Ultrasonic transducer with reference reflector

An ultrasonic transducer for a system for determining liquid levels by echo ranging composed of a housing including a piezoelectric crystal mounted within the housing to transmit acoustic waves, an impedance matching medium mounted within the housing adjacent the crystal and including a window layer and a diaphragm layer to transmit acoustic waves between the crystal and a gaseous environment which window layer is composed of a material having hollow glass spheres dispersed therein, and a dampening backing mounted in said housing to abut the crystal which backing includes a plurality of solid lead spheres; a tube mounted to extend from the housing to form a beam of acoustic waves propagated from said crystal, the tube having an end which is telescoped into the housing and spaced a short distance from the diaphragm to form a gap for flow communication; and a reference reflector assembly mounted to extend from adjacent said housing, which assembly includes a U-shaped member having two legs extending in slideable contact with the sidewall of the tube and further including a member connecting the two legs.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to systems for sensing liquid levels and, more 
particularly, to systems utilizing ultrasonic transducers for determining 
liquid levels. 
2. State of the Art 
U.S. Pat. No. 3,834,233 to Willis et al. discloses a system for determining 
liquid level by echo ranging. The system includes a first ultrasonic 
transducer mounted at the top of a tank to direct acoustic wave energy 
down into the tank and detect an echo from the surface of the liquid 
contents of the tank. The distance from the first transducer to the 
surface is determined from a time measurement. Willis et al., in order to 
compensate for inaccuracies due to changes in the velocity of sound over 
the path the wave travels, position a second ultrasonic transducer at a 
fixed distance from the first transducer to detect the transmitted wave. 
Signals from the two detectors are processed to cancel the effects of any 
variation in the speed of sound. 
Known transducers typically include a piezoelectric crystal sandwiched 
between a matching medium for improving energy transfer from the crystal 
to a gaseous environment and a backing for dampening ringing of the 
crystal (continued vibration of the crystal after excitation). The 
materials composing the medium and backing typically limit the temperature 
range at which the medium efficiently transfers acoustic energy and the 
backing efficiently dampens ringing. 
U.S. Pat. No. 2,430,013 to Hansel discloses a matching medium positioned 
between a crystal and a water environment. Hansel teaches a medium 
thickness of an odd multiple of quarter wavelengths and the adusting of 
the acoustic impedance of a medium material by adding other finely divided 
materials including glass. 
U.S. Pat. No. 3,995,179 to Flournoy discloses a backing composed of an 
epoxy resin having a plurality of pointed steel rods molded therein. 
OBJECTS OF THE INVENTION 
An object of this invention is to provide an ultrasonic transducer to 
accurately detect the level of a liquid surface in a vessel, independent 
of changes in the sound velocity characteristics of the gaseous 
environment in the vessel between the transducer and the liquid surface. 
Another object is to provide an improved matching medium which matches the 
impedance of a piezoelectric crystal to a gaseous environment, which 
medium has relative constant acoustic impedance and sound velocity 
characteristics over a large range of temperatures. 
Yet another object is to provide an improved dampening backing for a 
piezoelectric crystal capable of effectively eliminating excessive ringing 
of the crystal over a large range of temperatures.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION 
Referring to FIG. 1, an ultrasonic transducer 10 for use in a system for 
determining liquid level in a vessel, channel, etc. by the so-called echo 
ranging technique generally includes a housing 12 which contains a 
piezoelectric crystal to generate acoustic waves, an elongated tube fixed 
to the housing to form a beam of acoustic waves propagated from the 
crystal and a reference reflector assembly 14 which reflects a portion of 
the waves in the beam back to the crystal for calibration purposes. The 
housing 12 is composed of a material which prevents excessive ringing such 
as chlorinated polyvinyl chloride (CPVC). In practice, the transducer 10 
can be mounted to a structure by means of a flange 11 attached to the 
housing. 
Mounted within the housing 12 are acoustically active elements. The 
illustrated elements include a wafer-shaped piezoelectric crystal 15, 
acoustically transmissive layers 16 and 17 which match the acoustic output 
of the crystal 15 to the gaseous environment exterior to the transducer 
10, and a wave dampening backing 18. The wafer-shaped crystal 15 is 
mounted so that the primary direction of wave propagation from the crystal 
15, which is normal to the two faces of the crystal 15 is parallel to the 
longitudinal axis of the tube 13. A suitable material for the crystal 15 
is lead zirconate titanate having a characteristic radial frequency of 
about 50 kilohertz. In practice the faces are coated, at least in part, 
with a conducting material such as silver. As used herein, the term 
"crystal" includes both the piezoelectric substance and the conducting 
coating. 
