Method and apparatus for measuring the level of a fluent material in a container

In a method of measuring the level of a flowing material kept in a container there is employed a microwave signal that is fed from a transmitter through a tubular waveguide (7) that extends vertically downwardly through the container and communicates therewith so that the surface (10) of the material in the waveguide follows the level of the surrounding material. The signal is reflected from the surface back up through the waveguide and is conducted to a receiver to be employed, after signal processing in an electronic unit, for determining the level of material in the container. The microwave signal is fed to the waveguide by way of a mode generator (11) that produces only one dominant propagation mode of the signal, the wavelength of which is smaller than the diameter of the waveguide (7).

This invention relates to a method for measuring the level of a fluent 
material stored in a container by means of a microwave signal which is fed 
out of a transmitter through a tubular waveguide that extends vertically 
downwardly through the container and so communicates with it that the 
surface of the material in the waveguide follows the level of the 
surrounding material, and which signal is reflected by the surface back up 
through the waveguide and is conducted to a receiver, to be employed, 
after signal processing in an electronic unit, for determining the level 
of material in the container. The invention also relates to apparatus for 
practicing this method. 
Radar can be employed for measuring the level of a liquid or liquid-like 
material that is contained in a cistern, tank or the like, as described 
for example in U.S. Pat. No. 4,044,355. Because the velocity of radar 
waves in air or other gases is very stable, a good accuracy is obtained, 
and because the radar antenna can be made of very durable material, such a 
level measurer can be employed in environments that are very extreme with 
respect to temperature, chemical corrosion and mechanical stress. Since 
the radar antenna can be mounted in a hole in the top of the tank, its 
installation becomes simple and it also becomes simple to perform 
maintenance and eventual replacement. 
It has heretofore been a limitation on its employment that a radar beam 
requires a certain space among the existing struts, ladders, pipes, etc. 
in the tank. If a round radar antenna with a diameter D is employed, the 
usable width of the radar beam becomes about .lambda./D radians but the 
undisturbed zone, taking into consideration the diffuse boundaries of the 
radar beam, must be a cone with a top angle of about 2.lambda./D radians. 
Here, .lambda. designates the wavelength of the radar carrier wave, which 
can be, for example, 3 cm. On the basis of various practical 
considerations the antenna diameter must be held within certain limits and 
the wavelength of the carrier wave is in practice limited downwardly 
inasmuch as the radar transmitter and other components become expensive 
and critical in various respects with very high carrier frequency. Thus 
the radar beam cannot be arbitrarily narrow, and in several applications 
this is not at all desirable, for example when the apparatus is employed 
in a tanker ship with varying trim and list. In a practical case an angle 
of 5.degree.-15.degree. can be mentioned as a typical space requirement, 
and this means that many tanks do not allow a radar level measurer with 
free space propagation to be installed. This applies no less to cisterns 
or tanks with so-called floating roofs, that is to say, tanks with roofs 
that float directly upon the contents. 
A method for avoiding the above mentioned limitation is to guide the radar 
waves in a waveguide that extends downwardly through the tank. Level 
measuring according to this method has heretofore been tried (see for 
example U.S. Pat. No. 4,359,902) but has great practical limitations owing 
to the fact that a normal waveguide has a relatively small diameter, in 
order to be suitable for the radar frequency range to be employed. The 
waveguides that are referred to have comprised rectangular or circular 
cylindrical pipes of metal with dimensions that allow one-mode 
propagation. For a circular waveguide this implies that the wavelength 
.lambda. should be between 1.3 and 1.7 times the inside diameter of the 
pipe, and for typical radar frequencies the pipe diameter thus has a 
magnitude on the order of only a few centimeters. 
Such a waveguide presents these problems: 
If the tank contents is crude oil rich in wax, the pipe becomes clogged. 
The propagation of the radar waves is unacceptably influenced by the hole 
in the tubular waveguide that is needed for assuring free flow of liquid 
between the outside and the inside. 
Corrosion in the pipe causes unacceptable damping in transmission from top 
to bottom with normally occurring tank heights. It therefore becomes 
necessary to make the pipe of expensive material or to coat its inside 
with noble metal. 
The speed of propagation is powerfully influenced by the pipe dimension and 
the radar frequency and a good accuracy therefore imposes a very strong 
requirement for these magnitudes to be constant or accurately known. 
