The three Transducers

Natural Wet Bulb Temperature
Perspiration is one of the means the body controls its internal temperature. As the perspiration evaporates it has a cooling effect on the body. The natural wet bulb temperature simulates the effect evaporative heat loss has. The sensor achieves this by having an unshielded bulb covered with a wet cotton sock or wick. A 40 ml reservoir of distilled water ensures that the wick is kept moist. The evaporation from the wick cools the sensor in the same way that sweat cools the body. Because of this cooling effect the natural wet bulb temperature is normally lower than the air temperature. Note that the natural wet bulb temperature is not the same as a the psychrometric wet bulb temperature. The dimensions of the sensor are defined by ISO 7243 .

Globe Temperature
This temperature indicates the amount of heat exchanged by the body due to radiation. The transducer consists of a Pt100 temperature sensing element situated at the centre of the globe (150mm in diameter), which is made of 0.4 mm copper sheet coated with optically black lacquer that has an emission coefficient of 0.98. The temperature measured at the centre is an equilibrium temperature caused by radiative and convective heat exchanges between the globe and the environment. As a result of this, the globe temperature is influenced by air velocity, air temperature and radiant temperature. Size and construction of the globe are defined by ISO 7243 .

Air Temperature
Because the globe over-estimates the influence of direct sunshine, it is necessary to provide the true air temperature as well. This is measured by a platinum (Pt100) sensor which is radiantly shielded. The sensor is surrounded by an open ended aluminum-foil cylinder. This is highly polished to reduce the thermal radiation interference from any hot or cold bodies in close proximity of the transducer. The cylinder with its open ends enable a free flow of air to come in contact with the sensor.

Pt100 Sensor
This is a resistor sensor (resistance of 100 Ohm at 0°C) made of platinum, which provides excellent stability and accuracy. The actual sensors chosen for this transducer provide results within a narrow tolerance range. This enables the transducer to be moved around and connected to other instruments without requiring any pre-measurement adjustments to be made. Although the transducer will operate without ever requiring recalibration, it should be checked regularly for possible physical damage, which may impair its functionality.