NBSIR 73-346

RF TOTAL MASS GAUGING IN LARGE STORAGE CONTAINERS:

EMPTY TANK MODES

R.S. Collier

Doyle Ellerbruch

Cryogenics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

October 1973

Initial Report

Prepared for the

Maritime Administration

Department of Commerce

Washington, D.C. 20235

NBSIR 73-346

RF TOTAL MASS GAUGING IN LARGE STORAGE CONTAINERS:

EMPTY TANK MOOES

R.S. Collier

Doyle Ellerbruch

Cryogenics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

October 1973

Initial Report

Prepared for the

Maritime Administration

Department of Commerce

Washington, D.C. 20235

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS. Richard W Roberts Director

CONTENTS ^Page

INTRODUCTION 1 OPERATIONAL PROCEDURES 3

EXPERIMENTAL DATA 12

CONCLUSIONS .22

ACKNOWLEDGEMENTS 23

REFERENCES 23

List of Figures

Figure 1. LOX storage tank 4

Figure 2. LOX tank stair access 5 Figure 3. LOX tank outer manhole access . , 6

Figure 4. LOX tank inner manhole cover 7

Figure 5. RF loop antenna 8

Figure 6. Electronic equipment 9

Figure 7, LOX compatible tank entry , 11

Figure 8(a). Signal reflected from an open coaxial line 20 feet long 13

Figure 8(b). Same signal with the whip antenna connected 13

Figure 9(a). Signal from the whip antenna 14

Figure 9(b). Same signal as seen by a crystal detector 14

Figure 10(a). Signal from an open coaxial cable 100 feet long ... 17

Figure 10(b). Same signal with the whip antenna connected 17

Figure 11(a). Expanded view of signal centered on the first mode. . 18

Figure 11(b). Same signal as seen by a crystal detector ..... 18

Figure 12(a). Expanded view of the signal centered on the second

mode 19

Figure 12(b). Same signal as seen by the crystal detector 19

iii
CONTENTS (Continued)

Page

List of Figures (Continued)

Figure 13(a). Signal from the whip antenna extended 25 inches ... 20

Figure 13(b), Signal from the whip antenna extended 31 inches ... 20

Figure 14(a). Signal from the whip antenna extended 3 7 inches ... 21

Figure 14(b). Signal from the whip antenna extended 49 inches ... 21

iv
ABSTRACT

This report describes experiments to determine the feasibility

of radio frequency (RF) mass gauging for fluids stored in large containers.

The experiments were done at the NASA Mississippi Test Facility using

the 460, 000 gallon LOX Storage Tank as an electromagnetic resonant

cavity. The results show that the RF gauging technique is feasible for

large containers.

Key Words: LOX storage container; radio frequency; total mass gauging

V

INTRODUCTION

The Radio Frequency (RF) mass gauging method uses the fluid

container as a cavity which will contain electromagnetic waves. At

certain frequencies (called resonant frequencies) the electromagnetic

waves are stationary, forming resonant modes. At the resonant frequencies there is a good coupling between the RF antenna and the cavity

which allows the resonant frequencies to be easily measured. As the container is filled with a dielectric fluid the resonant frequencies shift downward because the speed of wave propogation is decreased in the presence

of the fluid. Hence, the possibility exists that the total mass of the fluid

may be measured by measuring the change in the resonant frequencies.

[1] Previous work at the National Bureau of Standards on Radio Frequency (RF) Total Mass Gauging (contained in NBSIR 73-318) has shown

the feasibility and accuracy of the RF method in small containers. Data

have been obtained for hydrogen, nitrogen, oxygen and LNG in which a predictable change in the resonant frequencies have been observed with

a change in mass.

We have proposed that the RF mass gauging technique be considered for large containers, in particular, for LNG shipboard and land

storage containers as well as for fuel and oxidizer storage containers

used in space flight.

The advantages of the RF technique are (1) the simplicity of the

hardware involved and (2) the RF antenna senses the entire volume of the

tank; this tends to have an integrative effect over all the fluid in the tank

which approaches a true total mass gauge.

This report describes initial RF experiments on the 460, 000 gallon

LOX storage tank located at the NASA, Mississippi Test Facility. These

experiments were done in cooperation with the NASA George C. Marshall

Space Flight Center.
The purpose of the experiment was to see if a resonant response

could be obtained from a large cavity tank and, if so, to answer the following questions:

(1) What are the frequencies of the resonant modes?

(2) Are the resonances sharp and well defined; i. e. , what is the

"Q" of the cavity?

(3) What is the coupling between the antenna and the container;

what are the effects of antenna size?

(4) Is the technique feasible for total mass gauging in large

containers ?

