Title of Invention

"AN IMPROVED DEVICE USEFUL FOR GALIBRATING HYDRAULIC PRESSURE GAUGES"

Abstract

An improved device useful for calibrating hydraulic pressure gauges which comprises a solid vertical column (1) having an inlet (2) for passing the liquid media the pressure of which is to be measured and an outlet (3), the outlet is connected to a hydraulic pressure gauge (17) to be calibrated, through a fine needle valve (14), characterized in that the said inlet and outlet are also connected to a coaxial opening (4) extending upto the top of the said vertical column, a cylinder (5) is fitted by a retaining nut (6) on to the upper end of the said column (1), the said cylinder is provided with a coaxial opening capable of accommodating a piston (7) to be capable of transferring the pressure of the liquid media through the coaxial opening (4) to the piston extending above the vertical column (1), the top of the said piston is connected to a weight table (8) by a cap (9), the said weight table being provided with a circumferential vertical cylinder (10) having a circumferential table (11) for loading weights, the said inlet (2) is provided with multi way connector for connecting to a known hydraulic circuit (12 & 13 needle valve, 15 pump and 16 reservoir).

Full Text

This invention relates to an improved device useful for calibrating hydraulic pressure gauges. More particularly the present invention relates to a device for generating and measuring accurate pressure from the ratio off the total downward known gravitational force applied on a cylindrical piston to its known area, fitted into a matching cylinder filled with fluid. The device of the present invention is particularly useful as a for calibration of hydraulic pressure measuring devices. It has hitherto been known to calibrate pressure gauges through the use of liquid column manometers shown in figure 1 of the drawing accompanying this specification. Here the pressure (P) is determined in terms of the height of a column of a liquid (A) of known density (B) under known gravitational force (g) P = AX-Bx g. However, the operating range of these instruments has been generally limited to pressure below 0.5 MPa, their optimum performance is subjected to a stringent controlled conditions during operation and they are not easily portable. It has hitherto been known to calibrate pressure using the Burdon dialgauge shown in figure 2 by measuring the expansion of a elliptical mechanical tube (C) whose one end (D) is connected to the pressure to be measured and other end (E) is engaged with the lever (F) and a needle (G). The applied pressure forces the tube to expand along its length, which in turn moves the lever, and hence the needle to give the value of applied pressure. This cannot generate or measure pressure with an accuracy better than 0.1%. Moreover a degradation in its accuracy is observed when it is used to measure pressure below 10% of its full-scale pressure value. It has hitherto also been known to calibrate pressure in terms of some suitable physical/electrical property of a material. A wide choice of variation of physical properties with pressure is available. But to date the gauges depend ultimately on the measurements of piezo resistance transducers in which the change in the resistance with pressure is measured. In another device strain gauge transducers are used to measure pressure whereas the change in voltage due to the pressure applied is calibrated in terms of the pressure. These transducers however, have inherent problems, e.g. their low pressure cut-off value, high temperature sensitivity and low pressure coefficients and hysteresis. The pressure is the ratio of the total downward gravitational force to the area of the piston has been known for a long time and are generally measured by a device known as dead weight gauges. In these gauges a piston rotates in a closely fitted circular cylinder. The pressure at the base of the piston is defined as the ratio of the total downward force on the piston to its effective area when floating at its operating level. The accuracy with which a pressure measurement can be made using these gauges depends on the accuracy with which measurements of both force and pressure dependent effective area can be made. It is known to use a piston and cylinder in controlled clearance configuration to measure pressure and generally act as primary pressure standard and is shown in figure 3. In figure 3 a cylindrical piston (H) and the cylinder (I) is applied a jacked pressure (J) and the pressure to be measured (K) at the end of the cylindrical piston (H). The applied pressure pushes the piston upward and the amount of the force (L) applied at the top of the piston to counter balance the upward motion of the piston and in equilibrium condition, the applied pressure is the ratio of the force (L) to the area (M) of the piston. The use of the controlled clearance configuration is restricted as its operation is very cumbersome and needs an independent pressure line (J) to be applied across the cylinder and moreover it is very expensive, bulky and needs relatively more stringent control of environmental conditions. In other available pressure calibrating devices a simple piston and cylinder configuration in which the pressurizing fluid acts on the interior or some times at the end surface of the cylinder. The device when used in a complete system is capable of generating and measuring the pressure. There are some inherent limitations such as priming, a large diameter and long tube to transmit the pressure of the fluid, which takes long time for temperature and pressure stabilization, repeated filling up of the hydraulic pump, prevention of the isolation of the complete hydraulic circuit from the measuring system etc. which restricts its use for generating measuring and calibrating the applied pressure accurately and