US3842270A

US3842270A - Pressurized oil-in-water monitor

Info

Publication number: US3842270A
Application number: US00410850A
Authority: US
Inventors: M Gregory; J Stolhand; M Yost
Original assignee: Continental Oil Co
Current assignee: ConocoPhillips Co
Priority date: 1973-10-29
Filing date: 1973-10-29
Publication date: 1974-10-15
Anticipated expiration: 1991-10-15
Also published as: CA1006710A; FR2249326B3; FR2249326A1; DE2446594A1; JPS5075485A

Abstract

Method and apparatus for monitoring the presence in an aqueous medium of an oil which fluoresces when irradiated by ultraviolet light wherein the fluid stream being monitored does not come in direct contact with the light source or detection means.

Description

nite States Patent Gregory et al.

[451 Oct. 15, 1974 PRESSURIZED OlL-IN-WATER MONITOR Inventors: M. Duane Gregory; James E.

Stolhand; Marvin E. Yost, all of Ponca City, Okla.

Continental Oil Company, Ponca City, Okla.

Filed: Oct. 29, 1973 Appl. No.: 410,850

Assignee:

US. Cl 250/301, 250/365, 250/461 Int. Cl. G01n 21/34 Field of Search 250/301, 304', 365, 46l

References Cited UNITED STATES PATENTS Souther, Jr. 250/301 X 2,852,693 9/[958 Hughes et al. 250/301 FOREIGN PATENTS OR APPLICATIONS l,l6l,9l6 8/l969 Great Britain 250/30l Primary Examiner-Archie R. Borchelt Attorney, Agent, or FirmGerald L. Floyd [5 7] ABSTRACT 13 Claims, 4 Drawing Figures L Li 74 78 Ll 20 1? l Ill ,1

PAIENIEM H 3.842.270

' I sum 3 or a I40 I50 I60 AMPL/F/El? IIO OIL CONCENTRATION (PARTS PER MILLION (ScIINVO8OIW I 9NICIV38 83080038 1 PRESSURIZEI) OIL-IN-WATER MONITOR BACKGROUND OF THE INVENTION a. Field of the Invention The present invention relates to a method and apparatus for moniotring the presence in an aqueous medium of a material. which fluoresces when exposed to ultraviolet light. More particularly, it relates to such a method and apparatus which operate under pressurized conditions.

b. Description of the Prior Art Various aqueous base liquid streams which are handled in industrial processes also contain minor proportions of oleaginous base materials. It is oftentimes desirable to determine the presence of and concentration of such oleaginous base materials. Many oleaginous base materials fluoresce when exposed to ultraviolet light, emitting visible light. Thus, detection methods and apparatus have developed wherein a beam of ultraviolet light is cast on a liquid stream and the amount of visible light emitted, if any, detected. In one type of apparatus, the liquid stream is passed through a glass cell with the ultraviolet light beam and the emitted visible light passing through the glass. However, in such an apparatus, the liquid continually contacts the glass and the oleaginous materials tend to gradually coat the glass surface causing the apparatus to give a falsely high reading for the oleaginous materials. There also has beenused a similar apparatus employing light detection in which the stream of liquid being monitored does not touch the ultraviolet light source or visible light detection means. Such an apparatus is described in U.S. Pat. No. 3,581,085 issued May 25, l97l, to Barrett. In this apparatus, the liquid stream to be monitored falls down a tube without touching the sides of the tube, collects in the bottom of the tube and is discharged. The space between the falling liquid stream and the sides of the tube is occupied by air. The ultraviolet light source and visible light detection means are mounted outside the tube opposite holes in the tube. This apparatus functions during its initial phase of operation without any coating of the light source or detection means by the liquid being monitored. However, it has been found that the falling stream of liquid tends to entrain and dissolve some of the air occupying the space between the stream of liquid and the sides of the tube, carries this air to the bottom of the tube and on out of the tube.

Thus, as operation of the apparatus proceeds, the amount of air in the tube steadily decreases. The height of the pool of liquid collecting in the bottom of the tube rises to fill the space formerly occupied by the air until liquid reaches the level of the light source and detecting means, contaminates the same with oleaginous material and renders further monitoring possible.

It is an object of this invention to provide a method and apparatus for monitoring the presence of an oleaginous fluorescing material in an aqueous liquid.

It is a further object to provide such a method and apparatus which provide continuous monitoring on a stream of such liquid.

