Multiple Channel Pressure Recorder

This invention relates to a multiple channel (particularly a two channel) microcomputer-based pressure recorder having internal pressure transducers that are intrinsically transportable with the recorder. The pressure recorder, specifically adapted for portable use at the surface of an oil or gas well, also has an internally stored data base which includes calibration factors that enable automatic correction of transducer-responsive signals into signals representing the correct magnitudes of sensed pressures.

Highly accurate quartz pressure transducers have been used in memory recorders lowered into oil or gas wells to sense pressures down in the wells and to provide electrical signals representative of the sensed pressures. Such signals have been stored downhole or transmitted to the surface for storage or display. These devices typically are not adapted for use at the surface of a well outside the well bore.

Although there are devices that can be used at the surface of such wells to record pressure, those that we are aware of typically have only one internal pressure sensor and/or do not provide outputs that are automatically corrected with calibration factors and locally displayed through integral real-time displays.

In view of the foregoing, there is the need for a pressure recorder specifically adapted for use at the surface of an oil or gas well that automatically provides corrected pressure values without requiring operator intervention. There is also the need for such a device to be able to simultaneously monitor at least two pressures with internal pressure transducers. For example, both tubing pressure and annulus pressure sometimes need to be monitored at the wellhead. These pressures cannot be monitored at the same time with a device that can be connected to only one pressure source at a time. There is the further need for a pressure recorder that has multiple highly accurate quartz pressure transducers as integral parts so that they are transportable with the recorder as a single self-contained unit. Such a pressure recorder should also be capable of locally displaying the corrected pressure values in real time and of transmitting the data in real time to a remote location.

The present invention overcomes the above-noted and other shortcomings of the prior art, and meets the needs mentioned above, by providing a novel and improved multiple channel pressure recorder. The pressure recorder can internally sense and record, and locally display and transmit to a remote location in real time, automatically corrected pressure values from at least two simultaneously monitored pressure sources connected to the invention at the surface of an oil or gas well. The pressure recorder of the present invention is a single, compact, portable, self-contained unit having internally contained quartz pressure transducers.

The present invention provides a multiple channel pressure recorder, comprising: a portable housing adapted to be used at the surface of an oil or gas well; a first quartz pressure transducer disposed in the housing; a second quartz pressure transducer disposed in the housing; a microcomputer disposed in the housing; first circuit means, disposed in the housing and connected to the first quartz pressure transducer and the microcomputer, for communicating to the microcomputer a signal responsive to a pressure sensed by the first quartz pressure transducer; second circuit means, disposed in the housing and connected to the second quartz pressure transducer and the microcomputer, for communicating to the microcomputer a signal responsive to a pressure sensed by the second quartz pressure transducer; and wherein the microcomputer includes a memory programmed with a data base including predetermined calibration factors for converting the signals responsive to the sensed pressures into corrected signals representing the magnitudes of the sensed pressures.

The present invention more particularly provides a multiple channel pressure recorder, comprising: a portable housing, which housing is sealed against contamination by the environment at the surface of an oil or gas well; a first quartz pressure transducer gauge assembly disposed in the housing; a second quartz pressure transducer gauge assembly disposed in the housing; a first pressure transducer circuit connected to the first quartz pressure transducer gauge assembly and disposed in the housing adjacent the gauge assemblies; a second pressure transducer circuit connected to the second quartz pressure transducer gauge assembly and disposed in the housing adjacent the gauge assemblies; a first temperature sensing circuit disposed in the housing adjacent the first pressure transducer circuit; a second temperature sensing circuit disposed in the housing adjacent the second pressure transducer circuit; and a microcomputer disposed in the housing and connected for receiving signals from the first and second pressure transducer circuits and the first and second temperature sensing circuits, which microcomputer includes means for automatically computing corrected pressure magnitude signals in response to signals from the first and second pressure transducer circuits, the first and second temperature sensing circuits and predetermined calibration factors entered in the microcomputer.

Therefore, from the foregoing, it is a general object of the present invention to provide a novel and improved multiple channel pressure recorder. Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art when the following description of the preferred embodiment is read in conjunction with the accompanying drawings.

FIG. 1 is an exterior perspective view of the pressure recorder of the preferred embodiment of the present invention.

FIG. 2 is an interior view of the pressure recorder of FIG. 1.

