743. The WBGT index is also increasingly being used to assess risks at outdoor sporting events. Organizers and other responsible parties can use defined limit values and a heat stress measuring device to assess the current risk situation during an event and, if necessary, issue warnings and initiate the necessary precautionary measures. WBGT Index was developed for the US military to be used in training camps. It is as well used at workplaces and sport events and other physical activities. It is suggested in ISO 7243, Japan Society for occupational Health and the SMA (Sports Medicine Australia) and the OSHA (US Occupation Safety).

It is possible to use a heat stress tracker (HST) to continuously record the WBGT index over longer periods of time and evaluate it later on a PC. The device can be operated with batteries or, when measuring in areas with a power supply, also via a plug-in power supply unit. A heat stress tracker that allows a limit value to be set for the WBGT value and that gives a visual or acoustic alarm when this limit value is reached is particularly suitable for the continuous monitoring of areas where there is a risk of heat.