It is a still further object to provide an apparatus for such monitoring utilizing a light source and detection means wherein the liquid to be monitored initially does not directly contact the light source detection means.

It is another object to provide means for keeping the liquid out of direct contact with the light source and de- BRIEF SUMMARY OF THE INVENTION This invention involves a method and apparatus for continually monitoring an oleaginous material which fluoresces when exposed to ultraviolet light, which oleaginous material is present in an aqueous base liquid comprising:

a. a tubular housing through which falls a stream of liquid to be monitored, such housing having an internal diameter greater than the diameter of the falling stream of liquid to provide an air annulus therebetween, a bottom section of reduced diameter in which the stream of liquid collects before passing out of the housing and two sealed windows at the same horizontal level in the upper portion of the lateral sidewall of the housing,

b. means for introducing a stream of liquid into the upper portion of the housing so that such liquid falls through the housing without touching the lateral sidewalls thereof,

c. means for introducing a beam of ultraviolet light through one of the windows in the housing and into the falling stream of liquid,

d. means for detecting fluorescence radiation mounted outside the housing opposite the other window in the housing,

e. means for detecting the height of the column of liquid which collects in the bottom portion of the housing, and

f. means for injecting a gas into the air annulus in the housing before the height of the column of liquid which collects in the bottom portion of the housing reaches the level of the windows in the lateral sidewalls of the housing.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross sectional schematic view of the apparatus of this invention.

FIG. 2 is a cross sectional schematic view of the main body of the'subject apparatus of FIG. 1 taken along the Plane 2-2 of FIG. 1.

FIG. 3 is a cross sectional schematic view of an alternate embodiment of the fluid level control means.

FIG. 4 is a graph illustrating the calibration of the apparatus of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1 and 2, the apparatus, generally denoted by Reference 10, comprises means for supplying a stream of fluid to be monitored such as Valve 12 through which the composition to be monitored enters and preferably passes through inlet tube 14. The main body of the composition passes through flow controller 16 and tubing 18 into rotameter 20 which measures the rate of flow.

From rotameter 20, the main body of composition passes through tubing 22 into the top of housing 24. The composition then falls through housing 24 as stream 26 collects in the bottom of housing 24 and forms fluid column 28. The composition drains from the bottom of housing 24 and is withdrawn from the apparatus via tubing 30, valve 32, tubing 34, and valve 36. it is preferred that a small portion of the composition be withdrawn from tubing 14 via tubing 38 and valve 40, passed through constant temperature jacket 42 and then through tubing 44 which discharges into tubing 34.

Housing 24 is conveniently made in upper section 46 and lower section'48 connected by flange 50 and sealing mecnanism 52 such as an O-ring. Housing 24 is provided with first aperture 54, covered by transparent window 56 which may be made of quartz, glass or other solid transparent to light. Sealing mechanism 58, such as an O-ring, is provided between housing 24 and transparent window 56. Transparent window 56 is held in position by locking mechanism 59, such as a locking nut. A source of ultraviolet radiation 60 is positioned outside housing 24 and located so as to pass a beam of ultraviolet radiation through the transparent window 56 and through composition stream 26.

Housing 24 is also provided with second aperture 62 positioned at the same height as first aperture 54 and at any position around the circumference of housing 24, preferably at right angles to firstaperture 54.

This universal positioning of aperture 62 with respect to aperture 54 is made possible by a filter 110 in the photocell detector 70 that keeps the incident ultraviolet light from activating the photocell. The only light that may enter the photocell detector 70 is long wavelength fluorescent light the intensity of whichis proportional to the contaminant concentration in the stream to be monitored. Thus, photocell detector 70 detects the fluorescence radiation created by ultraviolet light striking composition stream 26. This is generally visible light.

Second aperture 62 is covered by transparent window 64. Sealing mechanism 66, such as an O-ring, is provided between housing 24 and transparent window 64. Transparent window 64 is held in position by locking mechanism 68, such a locking nut. Visible light detecting means 70, such as a photocell, is positioned outside housing 24 behind transparent window 64. Visible violet light detecting means 70 is surrounded by constant temperature jacket 42 which decreases fogging of transparent window 64 which can occur during operation of the apparatus in the absence of constant temperature jacket 42. The fluorescent light transmitted from stream 26 is picked up by visible light detecting means 70 and the results observed visually or recorded on a suitable recording means, such as wires 72 which connect visible light detecting means 70 to amplifier 74 which in turn is electrically connected by wires 76 to recorder 78.