The present invention proceeds from the realization that these problems can 
be solved by employing a powerfully over-dimensioned waveguide to which 
radar radiation is so conducted that all undesired waveguide modes are 
suppressed. The waveguide, in the majority of practical cases, can be 
assumed to consist of an existing pipe in the cistern or tank, which means 
that a useful construction must be able to tolerate substantial variations 
of the dimensions of the pipe from case to case. It is also necessary that 
a reasonable mass of rust and oil coating be acceptable. 
A pair of calculations of practical cases can illustrate the significance 
of employing the overdimensioned circular waveguide. If the distance is 
measured through the waveguide, there is obtained an apparent distance 
L.sub.s that is greater than the real distance L, and the quotient can be 
expressed by the formula 
##EQU1## 
where .lambda. can, as above, be set equal to 3 cm. and .lambda..sub.c is 
the limit wavelength (cutoff wavelength) of the waveguide, which for the 
basic mode is 1.71 times the diameter of the pipe. From the formula it can 
be seen that if .lambda..sub.c is large compared with .lambda. (that is, 
if the pipe has a diameter that is large compared with wavelength 
.lambda.) there is obtained a measurement value L.sub.s that is near the 
real L. If, on the contrary, the waveguide has one-mode propagation, 
.lambda. is 75-100% of .lambda..sub.c, and L.sub.s becomes substantially 
larger than L. If now the carrier frequency and with it .lambda. is 
changed, the quotient L.sub.s /L will be changed, and if the changes are 
small the relative change of L.sub.s /L becomes proportional to the 
relative change of .lambda. or .lambda..sub.c and the proportionality 
constant, after derivation and simplifying, can be given as 
##EQU2## 
For a normal waveguide with one-mode propagation the factor is typically 
2/3. For measuring with crude oil there is required a measuring accuracy 
of about 10.sup.-4 which means a maximum error of 2 mm in a 20 m distance, 
and the same accuracy would then be required of the pipe diameter and 
frequency, which is not practically possible. If, instead, one employs a 
pipe with for example a 25 cm diameter and a wavelength of 3 cm., the 
factor sinks to 5/1000. There is thus obtained a relative accuracy of 
10.sup.-4, provided that the accuracy of diameter and of frequency are on 
the order of one percent, which is reasonable. 
Damping in a waveguide depends upon the resistive loss in the walls, and 
the calculation thereof is found in several handbooks about waveguides, 
for example Marcuvicz: "Waveguide Handbook," McGraw Hill, 1951. For a 
one-mode waveguide of copper, the diameter of which is about 2 cm, with 
.lambda.=3 cm., the damping through a 25 m long waveguide forwardly and 
rearwardly is about 10 dB. If stainless steel is employed the damping 
becomes about 10 times higher (about 100 dB), which is too much to permit 
accurate level measuring. 
If signals of the same wavelength are instead led through a pipe with for 
example a diameter of 25 cm, the damping becomes 40 times lower, and even 
with a steel pipe the damping then stands at 2.5 dB. In practice the 
damping will become larger by reason of an oil deposit on the inside of 
the pipe, but the low damping in the ideal case allows a sufficient margin 
for deterioration in operation. 
The reason for the damping decreasing when the pipe diameter is increased 
is, as is known, that the surface current on the walls decreases, with a 
similarly large transferred power in the waveguide. For the same reason, 
therefore, significantly more holes can be tolerated in the envelope 
surface of a very large pipe than would be permitted in a waveguide with 
normal diameter. 
In prior reasoning it has been accepted to employ the basic mode of the 
waveguide, that is, H.sub.11 according to the designation system in the 
above mentioned handbook. To improve tolerance to the influence of rusty 
walls, holes and the like, it would however be preferable to employ the 
H.sub.01 propagation mode, which yields a significantly lower current in 
the walls of the waveguide and thereby lower losses. In addition to the 
low losses, an important characteristic of the H.sub.01 mode is that all 
current in the wall of the pipe flows in the peripheral direction, so that 
disturbances from existing pipe joints are insignificant. 
For obtaining an accurate distance measurement in a pipe in accordance with 
the present inventive idea, it is necessary that all undesired propagation 
modes be suppressed. If that is not the case, a normal echo will be 
interpreted as plural echoes from different distances, because the 
different propagation modes in general have different speeds in the pipe. 
