Name: Duane
L. Deardorff
Institution: The University of North Carolina at Chapel Hill
Address: CB# 3255
Chapel
Hill, NC 27599-3255
E-mail: duane.deardorff@unc.edu
Apparatus Title: Manometer
for Measuring Lung Pressure
Abstract:
This simple
gauge-pressure device measures lung pressure by viewing the height of a column
of water supported by air pressure from blowing into a tube. The device can
also measure negative pressures, and is surprisingly precise and accurate
(0.0001 atm resolution, ± 0.5% accuracy).
Pressure ranges are approximately ± 0.2 atm, but can easily be extended
by increasing the length of tubing.
Equipment and
costs required to construct apparatus:
|
Item |
Source |
Part number |
Cost |
|
Erlenmeyer
flask, 500 mL |
Fisher
Scientific |
10-040H |
$5.40 |
|
Vinyl tubing,
3/16” ID, 10’ |
Fisher
Scientific |
14-169-7B |
$4.32 |
|
2-hole
#7 rubber stopper |
Fisher
Scientific |
14-140J |
$1.50 |
|
glass
tubing, 5 mm OD, 1’ |
Fisher
Scientific |
11-365C |
$0.50 |
|
drinking
straws, 3/16” OD |
Dollar Store |
|
$1.00 |
|
meter
stick or tape measure |
common lab
supply |
|
--- |
|
ring
stand and clamp |
common lab
supply |
|
--- |
Total Cost |
$12.72 |
||
Description:
This
manometer consists of an Erlenmeyer flask that is half-filled with water and
has two open-ended tubes passing through a 2-hole stopper in the top of the
flask. One tube extends down into the
water to the bottom of the flask while the other tube extends just below the
bottom of the 2-hole stopper into the air space above the water. When positive pressure is applied to the
short tube, water is forced up the long tube, and the gauge pressure can be
calculated from the height of the column of water above the water
reservoir. When a partial vacuum is
applied to the long tube, water is drawn up the tube (just like drinking from a
straw).
Although
similar to J-tube manometers, this device does not require a height-difference
measurement. The only caution is that
the level of the water reservoir changes slightly as water rises up the long
tube, but this change is only about 0.5 cm for a water column of 1 m, which can
result in a 0.5% error if not corrected.
Otherwise, this device provides precise and accurate readings for
pressures near 1 atmosphere.
Pressure
Theory: Gauge
pressure: P = Dgh
D = density of fluid (water in this
case)
g = acceleration due to gravity
h = height of fluid
Useful
conversions:
1 atm = 101.3 kPa = 14.7 psi = 760 mm
Hg = 10.3 m water
Density of water = 1,000 kg/m3
Density of mercury = 13,600 kg/m3
Discoveries
learned from this device:
· Typical
lung pressure ~ 1 m water = 74 mm Hg = 9.8 kPa = 0.097 atm
· The
maximum lung pressure of about 1 m of water explains why breathing under water
(even with SCUBA gear) is difficult for depths greater than a couple of meters.
· Even
Superman could not drink from a straw that is taller than 10.3 m since this is
the maximum height of a column of water supported by 1 atm and a perfect
vacuum.
· The gauge
pressure inside a 9” latex balloon is approximately 20 cm of water (0.02 atm),
and is roughly independent of the balloon’s diameter except for the initial
inflation. This manometer could be used
in an experiment where student take measurements to examine the pressure as a
function of diameter.
Sketches:
Below is list of
resources for other types of gauge pressure devices that are available
commercially, but most of these are digital meters that do not give students a
visible or conceptual feel for the pressure that their lungs can exert.
Pressure Gauge Directory: http://www.pressure-gauges.com/
1 meter J-tube open on both ends. $110.
http://www.sciencelab.com/laboratory-equipment/Manometer_-_Vacuum_and_Pressure_Gauge.html
Differential Pressure Gauge: -15” to +15” with 2% accuracy. $96.
http://www.sargentwelch.com/ec//products/index.cfm?categoryID=23116
Digital Manometer: http://www.control3.com/3460p.htm
Several different pressure ranges. $149.
Digital Manometer: http://members.nova.org/~dsaum/inf-dm5s.htm
http://www.infiltec.com/inf-dm1s.htm
Portable Manometer: http://www.bacharach-inc.com/pressure_gauge.htm
Digital Manometer: http://www.guemisa.com/mano/presion/gdh13ane.htm
0 to 2 bar with 1 mbar resolution, 0.2% accuracy