GB2228083A - Fluid monitor utilizing multiple internal reflectance crystal - Google Patents

Description

Fluid Monitor Utilizing MIR Crystal The present invention relates to a

fluid monitor using a thin multiple internal reflectance (MIR) crystal sealed in a fluid chamber wall, with the crystal optically monitoring a fluid (preferably at high temperature andlor high pressure) contained in the fluid chamber.

This invention is principally useful in a non-destructive reaction monitoring system using infrared spectroscopy. Such a system may utilize an optical system operative to reflect source infrared radiation a number of times off an MIR crystal surface in contact with a fluid under analysis and then to direct the remaining radiation (as modified by the infrared absorbtion characteristics of the reaction) to a detector for analysis of the fluid. In its preferred form, the invention is directed to an optical system including a reinforced thin MIR crystal element for incorporation with an infrared spec trophotome ter for IR spectroscopic analysis of reactions being conducted under or fluids being subjected to elevated pressures and/or temperatures.

In the infrared range, practically all organic (and many inorganic) molecules have characteristic spectra that can positively identify them. In one such identification method, infrared energy is reflected along the length of a crystal by the physical phenomenon of total internal reflection. A fluid sample or reaction placed in contact with the crystal selectively absorbs different frequencies of IR energy from the crystal. The energy that is'not absorbed exits the crystal and is directed to a detector which measures the distribution of energy absorbed by the fluid or reaction so as to obtain and display its infrared spectra. Two of our US patents illustrate different means for employing an MIR crystal to analyze a fluid or solid in contact therewith.

Sting U.S. Patent No. 4.595,833 discloses reflaxicon optics for directing infrared radiation from a source into the cone shaped entry end of a cylindrically shaped MIR crystal, as well as for directing radiation from the cone shaped exit end of the element towards a detector. The cylindrically shaped MIR element is sealed into a tubular member in order to provide a sample chamber or cell for the fluid and fluidized samples being analyzed.

Messerschmidt U.S. Patent No. 4,730,882 discloses an elongated flat MIR crystal having a first surface, a slightly longer second surface and beveled entry and exit end surfaces interconnecting the same. The radiant energy enters at right angles through the second surface, reflects off the beveled entry end surface, reflects between the second and first surfaces in multiple reflections along the length of the crystal, and reflects off the beveled exit end surface through the second surface to a detector.

The circular MIR crystal of U.S. Patent No. 4,595,833 and the flat, bevel ended MIR crystal of U.S. Patent No. 4,730,882 have been successfully commercially sold in sampling assemblies to analyze fluid and solid samples. These MIR crystal elements require special assembly, disassembly and maintenance procedures within the analysis cell. The MIR crystals, as currently mounted, are not preferred for high pressure andlor high temperature fluid monitoring because of the possibility of the MIR crystal breaking. In addition, the crystals, as currently mounted, are positioned in the chamber or cell presenting obstructions to mixing or fluid flow.

The principal object of the present invention is to provide a thin MIR crystal element adapted to monitor fluids and especially those fluids under high temperature and/or high A, 1 pressure. This object is accomplished by sealingly mounting flat MIR crystal in a wall forming or cooperating to form the fluid chamber. The mount in such wall is adapted to place a monitoring section of a first surface of the flat MIR crystal in contact with the fluid, to provide seals for the MIR crystal in the wall structure and to reinforce the opposed second side of the MIR crystal along a longitudinal extent greater than the length of the monitoring section of the MIR crystal. This sealed mount allows a thin MIR crystal to monitor high pressure andlor temperature fluids or reactions.

It is another object of the present invention to provide a flat MIR crystal for monitoring high temperature and/or high pressure reactions or fluids, which crystal can be temporarily removed for easy crystal cleaning and/or can be easily replaced with a different crystal to vary the optical path length. This object is accomplished by embedding the thin MIR crystal in a wall that is sealed to and selectively removable from the rest of the fluid cell body. Fo example, the MIR crystal can be embedded in and sealed to a wall which becomes the base wall, side wall or top wall of the f luid chamber when connected to and sealed with the fluid chamber body. When the wall is removed from the fluid chamber body, the exposed monitoring section of the first crystal surface can be cleaned or polished. The wall with the cleaned or polished crystal surface assembled'therein can then be remounted on the fluid chamber body to form the complete fluid chamber. Alternatively, a wall having an MIR crystal element embedded and sealed therein having different optical path length characteristics can be assembled on and sealed to the fluid chamber body to provide ready adaptability for monitoring different types of reactions and/or utilizing different radiant energy forms.

