3.4 Forces on Materials
53
(p ≈ 40 mmHg), our diaphragm moving air in our lungs (p ≈ 10 mmHg), or
our alimentary canal peristaltically moving material in our digestive track (p ≈
5 mmHg). All of these process change internal pressures by less than 6%. Assuming
regular metabolic processes occur both isobarically and isothermally is a good
approximation.
Atmospheric lows and highs have a range at sea level from 635 to 813 mmHg,
i.e. a 23% variation, but the changes occur over hours, not seconds. For those who
live in the Andes (7 km high), the atmospheric pressure can be as low as 40% of sea
level pressure. People who live at such heights develop more hemoglobin to better
utilize oxygen, whose partial pressure has been reduced by 40%.
Measuring Pressure
Below is a list of some devices used to measure pressure using the elasticity of
materials responding to pressure:
• A capacitor with an elastic dielectric will have a capacitance which increases
with pressure acting to squeeze it. Very small such capacitors can be placed in
the body and in the blood stream.
• A closed gas chamber with an elastic diaphragm acts as a barometer by placing
a lever on the diaphragm.
• A column of mercury in a closed glass vertical tube with an open bottom end
placed in a dish of mercury will have a height proportional to the atmospheric
pressure. (Mercury is used because of its high density, and its low vapor
pressure.) Sea-level atmospheric pressure will support a column 76.0 cm high.
• A sphygmomanometer measures blood pressure in the arm by determining the
pressure in an inflatable cuff.
• Strain gauges measure pressure by the change of the electrical resistance in a
stretched or compressed conductor, such as a metal foil, or carbon granules,
placed in stress.
• A carbon microphone has a resistance which depends on how much the carbon
granules are compressed by air pressure.
• An inductor with a core whose position varies with an external stress will have
an inductance depending on that external pressure.
• A piezoelectric crystal, such as quartz, will produce an electric field when under
strain. This strain can be caused by pressure. (Piezo means squeeze in Greek.)
• An ocular tonometer is a device to measure the intraocular pressure (normally
12–20 mmHg above atmospheric). The instrument invented by Hans Goldmann
measures the force needed to flatten the cornea within a ring of 3.06 mm diameter.
Assuming only very soft media between the cornea and the vitreous humor in the
eye, that force divided by the area of the ring will be the intraocular pressure. The
modern instrument corrects for tear surface tension and cornea elasticity.
53
(p ≈ 40 mmHg), our diaphragm moving air in our lungs (p ≈ 10 mmHg), or
our alimentary canal peristaltically moving material in our digestive track (p ≈
5 mmHg). All of these process change internal pressures by less than 6%. Assuming
regular metabolic processes occur both isobarically and isothermally is a good
approximation.
Atmospheric lows and highs have a range at sea level from 635 to 813 mmHg,
i.e. a 23% variation, but the changes occur over hours, not seconds. For those who
live in the Andes (7 km high), the atmospheric pressure can be as low as 40% of sea
level pressure. People who live at such heights develop more hemoglobin to better
utilize oxygen, whose partial pressure has been reduced by 40%.
Measuring Pressure
Below is a list of some devices used to measure pressure using the elasticity of
materials responding to pressure:
• A capacitor with an elastic dielectric will have a capacitance which increases
with pressure acting to squeeze it. Very small such capacitors can be placed in
the body and in the blood stream.
• A closed gas chamber with an elastic diaphragm acts as a barometer by placing
a lever on the diaphragm.
• A column of mercury in a closed glass vertical tube with an open bottom end
placed in a dish of mercury will have a height proportional to the atmospheric
pressure. (Mercury is used because of its high density, and its low vapor
pressure.) Sea-level atmospheric pressure will support a column 76.0 cm high.
• A sphygmomanometer measures blood pressure in the arm by determining the
pressure in an inflatable cuff.
• Strain gauges measure pressure by the change of the electrical resistance in a
stretched or compressed conductor, such as a metal foil, or carbon granules,
placed in stress.
• A carbon microphone has a resistance which depends on how much the carbon
granules are compressed by air pressure.
• An inductor with a core whose position varies with an external stress will have
an inductance depending on that external pressure.
• A piezoelectric crystal, such as quartz, will produce an electric field when under
strain. This strain can be caused by pressure. (Piezo means squeeze in Greek.)
• An ocular tonometer is a device to measure the intraocular pressure (normally
12–20 mmHg above atmospheric). The instrument invented by Hans Goldmann
measures the force needed to flatten the cornea within a ring of 3.06 mm diameter.
Assuming only very soft media between the cornea and the vitreous humor in the
eye, that force divided by the area of the ring will be the intraocular pressure. The
modern instrument corrects for tear surface tension and cornea elasticity.