CA1069727A - Densitometer probe shield and well - Google Patents

Abstract

DENSITOMETER PROBE SHIELD AND WELL

ABSTRACT OF THE DISCLOSURE
A vibration densitometer having a probe with a thin vibrating vane, and a feedback loop for vibration of the vane at a resonance varying with the density of a fluid in which the vane is immersed. With conventional lineari-zation, the densitometer output can indicate fluid density, operate a process control system or otherwise. A probe shield and well have solved a perplex-ing problem. In the past, the densitometer would take a calibration shift if the probe were placed in different gases or in the hollow interiors or pipelinesof different sizes or geometries or the probe was placed in the same container or pipeline with different orientations. The shield and well have solved the calibration shift problem in all of these cases. It is also an advantage of the shield and well that they improve densitometer accuracy over broad density -and flow rate ranges and decrease temperature instability and long start-up times. Perplexing phase shifts caused Inaccuracies. The well and shield have also solved another serious problem where oscillation failed. In other words, before the shield was used, a densitometer "lost lock." Unusual restricted shield perforations and ports improve operation and accuracy by limiting pitot (ram pressure), turbulence and other undesirable flow effects when the densito-meter probe and shield is immersed in either compressible or "incompressible"
fluids, e.g. gases and liquids, respectively. The shield enables operation under conditions of high amplitude flow noise where operation therein is not possible without the shield.

Description

106~7Z7 M. H. November 3ZA
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BACKGROUND OF THE INVENTION
This is a division of copending Canadian patent application ; Serial No. 227,088, which was filed on May 15, 1975.
This invention relates to densitometers, and more particularly to vibration densitometer probe accessories.
In the past, a vibration densitometer would take a frequency calibration shift when used in different environments. This has been a very serious problem because recalibration is time consuming, requires expert personnel and is often required at an inconvenient place, i.e.
in the field.
It has also been impossible to obtain good accuracy over a broad range in the use of conventional vibration densitometers. Per-plexing phase shifts and other phenomena not traceable have caused inaccuracies. At times, a vibration densitometer has completely failed to achieve lock or has "lost lock." A vibration densitometer is essen-tially an electromechanical oscillator. When it fails to oscillate, the densitometer is commonly described as failing to achieve lock or losing lock.
Vibration densitometer accuracy has also been impaired by the pitot (ram pressure) effect, turbulence and other undesirable flow characteristics.
Still further, high amplitude flow noise has prevented or made extremely difficult the operation of conventional vibration densito-meters.

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In the past, vibration densitometer outputs have had erroneous readings over large ~low rate ranges in pipelines. The same is true over large density ranges. Further, prior art vibration densitometers have had troublesome temperature instability and unduly long start-up times.
S SU~ARY OF THE INVENTION
In accordance with the present invention there is provided a densitometer comprising: A probe including a body havin~ a shank portion;
and assembly mounted on the end of said shank portion; said assembly including a vibratable member; a shield fixed to the exterior of said body encasing said assembly, said shield having a hollow interior in which said assembly is located, said shield having at leasttwo openings therethrough providing a restricted communication from the exterior thereof to the hollow interior thereof; a feedback loop connected from and to said probe to vibrate said member, said loop producing an output which is a function of the density of any fluid in which said assembly is immersed, said shank having an outer cylindrical surface with a first axis, said assembly having external and internal surfaces, both of which are approxi-mately cylindrical and concentric with a second axis, said member including a thin vane which is an approximate right prism having a uniform rectangular cross section throughout its length, said shank having a fluid tight bond to said assembly at its lower end, said axes being perpendicular to each other, said vane being fixed to the interior of said assembly generally in a diametral plane perpendicular to said first axis, said shield being constructed in two vertically split substantially identical halves, means to clamp the bottom portions of said halves together, means to clamp said halves to said shank a short distance above said assemblyj said assembly having a hole all the way therethrough defined by said internal surface thereof and uninterrupted except for said vane, said hole being open at both ends, said shield having first and second intersecting internal bores generally conforming to the shape of respective corresponding portions of

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the external cylindrical surfaces of said shank and said assembly, respec-tively, said first and second bores being generally concentric with said first and second axes, respectively, but being slightly, but everywhere spaced from said shank and said assembly, respectively, so as not to touch the same, said shield having first and second pairs of holes through the wall thereof near first and second opposite ends of the external cylindri-cal surface of said assembly, said holes being located generally in the plane of said vane, said holes having areas small in comparison to the cylindrical surface area plus the end surface areas of said second bore.