It is preferred that housing 24 be provided with splash guard means 80 and 82 positioned between the lateral sidewall of housing 24 and stream 26. Upper splash guard means 80 is preferably a hollow cylinder attached to the top of housing 24 and provides a shroud around the upper portion of stream 26. The bottom of upper splash guard 80 is just above apertures 54 v and 62. Lower splash guard 82 is preferably a hollow cylinder attached to the lateral sidewall of housing 24 at a point below

apertures

54 and 62. The top of lower splash guard 82 extends to just below

apertures

54 and 62. The function of

splash guards

80 and 82 is to reduce the amount of liquid from stream 26 and fluid column 28 that splashes onto

transparent windows

56 and 64 during operation of the apparatus.

It was experienced during operation of the apparatus as described above that falling stream 26 dissolved and entrained a portion of the air in the housing and carried this air with it into fluid column 28 and then on out of housing 24. Thus, as the quantity of air in housing 24 decreased, the height of fluid column 28 would gradually rise in housing 24 until it reached the level of

transparent windows

56 and 64 and cover the same, thus disrupting the desired observations. To prevent housing 24 from filling with liquid in this manner, it is necessary to inject gas into housing 24. This can be done by constantly bleeding gas from gas supply 84 which is at a pressure above the pressure in housing 24 into housing 24 via tubing 86 and control valve 88. Preferably gas is intermittently injected into housing 24 before the height of fluid column 28 gets dangerously close to

transparent windows

56 and 64. The intermittent injection can be achieved by a means for detecting the height of fluid column 28 in housing 24 which controls means for injecting gas into housing 24. The means for detecting the height of fluid column 28 in housing 24 can be lower level conductance probe 90 and upper level conductance probe 92. These level probes extend through the side of housing 24. Lower level probe 90 is positioned near the bottom of housing 24. Upper level probe 92 is vertically positioned therefrom at a point below

transparent windows

56 and 64. Lower level probe 90 and upper level probe 92 are electrically connected by wire 94 to level controller 96. Level controller 96 is electrically connected by wire 98 to solenoid valve 88 which controls flow of gas from gas supply 84 into housing 24. Pressure gauge 100 indicates the pressure in the housing. The pressure inside housing 24 should be maintained about 10 psi above the pressure downstream of monitor 10. When the height of fluid column 28 rises in housing 24 and touches upper level probe 92, level controller 96 opens solenoid valve 88 allowing gas to flow into housing 24, thus depressing the height of fluid column 28'. When the height of fluid column 28 falls in housing 24 below lower level probe 90, level controller 96 closes solenoid valve stopping flow of gas into housing 24.

An alternate means for detecing theheight of fluid column 28 in housing 24 comprises a sight gauge means such as shown in F l6. 3 wherein hollow sight gauge column 102 connected to the interior of housing 24 at its upper and lower end is provided with lower light source and photocell combination 104, upper light source and photocell combination 106 and opaque ball 108 which floats on fluid column 28 extending into the sight gauge means. Lower light source and photocell combination 104 and upper light source and photocell combination 106 are electrically connected by wire 94 to flow controller 96. When the height of fluid column 28 rises so that opaque ball 108 interrupts light between upper light source and photocell combination 106, level controller 96 opens solenoid valve 88 allowing gas to flow into housing 24. When the height of fluid in column 28 falls so that opaque ball 108 interrupts light between lower light source and photocell combination 104, level controller 96 closes solenoid valve 88 stopping flow of gas into housing 24.

The apparatus of this invention can be used to monitor the presence in an aqueous medium of small concentrations of an oleaginous material which fluoresces when irradiated by ultraviolet light (220 to 35.0 nanometers. The oleaginous material may be a petroleum oil such as crude oil or any fraction or refined product thereof. The compounds most responsible for fluorescence in oil are the polynuclear aromatics. The fluorescent yield of an oil depends both upon the polynuclear aromatic content and upon the kinds of molecules comprising the polynuclear aromatics. The size of oil particles injected into the optical path has an effect on fluorescent intensity. Above 5 microns variation in particle diameter will result in fluctuations of the output signal due to surface effects. Below 5 microns in diameter, particles become essentially transparent and fluorescence becomes independent of particle size. If the fluid to be monitored contains oil particles having a di ameter of more than 5 microns, it is preferred to pass the fluid through a high shear pump positioned upstream of the monitor to create smaller dispersed oil droplets with a narrower drop size distribution.