It should be stated here that this work and also the work reported

in NBSIR 73-318 is oriented toward spherical containers; this is because

a sphere is a common container shape and also it is easier to study the

modes of the sphere from a theoretical point of view. In principle, the

technique could be extended to an arbitrary shaped container. Questions

on container shape must be answered as the need arises; in particular

large containers could be scale modeled in the laboratory if the scaling

laws between large and small containers were well understood; this report

will show that further work needs to be done in this area, particularly in

establishing values for the resonant frequencies of large containers.

Also, if RF gauging is to be applied to LNG, it will be necessary to

understand the relationship between the dielectric constant and the density

(or effective heating value) of LNG mixtures. It is possible that the RF

technique may have an integrative effect over the constituents of the mixture. Also, it is anticipated that the RF antenna will not be adversely

affected by heavy hydrocarbon contamination as experienced on capacitive type probes.

2
OPERATIONAL PROCEDURES

The external views of the sphere are shown in Figures 1 and 2. The sphere is stainless steel; the inner diameter is 49 feet and there is

5 feet of insulation between the inner sphere and the outer sphere. The

sphere is part of the LOX supply system at the NASA Mississippi Test

Facility. At the time of the experiment the sphere was at ambient temperature and had been purged with dry air.

Access to the inner part of the sphere is obtained through double

manhole covers on the inner and outer sphere is shown in Figures 3 and

4. A positive purge was maintained on the vessel to help maintain the

LOX compatibility of the inner sphere.

The RF antenna was mounted on a 1/4-inch aluminum plate as shown in figure 5. This plate replaced the inner manhole cover shown in

Figure 4 and just rested without bolting on the inner manhole rim. Two

different types of antenna were used: the u- shaped antenna shown In Figure 5 was 18 inches x 18 inches, had one side grounded to the aluminum plate and the other side led through an insulated feed-through to a coaxial connector; also, a whip-type telescoping (dipole) antenna was

used, this was, in fact, an automobile radio antenna of adjustable length.

The electronic equipment as shown in Figure 6 consisted of a 10

milliwatt RF generator which could be hand tuned or swept in frequency on

a linear ramp between two fixed frequency values. The RF signal is fed

on coaxial cable through a directional coupler to the antenna; the reflected

signal is fed back from the same antenna through the directional coupler to

the oscilloscope. Most of the data was taken on the platform on top of the

sphere; some data was taken at the bottom of the sphere with a 120 foot

coaxial cable from the antenna; a van was provided, as shown in Figure 1,

for taking data in inclement weather but was never used.

3
4
5
6
7
8
9
Figure 7 shows a LOX compatible entry into the inner container

in which visual observation indicated that the inner geometry was truly

a sphere with only three minor tubular intrusions into the tank. This was

born out by the data which showed no splitting of the spherical degeneracy

of the modes.

10
11
EXPERIMENTAL DATA

The initial data was taken with the dipole antenna extended to its

fall length of 54 inches. After some initial adjustments the RE generator was

set to sweep on a linear ramp between 20 MHz and 50 MHz. Figure 8 shows

the signal at the scope; the horizontal scale is frequency 20-50 MHz, left to

right, and the vertical scale is the signal amplitude as reflected back

through the directional coupler.

Eigure 8(a) is the signal reflected back from the end of the open

coaxial line; that is, the antenna was not connected. The wavy response

is characteristic of the 20 foot coaxial line representing standing waves

set up in the line.

Figure 8(b) is the same signal with the antenna connected; the

absorption lines of the first seven TM modes of the cavity are seen to be

superimposed on the general background response.

Figure 9 shows the effect of inserting a crystal detector between

the directional coupler and the scope; the wavy response is filtered out and

the absorption lines become spikes at the detector output. These spikes

can then be used to trigger digital timing circuits to convert the frequency

measurements to the time domain as described in NBSIR 73-318. The

important feature of the spike is that the leading edge (left hand edge) of

the spike is very steep; this contributes to precise timing measurements

and greatly enhances the accuracy of the technique as a mass gauge.

The sweep on the RE generator was turned off and the generator

was hand tuned to each of the spikes in succession and the frequencies

were read directly with a counter. The frequencies of the first seven

TM modes were as follows:

12
Figure 8(a). Signal reflected from an open coaxial line 20 feet long.

Horizontal scale 20 - 50 MHz,

Vertical scale 0. 1 volt/cm.
Figure 9(a), Signal from the whip antenna.

Horizontal scale 20 - 50 MHz.

Vertical scale 0. 1 volt/cm.
Mode Frequency (MHz)

TMon

TMosi

TMo3i

TMo4i

TMois

TMoBi

TMo2 2

24. 09

29. 36

34. 19

39. 07

40. 38

46. 80

47. 59

These measurements were repeatable to ± . 02 MHz. In addition to these

data, the loop antenna picked up an additional mode; the TEon at 31. 62

MHz.

Figure 10 shows the effect of a remote measurement taken at the

end of a 120 foot coaxial line leading to the bottom of the sphere. Figure

10(a) shows the open circuit response; the wavy nature of the response

has changed due to the increase in line length. The cavity response is

shown in figure 10(b) and a crystal detector gives the same spike pattern

as shown in Figure 9(b).

