A liquid level sensing system includes waveguides disposed in a liquid and distributed along a path with a gap between adjacent waveguides. A source introduces electromagnetic energy into the waveguides at a first end of the path. A portion of the electromagnetic energy exits the waveguides at a second end of the path. A detector measures the portion of the electromagnetic energy exiting the second end of the path

Duffell, Amanda G.

Korman, Valentin

Wiley, John T.

NASA Technical Reports Server

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(12) United States Patent 
Korman et al. 
(54) LIQUID LEVEL SENSING SYSTEM 
(75) Inventors: Valentin Korman, Huntsville, AL (US); 
John T. Wiley, Huntsville, AL (US); 
Amanda G. Duffell, Gurley, AL (US) 
(73) Assignee: The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, DC (US) 
(*) Notice: 	 Subject to any disclaimer, the term of this 
patent is extended or adjusted under 35 
U.S.C. 154(b) by 279 days. 
(21) Appl. No.: 12/827,598 
(22) Filed: 	 Jun. 30, 2010 
(51) Int. Cl. 
GOLF 23100 	 (2006.01) 
(52) U.S. Cl. 
USPC 	 ............................................................ 73/293 
(58) Field of Classification Search 
USPC ......... 340/619; 250/577, 901, 277; 73/290 V, 
73/293 
See application file for complete search history. 
(1o) Patent No.: 	 US 8,695,420 B1 
(45) Date of Patent : 
	