The gas injected into the housing to supress the column of liquid in the housing can be any gas which is inert to the falling stream of liquid. Suitable gases include air, oxygen, carbon dioxide, combustion gases, nitrogen, argon, and similar inert gases. The aqueous base fluid may be water or any brine.

EXAMPLE An apparatus was assembled as described above using /2-inch diameter tubing 22 to introduce the fluid stream into housing 24 at the rate of 1 gallon per minute. The pressure in housing 24 was regulated by ball valve 32 to a pressure pounds per square inch above the system into which the fluid is being discharged through valve 36. Conductance probes 90 and 92 were insulated from the metal of housing 24 by drilled-out fittings of nylon, a non-conductor. Conductor probes 90 and 92 were coated by a thin coating of epoxy resin, a non-conductor, except for one-half inch on each end.

Probes

90 and 92 were O-ring sealed in the nylon fittings. As the signal from the photocell is sometimes weak if the concentration in the aqueous medium of materials which fluoresce is small, it is preferred to use an amplifier. In these tests, three-decade amplifier 74 which contained a potentiometer for setting the zero point recorder 78 was used. LOGO-ohm single channel micro-ampere recorder 78 was used with a recorder chart speed of one inch per hour to give a one-month life expectancy to each roll of chart paper. The mercury lamp ultraviolet light source exhibits its greatest intensity at 254 nm with less intense spectral lines occurring up to 577 nm. To initiate fluorescence in a molecule, the exciting wavelength must be within a certain energy range characteristic of the molecule. Most dark crude oils fluoresce most intensely when excited with energy in the 300 to 400 nm range. Thus, the spectrum of the mercury lamp was altered by using two filters. One filter was used to shift the 254 nm peak to 366 nm. A band pass filter was used to limit the emiitted light to the 300 to 400 nm range.

The monitor was calibrated by passing a series of streams of fluid of varying oil concentration through the monitor and plotting the results of the recorder reading for each versus oil concentration as determined by some standard analytical technique. The monitor was installed on a water line entering a waterflood injection tank. The water was water which had been produced from an oil producing well, separated as nearly 6 as possible from the oil and was being injected into a subterranean formation via another well to serve as a drive fluid in an effort to recover additional oil from the formation. The water contained some oil which had not been completely separated therefrom. Readings were obtained with amplifier set on 1 and 2. When fluctuations in the readings were observed, samples of the water passing through the monitor were obtained and the oil concentration thereof determined using a standard colorimetric method of analysis. FIG. 4 shows a plot of recorder readings versus oil concentrations obtained in the colorimetric analysis. For an amplifier setting of 2, the slope of the calibration curve was very nearly unity showing that each part per million of oil caused recorder'response of l microamp. When the type of oil in the fluid being monitored changes, the monitor must be recalibrated.

Even though the foregoing discussion has described certain specific embodiments of this invention, it is to be understood that numerous other arrangements will be obvious to those skilled in the art which embody the principles of the invention and fall within the scope thereof.

We claim:

1. An apparatus for monitoring in an aqueous base fluid an oleaginous material which fluoresces when irradiated by ultraviolet light comprising:

a. a vertically positioned tubular housing means for receiving a stream of fluid to be monitored, said tubular housing means having an upper portion having an internal diameter greater than the diameter of the stream of fluid to provide an air annulus therebetween, a bottom section of reduced diame' ter in which the stream of fluid collects before passing out of the housing and a first and second sealed transparent windows at the same horizontal level in the upper portion of the lateral sidewall of the housing,

b. means for introducing a stream of fluid into the upper portion of said tubular housing means so that such fluid falls through said upper portion without touching the lateral sidewall thereof,

c. means for introducing a beam of ultraviolet light through one of the said sealed windows and into the faTlin g stream of fluid,

d. means for detecting fluorescence radiation mounted outside said tubular housing means opposite the other of the said sealed windows,

e. a supply of gas at a pressure above the pressure in the tubular housing connected to the upper portion of the tubular housing.