FIG. 3 is a portion of the interior view of FIG. 2, but with pressure transducer and temperature sensing circuit boards and a transducer gauge assembly clamp removed.

FIG. 4 is a block diagram of electrical circuits of the preferred embodiment pressure recorder.

FIGS. 5A and 5B are a schematic circuit diagram of one pressure transducer circuit of the preferred embodiment.

FIG. 6 is a schematic circuit diagram of one temperature sensing circuit of the preferred embodiment.

FIGS. 7A-7G are schematic circuit diagrams of the microcomputer circuit and auxiliary circuits of the preferred embodiment.

FIG. 8 is a sectional view of one of the transducer gauge assemblies.

FIGS. 9A-9I are flow charts of the program stored in memory of the microcomputer for controlling its operation.

Referring to FIG. 1, the portable self-contained two channel pressure recorder of the preferred embodiment includes a single compact housing 2 that can be hand-carried by an individual. The housing 2 contains all the components of the invention.

The housing 2 has two bodies 4, 6 that are releasably connected together by four screws 8. The connection is made so that a seal is provided between the coupled bodies 4,6 to protect the internal components inside the hollow interior of the housing 2 from the surrounding environment outside the housing 2. The sealing and the material of which the housing 2 is made are such as to particularly adapt the housing 2 to be used at the surface of an oil or gas well. For example, the seal can include mating groove 7a and rib 7b shown in FIG. 2, and the material of construction can include any suitable substance (e.g., polyurethane, neoprene, silicon, RFI).

The housing body 4 has an aperture 10 through which a liquid crystal display 12 is visible outside the housing 2. The housing body 4 has another aperture through which a single electrical communication coupling 14 is mounted. The coupling 14 has a plurality of electrical conductors, such as metallic pins or sockets, over which all external electrical communications with the pressure recorder are made by connecting a mating coupling as known in the art. Referring to FIG. 2, attached to the inside of the housing body 4 are two printed circuit boards 16, 18 containing the main processing circuits and the display circuit, respectively, which circuits will be more particularly described hereinbelow.

Referring to FIGS. 1 and 2, the housing body 6 has two apertures through which two pressure couplings 20, 22 are mounted. The pressure couplings 20, 22 include autoclave bulkhead fittings mounted outside the housing, for use with 1/4" O.D. (outer diameter) high pressure tubing. The tubing connects these fittings to 1/4" autoclave collars and glands at respective transducer gauge assemblies 24, 26 inside the housing body 6 (see FIG. 3). The outside bulkhead fittings can be connected to 1/4" high pressure tubing, which can then be connected to autoclave valves, capillary tubing, various adapters, etc. The pressure couplings 20, 22 are filled with an electrically non-conductive liquid, such as sebacate, in communication with the same type of liquid inside the quartz pressure transducer gauge assemblies 24, 26.

Referring to FIGS. 2 and 3, the gauge assemblies 24, 26 are held in place within the housing body 6 by a bulkhead 28 and upper and lower clamps 30, 32 secured to the housing body 6 by screws 34a, 34b, 34c, respectively. The bulkhead 28 extends perpendicularly from the back wall of the housing body 6 and has two parallel lateral holes defined in it for receiving respective ends of the gauge assemblies 24, 26 and for permitting wires to pass therethrough for connecting to electrical circuits subsequently described. The clamp 32 is adjacent the back wall of the housing body 6, and the clamp 30 is spaced parallel to the clamp 32. The clamps 30, 32 are defined with two notches each to receive respective circumferential portions of the gauge assemblies 24, 26.

Referring to FIG. 8, each gauge assembly 24, 26 has a quartz crystal pressure transducer 27 sensitive to pressure changes. The crystal of the transducer is partially enclosed in a Teflon housing 29, where wires 31 (one shown) from the crystal are connected to electrical sockets 33 (one shown) at one end of the housing 29. This Teflon housing 29 is placed in a high pressure chamber defined by a body 35 and a cap 37. When assembled, a high pressure electrical feedthrough 39 on the cap 37 of the pressure chamber is connected to the sockets 33 so that the crystal can be connected to an electrical oscillator circuit outside the pressure chamber. An O-ring 36 is mounted on the cap 37 to seal against the body 35. A Kapton washer is used with the high pressure feedthrough. The end of the pressure chamber body 35 opposite the end that receives the cap 37 threadedly receives a coupling for connecting tubing from the respective pressure coupling 20, 22 (see tubes 58, 60 in FIG. 2). Liquid is contained in the chamber or compartment defined within the pressure chamber body 35, and it is in pressure communication with liquid in the respective tube 58, 60 when the present invention is assembled.

