Page 139 - Biomedical Engineering and Design Handbook Volume 2, Applications

118 MEDICAL DEVICE DESIGN

Reynold’s numbers greater than 2000 are predictive of significant turbulence. It should be
noted that while turbulent flow depends on both gas density and viscosity, laminar flow depends
only on viscosity.

4.4 DEVICE COMPONENTS

4.4.1 Volume

Gas volumes may be measured directly using one of several volume-displacement spirometers. The
simplest and oldest design, the water-sealed spirometer, uses a hollow cylinder, or bell, which is
inverted and lowered into a bucket of water containing a tube for the movement of gas (Fig. 4.4). The
bell rises or lowers as gas moves into or out of the gas space trapped between the bell and the water.
In order to prevent excess compression of the gas within the bell, earlier models used a chain and
counter-weight, although newer models are designed with lightweight bells in which gas compres-
sion is not significant. A pen is often attached to the bell, which may graph the volume-time tracing
on an attached kymograph, a rotating drum with recording paper. Many spirometers also incorporate
a linear potentiometer attached to the bell for easy electrical recording via a simple amplifier. The
basic principle of operation is that the height of the bell is related to its volume by the formula for
the volume of a cylinder:
V =π r h (4.7)
2
A similar approach is used in the dry-seal spirometer. In this case, the bell is sealed to its base
with a thin layer of latex (or some other thin and flexible material). As gas is introduced into the bell,
the latex prevents its escape and forces the bell to move, as with the water-sealed spirometer. Dry-
seal spirometers may be mounted horizontally, and may employ a moving piston instead of a mov-
ing bell. Manual and electrical recording are achieved as with the water-sealed spirometer. A third
type of volume-displacement spirometer, somewhat less common recently, is the bellows, or wedge,

Floating bell

Recording
kymograph

To subject
Water-filled
cylinder

FIGURE 4.4 Schematic drawing of the water-sealed spirometer. The kymograph rotates at
a constant speed, allowing for the inscription of a volume (y axis) versus time (x axis) plot as
the bell moves up and down in response to gas movement at the outlet.