2. An apparatus for monitoring in an aqueous base fluid an oleaginous material which fluoresces when irradiated by ultraviolet light comprising:

a. a vertically positioned tubular housing means for receiving a stream of fluid to be monitored, said tubular housing means having an upper portion having an internal diameter greater than the diameter of the stream of fluidid to provide an air annulus therebetween, a bottom section of reduced diameter in which the stream of fluid collects before passing out of the housing and a first and second sealed transparent windows at the same horizontal level in the upper portion of the lateral sidewall of the housing,

0. means for introducing a beam of ultraviolet light through one of the said sealed windows and into the falling stream of fluid,

e. means for detecting the height-of the column of fluid which collects in the bottom section of said tubular housing, and

f. means for injecting a gas into the air annulus in said tubular housing whensaid height detecting means indicates that the height of said column of fluid is approaching the level of said windows.

3. The apparatus of claim 2 wherein the means for detecting the height of the column of fluid which collects in the bottom section of said tubular housing comprises two conductance probes mounted through the lateral sidewall of said tubular housing vertically spaced apart between the bottom section of said tubular housing and the said windows.

4. The apparatus of claim 2 wherein the means for detecting the height of the column of fluid which collects in the bottom section of said tubular housing com,- prises a sight gauge 'mounted in the lateral sidewall of said tubular housing between the bottom section of said tubular housing and the said windows.

5. The apparatus of claim 2 wherein the means for detecting the height of the column of fluid which collects in the bottom section of said tubular goods con trols a solenoid valve which admits gas into the air annulus in said tubular housing.

6. The apparatus of claim 2 wherein the means for introducing a stream of fluid into the upper portion of said tubular housing means includes a flow controller to regulate the rate at which the stream of fluid falls through the tubular housing.

7. The apparatus of claim 2 wherein the first and second sealed windows are mounted at a right angle to each other in the tubular housing.

8. The apparatus of claim 2 wherein the means for detecting fluorescence radiation of (d) comprises a photocell positioned behind a filter which filters out incident ultraviolet light.

9. The apparatus of claim 2 wherein the tubular housing is provided with splash guards positioned between the lateral sidewall of the tubular housing and the falling stream of fluid which splash guards, decrease splashing of the fluid onto the sealed transparent windows.

10. The apparatus of claim 2 wherein the fluorescence radiation detecting means of (d) is supplied with a constant temperature jacket maintained at the same temperature as the fluid stream.

11. A method for monitoring in an aqueous base fluid an oleaginous material which fluoresces when irradiated by ultraviolet light comprising:

a. injecting a fluid to be monitored into the upper portion of a vertically positioned tubular housing so that a stream of said fluid falls through the tubular housing without touching the lateral sidewall of said upper portion of the tubular housing,

b. collecting said fluid in the bottom portion of said tubular housing which bottom portion has a diameter which is smaller than the diameter of the upper section thereof,

c. discharging said fluid from said tubular housing,

d. irradiating said falling stream of fluid with ultraviolet light transmitted through &' first transparent sealed window in the upper portion of said tubular housing,

e. detecting fluorescence radiation from said irradiated falling stream of fluid through a second transparent sealed window in the upper portion of the housing, and

f. injecting a gas into the upper portion of said tubular housing to suppress the height of the column of fluid collected in the bottom portion of the tubular housing below the level of the transparent sealed windows.

12. A method for monitoring in an aqueous base fluid an oleaginous material which fluoresces when irradiated by ultraviolet light comprising:

c. discharging said fluid from said tubular housing,

d. irradiating said falling stream of fluid with ultraviolet light transmitted through a first transparent sealed window in the upper portion of said tubular housing,

f. detecting the height of the column of fluid collected in the bottom of the tubular housing, and

g. injecting a gas into the upper portion of said tubular housing when the height detection of (f) indicates that the height of said fluid column is approaching the level of the sealed transparent windows.

13. The method of claim 12 wherein the gas injection of (g) is maintained at a rate so that the pressure in the tubular housing is at least 10 pounds per square inch greater than the pressure of the discharged fluid of (e). l

Claims

1. An apparatus for monitoring in an aqueous base fluid an oleaginous material which fluoresces when irradiated by ultraviolet light comprising: a. a vertically positioned tubular housing means for receiving a stream of fluid to be monitored, said tubular housing means having an upper portion having an internal diameter greater than the diameter of the stream of fluid to provide an air annulus therebetween, a bottom section of reduced diameter in which the stream of fluid collects before passing out of the housing and a first and second sealed transparent windows at the same horizontal level in the upper portion of the lateral sidewall of the housing, b. means for introducing a stream of fluid into the upper portion of said tubular housing means so that such fluid falls through said upper portion without touching the lateral sidewall thereof, c. means for introducing a beam of ultraviolet light through one of the same sealed windows and into the falling stream of fluid, d. means for detecting fluorescence radiation mounted outside said tubular housing means opposite the other of the said sealed windows, e. a supply of gas at a pressure above the pressure in the tubular housing connected to the upper portion of the tubular housing.

