Dead Weight PDC PDF

Preview

Summary

PROCESS DYNAMICS AND CONTROL Experiment Number # 2 Calibration of Pressure Gauge Using a Dead Weight Tayyiba Zafarullah (2012-CH-61) Syed Hassan Zoraiz (2012-CH-71) Sadia Qamar (2012-CH-73) Haris Rana (2012-CH-85) Usama Jawaid (2012-CH-111) Date of the Experiment: October 8, 2015. Due Date: October ...

Description

Accelerat ing t he world's research.

Dead Weight PDC Mohsin Akbar

Related papers

Download a PDF Pack of t he best relat ed papers 

Brakes, Brake Cont rol and Driver Assist ance Syst ems Funct ion, Regulat ion and Component s … Mario Alaniz

Chapt er 18 Inst rument calibrat ion vinay sagar 240871 Deut sch Technical Terms Charles Conconi

PROCESS DYNAMICS AND CONTROL Experiment Number # 2

Calibration of Pressure Gauge Using a Dead Weight Tayyiba Zafarullah Syed Hassan Zoraiz Sadia Qamar Haris Rana Usama Jawaid

(2012-CH-61) (2012-CH-71) (2012-CH-73) (2012-CH-85) (2012-CH-111)

Date of the Experiment: October 8, 2015. Due Date: October 15, 2015.

Department of Chemical Engineering, University of Engineering and Technology, Lahore.

ABSTRACT In this experiment we calibrate a pressure gauge using the apparatus “Dead Weight Calibrator”. This apparatus works on the principle of Pascal’s Law. A change in pressure at any point in an enclosed fluid at rest is transmitted undiminished to all points in the fluid. A dead weight calibrator uses the weight pressure which are predetermined to calibrate the pressure gauge. By comparing readings from pressure gauge and weights that are placed on the apparatus one can easily determine the accuracy of the pressure gauge. Calibration defines the accuracy and quality of measurements recorded using a piece of equipment. Over time there is a tendency for results and accuracy to ‘drift’ particularly when using particular technologies or measuring particular parameters such as temperature and pressure. To be confident in the results being measured there is an ongoing need to service and maintain the calibration of equipment throughout its lifetime for reliable, accurate and repeatable measurements. Dead weight calibrator is one method to calibrate the pressure gauge. There are numerous ways to accomplish this. It is one of the old and basic method of calibration.

INTRODUCTION Deadweight testers can be calibrated using either the “Fundamental” or “Calibrated” methods to measure the pressure and effective area. A fundamental calibration involves having the effective area of the gauge determined using only measurements of the SI base units (e.g. mass, length) plus a suitable model. A calibrated calibration has the effective area determined via calibration against a gauge for which the effective area or generated pressure is already known. The base formula for deadweight testers is P = F/A. Where F equals the force or amount of weight, A equals the area over which the force is applied and P equals the resulting pressure. Two general types of deadweight testers exist. First, a closely fit piston and cylinder with weights applied to the piston. Second, a precision ceramic ball within a tapered nozzle, with weights applied to the ball. Many different types of deadweight testers are available. In order for a user to select the correct tester, several aspects of the task to be performed should be considered. The first item that should be considered is the required accuracy of the tester. Accuracy of most instruments is expressed as "percent of full scale". Another important consideration is the test fluid. Since the test fluid will enter the pressure sensing element of the instrument being tested, the test fluid must be compatible with the process fluid to which the instrument will be attached. Otherwise, all instruments must be cleaned after testing, an expensive operation. The most common test fluid is instrument grade oil. Where usable, oil provides an outstanding combination of corrosion resistance with lubrication of the close fitting piston and cylinder. Distilled water provides an excellent test fluid that is inert to most process fluids. Clean dry air or nitrogen gas; however, eliminates the problem altogether. The user must be particularly careful that the tester selected is designed for operation with the intended test fluid. The third consideration is the pressure range. A survey should be made of the pressure range of all instruments to be tested. The tester should produce pressures in excess of the highest instrument to be tested. Deadweight testers are available with accuracy ranging from +/- .015% to +/- 0.1% of indicated reading. This range of accuracy can be appropriately matched with the various types of process