It is still another object of the present invention to provide an MIR crystal monitor for high pressure and/or high temperature reactions or fluids having adaptability to different types of crystal configurations or to different types of fluid monitoring. For example, the mounting structure utilized in the fluid chamber wall allows the use of a flat MIR crystal element having either the first reflective surface shorter than the second reflective surface or vice versa. In addition, the mounting structure allows the crystal to be used for absorption spectroscopy or emission spectroscopy.

The invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims, the following description and annexed drawings setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the invention may be embodied.

DESCRIPTION OF THE DRAWINGS In the drawings:

Fig. 1 is a bottom plan view of a f luid chamber utilizing an elongated flat MIR crystal mounted in and sealed to the base member of the fluid chamber so as to optically monitor and identify the fluid; Fig. 2 is a vertical cross section taken generally in the plane 2-2 of Fig. 1 schematically showing the optical path through the elongated MIR crystal; Fig. 3 is a vertical cross section taken generally along the plane 3-3 of Fig. 1 showing the inverted T-shape slot in the base member and the inverted T-shape backup member tightly received therewithin; Fig. 4 is a fragmentary cross section of the base portion of a fluid chamber similar to that shown in Fig. 3 but showing an alternative seal arrangement and an alternative elongated MIR crystal configuration providing a different optical path; 4 i Fig. 5 is a fragmentary elevation of a tubular body having an elongated flat MIR crystal embedded in its wall to monitor a reaction occurring therewithin or to monitor a fluid flowing therethrough; Fig. 6 is a vertical cross section taken generally along the plane 6-6 of Fig. 5 showing the elongated MIR crystal embedded in a slot extending through the fluid chamber wall along most of its-length, with semicircular straps cooperatively defining an annular backup member extending around the tubular body to reinforce the embedded crystal; and Fig. 7 is a cross section taken generally along the plane 7-7 of Fig. 6 showing the elongated crystal mount in the fluid chamber wall and schematically illustrating the optical path through the elongated MIR crystal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now in more detail to the drawings and initially to the embodiment illustrated in Figs. 1 through 3, a fluid chamber body, indicated generally at 1, includes a cylindrical side wall 2 and a radially outwardly directed annular flange 3 at its lower end. The cylindrical wall 2 defines therewithin an internal hollow bore. Although not illustrated, the fluid chamber 1 either has an integral or removable top wall further cooperating with the cylindrical side wall 2 to close off the top end of the bore to define a cavity 4. The cavity 4 is fully enclosed by a selectively removable base member assembly, indicated generally at 6, to form the sealed fluid chamber or cell.

The base member assembly 6 includes a base member 7 having an outer diameter substantially equal to the outer diameter of flange 3 on reaction chamber body 1. The circular base member 7 has an inverted T- shape slot, indicated generally at 8, extending therethrough along most of its length, as best shown in Figs. 2 and 3. The inverted T-shape slot includes a generally vertically extending leg section 10 and a generally horizontally extending base section 11. As best shown in Figs. 1 and 3, the inverted T-shape slot 8 has a length greater than the internal diameter of the cavity 4 but less than the outer diameter of base member 7.

At its upper outer ends, the vertical leg 10 of the inverted T-shape slot 8 does not extend entirely through the base member 7. The outer ends of the vertical leg 10 of the inverted T-shape slot 8 are respectively flared radially outwardly from top to bottom to form tapered end surfaces 14 and 15. The lower outer ends of the inverted T-shape slot 8 may be enlarged to assist in directing the radiant energy into and out of the base member assembly 6 and to assure clear apertures to avoid vignetting the radiant energy beam. As shown, the lower outer ends of the inverted T- shape slot may be enlarged by spaced, frusto-pyramidal openings 17 and 18 in the base member 7.

