Abstract:
The invention covers a procedure for measuring hydrostatic pressure, especially that of ground water, with the particularity that air is bubbled into the ground water, the pressure in the measuring pipe is fed to an absolute pressure cell and atmospheric pressure is then applied to the same measurement cell.

Description:
FIELD OF THE INVENTION 
     The invention covers a procedure for measuring hydrostatic pressure, especially that of ground water. 
     The invention also includes a ground water measuring device with a data collector, a measured data recorder and an active link between the measured data recorder and the data collector. 
     BACKGROUND OF THE INVENTION 
     A very important aspect of hydrological measurement practice is continuous monitoring of the water table, since this is needed for resolving environmental problems caused by manmade developments. 
     Water occurs naturally in the form of surface water and ground water. In the surface water category are rivers, streams, lakes and oceans. The water level of surface water is normally easily accessible which makes installation of equipment for measuring and recording this level relatively easy. 
     By contrast boreholes have to be sunk in order to monitor ground water feeds, and these are normally constructed using 2&#34; to 6&#34; pipes. These pipes are normally capped to prevent unauthorised access. 
     The requirements of a ground water measuring system are very exacting because of local environmental conditions and specifications imposed by measurement network operators. Some of these requirements are listed below. 
     Total installation inside 2&#34; to 6&#34; pipes 
     Operating temperature -20° C. to ±60° C. 
     Resistance to water condensate 
     Stable long-term measurement accuracy of +1 cm over 10 m measurement run 
     Independent power supply for at least 1 year 
     Low acquisition costs 
     Low operating costs 
     Nowadays a large number of measurement systems are available for measuring water levels. But in the light of the overall requirements outlined above not all measurement procedures are suitable for measuring ground water in conjunction with electronic data collectors. 
     Measurement procedures so far invented can basically be placed in two categories, those measuring length and those measuring pressure. Those measuring length include all measurement systems operated by means of floats with angle encoders, acoustical and optical procedures and measurement using conductivity levels. Those measuring pressure include all systems such as pneumatic pressure recording (introduction of air bubbles) and electronic pressure measurement (piezo resistant, capacitative etc.) 
     For the purposes of ground water measurement the only systems to have proven themselves in practice are measurement of length by means of floats with angle encoders and electronic pressure measurement. There is a DVWK paper, Issue 107: Ground Water Measurement Devices, Bonn, 1994 which provides a current overview of the use of ground water measurement systems. 
     SUMMARY OF THE INVENTION 
     Based on a process for measuring hydrostatic pressure, particularly of ground water, as described at the outset of this document, the task performed as the basis of this invention is to implement such a process in a container with a very small diameter compared with general technological standards, and to do so in a manner which is resistant to water condensate, saves a great deal of power, works independently and costs very little to actually produce measurements. 
     The ground water measurement device outlined above has to operate at the lowest possible diameter independently of any source of energy which is not part of the system and must be designed to resist water condensate and to be energy efficient, whilst being located in a small-diameter pipe, for example 2 inches. 
     The surprising way of achieving this with the present process is for air to be bubbled into the ground water with the measurement pipe pressure being relayed to an absolute pressure cell and with atmospheric pressure then being applied to the same measuring cell. 
     This is achieved with the deployment of a ground water measurement device of the above type with a data collector, measured data recorder and an active link between measured data recorder and data collector by means of a piston-driven pump which bubbles air into the water together with an integrated valve function to open and close the measurement pipe while simultaneously controlling pressure in the measurement pipe and atmospheric pressure. 
     It has not been overlooked that the process of bubbling air into water has been used highly successfully for the measurement of water levels in surface waters. For this the outlet end of an air feeder pipe or tube is fed to a point just below the lowest water level to be measured. Air is fed through the pipe continuously (pressure reducer, quantity controller) to ensure that air bubbles come out of the outlet end of the air feeder pipe. In this way air pressure at the outlet end of the feeder pipe is the same as the pressure of the liquid at this point. Taking the density of the water one is left with a linear connection between the desired water level reading and air pressure in the pipe. Air pressure in the measurement pipe is measured outside the water using a compensation process (sprung bellows, sliding weight) or a reference pressure measurement cell. Recording of the water level is by means of a mechanical marking device or a data collector. A description of this measurement system can be found in the company brochure issued by Messrs Ott Measurement Technology GmbH &amp; Co KG, Ludwigstrasse 16, 87437 Kempten, Compact Pneumatic Level Device R25, 20.502.000.P.D. 
     In addition measurement systems using the introduction of air bubbles are known through patent descriptions DL-PS 7733 - IPC G 01 c, German Offenlegungsschrift to Rhieische Braunkohlenwerke, P 22 48 315.4-52 and Eastern German (DDR) patent specification WP G 01 F / 198 683 leading to German Patent Number 132278, issued Sep. 13, 1978. However none of these air-fed measurement systems are housed completely in a 2&#34; diameter level-detecting tube, nor can they be operated for a whole year using their own independent power supply (6V, 1.5 Ah). Further distinguishing features are that they all require a continuous feed of air and therefore have to be supplied from a volume of air in a spare tank, involving a considerable requirement for space and/or energy resources. 