Apr. 15, 2014 
(56) References Cited 
U.S. PATENT DOCUMENTS 
4,505,542 A 3/1985 Clarke 	 .......................... 385/127 
6,272,060 131 8/2001 Sheen et al . 	 .................. 365/221 
6,965,708 132 11/2005 Luoetal ......................... 385/12 
7,294,458 132 11/2007 Philpott et al ..................... 435/5 
7,344,830 132 3/2008 Philpott et al ..................... 435/5 
7,710,567 131 * 5/2010 Mentzer et al . 	 ............... 356/436 
2004/0021100 Al 2/2004 Gouzman et al .............. 250/573 
20 04/00 27 13 7 Al 2/2004 Sherrard 	 ....................... 324/644 
2008/0089367 Al 4/2008 Srinivasan et al ............... 372/19 
2009/0129721 Al 5/2009 Chen et al ....................... 385/12 
* cited by examiner 
Primary Examiner Peter Macchiarolo 
Assistant Examiner Tamiko Bellamy 
(74) Attorney, Agent, or Firm Peter J. Van Bergen; James 
J. McGroary 
(57) ABSTRACT 
A liquid level sensing system includes waveguides disposed 
in a liquid and distributed along a path with a gap between 
adjacent waveguides. A source introduces electromagnetic 
energy into the waveguides at a first end of the path. A portion 
of the electromagnetic energy exits the waveguides at a sec-
ond end of the path. A detector measures the portion of the 
electromagnetic energy exiting the second end of the path. 
12 Claims, 2 Drawing Sheets 
4 ~ 4 
4, 
>8 
14 
140 
180 
40 
f 82 
https://ntrs.nasa.gov/search.jsp?R=20150003258 2019-08-31T11:18:11+00:00Z
U.S. Patent 	 Apr. 15, 2014 	 Sheet 1 of 2 	 US 8,695,420 B1 
102 
>2 
Electromagnetic 
Energy 
Source 
r>20 
/ 10 
200 
> OOA 
Waveguide 
	 >4 
Segments 
Sensor 	 >00 
>60 
Electromagnetic 
Energy 	 16 
Detector 
FIG. 1 
U.S. Patent 	 Apr. 15, 2014 	 Sheet 2 of 2 US 8,695,420 B1 
1 
142 
1 
' OOA 
142 
00 
00 
14 120 1 144 . ... 
160  
144 
FIG. 2 
140 182 
180 
140 
180 
FIG. 3 
f 82 
14, 
18 
14, FIG. 4 
40 
US 8,695,420 B1 
2 
LIQUID LEVEL SENSING SYSTEM 	 end of the path. The waveguides are disposed in a liquid 
whose level is to be measured. 
ORIGIN OF THE INVENTION 
The invention described herein was made in the perfor-
mance of work under a NASA contract and by an employee of 
the United States Government and is subject to the provisions 
of Public Law 96-517 (35 U.S.C. §202) and may be manu-
factured and used by or for the Government for governmental 
purposes without the payment of any royalties thereon or 
therefore. In accordance with 35 U.S.C. §202, the contractor 
elected not to retain title. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to liquid level sensing. More specifi-
cally, the invention is a liquid level sensing system using a 
form of electromagnetic energy such as optical energy. 
2. Description of the Related Art 
Typical liquid level sensing systems that measure the 
amount of cryogenic liquid in a tank use differential pressure 
measurement, capacitance measurements, or measurements 
made with a series of temperature or thermal resistance sens-
ing devices in a rake or array configuration. Differential pres-
sure measurements only work well in non-flowing condi-
tions. Capacitance measurements do not work well when 
used in thermal gradients and dynamic flow conditions. Ther-
mal measurement systems utilize the thermal change between 
a cryogenic liquid's gas-to-liquid fluid phases, and are limited 
by thermal latency or the time it takes for the thermal sensing 
elements to respond to a temperature change. In addition, the 
surface boundary in cryogenic liquid storage (i.e., the ullage 
layer) transitions from liquid to saturated vapor to gas. The 
temperature of the saturated vapor layer is very close to that of 
the liquid layer. This makes it difficult for a temperature-
based system to resolve the true liquid boundary in a tank. 
Still further, the accuracy and resolution of conventional liq-
uid level sensing systems are determined by the number of 
sensing elements. Since the number of sensing elements is 
typically tied to individual data channels, substantial data 
acquisition systems are usually required to achieve accurate 
results. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to 
provide a liquid level sensing system. 
Another object of the present invention is to provide a 
liquid level sensing system having a reduced number of data 
sensing elements and data acquisition channels. 
Still another object of the present invention is to provide a 
liquid level sensing system that does not rely on the use of 
pressure, capacitance or thermal sensing technology. 
Other objects and advantages of the present invention will 
become more obvious hereinafter in the specification and 
drawings. 
In accordance with the present invention, a liquid level 
sensing system includes a plurality of waveguides distributed 
along a path with a gap between adjacent waveguides. The 
path has a first end and a second end. A source introduces 
electromagnetic energy into the waveguides at the first end of 
the path. A portion of the electromagnetic energy exits the 
waveguides at the second end of the path. A detector measures 
the portion of the electromagnetic energy exiting the second 