2. An apparatus for monitoring in an aqueous base fluid an oleaginous material which fluoresces when irradiated by ultraviolet light comprising: a. a vertically positioned tubular housing means for receiving a stream of fluid to be monitored, said tubular housing means having an upper portion having an internal diameter greater than the diameter of the stream of fluidid to provide an air annulus therebetween, a bottom section of reduced diameter in which the stream of fluid collects before passing out of the housing and a first and second sealed transparent windows at the same horizontal level in the upper portion of the lateral sidewall of the housing, b. means for introducing a stream of fluid into the upper portion of said tubular housing means so that such fluid falls through said upper portion without touching the lateral sidewall thereof, c. means for introducing a beam of ultraviolet light through one of the said sealed windows and into the falling stream of fluid, d. means for detecting fluorescence radiation mounted outside said tubular housing means opposite the other of the said sealed windows, e. means for detecting the height of the column of fluid which collects in the bottom section of said tubular housing, and f. means for injecting a gas into the air annulus in said tubular housing when said height detecting means indicates that the height of said column of fluid is approaching the level of said windows.

4. The apparatus of claim 2 wherein the means for detecting the height of the column of fluid which collects in the bottom section of said tubular housing comprises a sight gauge mounted in the lateral sidewall of said tubular housing between the bottom section of said tubular housing and the said windows.

5. The apparatus of claim 2 wherein the means for detecting the height of the column of fluid which collects in the bottom section of said tubular goods controls a solenoid valve which admits gas into the air annulus in said tubular housing.

9. The apparatus of claim 2 wherein the tubular housing is provided with splash guards positioned between the lateral sidewall of the tubular housing and the falling stream of fluid which splash guards decrease splashing of the fluid onto the sealed transparent windows.

11. A method for monitoring in an aqueous base fluid an oleaginous material which fluoresces when irradiated by ultraviolet light comprising: a. injecting a fluid to be monitored into the upper portion of a vertically positioned tubular housing so that a stream of said fluid falls through the tubular housing without touching the lateral sidewall of said upper portion of the tubular housing, b. collecting said fluid in the bottom portion of said tubular housing which bottom portion has a diameter Which is smaller than the diameter of the upper section thereof, c. discharging said fluid from said tubular housing, d. irradiating said falling stream of fluid with ultraviolet light transmitted through a first transparent sealed window in the upper portion of said tubular housing, e. detecting fluorescence radiation from said irradiated falling stream of fluid through a second transparent sealed window in the upper portion of the housing, and f. injecting a gas into the upper portion of said tubular housing to suppress the height of the column of fluid collected in the bottom portion of the tubular housing below the level of the transparent sealed windows.

12. A method for monitoring in an aqueous base fluid an oleaginous material which fluoresces when irradiated by ultraviolet light comprising: a. injecting a fluid to be monitored into the upper portion of a vertically positioned tubular housing so that a stream of said fluid falls through the tubular housing without touching the lateral sidewall of said upper portion of the tubular housing, b. collecting said fluid in the bottom portion of said tubular housing which bottom portion has a diameter which is smaller than the diameter of the upper section thereof, c. discharging said fluid from said tubular housing, d. irradiating said falling stream of fluid with ultraviolet light transmitted through a first transparent sealed window in the upper portion of said tubular housing, e. detecting fluorescence radiation from said irradiated falling stream of fluid through a second transparent sealed window in the upper portion of the housing, and f. detecting the height of the column of fluid collected in the bottom of the tubular housing, and g. injecting a gas into the upper portion of said tubular housing when the height detection of (f) indicates that the height of said fluid column is approaching the level of the sealed transparent windows.

13. The method of claim 12 wherein the gas injection of (g) is maintained at a rate so that the pressure in the tubular housing is at least 10 pounds per square inch greater than the pressure of the discharged fluid of (e).