A typical demand upon power overweight for the desired mode can be 25 dB, 
and this presents a large demand upon the measurement apparatus in the 
upper end of the pipe. 
The demand is in part that the emission power in the undesired modes be 
sufficiently low, in part that the sensitivity for incoming power in 
undesired modes be sufficiently low. This latter demand must be posed to 
avoid having the power reach the receiver that is spread to undesired 
modes by way of holes in the pipe walls. 
The object of the present invention is to provide a method and apparatus 
for accurately determining with radar the level of a liquid or other 
fluent material that is held in a container. In this the invention 
especially aims to solve the above discussed problems that arise when a 
pipe extending through the container is to be employed as a waveguide. For 
application of the method to tanks or cisterns on land, comprising 
so-called floating roofs, it is in this respect a special objective to 
have a simple installation of radar apparatus that does not make necessary 
extensive and expensive remodeling of the tank or the cistern. 
This objective is achieved according to the invention because the method 
and apparatus have the characteristics set forth in the patent claims 
below.

The application of the invention that is illustrated in FIG. 1 is for 
carrying out level measuring in a cistern 1 that can be built up on a 
foundation on the ground 2 and wherein there can be stored a large 
quantity of oil or other fluent material 3 which, while in storage, is 
protected by a so-called floating roof 4. Such a cistern can be very 
large, with a diameter on the order of 100 m; and because every millimeter 
of height represents a substantial volume and a large economic worth, it 
is required that the level of material shall be measured exactly, for 
determining the contents of the tank as correctly as possible. 
At the top of the cistern there is a platform 5 to which leads a stairs 6 
and to which is fixed the upper end of a pipe 7 provided for level 
measuring. Through an opening 8 in the floating roof 4 the pipe 7 extends 
vertically downward to the bottom of the cistern, where it is fixed. Along 
the whole of its length the pipe is perforated with sufficiently large and 
closely spaced holes 9 so that the interior of the pipe is communicated 
outwardly and the liquid surface 10 in the pipe can follow the level of 
the surrounding liquid, that is, the underside of the roof 4, see FIG. 2. 
A similar pipe in existing tanks was originally intended for housing a 
float belonging to a mechanical measuring device and its diameter is 
therefore usually as opportunely large as 20 to 30 cm. 
It will be appreciated that it is of great value if, in converting to a 
radar measuring system with such a storage structure, the installation can 
be based upon the existing cistern construction and, with this, also 
continue to employ the large pipe that had been provided for float 
measuring. A remodeling of an oil cistern of the size here suggested with 
a view to instead base a radar measuring system upon a free antenna 
radiation would involve such large costs and be so hard to carry out, 
especially if the cistern had a floating roof, that it would not be a 
realistic alternative. 
The solution that the invention contemplates to the problem of providing a 
measuring system that can be installed in cisterns so that they can 
maintain a performance as set forth above principally involves employing 
the cistern pipe 7 as a waveguide and feeding it with a microwave signal 
by a mode generator generally designated by 11, which is applied to the 
pipe and is arranged to produce only one dominant mode of propagation of 
the signal. 
In the illustrated example the mode generator 11 comprises a cylindrical 
waveguide 12 which is coupled by means of a coaxial conductor 13 to a 
transmitter (not shown) that is included in an electronic unit which is 
suitably mounted in a housing 14 above the platform 5. The waveguide 12 
should have such a diameter in relation to the wavelength of the supplied 
signal that only modes H.sub.11 and E.sub.01 can be transmitted, and with 
the help of symmetry it can be brought about that only the latter mode 
will be found in the signal. The waveguide passes over into a downwardly 
directed primary radiator 15 which can be formed as an antenna horn and 
which produces an antenna radiation with for example a 60.degree. lobe 
width and a field image of E.sub.01 character so that the electrical field 
is radially directed. 
The mode generator in the illustrated example is of the double reflector 
type and here comprises two reflectors 16, 17 disposed one after the other 
in the direction of radiation. The first mentioned, which can be planar or 
parabolic, consists of a dielectric shell, for example of plastic, which 
on one side--preferably its upper side--is provided as shown in FIG. 3 
with a system of radially extending conductors 18, preferably produced as 
a printed pattern of leads. The mutual distance between the conductors 
should be so small that the E.sub.01 mode emitted from the antenna horn 15 
is reflected, in the main, just as well as in a continuous metal surface. 
