To check the size of small holes in injectors, flow control orifices, filters, and similar hardware, a surface tension technique is used. The liquid surface tension causes it to act as a membrane when pressure is applied. This bubble pressure is a function of hole diameter and surface tension

Hines, W. J.

English

NASA Technical Reports Server

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November 1969	 Brief 69-10595 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
A Method for Using Surface Tension to Determine

the Size of Holes in Hardware 
The Functioning of Bubble Pressure in Propellant Retention 
One method which is presently used to measure 
hole diameter is the use of precision air gauges. 
This Tech Brief describes a simpler and more inex-
pensive process for accomplishing these measure-
ments. The concept can be used to check the size of 
small holes in injectors, flow control orifices, 
filters, and similar hardware. 
The surface tension of a liquid causes it to act as 
a membrane when pressure is applied. The amount of 
preure a membrane can support ithout breaking
is called the bubble pressure, which is a function of 
the hole diameter and surface tension. It has been 
determined that for a hole of 0.01 inch diameter the 
bubble pressure is 4.6 inches of water, and for a hole 
of 0.001 inch diameter, 46 inches of water. This tech-
nique can be used for holes in the range between 0.010 
and 0.001 diameter. 
In general, capillary barriers derive their effectivity 
from the phenomenon of bubble pressure. That is, a 
pressure dIfferentIal is required to force a bubble 
inhin tied overleaf) 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19690000550 2020-03-16T18:26:29+00:00Z
through a hole in a plate wetted by a liquid. Classically, 
for any curved liquid-gas interface, this pressure 
difference is given by the capillarity equation 
B =11R 1 + l/R2) 
where a is the liquid surface tension and RI and R2 
are the local principle radii of curvature. For a per-
fectly wetted plate where the hole geometry is regular, 
the maximum pressure differential (which occurs for 
a hemispherically shaped bubble) may be expressed as 
= 77 aid. 
where d is the hole diameter and 11 has a value of 4. 
The figure shows the way in which the bubble pres-
sure principle functions in capillary-barrier pro-
pellant retention and feedout devices. Bubble 
pressure prevents liquid from flowing out of a 
compartment despite an acceleration parallel to the 
barrier. Liquid remains in the compartment as long
as the bubble pressure exceeds the hydrostatic pres-
sure, ö gh. When the hydrostatic pressure is greater, 
gas bubbles enter at the "top" permitting liquid to 
leave the compartment. 
Notes: 
1. This concept may be of interest to manufacturers 
of injectors, flow control orifices, filters, and 
similar hardware. 
2. This development is in conceptual stage only, and 
as of date of publication of this Tech Brief, neither 
a model nor prototype has been constructed. 
3. This Tech Brief is'complete in itself. No additional 
information is available. 
Patent status: 
No patent action is contemplated by NASA. 
Source: William J. Hines 
Space Division 
North American Rockwell Corporation 
under contract to 
Manned Spacecraft Center 
(MSC-15194)
LI 
Li 
., 
Brief 69-10595	 Category 03


A method for using surface tension to determine the size of holes in hardware

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