Figure 11 shows an expanded scale of the absorption line and the

spike for the TMqh mode. The full scale sweep is about 1 MHz; from

this, it is estimated that the width of the line at the half power point is

on the order of 0. 01 MHz. This is roughly the accuracy of the frequency

measurement. The Q of the cavity is this width divided by the center

frequency giving a lower limit on Q of approximately 2000. This is very

encouraging from an accurate gauging point of view. Figure 12 gives the

same conclusion for the TMogi mode.

Figures 13 and 14 give a qualitative picture of the effect of antenna

size on the coupling to the cavity. Figure 13(a) is the response from a

15
25 inch long dipole antenna; the absorption lines are barely detectable.

In Figure 13(b), the antenna is 31 inches long; 14(a), 37 inches long;

14(b), 49 inches long. From this study, it appears that the antenna size

should be roughly 1/20 the diameter of the cavity. This is consistent

with the previous work on 18 the inch diameter cavity where the antenna

size is on the order of one inch.

16
Figure 10(a). Signal from an open coaxial cable 100 feet long

Horizontal scale 20 - 50 MHz.

Vertical scale 0. 1 vo4t/cm.
Figure 11(a). Expanded view of signal centered on the first mode.

Horizontal scale

1 MHz full scale

Pulse frequency 24. 09 MHz

Vertical scale 0. 1 volt/cra

Figure 11(b). Same signal as seen by a crystal detector.

18
Figure 12(a). Expanded view of the signal centered on the second mode.

Horizontal scale.

1 MHz full scale.

Line frequency 29. 36 MHz.

Vertical scale 0. 1 volt/cm.

Figure 12(b). Same signal as seen by the crystal detector.

19
Figure 13(a). Signal from the whip antenna extended 25 inches

Horizontal scale 20 - 50 MHz.

Vertical scale 0. 1 volt/ cm.

Figure 13(b). Signal from the whip antenna extended 31 inches

20
Figure 14(a). Signal from the whip antenna extended 37 inches

Horizontal scale 20 - 50 MHz.

Vertical scale 0. 1 volt/ cm.

21
CONCLUSIONS

These experiments show that it is possible to excite the resonant

modes of a large tank; that the response is sharp and well defined; and

hence, the RF technique is a feasible method for large tank total mass

gauging. The frequencies of the modes were somewhat higher than expected; the differences between the experimental and theoretical values for the

resonant frequencies are as follows:

(freq) - (freq) MHz Mode exp calc

TMon 6.59

TMo2i 4. 70

TMo3i 2. 49

TMo4i 0-47

TMois 1-38

TMoBi 2. 77

TM022 0. 19

It is seen that the agreement becomes better for the high frequency modes.

This difference produces no conceptual problems to the mass gauging

process. However, if large tanks are to be modeled in the laboratory,

this effect needs to be more clearly understood. The calculated frequencies were obtained assuming perfect cavity walls; it is possible that

at the lower frequencies, the wall permeability losses become important

to the calculations. This point should be investigated.

22
ACKNOWLEDGEMENTS

The authors wish to thank Mark Payne and the personnel of the

NASA Mississippi Test Facility for their interest and technical support

of this experiment; and also D. B. Mann of the NBS Cryogenics Division

for his encouragement and profitable discussions concerning this work.

REFERENCES

[1] Mass Quantity Gauging by RF Mode Analysis, NBSIR 73-318,

June 1973, R. S. Collier, Doyle EUerbruch, J. E. Cruz, Robert

W. Stokes, Philip E. Luft, R. Gordon Peterson and A. E. Hiester.

23
FORM NBS-114A (1-71) U.S. DEPT. OF COMM. 1. PUBLICATION OR REPORT NO. [2, Gov't Acce«si«tt BIBLIOGRAPHIC DATA No. SHEET NBSIR 73-346 |

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

RF Total Mass Gauging in Large Storage Containers:

Empty Tank Modes

5, Publication Date October 1973

6. Perfonniag OrgaaizLation Code

7. AUTHOR(S)

R. S. Collier and Dovle EUerbruch

t. Perforimng Ofganization

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

Boulder, Colorado 80302

"10. Project/Task/Work Unit No. 2750401

11. Contract/Grant No.

12, Sponsoring Oganization Name and Address Maritime Administration

Department of Commerce

Washington, DC 20235

13. Type of Report & Period Covered

Initial Report

14. %>onsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

This report describes experiments to determine the feasibility of radio

frequency (RF) mass gauging for fluids stored in large containers. The experiments were done at the NASA Mississippi Test Facility using the 460, 000 gallon

LOX Storage Tank as an electromagnetic resonant cavity. The results show

that the RF gauging technique is feasible for large containers.

17. KEY WORDS (Alphabetical order, separated by semicolons)

LOX storage container; radio frequency; total mass gauging

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