moreover the total applied force is kept at the top end of the piston and as its center of gravity lies above the piston which slows down its rotary motion during pressure measurements and hence more friction between piston and cylinder and ultimately the additional error in the pressure measurements. Not only this due to its low rotary motion one has to rotate the piston by hand frequently, which adds force to the applied known force and ultimately gives the additional error in the pressure generated or measured by the device, pressure being directly proportional to the force. The object of the present invention is to provide an improved device useful for calibrating hydraulic pressure gauges, which obviates the drawbacks of the hitherto known devices. Another object of the present invention is to provide a device which will be accurate, inexpensive and which will also allow to generate, measure and calibrate pressure over a wide range 0.1-80 MPa with an accuracy of 6 0.03% or better. In figure 4 a schematic diagram of an embodiment of the device of the present invention is shown. In the drawings the various parts are pressure column (1), having an inlet (2) and outlet (3) which are connected to the co-axial opening (4) and a cylinder (5) being fitted by a retaining nut (6) on to the upper end of the vertical column and a piston (7) is fitted into the coaxial opening of the cylinder and the top of the piston being connected to a weight table (8) by a cap (9), the said weight table being provided with a circumferential vertical cylinder (10) having a circumferential table (11). In figure 5 the layout of an embodiment of the device of the present invention in actual use is depicted. In the drawing, the various parts are device of the present invention (1), fine needle valves (12,13,14), screw type hydraulic pump (15), oil reservoir (16), gauge to be calibrated (17). Accordingly, the present invention provides an improved device useful for calibrating hydraulic pressure gauges which comprises a solid vertical column (1) having an inlet (2) for passing the liquid media the pressure of which is to be measured and an outlet (3), the outlet is connected to a hydraulic pressure gauge (17) to be calibrated, through a fine needle valve (14), characterized in that the said inlet and outlet are also connected to a coaxial opening (4) extending upto the top of the said vertical column, a cylinder (5) is fitted by a retaining nut (6) on to the upper end of the said column (1), the said cylinder is provided with a coaxial opening capable of accommodating a piston (7) to be capable of transferring the pressure of the liquid media through the coaxial opening (4) to the piston extending above the vertical column (1), the top of the said piston is connected to a weight table (8) by a cap (9), the said weight table being provided with a circumferential vertical cylinder (10) having a circumferential table (11) for loading weights, the said inlet (2) is provided with multi way connector for connecting to a known hydraulic circuit (12 & 13 needle valve, 15 pump and 16 reservoir). The constructional features of the improved device of the present invention is described below: The pressure in the range 0.1-80 MPa can be measured on generated through the fluid in a hydraulic circuit connected to a solid vertical column where the pressure is applied at the bottom of the piston fitted into a matching cylinder filled with fluid and then by applying the known downward gravitational force through a set of weights stacked together in a circumferential vertical cylinder resting on top of the piston to equalize the upward thrust of the pressurizing fluid at its bottom when the piston is not at rest and the ratio of the total downward force applied through the set of weights to the accurately determined area of the piston is the pressure. Keeping a suitable clearance between the piston and the cylinder, mounted in the solid vertical column filled with the fluid and using the known downward gravitational force acting on the weight table connected to the top of the piston, a device which is efficient, inexpensive and simple in design can be made. The solid vertical column can be of Brass, Aluminum, Stainless steel, Mild steel in pure form or their alloy, preferably stainless steel and/or mild steel and the length of the column can be between 150 mm to 400 mm and preferably between 250-350 mm and the outer diameter may vary from 20 mm-100 mm preferably in the range 30-70 mm and the inner diameter can be between 2-80 mm preferably in the range 10-60 mm. The clearance between the piston and the cylinder can be between 0.001 to 0.01 mm and preferably between 0.005 to 0.008 mm and the diameter of the cylindrical piston can be between 0.5 to 20.0 mm preferably 3.0 to 10.0 mm and the diameter of the cylinder can be between 10 mm to 50 mm preferably between 15mm to 40mm and the inner diameter of the cylinder can be between 0.5-20mm and preferably 2-10.0 mm depending upon the magnitude of the generated or measured pressures. The length of the piston can be between 30-100mm and preferably between 40-60 mm and the length of the cylinder can be between 30-70mm preferably in the range of 20-50mm. The thickness of the weights used to generate the downward can be between 0.5mm to 20mm and preferably between 2mm to 15mm and the distance between the two weights (surface to surface) may vary from 0.01mm to 0.5mm and preferably between 0.05mm - 0.1mm and their diameter can vary from 50mm-350mm and preferably between 150-300mm. The piston is made up of brass, steel, tungsten carbide in pure form or their alloys and preferably steel or tungsten carbide where as the cylinder material can be of same material as that of the piston and preferably it can be of steel or tungsten carbide. The weights can be of brass, aluminum, copper, steel in pure form or their alloy