Referring to FIG. 2, connected outwardly of the gauge assemblies 24, 26 by nut and bolt combinations 40 is a support plate 41. Mounted on the plate 41 are four printed circuit boards 42, 44, 46, 48. The printed circuit boards 42, 44 contain the circuitry for respective pressure transducer circuits, and the printed circuit boards 46, 48 contain respective temperature sensing circuits. These circuits will be further described hereinbelow.

Defined in the housing body 6 on the side of the bulkhead 28 opposite where the gauge assemblies 24, 26 are disposed is a cavity 50 in which a rechargeable battery 52 is retained by a removable bracket 54 bolted to the bulkhead 28. The battery 52 is the primary power supply for the pressure recorder of the present invention. Defined in the housing body 6 on the side of the bulkhead 28 where the gauge assemblies 24, 26 are disposed is a cavity 56.

To make it easy for one person to carry the pressure recorder, a handle 62 is attached to the top of the housing body 6.

The pressure recording achieved with the present invention occurs through the operation of the various electrical circuits generally referred to above. This recording occurs in response to pressure communicated to the pressure transducers in the gauge assemblies 24, 26 from external pressure sources connected to the pressure couplings 20, 22 in known manner and as generally represented in FIG. 4. The gauge assembly 24, the pressure transducer circuit on the printed circuit board 42 and the temperature sensing circuit on the printed circuit board 46 make up the combined pressure and temperature sensing circuits 64 depicted in FIG. 4. The gauge assembly 26, the pressure transducer circuit on the printed circuit board 44 and the temperature sensing circuit on the printed circuit board 48 are included in the combined pressure and temperature sensing circuits 66 of FIG. 4. Each of the circuits 64, 66 outputs two respective electrical signals. One of the signals (the one from the pressure transducer circuit) has a frequency proportional to pressure and temperature, and the other signal (the one from the temperature sensing circuit) has a frequency proportional to temperature.

The signals from the circuits 64, 66 are individually selected through a multiplexer 68 controlled by a microcomputer 70 as represented in FIG. 4. The selected signal is processed through a window timer 72 which also operates in response to control from the microcomputer 70. The window timer provides a single pulse (i.e. a window) that is proportional to the selected signal during which the microcomputer counts internal microcomputer pulses proportional to its clock frequency. The number of internal pulses that the microcomputer counts is proportional to the frequency of the selected signal.

The microcomputer 70 comprises a microprocessor and a memory programmed for operating the microprocessor. Flow charts of the program for the preferred embodiment are shown in FIGS. 9A-9I, which will be further described hereinbelow. In general, however, the program causes the microcomputer 70 to receive the pulses from the window timer 72 and to convert those responsive to the sensed pressure into an absolute pressure magnitude. The conversion occurs in response to the corresponding sensed temperature and to predetermined calibration factors entered in the memory.

The various data received and computed by the microcomputer 70 are stored in data storage memory 74 depicted in FIG. 4. The data can also be displayed through the display 12 identified in FIG. 1 and also represented in FIG. 4, and the data can also be transferred out of the recorder through the electrical coupling 14 which has conductors connected to an RS-232 interface 76 depicted in FIG. 4.

Particular implementations of the circuits referred to in FIG. 4 are more particularly shown in FIGS. 5-7. These particular implementations are self-explanatory to those skilled in the relevant arts so they will be described only briefly.

A particular implementation of the pressure transducer circuit contained on each of the printed circuit boards 42, 44 and included in each of the circuits 64, 66 of FIG. 4 is shown in FIGS. 5A and 5B. The pressure input called for in FIG. 5A is the pressure sensitive crystal of the respective gauge assembly 24, 26. As the crystal connected to these input terminals responds to pressure communicated through the respective pressure coupling 20, 22 and tubing 58, 60, the frequency of an oscillator 78 shown in FIG. 5A changes (the respective pressure transducer crystal is part of the oscillator 78). The output of the oscillator 78 drives a transistor 80 to gate a reference signal from a reference oscillator 82 (FIG. 5B). The gated signal passes through a signal conditioning and amplifying output circuit 84 (FIG. 5A), and the resulting 7-90 kHz output signal indicated in FIG. 5A is one of the pressure-and-temperature responsive signals provided to the multiplexer illustrated in FIG. 4. The reference oscillator 82 of the FIG. 5B implementation is set up for a particular SC-cut quartz pressure transducer used by Halliburton Services; for an AT-cut quartz pressure transducer used by Halliburton Services, the following component values are preferred: R21, 82.5 k; R22, 33.2 k; R23, 18.0 k; C15, 100 pf.