The acoustically transmissive layers 16 and 17 form a matching medium which 
abuts the face of the crystal 15 towards the elongated tube 13. The nearer 
of the layers to the crystal 15 is a window layer 16 and the other is a 
diaphragm layer 17. To maximize energy transfer the medium has a thickness 
along the direction of wave propagation equivalent to an additive wave 
delay in the medium of about an odd multiple, preferably one, of 
one-quarter of the characteristic period of the crystal 15. 
The window layer 16 of the medium is composed of a synthetic polymer 
material, preferably a polyurethane, having hollow glass spheres 
homogeneously dispersed therein to provide a substance having relatively 
constant acoustic impedance and sound velocity characteristics over a 
temperature range varying over about a hundred degrees celsius, for 
example from about -30.degree. C. to 70.degree. C. Preferably the glass 
spheres compose about 10% by weight of the window layer 16 and have 
diameters in the range of 20-300 microns. A preferred material for the 
glass spheres is sodium borosilicate. A preferred wall thickness for the 
spheres is about 2 microns. To provide the overall matching medium with 
relatively constant acoustic impedance and sound velocity characteristics, 
the window layer 16 accounts for about 90% of the total thickness of the 
medium or about nine times the thickness of the diaphragm layer 17. 
The diaphragm layer 17 of the medium is laminated to the opposite side of 
the window layer 16 from the crystal 15. The primary purpose of the 
diaphragm layer 17 is to protect the inner parts of the housing 12 from 
the vessel environment. The diaphragm layer 17 is preferably composed of 
the same material as the housing 12. 
The dampening backing 18 is positioned to abut the opposite face of the 
crystal 15 from the matching medium. Preferably, the backing 18 extends 
around the edges of the upper face of the crystal 15 to about one-half the 
thickness of the crystal 15 to also dampen propagation from the sidewall 
of the crystal 15. A preferred composition for the backing 18 includes a 
plurality of solid lead spheres 21 having diameters in the range of about 
2 mm to 3 mm arranged in contact or close proximity with the crystal 15 
and one another to absorb acoustic energy from the crystal 15. The 
interstices between the spheres are filled with a synthetic polymer 
material such as a polyurethane. In practice, this backing 18 prevents 
excessive ringing of the crystal 15 over a temperature range varying over 
about a hundred degrees Celsius, for example from about -30.degree. C. to 
70.degree. C. 
In the illustrated embodiment, the top of the housing 12 is sealed by a cap 
25. Two wires 23 and 24 are threaded through the cap 25 to connect to the 
faces of the crystal 15. Additionally, the otherwise unfilled spaces in 
the housing 12 are filled with polyurethane foam 26 and solid polyurethane 
27. 
The housing 12 has a tubular sidewall which extends below the diaphragm 17 
to provide means for mounting the tube 13. Through the sidewall are formed 
two slots 19 at diametrically opposed locations and three circular 
apertures 20 spaced at equal intervals around the sidewall. 
The elongated tube 13 of the transducer 10 is mounted to extend vertically 
down from the housing 12. The tube 13 is composed of a material which 
prevents excessive ringing of the tube 13. Plastics, such as CPVC, are 
suitable. The upper tapered end 30 of the tube 13 is telescoped into the 
housing 12 and spaced a short distance below the diaphragm layer 17 to 
form a gap 31 about the top of the tube 13. The gap 31 provides a passage, 
in cooperation with the tapered end 30 and the three apertures 20 in the 
sidewall of the housing 12, for flow communication between the gas space 
enclosed at the end 30 of the tube 13 and the environment outside the tube 
13. 
The reference reflector assembly 14 of the transducer 10 includes a 
U-shaped rod attached to extend vertically downward from adjacent the 
housing 12 so that the tube 13 and the reflector assembly 14 can 
independently expand and contract with temperature. The rod includes two 
legs 32a and 32b connected at one end at the slots 19 to the housing 12 
and tube 13. The legs 32a and 32b are in slideable contact with the 
exterior sidewall of tube 13 which is formed with two diametrically 
opposed grooves 33 into which the legs 32a and 32b fit for lateral 
support. 