An elongated, multiple internal reflectance (MIR) crystal, indicated generally at 20, is tightly received in and sealed to the inverted Tshape slot 8. The flat MIR crystal 20 includes a first flat top surface 21, a second flat bottom surface 22, two beveled end surfaces 23 and 24 that taper outwardly from the top surface 21 to the ends of the crystal to form truncated ends 23A and 24A and two opposed vertical side walls 25 and 26. The second flat bottom surface 22 of crystal 20 may be aluminized or gdld coated or may have a low index of refraction material positioned thereagainst to enhance optical path efficiency. The side walls 25 and 26 of crystal 20 may be aluminized or gold coated to be secured to and sealed in the walls of the vertical leg 10 of inverted T-shape slot 8 by a low temperature solder andlor a ceramic adhesive (preferably one for high temperatures).

As best shown in Figs. 2 and 3, the MIR crystal 20 is mounted in the upper portion of the vertical leg 10 of inverted T-shape slot 8. The first surface 21 of crystal 20 has an 1 elongated central monitoring section extending substantially the full diameter of the cavity 4 and being directly exposed to cavity 4. The first surface 21 of crystal 20 as mounted is flush with the upper surface of base member 7. The tapered end surfaces 23 and 24 of the MIR crystal are slightly spaced from the tapered surfaces 14 and 15 in the base member 7. The MIR crystal is held in place and reinforced by a backup member indicated generally at 27.

The backup member 27 has a generally inverted T-shaped configuration including a vertical leg 28 and a generally horizontal base member 29. The inverted T-shaped backup member 27 is configured to be tightly frictionally received in the inverted T-shape slot 8 in base member 7. The vertical leg 28 on backup member 27 is shorter than the length of the vertical leg 10 of inverted T-shape slot 8 allowing the upper end 30 of the vertical leg 28 to engage the second lower surface 22 of MIR crystal 20, as best shown in Figs. 2 and 3. The backup member 27 is removably held in place by a plurality of fasteners 31 extending through the horizontal base member 29 of inverted T-shape backup 27 into the base member 7.

The upper end 30 of the inverted T-shape backup member 27 has a length greater than the diameter of the cavity 4 but less than the total length of the slot 8. With such length, the backup member 27 abuts and reinforces the MIR crystal 20 along most of its length. If desired, the upper end 30 of backup member 27 can be connected to the second bottom surface 22 of crystal 20 to effect a seal therebetween.

The backup member 27 cooperates with the ends of the slot 8 and the frusto pyramidal openings 17 and 18 respectively to define a radiant energy inlet area or aperture, indicated generally at 33, and a radiant energy outlet area, indicated generally at 34. The backup member 27 may be tapered radially inwardly from top to bottom at its opposite ends as shown at 36 and 37 to enlarge the radiant energy inlet and outlet areas to provide flexibility in selecting optical paths.

The base member assembly 6, with the MIR crystal 20 and backup member 27 assembled therein, is removably secured to the reaction chamber body 1. For this purpose, an annular layer 39 of gasket material (or 0-rings) may be sandwiched between the annular flange 3 on reaction chamber body 1 and both the base member 7 and crystal 20. The base member assembly 6 is then secured to the flange 3 by a plurality of circumferentially spaced fasteners 40. The fasteners 40 draw the base member 7 tightly against the annular flange 3 to compress the gasket material layer 39 positioned therebetween. The compressed gasket material 39 seals the upper outer ends 41 and 42 of the crystal 20 to the body 1 and also seals the base member 7 to the body 1.

With the base member assembly 6 thus secured and sealed to body 1, the fluid chamber cavity 4 is enclosed fully to define the fluid chamber or cell. As thus assembled, the monitoring section of first surface 21 of crystal 20 in direct contact with the fluid or reaction extends the full diameter of chamber 4, but does not extend into the chamber or cell itself. The backup member 27 has a length along its upper surface 30 greater than the diameter of chamber 4 and thus spacially overlaps but is vertically spaced from body wall 2 at both of its ends. Since backup gurface 30 abuts crystal surface 22 along its entire length and spacially overlaps the fluid chamber body at both of Its ends, backup member 27 reinforces crystal 20 against elevated fluid temperatures or pressures, thereby to retain the crystal in compression and minimize Any bending moments on the crystal. The term elevated temperature, as used herein, means (by way of example but not of limitation) any temperature above ambient and below 500F (260IC). The term elevated pressure, as used herein, means (by way of example but not of limitation) any pressure above atmospheric and below -2), 5,000 psi (34.5MNM with rnany analyses being conducted at approximately 1,500 psi (10.3MN2) With the reaction or fluid chamber completed, the fluid or reaction materials are inserted into the chamber through an opening in the side or top wall thereof, with the opening thereafter being tightly covered and sealed. The reaction occurring in or the fluid contained in the chamber is continuously monitored and identified by the optical system incorporating the MIR crystal 20.

