Biomedical Engineering Reference
In-Depth Information
TABLE 2-7
Conversion Among Different Pressure Measurement Units
Unit
kPa
psi
in H
2
O m
2
O
in Hg
mm Hg
mbar
kPa
1
0.145
4.015
10.2
0.2593
7.501
10
psi
6.895
1.0
27.68
70.31
2.036
51.72
68.95
in H
2
O
0.2491
0.03613
1.0
2.54
0.07355
1.868
2.491
cm H
2
O
0.09806
0.01422
0.3937
1.0
0.02896
0.7355
0.9806
in Hg
3.386
0.4912
13.6
34.53
1.0
25.4
33.86
mm Hg
0.1333
0.01934
0.5353
1.36
0.03937
1.0
1.333
mbar
0.10
0.01450
0.04015
1.020
0.02953
0.7501
1.0
The conventional sphygmomanometer is an indirect method of measuring blood pres-
sure. It consists of an inflatable cuff and a mercury manometer and is considered to be
the gold standard for blood pressure measurement because it cannot be in error if oper-
ated correctly. However, its main disadvantage is that it is unable to provide a continuous
reading of pressure variations, and its update rate is limited. In addition, only the sys-
tolic and diastolic arterial pressures can be measured. The operational principles of the
sphygmomanometer are quite simple. A cuff is placed around the upper arm and inflated,
and arterial blood can flow past it only when the arterial pressure exceeds that of the cuff.
At this stage, blood squeezing through the reduced aperture of the brachial artery gener-
ates turbulence that can be heard by a practitioner with a stethoscope. As the pressure in
the cuff is reduced and monitored using a manometer, the first indication of this sound
(called Korotkoff sounds) gives an indication of the systolic pressure. This reduction in
cuff pressure continues until Korotkoff sounds disappear, and that is an indication of the
diastolic pressure.
Direct pressure measurement devices consist of a chamber with a flexible diaphragm
making up a portion of one wall with the other side of the diaphragm at atmospheric
pressure. A pressure differential across the diaphragm will cause it to deflect, and this
deflection can be measured using a displacement sensor. Capacitance was the method
of choice for measuring small displacements. If a second plate was placed within a few
hundred microns of the diaphragm, changes in the relative separation resulted in changes
in capacitance, as discussed earlier in this chapter.
Today, such small displacements are generally measured using strain gauges in a
bridge configuration. In the past, a mechanical assembly comprising the diaphragm, a
force rod, and four unbonded strain gauges was used. With increasing deflection, two
of the strain gauges become progressively more relaxed and two are stretched (Gregory,
1975). Modern pressure transducers are generally manufactured onto a single MEMS
silicon chip where a portion of the chip is formed into a diaphragm and semiconductor
strain gauges (piezoresistive bridge) are incorporated directly onto the diaphragm to form
small inexpensive pressure sensors, as shown in Figure 2-55. Such sensors are sufficiently
low cost, so they can be used as disposable, single-use devices for measuring blood pressure
without need for additional sterilization (Bronzino, 2006).
Wire strain gauges usually have gauge factors of between two and four, while the
semiconductor based units have gauge factors of between 50 and 200. For silicon, the
gauge factor is typically 120. However, in measuring pressure sensitivities are generally
quoted in microvolts per volt (applied to the strain gauge) per millimeter of mercury
(Cromwell, Weibell et al., 1973). Signal conditioning and display instruments for these
sensors consist of a method of exciting the strain gauge bridge, a method of zeroing or
balancing it, followed by an amplifier and a display device or data logger.