To insure adequate temperature compensation of the foregoing signal that is affected both by pressure and temperature, the temperature sensing circuits on the printed circuit boards 46, 48 are used (one for each channel of pressure sensing). Each of these circuits is the same, and a particular implementation for one of these is shown in FIG. 6. A temperature responsive crystal 86 is mounted on the respective circuit board in an oscillator circuit 88. In the illustrated particular implementation, the crystal 86 is a torsional mode tuning fork MX-IT 190.5 kHz .+-.1 kHz temperature sensitive crystal that is in a cylindrical can roughly 1/4" in length and 1/16" in diameter. This temperature sensor is mounted inside the feedthrough cavity of the respective pressure cap 37. This allows the temperature of the pressure crystal to be more closely monitored. As the response of the crystal 86 changes with sensed ambient temperature, the frequency of the oscillator output signal varies. This signal is conditioned and amplified and provided as an 8-12 kHz signal to one of the inputs of the multiplexer 68.

Particular implementations of the multiplexer 68 and the window timer 72 are shown in FIG. 7A. The output from a counter 89 implementing the window timer 72 is provided to an input of the microcomputer 70 having a particular implementation shown in FIG. 7B.

The microcomputer 70 includes a microprocessor 90 that operates in response to a program stored in a program memory 91 shown in FIG. 7B. Control and data transfers occur over the designated address, data and control lines of the microprocessor 90 and a latch 92 and a decoder 94 shown in FIG. 7B.

Operating memory space is provided in a random access memory 96, and data storage memory 74 is provided in random access memory 98, both shown in FIG. 7C. Also shown in FIG. 7C is a real time clock circuit 100.

Data to be communicated through the display 12 is output through a display interface circuit 102 shown in FIG. 7D. Data to be communicated through the electrical connector 14 (FIG. 1) is output to the connector 14 through an interface circuit 104 (FIG. 7E) implementing the interface 76 of FIG. 4.

To convert the level of the primary power supply battery 52 to the proper level for the circuits of the particular implementation, a power level converter circuit 106 is provided as shown in FIG. 7F. The pressure recorder is energized by this power when either an internal switch 108 or an external switch (not shown) connected through the electrical coupling 14 is closed. Backup power is provided by a battery 110 shown in FIG. 7F. The battery level of the off-board rechargeable battery 52 is monitored through a battery monitoring circuit 112 shown in FIG. 7G.

The components described above with reference to FIG. 7 are mounted on the printed circuit board 16 connected inside the housing body 4.

Mounted on the printed circuit board 18 also attached inside the housing body 4 is the particular implementation of a display circuit. The display unit is a liquid crystal display (LCD). The display of the preferred embodiment is an already assembled LCD module with controller, memory, etc. This type of module can be purchased from several LCD manufacturers. A specific model is a Densitron LM2434A4C20SNY.

The flow charts representing the program for controlling the microcomputer 70 will next be described with reference to FIGS. 9A-9I. The main program flow chart is shown in FIG. 9A, with the remaining flow charts of FIG. 9 showing those aspects of FIG. 9A which are not otherwise readily apparent.

Referring to FIG. 9A, the program controls the microcomputer to insure that the power is stable, to initialize the various components of the circuitry shown in FIGS. 5-7, to read the calibration factors stored in the memory 91, and to initialize the state machine. Initializing the state machine means placing the "state" of the microcomputer in state A state (the microcomputer of the preferred embodiment has three main states) is where the microcomputer is at in its program execution. State 1 is a power up state, which first occurs as the microcomputer is turned on. State 2 is where a smart terminal (to be explained later) is connected to the microcomputer. This occurs when the right data is received from the microcomputer's serial port. State 3 is the data acquisition mode, where pressure and temperature are measured for two pressure sources.