instrumentation (and their respective performance) which are to be calibrated. Ideally, a calibration device should be 4x as accurate as the test device. However, the improved performance in process instrumentation has resulted in a 2:1 ratio as being minimally acceptable. Piston and cylinder testers are most responsive within the upper 90% of the operating range. If a broad range of pressures are to be tested, the user should consider dual range testers which contain piston and cylinders of more than one size. A second consideration is two types of testers, pneumatic at low pressures and oil or water at higher pressures. A final consideration should be the task to be performed. If most of the instruments to be tested are fixed in place, such as recorders, the portability of the tester is important. If many instruments are to be tested, dual column testers that change test range quickly are helpful. If many technicians will use the tester, the tester should be rugged and relatively independent of operator technique. High performance tasks, such as testing of instruments at manufacture, require custom designed testers. Types: In general, there are three different kind of DWT's divided by the medium which is measured and the lubricant which is used for its measuring element: 1.

Gas operated gas lubricated PCU's

2.

Gas operated oil lubricated PCU's

3.

Oil operated oil lubricated PCU's

Principle: Dead weight calibration unit works on Pascal’s Law. Pascal’s law or the principle of transmission of fluid-pressure states that pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid such that the pressure variations (initial differences) remain the same. Or Pascal’s principle is defined as A change in pressure at any point in an enclosed fluid at rest is transmitted undiminished to all points in the fluid. ∆� = ��∆ℎ

∆� = Is the hydrostatic pressure or the difference in pressure at two points within a fluid column, due to the weight of the fluid.

∆� = Is the height of fluid above the point of measurement, or the difference in elevation between the two points within the fluid column. Pascal’s principle applies to all fluids, whether gases or liquids Applications:   

Hydraulic Jack. Braking system. Scuba divers.

Braking system: The hydraulic brake is an arrangement of braking mechanism which uses brake fluid, typically ethylene glycol, to transfer pressure from the controlling mechanism to the braking mechanism The most common arrangement of hydraulic brakes for passenger vehicles consist of the following:     

Brake pedal or lever A pushrod (also called an actuating rod) A master cylinder assembly containing a piston assembly. Reinforced hydraulic lines Brake caliper assembly

Operation: In a hydraulic brake system, when the brake pedal is pressed, a pushrod exerts force on the piston in the master cylinder, causing fluid from the brake fluid reservoir to flow into a pressure chamber through a compensating port. This results in an increase in the pressure of the entire hydraulic system, forcing fluid through the hydraulic lines toward one or more calipers where it acts upon one or two caliper pistons sealed by one or more seated O-rings (which prevent leakage of the fluid). The brake caliper pistons then apply force to the brake pads, pushing them against the spinning rotor, and the friction between the pads and the rotor causes a braking torque to be generated, slowing the vehicle. Heat generated by this friction is either dissipated through vents and channels in the rotor or is conducted through the pads, which are made of specialized heat-tolerant materials such as kevlar or sintered glass.

Subsequent release of the brake pedal/lever allows the spring(s) in the master cylinder assembly to return the master piston(s) back into position. This action first relieves the hydraulic pressure on the caliper, then applies suction to the brake piston in the caliper assembly, moving it back into its housing and allowing the brake pads to release the rotor. The hydraulic braking system is designed as a closed system: unless there is a leak in the system, none of the brake fluid enters or leaves it, nor does the fluid get consumed through use. Hydraulic Fluids: Hydraulic fluid(s), also called hydraulic liquid(s), are the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on waste, mineral oil or water. Examples of Equipment using Hydraulic Fluids: Excavators and backhoes, hydraulic brakes, power aircraft, lifts, and industrial machinery.

steering systems, transmissions, trucks,

Functions and Properties: The primary function of a hydraulic fluid is to convey power. In use, however, there are other important functions of hydraulic fluid such as protection of the hydraulic machine components.

