Technique provides nondestructive method of making quick, accurate determination by measuring electrical resistance of capillary tube when it is filled with electrolyte of known conductivity. Apparatus consists of conductivity cell and equipment for measuring resistance and for monitoring and controlling temperature

Ho, W. W.

Love, A. W.

Vanmelle, M. J.

English

NASA Technical Reports Server

NASA TECH BRIEF 
Langley Research Center 0
PAR HR-8 
^  R,
/ RT	 Sample Tube
Rx = or V 
= 
A 1
 = A 2 = 100,000 ohms 
A 3 = Known Resistance 
RT = R2R3/R1 
August 1975
	
B75-10156 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Nondestructive Measurement of Capillary Tube Internal Diameter 
is
A new technique for measuring the internal 
diameter (ID) of a capillary tube has been developed. 
The technique provides a nondestructive method of 
making a quick, accurate determination by meas-
uring the electrical resistance of a capillary tube when 
it is filled with an electrolyte of known conductivity. 
The measurement apparatus is shown in the 
illustration and consists of a conductivity cell and 
50 Hz
equipment for measuring resistance and for monitor-
ing and controlling temperature. 
The conductivity cell has three separate chambers, 
in series, with electrodes in each of the end chambers. 
The two end chambers and the sample tube 
connecting these chambers are filled with a known 
solution 0 N KC1). The middle chamber, electrically 
isolated from the other two, is filled with an 
ice-and-water mixture which surrounds the capillary 
tube. 
The entire apparatus is immersed in an ice-and-
water bath to maintain an electrolyte temperature of 
00 C. The two electrodes are placed near each end of 
the sample tube and are connected to a Wheatstone 
bridge arrangement as the unknown resistance. This 
resistance can be expressed as 
.
KCI	 Ice and Water	 KCI 
at0°C	 at0°C 
Apparatus for Nondestructive ID Measurement of Capillary Tube
(continued overleaf) 
This document was prepared under the sponsorship of the National	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States	 will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19750000156 2020-03-19T20:26:12+00:00Z
where: L = length of the sample tube, 
A = cross-sectional area of the tube, 
= conductivity of the KCI electrolyte 
(0.06541 mhos per cm at 00 C for 1 N 
solution), and 
V = volume of the sample in the tube. 
For tests with the apparatus prototype, the bridge 
was excited with ac at approximately 50 Hz to avoid 
polarization due to electrolysis at the electrodes. A 
lock-in amplifier was used as a detector, along with 
two precision resistance boxes (100,000 ohms) and a 
precision variable-decade resistance box in the other 
bridge arms. 
The resistance of the empty tube, measured by 
trapping a section of air in the tube, was on the order 
of 1,000 megohms. Stray effects occurring between 
the ends of the tube and the platinum electrodes were 
found to be negligible (less than 10 ohms). The length 
of the tube was measured with a traveling microscope 
to better than 0.05 percent. The resistance measure-
ments were measured to better than 1 part in 104 and, 
typically, would be in the 100,000-ohm range for 
capillary tubes of 10 cm in length and 0.04 cm ID. 
The overall accuracy achieved for V or A was better 
than 0.1 percent. 
Measuring extremely nonuniform tubes with this 
method gives a smaller volume measure than the true 
value because the smaller cross sections are more 
heavily weighed. A tube with a 5-percent taper has a 
volume error of only 0.02 percent, whereas for a 
10-percent taper, the error is of the order of 0.08 
percent.
A sample tube was measured by the conductivity 
method. Then the tube was cut into approximately 10 
sections, and the diameter was measured. The 
averaged cross-sectional area was within 0.15 percent 
of the conductivity measure. This error is approxi-
mately the uncertainty expected in the destructive 
measuring method. The conductivity technique is 
useful for measuring cross-sectional area or volume of 
small capillary tubes, and it has been found to have 
advantages over all other ways of obtaining these 
measurements. 
Note: 
No further documentation is available. Specific 
questions, however, may be directed to: 
Technology Utilization Officer 
Langley Research Center 
Mail Stop 139-A 
Hampton, Virginia 23665 
Reference: B75-10156 
Patent status: 
NASA has decided not to apply for a patent. 
Source: William W. Ho, Allan W. Love, and

Marinus J. Van Melle of

Rockwell International Corp.

under contract to

Langley Research Center

(LAR-1 1647)
0 
. 
Categories: 02 (Electronics Systems) 
04 (Materials) 
06 (Mechanics) 
05 (Life Sciences)
fl 
B75-10156


Nondestructive measurement of capillary tube internal diameter

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