BRIEF DESCRIPTION OF THE DRAWING(S) 
5 
Other objects, features and advantages of the present 
invention will become apparent upon reference to the follow-
ing description of the preferred embodiments and to the draw-
ings, wherein corresponding reference characters indicate 
to corresponding parts throughout the several views of the draw-
ings and wherein: 
FIG. 1 is a top-level schematic view of a liquid level sens- 
ing system in accordance with the present invention; 
FIG. 2 is an isolated schematic view of a waveguide seg- 
15 ments sensor used in the present invention; 
FIG. 3 is a plan view of a waveguide support with a plu-
rality of waveguide segments mounted therein in accordance 
with an embodiment of the present invention; and 
FIG. 4 is an end view of the waveguide support taken along 
20 line 4-4 in FIG. 3. 
DESCRIPTION OF THE PREFERRED 
EMBODIMENT(S) 
25 	 Referring now to the drawings and more particularly to 
FIG. 1, a liquid level sensing system in accordance with the 
present invention is shown and is referenced generally by 
numeral 10. System 10 is used to determine the level of a 
liquid 100 in a reservoir or tank 200. Liquid 100 can be a 
30 cryogenic on non-cryogenic liquid with the space 102 above 
the surface 100A of liquid 100 being non-liquid (i.e., a satu-
rated vapor or gas). It is to be understood that liquid 100 and 
tank 200 are not limitations of the present invention. 
Liquid level sensing system 10 includes an electromag- 
35 netic energy source 12 (e.g., an optical energy source such as 
a laser), a waveguide segments sensor 14, and an electromag-
netic (e.g., optical) energy detector 16. Source 12 and detector 
16 are tuned to the same wavelength(s) of operation. The 
particular wavelength(s) should be such that the electromag- 
4o netic (e.g., optical) energy will not be absorbed by the par-
ticular liquid 100 whose liquid is being sensed. In general, 
waveguide segments sensor 14 is disposed in liquid 100 along 
a path (e.g., straightline vertical path in the case of a tank/ 
liquid subject to the forces of gravity) between source 12 and 
45 detector 16 such that sensor 14 will be sensitive to relevant 
levels of liquid 100 in tank 200. 
Referring additionally now to FIG. 2, sensor 14 is illus-
trated in an isolated schematic view in order to explain the 
operating principles of the present invention. Sensor 14 
50 includes a number of waveguides 140 separated from one 
another by gaps 142. Sensor 14 is configured such that gaps 
142 fill with liquid 100, the non-liquid in space 102, or a 
combination of the two, depending on their position in tank 
200 and the level of liquid 100 therein. Waveguides 140 and 
55 gaps 142 are distributed along a substantially straightline path 
designated by dashed line 144. In a normal gravity environ-
ment, sensor 14 is positioned such that straightline path 144 is 
in a substantially vertical orientation in tank 200 in order to 
measure the level liquid 100 in tank 200. However, it is to be 
60 understood that the orientation of sensor 14 can be changed to 
satisfy the needs of a particular application without departing 
from the scope of the present invention. 
Each of waveguides 140 will generally by the same type of 
waveguide (e.g., rigid light pipe, flexible optical fiber, optical 
65 lens, or any other waveguide that can contain the electromag- 
netic energy produced by source 12) with known transmis- 
sion properties. For processing simplicity, the length L w of 
US 8,695,420 B1 
3 
each waveguide 140 should be the same as should the length 
L, of each gap 142, although L,, does not need to equal L,. 
Typically, L, is much smaller than Land is dependent on the 
amount of energy loss caused thereby for the wavelength(s) of 
the electromagnetic energy. The resolution of sensor 14 is 
defined by the length of waveguides 140 and gaps 142. 
By way of example, operation of the present invention will 
be explained for the case where source 12 is an optical energy 
source. A known amount of optical energy 120 (of a wave-
length that will not be absorbed by liquid 100) is introduced at 
one end of sensor 14. As optical energy 120 propagates along 
path 144, it passes through successive ones of waveguides 
140 and gaps 142. When a gap 142 resides in space 102 above 
the surface 100A of liquid 100, attenuation of optical energy 
120 will be different than when a gap 142 is in liquid 100. 
Accordingly, when the attenuated portion of optical energy 
120 exits sensor 14 as optical energy 160, such attenuated 
optical energy 160 is indicative of the level of liquid 100 in 
tank 200 once sensor 14 has been calibrated for a particular 
optical energy 120 and liquid 100. Methods of calibration 
would be readily understood by persons of ordinary skill in 
the art. Attenuated optical energy 160 is provided to detector 
16 where the level of optical energy 160 can be converted to 
a level of liquid 100 based on the calibration of sensor 14. 
Sensor 14 in its straightline orientation can be realized by 
a variety of embodiments without departing from the scope of 