A bar 34, sheathed with a teflon tube 35 to provide a larger surface for 
reflecting acoustical waves, connects between the distal ends of the two 
legs 32a and 32b. The bar 34 extends normal to the direction of the wave 
propagation in the tube 13 at a typical distance of about 36 cm from the 
crystal. An additional minimum distance of about 9 cm between the bar 34 
and a liquid surface is needed to detect the surface and distinguish it 
from the bar 34. The legs 32a and 32b are composed of a material having a 
low coefficient of thermal expansion so that the bar 34 remains at a 
relatively constant known distance from the crystal 15 over a temperature 
range varying over about a hundred degrees Celsius, for example from about 
-30.degree. C. to 70.degree. C. Most metals such as a stainless steel are 
suitable. 
Electronic circuitry for operating the transducer 10 includes means 40 for 
transmitting acoustic waves from the crystal 15 as periodic pulses, means 
41 for detecting the pulses as reflected by the bar 34, means 41 for 
detecting the pulses as reflected by a surface, first digital means 43 for 
counting initiated by the periodic pulse, means 44 for adjusting the speed 
of the count of the first digital means 43 so that the time interval 
during which the pulse travels from the crystal to the bar 34 and returns 
represents a constant count and a second digital means 45 for counting at 
a speed varying in proportion with the speed of the count of the first 
digital means 43 during the interval between the detection of the 
reflected pulse from the bar 34 and the detection of the reflected pulse 
from the surface. 
For example, the electronic circuitry can include a common amplifier chain 
41 for processing the reflected pulses from the bar 34 and the surface. 
The adjustment of the first digital means can be provided by a phase lock 
loop 44 in which the first digital means counts up to a constant. A single 
digital clock can provide the first and second digital means. 
In practice, the ultrasonic transducer 10 is mounted on the top of a sealed 
tank holding a liquid. The tube 13 is positioned to extend vertically 
below the housing 12 in the gaseous environment of the sealed tank. The 
electronic circuitry periodically applies an electrical potential to 
excite the crystal to transmit a pulse of acoustic waves through the 
matching medium and down the tube 13 towards the surface of the liquid in 
the tank. The backing 18 dampens the acoustic waves transmitted from the 
top face of the crystal 15. Reflections of the pulse from the bar 34 and 
the liquid surface are detected by the crystal 15 before the crystal 15 is 
again excited by the circuitry. 
As the pulse moves down the tube 13, a portion of the wave front strikes 
the bar 34 and is reflected back towards the crystal 15. This reflected 
pulse, upon striking, excites the crystal 15 to generate an electrical 
potential which is detected by the circuitry. When detected, the first 
digital means, which was previously initiated when the crystal 15 was 
initially excited, has or has not reached a predetermined constant. If the 
constant is reached by the first digital means 43 before the detection, 
the speed of the count is decreased and if the constant is reached after 
the detection, the speed of the count is increased. As adjusted the count 
is immediately initiated again from zero. The pulse continues downward 
past the bar 34, strikes the liquid surface and a portion of the wavefront 
is reflected back. Assuming the angle of the tube 13 is within several 
degrees of vertical a sufficient reflected pulse from the liquid surface 
reenters the tube 13 and excites the crystal 15 by striking it to generate 
an electrical potential which is detected by the circuitry. The count is 
again stopped and this second count is proportional to the unknown 
distance between the crystal 15 and the bar 34. The count can then be 
converted to a voltage and displayed on a voltmeter 46 to indicate a 
particular surface level. Regardless of changes in the speed of sound in 
the tank the same level produces the same second count. Changes in the 
velocity of sound are compensated for by the adjustments made in the speed 
of the first and second counts. 
The flow communication through the gap 31 and apertures 20 prevents less 
dense components of the gas environment from becoming trapped in the tube 
13 and producing sound velocity characteristics in the tube 13 which are 
not representative of the environment and as a consequence preventing the 
adjusting means 44 from accurately compensating for changes in the sound 
velocity characteristics of the environment. 
The bar 34 of the reference reflector assembly 14 remains at a relatively 
constant distance from the crystal 15 regardless of temperature changes 
because of the low coefficient of thermal expansion characteristic of the 
legs 32a and 32b and their attachment adjacent the housing. The tube 13 is 
allowed to expand and contract with temperature changes according to its 
coefficient of thermal expansion which for plastics is typically higher 
than is acceptable for the legs 32a and 32b if high accuracy is to be 
achieved. 
The transducer 10 can also be used to measure the distance to a solid 
object. When so used, vertical orientation of the tube 13 is not required.