Biomedical Engineering Reference
In-Depth Information
be connected to pressure transducers that measure blood pressure inside the right side of
the heart while the catheter is advanced into the right atrium.
After the catheter is inserted, a cold bolus is injected into the right atrium through the
proximal lumen of the catheter. The bolus solution mixes with the venous blood in the right
atrium and causes the blood to cool slightly. The cooled blood is ejected by the right ventri-
cle into the pulmonary artery, where it contacts a thermistor that is located in the wall of the
catheter near its distal tip. The thermistor measures the change in blood temperature as the
blood passes on to the lungs. An instrument computes the cardiac output by integrating the
change in blood temperature immediately following the bolus injection, which is inversely
proportional to cardiac output.
Thermocouples
Thermocouples are temperature transducers formed by joining together two dissimilar
metals, based on the discovery by Seebeck in 1821. When the two junctions of these dissim-
ilar materials are maintained at different temperatures, an electromotive force (EMF) is gen-
erated. The magnitude of the EMF is dependent on the temperature at the junctions and the
properties of the materials. This means that the thermocouple is only capable of recognizing
a temperature difference between two points and it cannot measure absolute temperature
directly. To determine the absolute temperature of the measured environment, the temper-
ature of the reference junction must be determined independently and factored into the
absolute temperature calculation. In principle, one junction can be maintained at a constant
reference temperature (for example, 0
C if the reference junction is placed in an insulated
ice water bath) while the other junction is used to measure the unknown temperature of
the environment. However, the use of an ice water bath is not very practical and compli-
cates the measurement considerably. To simplify this process, special integrated circuits
have been developed and are available commercially to perform “cold junction compensa-
tion.” If a closed circuit is formed, as shown in Figure 10.23, the current flowing in the
V
Metal B
Metal B
Metal A
Hot
Cold
FIGURE 10.23
Principle of a thermocouple-type temperature transducer.
