Biomedical Engineering Reference
In-Depth Information
FIGURE 10.19
Fleish airflow transducer.
obstruction in the middle that produces a slight pressure drop as the air is passed through
the tube. The pressure drop created across the screen is measured by a differential pressure
transducer. The signal produced by the pressure transducer is proportional to the air veloc-
ity. The tube is normally shaped in a cone to generate a laminar airflow pattern. A small
heater heats the screen so water vapor does not condense on it over time and produce
an artificially high pressure drop. Fleish-type pneumotachometers are used to monitor
volume, flow, and breathing rates of patients on mechanical ventilators.
10.3.3 Temperature Measurement
Body temperature is one of the most tightly controlled physiological variables and one of
the four basic vital signs used in the daily assessment of a patient's health. The interior (core)
temperature in the body is remarkably constant—about 37
C for a healthy person—and is
normally maintained within
0.5
C. Therefore, elevated body temperature is a sign of dis-
ease or infection, whereas a significant drop in skin temperature may be a clinical indication
of shock.
There are two distinct areas in the body where temperature is measured routinely:
the surface of the skin under the armpit or inside a body cavity such as the mouth or the
rectum. Several sensors exist to measure body temperature.
Thermistors
Thermistors are temperature-sensitive transducers made of compressed sintered metal
oxides (such as nickel, manganese, or cobalt) that change their resistance with temperature.
Commercially available thermistors range in shape from small beads to large disks, as
shown in Figure 10.20.
FIGURE 10.20
Common forms of thermistors.