     The considerable quantity of apparatus required on the surface has hitherto ruled out application of the air bubble feeder process to ground water measurement technology, and this has been reinforced by the professional view that continuous energy supply from an outside source was necessary over long periods. 
     The task outlined above was surprisingly simple to resolve using a combination of a compressor integral to the system (piston-driven pump) and determination of the measurement cell as an absolute pressure cell. 
     It is particularly appropriate given the small amount of energy available that air does not have to be fed in continuously, but at say fifteen minute intervals. 
     A particularly appropriate design feature is the small diameter measurement pipe which allows air to blow out into a large volume compared with the size of pipe. 
     In terms of the equipment fitted the diameter of the air feeder vessel can for example be 20 times as great as the diameter of the measurement pipe itself. This ensures that measurement error can be restricted to a very small degree as the water level rises. This principle derives from a simple physical law according to Boyle-Mariotte, namely: &#34;the product of pressure and volume is constant for an enclosed gas at constant temperature&#34;. The piston-driven pump used does not have any connecting links. The piston rod simply operates in a straight line. The piston has a tappet which instantly opens a valve in a certain position, namely the position of maximum pressure. The resulting loss of pressure firstly rapidly cleans the measurement pipe and the air feeder vessel of any remaining water. It also ensures that any condensate residues and any algae that may be present at the air outlet apertures are blown away. 
     The design allows the pressure drop extremity to be used as a trigger for the motor, say a DC motor, to be reversed and operated the other way. 
     Conical valves have shown themselves to be particularly appropriate. However the tappet can also be used for the instant dislodging of a cup from its valve seating. The counter-pressure spring is designed only to cut in over the Hook scale. It is tared and is triggered at a pressure of say 2 bar, at which the tappet of the piston hits the valve. 
     Data collector, piston-driven pump, absolute pressure cell, optical interface can all be inserted along with the power source (battery) into a two-inch pipe and lowered beneath the well cap into a frost-free area. This houses the whole assembly in frost-free ambient conditions. The great advantage of this is that extreme temperatures do not influence the measurement system (which consists of electronic and mechanical devices) in any way. 
     Unlike other equipment there is in this case no continuous supply of air, which saves on electricity. Since, however, as the water table rises (between measurements) air in the outlet feeder vessel and in the pipe is compressed, the barrier between water and air is displaced. What this means is that as the water table rises, water enters the outlet feeder vessel. The pressure measured thus no longer relates to the air feeder outlets but to the water table level in the outlet feeder vessel. In existing equipment this source of error is remedied by an adequately high continuous supply of air. But power consumption by such equipment is too high as it pumps the requisite quantity of air through the system in order to be able to meet the above requirement, an aspect which the present invention surprisingly solves by means of the volume and/or diameter ratios applied. 
     Overall the invention covers application of the introduction of air bubbles, a technique which is already well-known in the measurement of surface water, to the measurement of ground water levels, replacing the standard compressor with a connection-free piston-driven pump with a tappet fitted to the piston to operate a valve and with pressure being measured via a source of absolute pressure. 
     The features of this invention mean that if it is used to measure hydrostatic water pressure by means of measurement pipes sunk into the ground water: 
     the principle of the introduction of air bubbles can be applied; 
     a two inch pipe can be used to house the equipment; 
     reliable operation is assured at outside temperatures of -20° C. to +60° C.; 
     resistance to water condensate is assured; 
     stable long-term measurement accuracy is guaranteed of +/-1 cm over 10 m; 
     a self-contained power supply is provided for at least 1 year, something which has been inconceivable for other existing systems; 
     compared with other existing systems, the procurement cost of this equipment is up to 50% lower, and operating costs are low. 
     A simple raising and lowering cable can be used to bring up the mechanical and electronic devices. The way that the outlet feeder vessel is deployed, as described in more detail below, means that it can stay in place. 
     As described below, measurement accuracy is not affected even if there is zero point drift in the absolute pressure measurement cell. 
     Design features of this invention allow a lockable cap to be fitted, which can be locked and thus allow the device to be deployed in municipal areas without any risk of contamination, for example through accidental oil spillages. 
     There follows a more detailed description of one possible implementation of the invention, to be read in conjunction with the enclosed drawings, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic view of a system using the introduction of air bubbles to measure ground water levels as per this invention; 
     FIG. 2 is a diagram which makes clear the degree of measurement accuracy, and 
     FIG. 3 shows a schematic view of one possible implementation of the invention, depicted while taking measurements in a ground water measurement pipe. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows the outlet feeder vessel 1 located below the lowest water level to be measured. Outlet apertures 2 are located all over the outlet feeder vessel 1 at regular intervals. The outlet feeder vessel 1 is itself linked via a measurement pipe 3 to a piston-driven pump 4. 