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BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings which illustrate exemplary embod-iments of the present invention:
Fig. 1 is a block diagram of a vibration densitometer;
Fig. 2 is an exploded perspective view of a vibration densito-meter probe, a portion of its mount and a portion of a pipeline;
Fig. 3 is a perspective view of one-half of a probe shield con-structed in accordance with the present invention, both halves of the probe shield being substantially identical;
Fig. 4 is another perspective view of the probe shield shown in Fig. 3;
Fig. 5 is a top plan view of the probe shield, partly in section;
Fig. 6 is an elevational view of a probe shield half taken generally in the direction of the line 6--6 shown in Fig. 5;
Fig. 7 is a transverse setional view of the probe shield half taken on the line 7--7 shown in Fig. 6.
Fig. 8 is a vertical elevational view similar to that shown in Fig. 6 with the vibration densitometer probe inser~ed in one-half of the probe shield;
Fig. 9 is a vertical sectional view, partly in elevation, taken on line 9--9 of the structure shown in Fig. 8;
Fig. 10 is a vertical sectional view, partly in elevation, through a pipeline having a shielded densitometer probe mounted in a well therein in accordance with the present invention;

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Fig. 11 is a horizontal sectional view through the well taken on the line 11--11 shown in Fig. 10;
Fig. 12 is a broken away elevational view of the well;
Fig. 13 is a vertical sectional view through the densitometer probe illustrated in Fig. 10;
Fig. 14 i5 a side elevational view of one-half of a probe shield constructed in accordance with the present invention; and Fig. 15 is a transverse sectional view of the probe shield taken on the line 15--15 shown in Fig. 14.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In Fig. 1, a vibration densitometer 10 is shown including a probe 11, a feedback loop 12 connected from and to probe 11 via a lead 13, and utilization means 14 connected from another output 15 of loop 12. Densito- -meter 10 may be identical, if desired, to that disclosed in U. S. Patent No.

3,677,067. Attention is also invited to U. S. Patent No. 3,741,000.
Probe 11 contains a vane 16 shown in Fig. 8 which is vibrated. Vane 16 is vibrated because the probe has a piezoelectric crystal pick-up, not shown, the output of which is amplified and the vane 16 vibrated by a magnetostrictive driver, not shown. The resonant vibrational frequency of vane 16 is a known function of the density of the gas or liquid or other fluid in which the vane 16 is immersed.
If desired, loop 12 in Fig. 1 may have a linearization circuit so that the output signal on lead 15 may have a magnitude directly propor-tional to density.
Utilization means 14 may be a voltmeter or ammeter calibrated in density, a process controller, a gas flow computer, a net oil computer or otherwise.
In accordance with the foregoing, the word "densitometer" is hereby defined to include or not include utilization means 14. Note will be taken . ~, .

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~069727 M. H. Nov~mber 3ZA
that the densitometer in many cases will be manufactured and sold without any utilization means 14. Such utilization means 14 would be supplied by the customer.
The vibratioll densitometer 10 is essentially an electromechanical oscillator. The oscillator obviously has losses. Loop 12, therefore, includes at least one amplifier. Two amplifiers 17 and 18 are illustrated in loop 12 in Fig. 1.
Probe 11 is shown again in Fig. 2 for mounting in a pipeline 19.
Densitometer 10 may, alternatively, be, if desired, identical to that disclosed in said U. S. Patent No. 3,741,000.
The probe 11 may be identical to the probe shown in the- said U. S.
Patent No. 3,741,000 with certain exceptions~ All these exceptions are noted hereina~er.
The said U. S. Patent No . 3, 741, 000 is referred to hereinafter as the "la~er" patent.
- The probe 11 is identical to the probe of the said later patent except for the addition of conduits 20 and 21, and a pull box 22. Conduits 20 and 21 and pull bc)x 22 simply serve as enclosures for the output leads ~rom probe 11 to loop 12 shown in Fig. 1.
- 20 Conduit 21 is threaded to pull box 22 in a manner not shown. Conduit 20 is threaded to pull box 22 and to a body 23 of probe 11. Conduits 20 and 2~, pull box 22 and body 23 are, thus, all fixed ~ogether. A body 24 is fixed to body 23. Body 24 has an upper portion 25 of a larger diameter and a lower portion 26 of a smaller diameter that is externally threaded. A shank 27 is fixed ~o threaded portion 26 and to a cylinder 28. Vane 16 is mounted in a fixed position along its opposite edges to cylinder 28, as shown in both ~igs. 2 and 8.
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1069727 M. H. November 3ZA

