The air flow can be varied by regulating the butterfly valve situated at the blower outlet, and the flow rate calculated from the pressure drop across the blower inlet orifice as measured by the sloping manometer mounted against the back of the bench. The essential component of the apparatus is a large, stainless steel duct which is supplied with air from a small blower via a set of damping filters. A window is provided in the air duct to enable the various readings to be taken without disturbing the conditions inside. A further wet and dry bulb hygrometer of the aspirated type is provided in which the inclusion of an integral blower affords constant air velocity across the bulb. Instrumentation consists of two screen-type wet and dry bulb hygrometers, one giving a digital readout by means of thermistor probes and the other giving a direct readout via a glass thermometer. Finally, there are two further types of hygrometers included with the apparatus, the first being a hair hygrometer seen through the window and the second one being a Vaisala capacitance type instrument with digital readout shown on top of the bench. A sling hygrometer is included, to demonstrate a further, simple method of ventilation of the wet and dry bulb type.
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