preferably of steel. The weights to generate the force on the weight table may or may not have a hole at the center and diameter varying from 45mm - 90mm and preferably between 50mm to 80mm. The circumferential vertical column to stack the weights can be of outer diameter varying from 45mm to 100mm and preferably between 50mm-80mm and the length from 100mm to 350mm preferably between 120mm to 230mm the wall thickness may vary between 0.2 - 5mm and preferably between 1.5mm - 3mm and it can be made up of brass, aluminum, copper, steel in pure form or their alloys and preferably of steel. The cylinder can be retained into solid vertical column either by screwing the cylinder in to the column or by a retaining nut and preferably by the retaining nut. To keep the piston and cylinder vertical the solid vertical column is fixed vertically on a sturdy base having leveling screws for making the column vertical and hitherto the piston and cylinder. All the parts of the improved device useful for generating and measuring the static pressure of a liquid media are easily detachable for their proper maintenance. The cylindrical piston and the cylinder can be separated from the solid vertical column and similarly the weights (used to generate the force) can easily be loaded and unloaded on the circumferential vertical cylinder. The operation of the improved device of the present invention is described with reference to figures 4 & 5. The pressure P to be measured is applied to the solid vertical column (1) through an inlet (2) by a hydraulic pump (15), which transmit the liquid, the pressure of which is to be measured, and there is provided a cylinder (5) fitted by the retaining nut (6), the said cylinder being provided with a co-axial opening capable of accommodating a piston (7) in such a manner so as to be capable of transferring the pressure of the liquid media through the co-axial opening (4) to the top of the piston extending above the vertical column (1) being connected to a weight table (8) by means (9), the said weight table being provided with a circumferential vertical cylinder (10) having a circumferential table (11) for loading the weights to generate force (F). The pressure of the liquid media pushes the piston upwards and the force generated by the weight is adjusted to equalise the upward thrust of the liquid media and when the piston is not at rest than the ratio of the total downward force F to the areas Ae of the cylindrical piston gives the pressure, P = F / Ae The dial gauge (17), which is to be calibrated, is connected by means (14) to the device of the present invention. The pump is unscrewed to fill with oil by opening the valve 12 and both the valves 13 & 14 remain closed. Close the valve 12 and open the valve 13 and the piston is lifted by screwing the pump and the system was checked for its verticality. Now bring down the piston by unscrewing the pump and keep the weights on the present device to generate the pressure equivalent to the full scale pressure of the gauge which is to be calibrated. In a position when both the valves 13 & 14 are open and valve 12 is closed, the pump is screw in to lift the piston along with the weights. Close valve 13 and watch the system and the gauge under calibration for some time. If there is no fall in the pressure in a gauge under calibration and the piston of the present device is also in a float position indicating thereby that the system is leak proof. Take the reading of the gauge and note down all the weights along with their pressure value and the temperature of the present device. The total pressure of the gauge is divided into equal 8-10 parts. Open the valve 13 and bring down the piston to rest by unscrewing the pump. The required weights unloaded from the system to generate the pre-determined next lower pressure value. The same exercise is being repeated i.e. the piston of the present device is lifted and when it comes in float piston, close the valve 13 and wait for some time for pressure and temperature stabilization. Note down the reading of the dialgauge, the temperature and the weights kept on the present device. The same exercise is repeated for 8-10 predetermined pressure values. The observations are taken at the same pressure in decreasing as well as in increasing order of pressures. Three such pressure cycles were taken to calibrate one hydraulic pressure gauge. The arithmetic mean of all the three pressure values measured by the present device at a particular pressure gives the deviation in the pressure value read by the dial gauge at that particular pressure value. Number of weights used for a particular observation is the correct pressure measured by the dialgauge against the indicated pressure at that particular pressure value. The following examples are given by way of illustration and should not be construed to limit the scope of the present invention. Example 1: Calibration of the Burdon dialgauge A hydraulic pressure gauge of 80 MPa full scale pressure which is to be calibrated is connected to the device of present invention. The hydraulic pressure gauge was of 1% accuracy. The valves (13 & 14) are closed and the hydraulic pump (15) is charged by unscrewing the pump when the valve (12) is opened. Close the valve (12) and open the valve (13), piston is being lifted by screwing in the pump and the system is leveled for its verticality. The piston was brought back to its original position by again unscrewing the pump and then the valve (14) is also opened. The weight nos. 1-22 corresponding to a pressure of 80 MPa were kept on the weight table and on the.circumferential table. The pump is screwed in till the piston is lifted and then it is rotated by hand, a hydraulic gauge connected to the system was showing a pressure of 79.95 MPa. The valve (13) is now closed while the piston is still in floating position. Observed the dial gauge reading