Composition: Base Stock: Natural oils such as rapeseed (also called canola oil) are used as base stocks for fluids where biodegradability and renewable sources are considered important. Other base stocks are used for specialty applications, such as for fire resistance and extreme temperature applications. Some examples include: glycol, esters, organophosphate ester,polyalphaolefin, propylene glycol, and silicone oils. Other components

Hydraulic fluids can contain a wide range of chemical compounds, including: oils, butanol, esters (e.g. phthalates, like DEHP, and adipates, like bis(2-ethylhexyl) adipate), polyalkylene glycols (PAG), organophosphate (e.g. tributylphosphate), silicones, alkylated aromatic hydrocarbons, polyalphaolefins (PAO) (e.g. polyisobutenes), corrosion inhibitors (including acid scavengers), anti-erosion additives, etc. Biodegradable Hydraulic Fluids: Environmentally sensitive applications (e.g. farm tractors and marine dredging) may benefit from using biodegradable hydraulic fluids based upon rapeseed (Canola) vegetable oil when there is the risk of an oil spill from a ruptured oil line.

Calibration: Calibration is process of finding a relationship between two unknown (when the measurable quantities are not given a particular value for the amount considered or found a standard for the

quantity) quantities. When one of quantity is known, which is made or set with one device, another measurement is made as similar way as possible with the first device using a second device.The measurable quantities may differ in two devices which are equivalent. Proper calibration of an instrument allows people to have a safe working environment and produce valid data for future reference.

Why Do We Need to Calibrate?   

To ensure readings from an instrument are consistent with other measurements. To determine the accuracy of the instrument readings. To establish the reliability of the instrument i.e. that it can be trusted.

Methods of Calibration: Different calibration methods are used for calibrating different instruments. Methods for calibration of pressure and temperature measuring instruments are discussed below: Pressure Balances: The reference standards used for the calibration of pressure measuring instruments are pressure balances which are also referred to as deadweight testers. They measure the physical quantity pressure with respect to its definition as force per unit area. From that point of view, pressure balances are considered as primary standards. Pressure balances are the most accurate instruments for the calibration of electronic or mechanical pressure measuring instruments Pressure Cylinder Systems: To allow calibration in different pressure ranges, different piston/cylinder systems can be selected such as pneumatic systems for pressures from 1 bar up to typically 100 bar. For higher pressures of up to typically 1000 bar or in special cases up to 5000 bar, hydraulic systems are used. In this case, the oil used serves simultaneously as a lubricant for the piston/cylinder unit and optimizes the running characteristics. Similarly, for calibration of temperature measuring instruments some methods used are: Fixed Point Cells: Cells in which fixed points of high-purity substances can be set are more suitable for the calibration of temperature measuring instruments. As a function of temperature and pressure, substances exist in the three classical physical states − solid, liquid and gas. Phase transitions, for example from solid to liquid, can be used for calibration since at constant pressure, the temperature of a substance remains constant until the phase transition is complete, i.e. until all the ice of a water/ice mixture has become liquid.

Triple Point Cells: Apart from the phase transitions between two states of matter, for some substances triple points are also used as fixed points for calibration. At the triple point, the three conventional phases (solid, liquid and gas) of a high-purity substance are present in a thermal equilibrium. Triple points can be set very accurately and reproduced at any time. Moreover, they can be maintained over a longer period of time. Significance of Calibration: Calibration defines the accuracy and quality of measurements recorded using a piece of equipment. Over time there is a tendency for results and accuracy to ‘drift’ particularly when using particular technologies or measuring particular parameters such as temperature and humidity. To be confident in the results being measured there is an ongoing need to service and maintain the calibration of equipment throughout its lifetime for reliable, accurate and repeatable measurements. The goal of calibration is to minimize any measurement uncertainty by ensuring the accuracy of test equipment. Calibration quantifies and controls errors or uncertainties within measurement processes to an acceptable level. In summary, calibration is vitally important wherever measurements are important, it enables users and businesses to have confidence in the results that they monitor record and subsequently control. Remarks:  Calibration checks the accuracy of the instrument and it determines the traceability of the measurement. In practice, calibration also includes repair of the device.  Depending on the type of the instrument and the environment in which it is being used, it may degrade very quickly or over a long period of time. The bottom line is that, calibration improves the accuracy of the measuring device. Accurate measuring devices improve product quality.

Difference between Precision & Accuracy: Accuracy and precision are used in context of measurement.