Next, the microcomputer determines whether a smart terminal is connected. A smart terminal is any IBM PC or compatible computer that contains a hard drive with a program loaded onto it to communicate with the microcomputer and to issue commands, program, perform diagnostics, and download/upload data from the microcomputer. This is not part of the presently claimed invention. If one is connected, the microcomputer executes the smart terminal module shown in FIG. 9B. If one is not connected, it performs one of the routines shown in the boxes at the bottom of FIG. 9A.

Referring to FIG. 9B, if a smart terminal is connected, the microcomputer checks for a low battery condition and updates the liquid crystal display. If an instruction has then been received, it executes it in accordance with the respective box representing one of the applicable instructions shown at the bottom of FIG. 9B.

If a smart terminal is not connected, the flow chart of FIG. 9A shows that the microcomputer then determines the program status from memory and performs one of the six indicated functions. That is, the microcomputer can erase memory and display an error or display an error message if the pressure recorder is not programmed. It can also display memory. The remaining functions shown in FIG. 9A include recording data, which can begin upon a preset time being reached or upon completion of a programmed countdown. The record data subprogram is shown in FIG. 9C; the start recording on a preset time subprogram is shown in FIG. 9D; and the start recording upon completion of countdown subprogram is shown in FIG. 9E.

The steps set forth in FIG. 9C are self-explanatory except that the acquire data function is performed in accordance with the subprogram shown in FIGS. 9F-9G and except as to the decision "Has next data point exceeded pressure window or passed force record time?" This latter function refers to when data are recorded in the preferred embodiment, as explained as follows.

To prevent memory being used for recording a pressure that does not change very much over time, the preferred embodiment records when a sensed pressure changes some predetermined magnitude over a time interval (a pressure window), otherwise data points are taken at a slower rate. A slow record time and a fast record time are set via the smart terminal. The slow record time is the maximum amount of time that can occur between recorded, time stamped, data samples. The fast record time is the minimum time that can occur between recorded, time stamped, data samples. For example: fast record=10 seconds, and slow record=60 seconds. This means that for this example data points can be no closer together than 10 seconds or further apart than 60 seconds, but they can be apart any time between 10-60 seconds. If the slow record and fast record times are equal, a constant record time is achieved. However, if the slow and fast record times are different, variable recording occurs. If a pressure does not change very much over time, samples will be recorded every "SLOW RECORD TIME" seconds. However, once the "PRESSURE WINDOW" has been exceeded, data will be recorded instantly, providing that the last sample recorded was at least "FAST RECORD SECONDS" ago.

As to the flow chart of FIGS. 9F-9G describing the data acquisition routine, the steps are self-explanatory but the following further explanation about the preferred embodiment is given.

Every one second, the microcomputer attempts to acquire a frequency signal from the first temperature sensor. If it can, it determines the frequency by using the window counter, and then, using stored temperature coefficients, calculates the temperature. If a signal cannot be obtained, no pressure for the first transducer can be calculated, and so the microcomputer attempts to record from the second temperature and pressure sensors.

After obtaining a temperature, the microcomputer attempts to acquire a frequency signal from the respective pressure sensor. If it can, the microcomputer determines this frequency, and then, using a 4th order 2-dimensional least squares curve fit, along with the measured temperature frequency from above, the pressure is determined. If the microcomputer cannot measure the pressure frequency signal, it returns to check the other set of sensors.

Once an acquisition cycle is completed for one of the pressure sensing channels, the microcomputer then updates the display with the appropriate information. At the conclusion of a pair of cycles, the microcomputer dumps this data out the serial port. Therefore, both real-time display and transmission are provided.

Calibration for the foregoing is performed using a least squares curve fit. This is a way of taking over-determined data that can be non-linear and that represents some variable, such as temperature, and producing a set of coefficients that can be used in a polynomial equation to derive the unknown variable at points other than where the original data was taken. For example, the temperature of a temperature crystal can be said to be a function of the frequency of the temperature crystal, and using least squares, and several data points (at least n+1 points), an nth order equation can be used to describe the temperature given the temperature frequency. This method can be expanded to include a variable that is a function of two measured quantities. For example, pressure can be said to be a function of both temperature frequency and pressure frequency. In solving for the least squares coefficients, smaller numbers are desired to provide for easier mathematics. This can be provided for by using an offset. All this can be seen below in the following equations.

