Name:  Duane L. Deardorff

Institution:  The University of North Carolina at Chapel Hill

Address:      CB# 3255
Chapel Hill, NC 27599-3255
     
     
     

Phone:         (919) 962-3013

Fax:             (919) 962-0480

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