The second reflector 17, which in the example is supported by a metal pipe 
19 that constitutes an upward elongation of the cistern pipe 7, should 
have a parabolic or similar form so that the radar beam, which propagates 
upward from the reflector 16 in diverging directions, will at a second 
reflection become plane-parallel and directed vertically downward in the 
pipe 19. The reflector 17 can also comprise a plastic plate 20, which is 
metallized on the portion of its upper side that is radially outward of 
the horn 15, so that the whole of this portion of the plate 20 is covered 
by a metal film 21 (see the cross-section in FIG. 5). On the underside of 
the plastic plate 20, except for its central portion corresponding to the 
diameter of the horn, there is a printed conductor pattern consisting of 
spiral-form leads 22 that can be as closely spaced as the radial leads 18 
on the lower reflector 16. The lead pattern together with the thickness of 
the plate (about 0.25 .lambda. in the actual dielectric) has the 
characteristic that an electromagnetic wave that has its E-vector 
perpendicular to the spirals is reflected with 180.degree. lag compared 
with a wave that has the E-vector parallel with the spirals. The spiral 
form of the leads is such that a tangent to each at every point on it 
forms an angle .alpha.=45.degree. to the radius from the center of the 
spiral pattern through the same point; hence, the ultimate result is that 
the E.sub.01 mode, by reflection downward, is transformed to an H.sub.01 
mode, that is to say the electrical field now takes a peripheral 
direction. Other, undesired modes of the radiation are at the same time 
reflected away. The reflector 17 can be combined with the primary radiator 
15, as shown in FIG. 2. 
The H.sub.01 mode so produced by the mode generator in propagating downward 
will go through the lower reflector 16, and since the field of the 
H.sub.01 mode has the above mentioned direction the signal is not 
significantly influenced when it passes the leads 18 that extend 
perpendicular to the field and the shell that supports them. The microwave 
signal thereafter continues downward through the cistern pipe 7 to be 
reflected by the liquid surface when it meets the same and be returned to 
the antenna 15 and the coaxial cable 13. The echo signal is conducted by 
the cable to a receiver in the electronic unit 14, where there takes place 
in a conventional manner a mixing of the transmitted and received signals, 
whereupon a determination of the material level takes place based upon the 
travel time, that is, the distance to the surface 10. 
During the upward travel of the echo signal through the pipe 7, by reason 
of the hole 9, a part of its power will be converted to propagation modes 
other than the dominant H.sub.01 mode, and these will be returned in part 
to the lower reflector 16. The radial grating 18 on this will however 
prevent these false echoes from propagating farther and reaching the 
interior of the antenna 15. The reflector thus functions in this respect 
as a mode filter. 
The invention is not limited to the embodiment here shown and described. In 
an alternative embodiment the reflector 17 can comprise a metallic surface 
with corrugations in the form of a spiral pattern as described above. In 
another embodiment the mode generator can comprise, instead of the 
reflectors 16 and 17, a transmitting plate, both sides of which have a 
spiral pattern like that of FIG. 4, whereby the E.sub.01 mode signals 
coming from above it are transformed to H.sub.01, which thus becomes the 
dominant mode. For parallelization of the radiation beam from the plate a 
lens with the same diameter as the pipe 7 can be employed. 
A measuring system according to the illustrated or modified construction is 
preferred when the invention is applied to large tanks or cisterns where 
there can be employed as a waveguide an existing pipe with a diameter of 
0.2-0.5 m. In a smaller pipe it is possible to employ as a mode generator 
a conventional H.sub.01 -H.sub.11 transition, for example a Marie 
transition, in combination with a conical diameter adaptation, but because 
of the requirement for mode suppression such a combination becomes very 
long (many meters) for many practical cases that are of interest. A 
certain improvement can be obtained with a non-conical funnel such as is 
shown in "State of the Waveguide Art" in Microwave Journal, December, 
1982. In certain cases a funnel can offer a practical solution in 
combination with H.sub.01 as the dominant propagation mode, provided that 
the funnel can be hung down in the pipe. The length is then a lesser 
disadvantage, and through the special characteristics of the H.sub.01 mode 
the requirement for matching form between the pipe and the funnel becomes 
moderate.