Pipeline 19 has a hollow cylindrical projection 29 permitting probe 11 to be lowered thereinto, projection 29 having an axis perpendicular to the axis of pipeline 19. Projection 29 is internally threaded at 30.
Probe portion 26 is threaded into pro~ection 29 at the thread 30. Pro-jection 29 has an O-ring groove 31 with 0-ring 32 therein that seals with a shoulder, not visible in Fig. 2, at the bottom of probe portlon 25 where the diameter of the probe is reduced to the diameter of the threaded portion 26 thereof. The bottom surface of the probe portion 25 may be flat and in a plane perpendicular to the vertical axis of the probe 11 so as to rest on O-ring 32, 0-ring 32 thereby sealing probe 11 inside pipeline 19. At least that portion of probe 11 below the thread 26, thus, protrudes down-wardly inside pipeline 19 below the inside diameter thereof.
All the structure shown in Fig. 1 and 2 may be entirely conven-tional, if desired.
In accordance with the present invitation, the structures of Figs. 1 and 2 are modified by placing a shield around shank 27 and cylinder 28.
Both halves of the shield are substantially identical. One-half of the shield is indicated at 33 in Fig. 3. Shield half 33 has a generally cylin-drical external surface at 34. However, this cylindrical surface is inter-rupted by vertical slots 35 and 36. Holes 37 and 38 extend perpendicularly completely through shleld half 33. Holes 37 and 38 have axes which are normal to a flat surface 39. Holes at 40 and 41 similarly have axes per-pendicular to surface 39 and go completely through shield half 33.
The holes 37 and 38 are somewhat smailer than the holes 40 and 41.
However, all of the holes 37, 38, 40 and 41 serve the same purpose. An ' ' ' ' M. H. November 3ZA
Allen head screw is positiDned ~n each of the hs:~les 37, 38, 40 and 41 to hold the '~wo shield halves together. The screws for the holes 37 and 38 ~ause surface 39 therebetween to abut the corresp~nding surface in the other shield half.
S ~hield half 33 has a vertical half bore 42 which may be slightly smaller than the diameter ~f shank 27 so that the screws of holes 40 and 41 can clamp b~th shield halves against shank 27 and hold the shield in a fixed position relative thereto.
. Shield half 33 has another partial cylindrical bore 43, the axis of 10 which is normal to the axis of the bore 42. Bore 43 terminates in a-flat surface 44 which is generally circular except for the slots 35 and 36.
Surface 44 is parallel t~ sur~ace 39.
In manuf~cture, the openings 35 and 36 are conveniently provided . at the same time tha~ bore 43is .provided in that the bore 43 is extended to an extent such that the boring tool intermpts the exteMal sur~ace 34 of shield half 33 and thereby provides the apertures 35 and 36. However, boring is st~pped shs~rt of going completely through the shield half 33 leaving material, Dne surface of which is illustrated at 44.
Shield half 33 is again shown in Fig. 4. Note will be taken in both ~f~he Figs. 3 and 4 that shield half 33 has an upper flat surface 45 which is generally semi-circular and lies in a plane perpendicular to the axis of bore 42.
The entire shield is illustrated at 46 in Fig. 5. Shield half 33 is shown in ~ig . 5 with the other shield half 47 . A typical Allen he ad screw 25 . 48 is shown in Fig. 5. All four of the screws may be identical except for their diameters.