The optical system includes a radiant energy source 43 directing a beam 44 of radiant energy (preferably infrared energy) into the radiant energy inlet area 33 of base member 7. The radiant energy beam enters the MIR crystal 20 through and at right angles to the outer end of second flat lower surface 22 and then reflects off beveled entry end surface 23 of the MIR crystal. The beveled inlet surface 23, as well as the beveled outlet surface 24, may be metallically coated or covered with a low index of refraction material to enhance the efficiency of the radiant energy reflection. The radiant energy beam then successively reflects off the second and first surfaces of the crystal 20 in a multiple series of internal reflections along the length of the MIR crystal as schematically illustrated at 45. The monitoring section of the first crystal surface 21 is in direct contact with the fluid or reaction in the fluid cell.

Certain frequencies or bands of radiant energy are absorbed by the reaction or fluid when the radiant energy beam is reflected off the monitoring section of the first surface 21 of crystal 20. The radiant energy reaching the exit end of the MIR crystal 20 is reflected off the outlet beveled surface 24 and leaves the crystal 20 through and at right angles to the outer exit end of the second bottom surface 22. The emitted radiant energy beam passes through the outlet area 34, as indicated by arrow 46, to a detector 47. Based upon the frequencies or bands of radiant energy remaining in the radiant energy beam leaving the crystal, the detector 46 determines the distribution of frequencies or energy bands of infrared energy absorbed by the fluid in the chamber to provide a fingerprint or identity of the fluid in that chamber. This determination can be displayed with an infrared spectrum. The reaction or fluids can be continuously monitored with the infrared spectra being successively displayed or the information being successively recorded.

A slightly different embodiment is illustrated in Fig. 4, with like reference numerals including the suffix A in Fig. 4 identifying like parts to the embodiment illustrated in Figs. 1 through 3. In Fig. 4, two radially spaced 0-rings 49 and 50 are respectively carried in radially spaced grooves 52 and 53 in the bottom surface of flange 3A on body 1A. When the base member assembly 6A is connected to body 1A by fasteners 40A, annular 0-rings 49 and 50 are respectively compressed between the base member 7A and body 1A to form seals therebetween. Alternatively, the 0rings could be carried by the base member with the lower surf ace of the body 1A being flat. The radially spaced and compressed 0-rings 49 and 50 provide fluid tight seals between the base member 7A and body 1A to retain the reaction materials or f luid within the cell 4A.

The base member7A in the embodiment of Fig. 4 also has a slightly different configuration including a radially inwardly extending annular seat section 55 adjacent its upper surface. This seat section 55 directly overlies and is secured by solder or adhesive to the top surface 21A of crystal 20A to form a fluid tight seal therebetween.

As further shown in Fig. 4, the MIR crystal 20A has a different configuration at its ends wherein the first top surface 21A is longer than the second bottom surface 22A. The beveled ends of the crystal extend radially inwardly from top 1 Q - 11 to bottom, with the end 24A being shown in Fig. 4. The radially inward taper of the beveled ends of crystal 20A results in a different optical path for the radiant energy entering and leaving the crystal.

To accomodate this different optical path, the openings at the ends of the inverted T-shape slot 8 may have a larger radially outwardly directed taper from top to bottom (as illustrated at 18A) to accomodate the angled optical path for the radiant energy entering and leaving the crystal 20A. As shown at the exit end, for example, the radiant energy 46A may be leaving at an angle to the vertical, with the angle being accomodated by the tapered surface 18A on the base member 7. As is apparent from Fig. 4, the ends of the backup member 27A still overlap the annular seat section 55 of the base member 7A to reinforce the crystal 20A against the elevated pressures and temperatures of the reaction or fluid while cooperating to define the radiant energy inlet and outlet areas.