for some time. No change was found in its reading indicating thereby that the system is leak proof. Thereafter the piston was brought down by unscrewing the pump. The weight nos. 18, 19 and 20 were unloaded from the table. The piston was again lifted to float by screwing the pump and rotated, the valve (13) is closed and the gauge was showing the pressure of 69.95 MPa. Similarly the gauge has indicated the pressure values of 59.95, 49.95, 39.95, 29.95, 19.95 and 9.95 MPa with the weights 1-15,; 1-12,21;, 1-10; 1-7,21; 1-5 and 1,2,21 respectively. It was found that the maximum deviation in the indicated reading by the gauge from pressure measured by the device of the present invention is + 0.05 MPa in absolute term which is equivalent to a maximum error of + 0.5% through out the pressure range of the gauge under calibration. This is within the accuracy limit of + 1% of the gauge. Example 2: Calibration of the Dead weight Piston Gauge A dead weight piston gauge of 80 MPa, which is to be calibrated, is connected to a device of the present invention. By closing the valves (13 & 14) the pump was unscrewed to charge it and then open the valve (13) only with all others valves closed. The piston was lifted by screwing the pump and the system was leveled for its verticality. In a condition when the valve (14) was opened the piston of the device under calibration was also lifted using the same pump. It was also adjusted for its verticality. Now the gauge was loaded with all the weights (A-K) to generate the full scale pressure value of 80 MPa and the device of the present invention was loaded with the weights 1-22 to generate nominal pressure of 80 MPa. The valve (13 &14) were closed and both the system were kept floating for some time for leak checked. The fall of the piston of the present device is observed while opening the valve (14). It is found that the piston was going up indicating thereby the force applied on the device is less. The valve (14) was closed and an additional 1 gm was kept on the present device. Both the systems were in float position and 5 min was given for the temperature and pressure stabilization. The piston of the present device remains at the same position irrespective of the valve (14) was either closed or opened. The value of the pressure measured by the gauge was 80.25 MPa. The valve 13 and 14 were opened and both the systems were brought down to rest by unscrewing the pump. The weights 18,19,20 from the present device and A from the gauge were unloaded. The piston of both the systems were lifted to float by screwing the pump and rotated, the valve (13) is closed and the gauge was giving the pressure of 70.25 MPa. Similar observations were taken by unloading the weights B,C,D,E,F and G from the gauge and correspondingly the weights 1-15 and 1 gm, 1-12,21 and 1.5gm; 1-10 and 1gm, 1-7,21 and 5gm; 1-5 and 2gm; 1,2,21 and 3.5gm respectively were used on the present device to achieve the condition of equilibrium i.e. the piston of the present device remains at the same position irrespective of the valve 14 was either closed or opened. The pressures measured by the gauge were 60.20, 50.25, 40.20. 30.15, 20.10 and 9.95 MPa. It was found that the maximum deviation in the pressure measured by the gauge from the pressure measured by the present device is 0.20 MPa in absolute term which is equivalent to a maximum error of 0.5% through out the pressure range of the gauge under calibration. This is within the accuracy limit of 1% of the gauge. The following are the main advantages of the device of the present invention: 1. The device can be used for calibration. The device can generate, measure and calibrate pressure with an accuracy of + 0.03% over a pressure range of 0.1-80 MPa. As it uses very small volume of the oil between the device under calibration and the piston and hence equilibrium both in temperature and pressure is achieved with short interval of time. 2. The device is portable and can be used for at site calibration. 3. The device is reliable, easy to use and economical which will find its use in many industries for upgradation and quality control of their product. claim: 1. An improved device useful for calibrating hydraulic pressure gauges which comprises a solid vertical column (1) having an inlet (2) for passing the liquid media the pressure of which is to be measured and an outlet (3), the outlet is connected to a hydraulic pressure gauge (17) to be calibrated, through a fine needle valve (14), characterized in that the said inlet and outlet are also connected to a coaxial opening (4) extending upto the top of the said vertical column, a cylinder (5) is fitted by a retaining nut (6) on to the upper end of the said column (1), the said cylinder is provided with a coaxial opening capable of accommodating a piston (7) to be capable of transferring the pressure of the liquid media through the coaxial opening (4) to the piston extending above the vertical column (1), the top of the said piston is connected to a weight table (8) by a cap (9), the said weight table being provided with a circumferential vertical cylinder (10) having a circumferential table (11) for loading weights, the said inlet (2) is provided with multi way connector for connecting to a known hydraulic circuit (12 & 13 needle valve, 15 pump and 16 reservoir). 2. An improved device useful for calibrating hydraulic pressure gauges substantially as herein described with reference to figures 4 and 5 of the drawing accompanying this specification and the examples.

Documents:

964-del-1995-abstract.pdf

964-del-1995-claims.pdf

964-del-1995-correspondence-others.pdf

964-del-1995-correspondence-po.pdf

964-del-1995-description (complete).pdf

964-del-1995-drawings.pdf

964-del-1995-form-1.pdf

964-del-1995-form-2.pdf

964-del-1995-form-4.pdf

964-del-1995-form-9.pdf

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