In the temperature frequency to temperature equation below, the tx[t.sub.-- num] [0-4] are the coefficients calculated as discussed above. Their source is from data taken where the temperature crystal is subjected to five different temperatures, and its frequency is measured. The [0-4] represents the five coefficients. The [t.sub.-- num] is the number of the transducer being used.

Below are the equations in determining pressure. The coefficients gx[t.sub.-- num] [0-4], hx[t.sub.-- num] [0-4], etc., are determined by using different pressures at different temperatures (for example, calibrating at temperatures -20, 5, 30, 55, and 80 degrees Celsius, and at each of these temperature, pressures of 0, 5000, 10000, 15000, 20000, 15000, 10000, 5000, and 0 PSIG are exerted onto the pressure crystal). Using these coefficients, along with temperature frequency and pressure frequency, the pressure can be calculated as follows:

The subprograms depicted in FIGS. 9D and 9E pertaining to the two "start recording" functions called for in FIG. 9A are self-explanatory. In general, the program of FIG. 9D matches a programmed real time value to the real time retrieved from the real time clock 100 shown in FIG. 7C, and the program of FIG. 9E decrements an entered count until zero is reached. Upon the respective condition occurring, data is recorded in accordance with the program of FIG. 9C.

The program depicted in FIG. 9H shows how the data storage memory 74 (specifically random access memory 98 in FIG. 7C) is tested in response to the appropriate instruction shown in FIG. 9B. To test the data storage memory, a predetermined value (such as 0.times.55H and 0.times.FFH shown in FIG. 9H) is written to the selected memory location and it is then read back to determine whether a proper write and read have occurred. This is repeated until the end of the memory has been reached.

The program for implementing the transducer test instruction of FIG. 9B is shown in FIG. 9I. This testing occurs by selecting one of the transducers and attempting to acquire a signal from it. The frequency of an acquired signal is checked to determine if it is within the proper operating range for the transducer. This is repeated for each transducer, and if a bad one is detected, an appropriate error flag is entered in the software.

A listing of a particular program implementing the foregoing is submitted in the Appendix hereof. Source file MQPT.sub.-- CMD.C is the main program with: file MQPT.C providing lower level functions; file MQPT.sub.-- CVT.C providing data conversion functions; file MQPT.H providing definitions, constants and other modules common to the others; and file STARTUP.A51 providing an assembler file.

Through the combination of the microcomputer hardware and programming contained in the program memory 91, there is provided means for automatically computing corrected pressure magnitude signals in response to the signals from the two pressure transducer circuits, the two temperature sensing circuits and the predetermined calibration factors entered in the program contained in the memory 91. There is also provided means for selectably recording in response to a preset real time alarm value or a preset countdown as implemented with the programs of FIGS. 9D and 9E, respectively. There is also provided memory means for storing data in response to the sensed pressures and temperatures and means for testing the memory means where the data is to be stored. A particular storage memory means is the random access memory 98 shown in FIG. 7C, and the means for testing such memory includes the program shown in FIG. 9H. Also provided is a means for testing the transducers as implemented using the program of FIG. 9I.

Accordingly, the pressure recorder of the present invention is an electronic instrument used to simultaneously display and record pressure data for at least two pressure sources from atmospheric pressure to the maximum pressure which particularly implemented pressure transducers can accommodate. The pressure recorder includes a display that shows real-time absolute pressure of the respectively connected pressure sources. System status and time of day data can also be obtained. In the particular implementation described above, there is sufficient data storage memory to record 10,816 data records, each containing two pressures, a temperature and the time when the data were taken. These data records can be transferred out of the pressure recorder to a data analysis system connected to the single electrical coupling 14 shown in FIG. 1.

Applications of the pressure recorder of the present invention include, without limitation, hydraulic horsepower monitoring, pressure measurements over a length of pipe or pipes, simultaneous monitoring of tubing and annulus pressures to check for packer leakage, fluid transport efficiency studies, and monitoring for fracture closure in an oil or gas well.

Thus, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned above as well as those inherent therein. While a preferred embodiment of the invention has been described for the purpose of this disclosure, changes in the construction and arrangement of parts can be made by those skilled in the art, which changes are encompassed within the spirit of this invention as defined by the appended claims. ##SPC1##