Accuracy

Precision

Accuracy refers to the closeness of a measured value to a standard or known value. For example, if in lab you obtain a weight measurement of 3.2 kg for a given substance, but the actual or known weight is 10 kg, then your measurement is not accurate. In this case, your measurement is not close to the known value.

Precision refers to the closeness of two or more measurements to each other. Using the example above, if you weigh a given substance five times, and get 3.2 kg each time, then your measurement is very precise. Precision is independent of accuracy.

Remarks: Accuracy is the degree of correctness, while precision is how strict that correctness is (or is not). Accuracy can be improved by taking repeat measurements and taking an average. Conversely, precision cannot be improved by taking repeated measurements but it is impossible to quantify precision without experimental repeats. atm to Kg/cm2 Conversion: In 1954 the 10th Conférence Générale des Poids et Mesures (CGPM) adopted standard atmosphere for general use and affirmed its definition of being precisely equal to 1,013,250 dynes per square centimeter (101325 Pa). This value was intended to represent the mean atmospheric pressure at mean sea level at the latitude of Paris, France, and does reflect the mean sea level pressure for many industrialized nations that are at broadly similar latitudes. A kilogram-force per square centimeter (Kgf/cm2), often just kilogram per square centimeter (kg/cm2), or kilo-pound per square centimeter is a unit of pressure using metric units. Its use is now deprecated; it is not a part of the International System of Units (SI), the modern metric system. Still, kg/cm2 remains active as a measurement of force primarily due to older torque measurement devices still in use. The kilogram-force (Kgf or KgF),), is a gravitational metric unit of force. It is equal to the magnitude of the force exerted by one kilogram of mass in a 9.80665 m/s2 gravitational field. �=

Here,

�

m= mass = 1 Kg a= acceleration= 9.8 m/s2 �= �� �� �

∗ .

�� ∗

=

�∗

�

�� �� ( ��� ) = . � �=

� �

� �� =

�= .

�

���

��� = � � Apparatus:

.

�

=

�

= .

=

.

∗ ∗

��

��

��� �

��� �

1) The dead weight tester 2) Standard weights 3) Precision gauge if possible Procedure: 1) Close the needle valve 5, 6, 6’ while the valve 2 is kept open. Pour the hydraulic oil through the cup 3 into the tester to fill it. 2) Rotate the piston outwards to suck oil in it. 3) Close the valve 2 and open valve 5 and 6 after mounting the gauge to be tested on nipple provided for pressure gauge. In case a precision gauge is available mount it on the other position and open valve 6’ as well. 4) Put the standard weights on the pan. 5) While rotating slowly the weight pan turn the handle of the cylinder piston to transmit the pressure towards the weight and pressure gauges. 6) When the fluid pressure is equal to the pan, it is lifted as well as the gauge indicates the corresponding pressure. Note that the weight should be lifted only to the red line. Beyond white line it would be ineffective. 7) Mark the point on the gauge, release the pressure, put the next weight and repeat the same procedure for the next calibration. Precautions: 1) The relevant valves should be promptly closed and opened as mentioned above, otherwise the hydraulic oil might spill outside or on the face of the user. 2) The standard weights should be placed with ease not to damage the apparatus. Conclusion: Calibration is the set of operations that establish the relationship between the values of quantities indicated by a measuring instrument and the corresponding values realized by standards. The result of a calibration allows for the determination of corrections to be made with regards to the indicated values. Based on the experimental results obtained a deviation in the calibrated reading was compared to the theoretical values. Therefore the pressure gauge on the downwards pressure was observed to

be not appropriate for very low pressure levels; Especially when the supplied pressure is low & incapable of lifting the applied load. Results and Discussion: TABLE 1: Calibration of Pressure Gauge in Ascending Order Run 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Dead Weight Pressure (atm) 1 2 3 4 5 6 7 12 17 22 27 37 47 57

Gauge Pressure (atm) 0 0.5 1 2 3 4 5 10 16 22 27 37 47 57

TABLE 2: Calibration of Pressure Gauge in Descending Order Run 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Dead Weight Pressure (atm) 57 47 37 27 22 17 12 7 6 5 4 3 2 1

Gauge Pressure (atm) 57 47 37 27 22 16 10 5 4 3 2 1 0.5 0

The results show that Pressure Gauge is well calibr...