As is apparent from the embodiment of Figs. 1-3 and the embodiment of Fig. 4, the base members 7 or 7A may readily be removed from the reaction chamber body 1 or 1A to permit the monitoring sections of the first surfaces of crystals 20 or 20A to be cleaned or polished before the base member is reinstalled. Alternatively, given comparable seal forms, the base member 7A could be substituted for the base member 7 to change the crystal used to monitor a reaction andlor to change the optical path through the crystal. By reinforcing the crystal over a length exceeding the length of its monitoring section, a thin elongated crystal can be used in the optical system to monitor a fluid subjected to high pressures andlor high temperatures. In addition, even though the crystal has been shown for exemplary purposes as being embedded in the base member assembly, it will be apparent that the crystal could alternatively be embedded in the side wall or top wall, if appropriate for the application and fluid being monitored.

Turning now to the third embodiment shown in Figs. 5 through 7, the fluid chamber 58 is defined within a tubular body member, indicated generally at 59. The wall 60 of tubular body 59 has an elongated rectangular slot 61 extending therethrough along some of its length. The respective ends of the slot 61 do not extend entirely through the body wall 60, thereby to define longitudinally spaced scat sections 62 and 63 at the radially inner side of the slot 61. The outer ends of the slot 61 are preferably tapered outwardly from the radially inner side to the radially outer side thereof to form the tapered end wall surfaces 64 and 65.

An MIR crystal 20 may be received in slot 61. The opposite ends of the first surface 21 of crystal 20 bear against or abut seat sections 62 and 63 formed on the wall 60 and are secured thereto by low temperature solder or ceramic adhesive to form a fluid tight seal therebetween. The beveled inlet surface 23 of crystal 20 is slightly spaced from tapered end surf ace 64 of wall 60, and beveled outlet surf ace 24 of crystal 20 is slightly spaced from tapered end surface 65 of wall 60.

The MIR crystal is further held in the slot 61 and reinforced by a backup member indicated generally at 67. As illustrated, the backup member 67 includes two semicircular straps 68 and 69 respectively having radially outwardly directed flanges 71 and 72 at iheir diametrically opposed ends. The two straps are positioned about the tubular member 67 and are secured together by fasteners 73 passing through the two pairs of abutting flanges. As thus assembled, the two straps cooperatively define a backup member 67 engaging and embracing the tubular member 67 and crystal 20. Although a backup member 67 effectively having substantially 360 of extent is illustrated, other geometrical configurations can be employed for the backup member 67 as long as the backup member has 1 sufficient width to reinforce the MIR crystal 20 and to cooperatively define the radiant energy inlet and outlet areas.

For this purpose, as best shown in Figs. 5 and 7, the backup member 67 is wide enough to longitudinally overlap the seat sections 62 and 63 to reinforce the crystal 20 along second surface 22 for a longitudinal extent greater than the length of the monitoring section of the first crystal surface 21. In addition, the backup member 67 is not as long as the entire slot 61 to cooperate therewith in defining the radiant energy inlet area, indicated generally at 33, and the radiant energy outlet area, indicated generally at 34.

As is apparent from Fig. 7, the central monitoring section of the first crystal surface 21, which extends between the seat portions 62 and 63 of the tubular body wall 60, is in direct contact with the fluid flow occurring within or the fluid contained in the fluid chamber 58. The optical system of the present invention utilizes the monitoring section of the first surface 21 of crystal 20 to monitor and identify the fluid within the chamber 58.

In this regard, a radiant energy source 43 directs a beam of radiant energy (preferably infrared energy) through the radiant energy inlet area 33. The radiant energy enters the crystal 20 through and at right angles to the second surface 22 and is then reflected off the beveled entry end surface 23. The beveled entry end surtace may be aluminized and gold coated or may have a low index of refraction material positioned thereagainst to enhance the efficiency of the optical system. The radiant energy is then successively reflected between the second surface 22 and first surface 21 in multiple reflections along the length of the MIR crystal as schematically indicated at 45. Certain frequencies or bands of the radiant energy reflecting off the first surface 21 are absorbed by the fluid flowing within fluid chamber or cell 58. The radiant energy reaching the end of the MIR crystal 21 is reflected off beveled outlet surface 24 and through the bottom surface 22 at right angles to such surface. The radiant energy remaining in the beam leaving the crystal is then directed to a detector 47 for sequentially monitoring the fluid and identifying its contents as described above.