the present invention. By way of example, one such embodi-
mentis illustrated in FIGS. 3 and 4 where a rigid supportbody 
18 is used to support and position waveguides 140 therein 
with gaps 142 defined therebetween. More specifically, sup-
port body is a rigid housing 180 with a channel 182 formed 
along the length thereof. Waveguides 140 can be fitted and/or 
glued in channel 182. To minimize any effects of support 
body 180 on the electromagnetic energy propagating along 
and through waveguides 140 and gaps 142, support body 180 
is typically made from a material that is non-transmissive, 
non-reflective and non-absorptive with respect to the wave-
length(s) of electromagnetic energy propagating through 
waveguides 140/gaps 142. Another option is to coat channel 
182 with a non-transmissive, non-reflective and non-absorp-
tive coating. For example, for optical energy having a wave-
length in the approximate range of 350-750 nanometers (i.e., 
the visible spectrum), suitable coating materials for channel 
182 that will not affect the optical energy include, for 
example, certain powder coatings, graphite coating, black 
paint, etc. Another option is to enclose the waveguides in a 
suitable sheathing. 
The advantages of the present invention are numerous. The 
waveguide segments sensor defines a single data channel 
capable of detecting all liquid levels of interest in a reservoir. 
Accuracy/resolution are readily defined by waveguide and 
gap length. The use of optical energy and optical elements 
means that the system will be inherently safe for sterile liq-
uids, explosive liquids, toxic liquids, etc., and can be used for 
both cryogenic and non-cryogenic liquids. 
Although the invention has been described relative to a 
specific embodiment thereof, there are numerous variations 
and modifications that will be readily apparent to those 
skilled in the art in light of the above teachings. It is therefore 
4 
to be understood that, within the scope of the appended 
claims, the invention may be practiced other than as specifi-
cally described. 
What is claimed as new and desired to be secured by 
5 Letters Patent of the United States is: 
1. A liquid level sensing system, comprising: 
at least three waveguides distributed along a vertically-
oriented straight optical path with a gap between adja-
cent ones of said waveguides, said path having a first end 
10 	
and a second end; 
a source for introducing electromagnetic energy into said 
waveguides at said first end of said path wherein a por-
tion of said electromagnetic energy exits said 
waveguides at said second end of said path; and 
a detector for measuring said portion of said electromag- 
15 	 netic energy. 
2. A liquid level sensing system as in claim 1, wherein said 
waveguides are selected from the group consisting of rigid 
light pipes, flexible optical fibers, and lenses. 
3. A liquid level sensing system as in claim 1, wherein each 
20 said gap is identically sized along said path. 
4. A liquid level sensing system as in claim 1, wherein each 
of said waveguides is identically sized along said path. 
5. A liquid level sensing system as in claim 1, wherein each 
of said waveguides is defined by a first length along said path 
25 and each said gap is defined by a second length along said 
path. 
6. A liquid level sensing system as in claim 1, wherein said 
waveguides are adapted to be disposed in a liquid, and 
wherein said electromagnetic energy has a wavelength that is 
30 not absorbed by the liquid. 
7. A liquid level sensing system, comprising: 
a rigid support; 
at least three optical waveguides coupled to said support 
along a straight path with a gap between adjacent ones of 
35 	 said waveguides, said path having a first end and a sec- 
ond end; 
said rigid support with said waveguides coupled thereto 
positioned to place said path in a vertical orientation; 
a source for introducing optical energy into said 
40 	 waveguides at said first end of said path wherein a por- 
tion of said optical energy exits said waveguides at said 
second end of said path; and 
a detector for measuring said portion of said optical energy. 
8. A liquid level sensing system as in claim 7, wherein said 
45 waveguides are selected from the group consisting of rigid 
light pipes, flexible optical fibers, and lenses. 
9. A liquid level sensing system as in claim 7, wherein each 
said gap is identically sized along said path. 
10. A liquid level sensing system as in claim 7, wherein 
50 each of said waveguides is identically sized along said path. 
11. A liquid level sensing system as in claim 7, wherein 
each of said waveguides is defined by a first length along said 
path and each said gap is defined by a second length along 
said path. 
55 	 12. A liquid level sensing system as in claim 7, wherein said 
support with said waveguides coupled thereto are adapted to 
be disposed in a liquid, and wherein said optical energy has a 
wavelength that is not absorbed by the liquid. 


Liquid Level Sensing System

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