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Shield half 33 is again shown in Fig. 6. Note that the upper and lower sur~aces 4S and 49, respectively, of shield half 33 in Fig. 6 are flat and parallel. Surface 49 has the exact shape of one-half of a circle.
Shield half 33 is again shown in Fig. 7 having openings allowing 5 ingress and egress of fluid in the direction of arrows 50 and 51.
In Fig. 8, screws are shown at 52, 53, 54 and 55. The view of Fig.
8 is quite similar to the view of Fig. 6 with shield 46 clamped onto probe shank 27. Shield half 33 is shown in Fig. 8.
In Fig. 8, note will be taken that a cylinder 56 is fixed inside cylinder 28. This construction is described in the said U. S. P~tent No. 3,677,067.
Cyl~nders 28 and 56 are substantially the same length and are substantially flush at each of their opposite ~nds. They are somewhat rounded at each of their Dpposite ends .
In Fig. 8, note will be taken that cylinder 28 is everywhere a distance A f~om bore 43. However~ shield halves 33 and 47 are clamped tightly upon probe shan~c 27, as shown in both Flgs. 8 and 9.
In Fig. 9, probe 11 is shown again with shield halves 33 and 47 clamped tightly ts~ probe shank 27. Shield half 47 has one opening indicated ~- at 56.
2~ In Fig . 9, it will be ns)ted that except for the three openings 35, 36 and 56' and the fourth symmetrical one, not shown, shield 46 encases an assembly which includes cylinder 28 and vane 16. The same is n~t fluid tight e~ccept for ~penings 35, etc., but it may be fluid tight, and it allows very little fluid flow into or out Df the shield 46 except for the apertures 35, 25 etc.

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Again, in Fig. 9, note will be taken that cylinder 28 is spaced from surface 44, a corresponding surface 57 of shield half 47, and completely around the external surface of cylinder 28 as shown in both Figs. 8 and 9, and as shown at A in both Figs. 8 and 9.
Although probe 11 may or may not be made of heavier materials such as stainless steel, shank 27, cylinder 28 and cylinder 56 may be made of stainless steel. Vane 16 m~y be made of Ni-Span-C, a trade mark. Shield 46 need not necessarily be made of a lighter material and need no~ neces-sarily be made of aluminum, but is preferably made of aluminum.
A modification of the invention is illustrated in Fig. 10. Fig. 10 includes a densitometer probe 60 having a vane 61 and a shield 62 located in a well 63. Well 63 is formed of a ring 64 having annular gaskets 65 and 66 bonded on opposite sides thereof. A cylinder 67 then has an upper open end sealed to ring 64 and a disk 68 sealing the lower end thereof except for a fluid exit hole 69.
A pipeline is illustrated at 70 having a hollow cylindrical projec-tion 71 which is welded at 72 to a fitting 73 that has a flange 74 bolted to a flange 75 of an assembly 76 at preferably three or more or, for example, eight places 77.
As shown in Fig. 11, cylinder 67 has circular holes 78 and 79 through the wall thereof. Holes 78 and 79 have axes 80 and 81, respectively, dis-posed at an angle A' of 40 degrees relative to the direction of fluid flow in pipeline 70. This fluid flow may be in either one of the directions indicated by arrows 82 or 83.

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Each of the holes 78 and 79 may have their respective axes 80 and 81 ls~cated in a horizontal plane perpendicular to the axis of cylinder 67 a distance B fr~m the bottom of cylinder 67 as indicated in Fig. 12, where B is 2.25 - inches. The dimensions shown in Fig. 12 are not critical, but are typical.
A vertical sectional view of probe 60 is shown in Fig. 13 where assembly 76 includes a nipple 84 threaded into a hollow cylindrical projection 85 of an end cap 86. End cap 86 is threaded to a body 87. Flange 75, end cap 86 and body 87 are welded or soldered together at 88. A hollow shaft 89 is externally threaded into a cylinder 90 that is solid except for a hole 91 which ~xtends completely therethrough and is in communication with the hollow ~nterior 92 s~f shaft 89~ Body 87 is welded at 93 to flange 75, and is provided with a thin web 94 whirh has an upwardly extending cylindrical projection 95 t~at is welded at 96 to shaft 89 and to cylinder 90. Body 87 may be provided with a pin hole 97, if desired, so that it may be held while end cap 86 is tumedor threaded theretl~.
Shaft 89 is, in turn, fixed to a ferrule 98 by bein~ threaded thereinto.
Ferrule 98, in turn, is fixed to a body 99 by being threaded thereinto.
A ring 100 is threaded into body 99. h magnetostrictive tube 101 wh~ch is hollow and open at both ends is press fit into a body 102. Body 102 2~0 is similar to a body disclosed in the said U . S. Patent No. 3, 741, 000, and , may be identical thereto, if desired. Alternatively, hody 102 may have one hole 103 to receive lead wires from a piezoelectric crystal 104, and a hole 105 ts) recelve lead wires from a drive coil 106 wound on a dielectric spool 107 press fit DntO tube 101. A ferrule 108 is welded at 109 to a cylinder 110.
Body 99 is threaded iinto ferrule 108 and welded thereto at 111. Tube 101 , .

