When a driving pressure is applied to the inlet of a<unk> , the ball rises in the tapered<unk>  until the flow rate creates an applied pressure on the ball equal to its weight. The<unk> 's shape, that of a slender cone, decreases the pressure behind the ball as it rises. A cylindrical<unk>  would not permit driving pressure to decrease with flow rate, resulting in the ball rising to the top of the<unk> , and allowing for no variance in readings. The flow rate of a specific gas necessary to cause the float to rise to a given height is precalculated in order to calibrate a<unk> . A variety of float shapes may be seen with older<unk> s, and all floats should be read from the top of the float, except for the ball float, which is read from its center. Floats should rotate in the airstream, and the absence of rotation may indicate faulty readings resulting from the float catching on the<unk> . The needle valve may be located proximal or distal to the inlet port; these two types of<unk>  are respectively called 'non-compensated' or 'compensated'. The original<unk>  is the non-compensated type: it works with a fixed orifice and variable pressure. The non-compensated type is more accurate for low flow rates, such as are used in neonatal units, laboratory experiments, or anaesthetic machines. Compensated<unk> s work with a variable orifice and fixed pressure. They read back pressure, and take into account resistance changes downstream from the needle valve. If pressure exceeds 50 psig downstream, flow ceases. The types can be distinguished by their response when gas starts to flow. In compensated<unk> s, the ball will initially jump as the gas flows through the<unk>  before being released through the needle valve just before the outlet. No result will be observed in the non-compensated<unk> , as the gas will release before reaching the tapered<unk>  containing the float. Although<unk> s are very similar in design and function to rotameters, the latter are more accurate. As a result,<unk> s are often checked against rotameters to ensure their accuracy. Rotameters are also used on anaesthesia machines, where precise measurement of gas delivery is crucial to the wellbeing of the patient.
Thorpe tube flowmeter
Carbon dioxide angiography
Plethysmograph
Intravascular fluorescence
Lung float test
CO-oximeter
Alveolar gas equation
Respiratory pressure meter