It will be apparent from the foregoing that changes may be made in the details of construction and configuration without departing from the scope of the invention as defined in the following claims. For example, the optical system incorporating an MIR crystal mounted in and sealed to a reaction chamber wall may also be used for emission spectroscopy analyses as well as for absorption spectroscopy analyses as described above. In emission spectroscopy, radiant energy source 43 would not be used, and the fluid itself would become the radiant energy source. Radiant energy emitted from the fluid would enter the monitoring section of the first surface 21, would be reflected along the crystal to beveled outlet end 24 and would then be reflected through the radiant energy outlet area to the detector for identification. If desired or if necessary for a given application, the backup member and/or base member could be modified to eliminate the radiant energy inlet area.

1 i

Claims (13)

1. A fluid monitor comprising:

a body having at least one wall defining or cooperating to define a fluid chamber containing fluid; a slot in said one wall extending through said one wall in its center portion to said reaction chamber and partially extending through said one wall at its respective opposite end portions; a multiple internal reflection (MIR) crystal tightly received in and sealed to said slot. a first inner surface of said MIR crystal having its central monitoring section directly facing the fluid chamber in contact with the fluid and having its respective opposite end portions out of contact with the fluid; means to seal the opposite end portions of the first internal surface of the crystal to the body to provide a fluid tight seal for the fluid chamber; a backup member secured to said wall agmainst a second outer surface of the MIR crystal, the backup member being longer than the central monitoring section to partially spatially overlap the end portions of the first crystal surface to reinforce substantially all of the second surface of the MIR crystal while defining with the ends of the slot at least a radiant energy outlet area at one of its ends; and an optical system including the crystal and a radiant energy detector, the optical system utilizing the monitoring section of the first crystal surface being in contact with the fluid and the multiple internal reflections of radiant energy in the MIR crystal to direct a resultant radiant energy beam through the radiant energy outlet area to the detector for identification of the fluid.

2. The f luid monitor of claim 1 wherein the one wall of the body is cylindrical and defines therein the fluid chamber.

3. The fluid monitor of claim 2 wherein the backup member encircles and embraces the cylindrical wall.

4. The fluid monitor of claim 1, 2 or 3 wherein the MIR crystal is elongated and flat and has beveled surfaces extending at least partially between its first and second surfaces at each end thereof.

5. The fluid monitor of claim 1,2,3 or 4 wherein the backup member also cooperates with the ends of said slot to form a radiant energy inlet area at its other end.

6. The fluid monitor of claim 5 wherein the seal means includes spaced seats formed in said wall to overlap opposite end portions of the first surface of the crystal, which end portions are secur ed to said seats to form a fluid tight seal therebetween.

7. The fluid monitor of claim 5 or 6 wherein the optical system includes a radiant energy source directing a radiant energy beam through the radiant energy inlet area of the crystal, said radiant energy beam reflecting off the first and second surfaces of the crystal in multiple internal reflections, with the fluid selectively absorbing certain radiant energy frequencies through the monitoring section of said first surface and then exiting the energy outlet area to the detector to identify the fluid based upon the unabsorbed radiant energy frequencies remaining in the radiant energy received at the detector.

8. The fluid monitor of any preceding claim wherein the one wall is selectively removable from the body to permit periodic maintenance on the MIR crystal embedded therein.

9. The f luid monitor of any preceding claim wherein the body has other walls cooperating with said one wall to def ine the fluid chamber, said one wall being selectively sealed to said body to provide a fluid tight fluid chamber.

10. The fluid monitor of claim 9 wherein the first inner surface of said crystal is flush with an inner surface of said one wall and said seal means includes seal material compressed between said body and the end portions of said first surface of said crystal to form a fluid tight seal therebetween.

i i 1 1 11. The fluid monitor of claim 9 or 10 wherein the one wall is the base member of the body, the base member having an inverted T-shape slot extending partially therethrough with the MIR crystal element being received in a portion of a vertical leg thereof.

12. The fluid monitor of claim 11 wherein the backup member is an inverted T-shape adapted to be tightly received in the inverted T-shape slot in the base member, the vertical leg of the backup member being shorter than the vertical leg of the inverted T- shaped slot to abut the second surface of the MIR crystal received in the rest of the vertical leg of the slot.

13. A fluid monitor substantially as hereinbefore described with reference to the accompanying drawings.

Published 1990 at The Patent Offce. State House. 66 71 IlighEolborn. London VIC1R4TP-purther copies mkv be obt-ed from The Patent OfficeSales Branch, St Mary Cray. Orpington. Kent BR5 3nD Printed by Multiplex techniques ltd. St Mary Cray. Kent, Con. 1'87