     It is important that the cross-sectional surface area of the outlet feeder vessel 1 is many times greater than the cross-sectional surface area of the measurement pipe 3. The ratio [R] between outlet feeder vessel cross-sectional surface area and measurement pipe cross-sectional surface area should, for example, exceed 400. 
     The piston-driven pump 4 consists of a piston 9, to which a rod 10 is attached and a cylinder sleeve 5, in which are located two boreholes 6 and 8. At the lower end of the piston 4 there is a valve-type mechanism, consisting of a return spring 11 and a cut-off cone 12. The piston 9 moves up and down on a piston rod 13 having trapezoidal thread thereon, linked to an electric motor 21. 
     When the piston is fully in the `up` position air enters via borehole 6 into the cylinder block. Pressure inside the cylinder equates exactly with atmospheric pressure. Atmospheric pressure is then measured in this in this position using an absolute pressure measurement cell 14. This absolute pressure measurement cell is connected to borehole 8 and is thus in contact with the atmospheric pressure in the cylinder block. 
     If the piston 9 is now driven down the enclosed air in the cylinder block is compressed to around 2 bar for example. Once a pressure of around 2 bar has been reached the rod 10 presses down on the cut-off cone 12 and opens the outlet 20. Air rushes into the measurement pipe 3. The pumping action is repeated until any water which has entered the outlet feeder vessel 1 has been forced out. This sudden blast of air in measurement pipe 3 has the additional great advantage of forcing out any water droplets present in measurement pipe 3. 
     The measured pressure drop, as measured between (from the piston lowest position) the maximum piston pressure and the value of the atmospheric pressure (the piston upmost position) is used for controlling the reversal moment of piston 9. 
     The time taken by the piston 9 to travel depends on the difference in water level (rise) and the desired measurement interval. 
     The piston 9 is in its bottom position when pressure in the measurement pipe is measured. Pressure in the measurement pipe 3 is thus relayed to the absolute pressure measurement cell 14. 
     Measurement of pressure in the measurement pipe and measurement of atmospheric pressure are required, since an absolute pressure measurement detector does not provide for any automatic compensation of atmospheric pressure. The electronic devices used for collection and evaluation of data are used to calculate hydrostatic water pressure. 
     The chosen configuration, a piston-driven pump with a valve outlet in conjunction with an absolute pressure measurement cell guarantees highly accurate and stable long-term measurement. The configuration is not in any way affected by moisture. A further advantage of the chosen configuration is that even if there is zero point drift of the absolute pressure measurement cell, measurement accuracy is not adversely affected as a result. 
     FIG. 2 shows what happens in practice. The characteristic line of a pressure measurement cell is in relation to the desired measurement reading (input reading) and the output signal received. This characteristic line is in the form of y=a+bx. When measuring with the help of this configuration po and po+pw are measured. Calculation of hydrostatic water pressure is then performed using the change of volume delta V, ΔV=V2-V1. If the characteristic line is now displaced (displacement of the zero point) the measurement is not adversely affected as a result. The [/ \] V=V21-V1 remains the same. 
     The pressure measured by pressure detectors is the product of the height of the water column and the density of the water plus the atmospheric pressure exerted on the water. In order to exclude the influence of the air pressure pressure detectors with differential pressure measurement are used in well-known and established equipment. To achieve this, atmospheric pressure is fed via a thin tube to the reverse side of the detector membrane. Moisture which has condensed into droplets inside this tube can hinder the passage of air and thus lead to incorrect measurements. 
     The readings measured (pressure in the measurement pipe and atmospheric pressure) are relayed to an electronic collection and evaluation device 30. This electronic collection and evaluation device 30 has a display 31 for displaying measurements taken at the site. There is an optical interface (infrared) 25 also connected to the electronic collection and evaluation device which allows data which has been collected to be read out as necessary from the electronic collection and evaluation device storage unit. 
     FIG. 3 illustrates additional advantages of measurement system 39. For a readout or to change the battery the measurement system has to be removed from the pipe. When this is done the position of the outlet feeder vessel 1 must not be altered. This is achieved by securing the outlet feeder vessel 1 to a thin wire 40 which is attached running upwards to a special securing device 41 beneath the lockable cap 42. Since the measurement system 39 is also connected to the outlet feeder vessel 1 via the measurement pipe 3, the upper section of the measurement tube is in the form of a spiral. This means that when the measurement system is taken out, the measurement pipe 3 can stretch for several meters without altering the position of the outlet feeder vessel 1. 
     The measurement system itself is also connected to the securing device 41 by means of an elastic cable. This elastic cable prevents any damage occurring to the measurement system from downwards pull.