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M. H. Novem~er 3ZA
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ex~ends, at the bot1Om thereof, through a circuIar hole in cylinder 110 and bears against the external cylindrical surface of a cylinder 112. A vane 113 is fixed inside cylinder 110 in a manner identical to that illustrated in the said U. S. P~tent No. 3,677,067. The same is true of crystal 104.
- The utility of a vibration densitometer employing the structure disclosed herein is described in detail in the last mentioned patent. The embodiment of the invention illustrated in Figs. 10, 11, 12 and 13 has additiDnal utilityin that erroneous readings are avoided over large density and flow rate ranges.
This embodiment also has superior temperature stability over that of the prior art, and has an unusually short start up time compared to the start up times o pr~r art of vibration densitometers.
Cylinders 110 and 112, vane 113, and crystal 104 may be identical to those disclosed in the last mentioned patent, if desired. Tube 101 is slidable thr~ugh the lower end Df body 99 and is slidable through thë sa;ld circular holethrough cylinder 110, as is kno~n from the said last mentioned patent.
A more detailed explanation ~f the operation of a vibr:ation densito-meter employlng the structure discl~sed herein is set forth in the said last men~tloned patent It is common t:~ use a preamplifier in the probe. Such a preamplifier may be empl oyed at 114 in Flg . 13, or at any other convenient location, as desired~

M. H. Nove}n~r 3ZA
- Fig. 14 is a view identical to that illustrated in Fig. 6 except for the enlargement thereof and the addition of holes 199 which extend completely through the portion of a shield half 33' normal to a flat surface 44' which may, if desired, be identical to surface 44 shown in Fig. 6.
Shield half 33' is a half of a shield of an alternative construction of the present invention.
If desired, shield half 33' may have holes 40', 41', 37' and 38' identical to holes 40, 41, 37 and 38 shown in Fig. 6.
The half of the shield to mate with shield half 33' would, in elevation, look identical to shield half 33'-except for the same differences illustrated inFig. 5. For example, a section taken on the line C~ in Fig. 5 would look the same as shield half 33' in Fig. 14 except that holes 40', 41', 37' and 38' would be omitted, and holes at 200, 201, 202 and 203 WDUld be provided.
Holes 199 or similar holes would be provided in both shield halves, of which shie1d half 33' would be a portion.
Note holes 199 in Fig. 15.
All the dimensions given herein are typical, but are not substantially critical. The diameters of holes 199 are preferably kept within the limits as set forth hereinafter.
Typically, holes 199 are located symmetrically about a vertical line through the center of shield half 33'. Typically, the distance between the center of one hole 199 and any hole adjacent thereto is 0.125 inch.
Preferably, the diameter of all the holes 199 is the same. Preferably, all of the holes 199 have a diameter of less than 0. 0625 inch and greater than 0.055 inch. Perhaps the most desirable value for the diameter of each of the holes 199 is about 0. 059 inch.
If desired, the construction shown in Fig. 14 may be considered to be drawn to scale. The width of surface 44' in Fig. 14 may be 1.25 inches.

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The emb~diment of Fig. 15 may be further characterized as follows.
As will be described, as between tw~ different fluids, such as nitrogen and methane, calibration can be saved if the holes 199 in Figs. 14 and 15 are used with well 63 in Fig. 10. The operating frequency over a density of zero to 4 pounds per cubic foot may vary only one or two Hertz out of 314 Hz. or more .
Nitrogen is a fluid helpful in calibration because it may be easily obtained in a high degree of purity.
The portiDn of shield half 33' in Figs. 14 and 15 having holes 199 therethrough may be described as "means providing an acoustical attenuator. "
This attenuator keeps Rb = Ra as described hereinafter. It may have about - an 80 percent transmission, if desired.
Well 63 may be described as "means providing an acoustical wave reflecting surface."
The differential equation defining the conditions for forced vibrations is as follows:

m d 2 ~ c dt I Kx = Q sin ~t (1) : .
where m is the effective mass of the vane, c is the damping factor, K is the vane spring constant, x is the vane displacement, t is time, ~ is the radian frequency of the vane vibration, and Q sin ~ t is the forcing function of amplitude Q .
See volume 4, page 353 of the McGraw-Hill Encyclopedia of Science and Technology.

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106g72~7 M. ~. November 3ZA

When there is a reflected wave from well 63 through, for example, nitrogen, the forcing function is Q sin~t ~ Qn sin (c~)t + S~n) where Qn is the amplitude of the reflected wave through nitrogen at the vane, and ¢~n is the phase of the reflected wave at the vane.
From (2) ~,; . . . .
(Q ~ Qn cos¢.n) sinc~t ~ Qn sin~n cos~t (3) If a calibration shift because of a change to methane is to be prevented, (Q ~ Qn cos ~n) s~n~t ~ Qn sin ~Sn cos~t = (Q ~ Qm cos ~m) sin~t ~ Qm sin ~m cos~t (4) :
where the m subscript denotes methane.
Thus, Q2 _ 2 QQn c~s ¢'n + Qn 2cos2¢~n + Qn2sin2~n = Q - 2 QQm cos ~m + Qm2cos2~m + Qm2sin2¢~m ~5) and Rn ~ 2 Rn cos ~n = Rm2 _ ZR m cos ¢'m (6) -where Rn = Q (7) and Rm = Qm (8) = arc taD Q OQ ~SO~_ . .

M. H. Novembe~ 3ZA

where the subscripts denote either n or m and that Sb should be the same for either case.
Thus, see (12) and Q ~ Qn cos çSn = Q ~ Qm cos ~m (10) Ra cos ~n = Rb cos ~Sm (11) From (6) and (11) R = + R (12) From (11) and (12), if Rm ~ Rn ~m ~n (13) 10 ` To compute the approximate inside radius R of well 63, R ~ PAn + 2 7r (14) where p is a design integer n f (15) ~ 2~7r (16) lS and vn is the sonic velocity of nitrogen.

Thus, q ~ m = p A ~ ~n ~n (17) 697Z'7 M. H. Nc>vember 3ZA

From (13) an d ( 17) q ~ m P 7~n = n 2m , n) (18) (2 ~r ) (q Am ~ P An) n . __ (19) From (14) and (19) R P A n + [ i~ m + A n][A n ¦ (2 0) R ¦~ n] [ P A m + An ] (21) n ][ A m + A ] (22) R _ Am A n (P + q) (23) For R of a reasonable size where nitrogen and methane are concerned p + q = s (24) where s is a positive integer. With nitrogen and methane, often p = 1 and q - 0, or vice versa.
Thus, s Am An (25) m ~ n ~06~727 M. H. November 3ZA
or R = A m + A (2 6) when s = 1 . (2 7) If R = -R (28) n in radians ~m = ~ r 7r + ~n (29) Thus, either [_ n Am ] I ]

or [ ~ n --i~m 1 [ . ] (31) where r is any positive odd integer.

Claims (2)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A densitometer comprising: a probe including a body having a shank portion, an assembly mounted on the end of said shank portion; said assembly including a vibratable member; a shield fixed to the exterior of said body encasing said assembly, said shield having a hollow interior in which said assembly is located, said shield having at least two openings therethrough providing a restricted communication from the exterior thereof to the hollow interior thereof; a feedback loop connected from and to said probe to vibrate said member, said loop producing an output which is a function of the density of any fluid in which said assembly is immersed said shank having an outer cylindrical surface with a first axis, said assembly having external and internal surfaces, both of which are approxi-mately cylindrical and concentric with a second axis, said member including a thin vane which is an approximate right prism having a uniform rectangu- .
lar cross section throughout its length, said shank having a fluid tight bond to said assembly at its lower end, said axes being perpendicular to each other, said vane being fixed to the interior of said assembly generally in a diametral plane perpendicular to said first axis, said shield being constructed in two vertically split substantially identical halves, means to clamp the bottom portions of said halves together, means to clamp said halves to said shank a short distance above said assembly, said assembly having a hole all the way therethrough defined by said internal surface thereof and uninterrupted except for said vane, said hole being open at both ends, said shield having first and second intersecting internal bores generally conforming to the shape of respective corresponding por-tions of the external cylindrical surfaces of said shank and said assembly, respectively, said first and second bores being generally concentric with said first and second axes, respectively, but being slightly, but everywhere spaced from said shank and said assembly, respectively, so as not to touch the same, said shield having first and second pairs of holes through the wall thereof near first and second opposite ends of the external cylindrical surface of said assembly, said holes being located generally in the plane of said vane, said holes having areas small in comparison to the cylindrical surface area plus the end surface areas of said second bore.

2. The invention as defined in claim 1, wherein said second bore is made in said shield halves so as to intercept and go part way through, but not all the way through said shield halves to provide said four holes therein.