Monthly Engineering Horizons

REVIEW OF LEAKS AND LEAK DETECTION METHODS

REVIEW OF LEAKS AND LEAK DETECTION METHODS

May 07
04:47 2020

By: Ch. Tariq Sattar, Pakistan Vacuum Society, G.P.O. Box 1880, Islamabad, Pakistan

Abstract

There is no vacuum system / container provides an absolute barrier to the passage of all gases. Containers/chambers are subject to permeation and are frequently penetrated by small holes, cracks or porosity, even when carefully designed and proficiently fabricated. With careful inspection of a given component or system and evaluation of the design, operating and maintenance procedures can be developed to minimize the effect of atmospheric leakage; when leakage is a problem, proper application of diagnostic leak detection method can minimize the time and cost of correction of the leakage. A primary consideration for efficient vacuum leak detection procedures is to distinguish real leaks from virtual leaks. The leak detection procedures are specially adapted to vacuum conditions. Detection of leaks in a vacuum enclosure can be completed according to the philosophy that is (a) all leaks detected are due to unacceptable defect which must be eliminated , or (b) A maximum acceptable leak level can be specified, which if not exceeded, will permit successful performance of the desired process inside the vacuum enclosure. This review is to discuss basic techniques and methods for systematically identifying real leaks with equipment most frequently available to the vacuum technologist.

Introduction

Like most technical fields, Leak Detection has a rapid growth since 1940 during World War II. The development of the coating and electronic industries and increased activities in space exploration enhanced the development of new equipment and processes requiring controlled environments that had to be free of significant leakage.

The philosophy of the leak test procedure directly influences the overall leak detection operation. Once a philosophy of leak detection is correctly/soundly established, the procedure developed depends upon  the leak testing equipment available, the object to be tested and its special characteristics, i.e., shape, size, material of construction, fabrication methods, history  and cleanliness and in many cases the ambient atmosphere in which the tests are to be performed.

Ultimate success of the procedure and test depends upon accurate interpretation of observations resulting from the test procedure. It must include consideration of the character of the leaks possible, response of the leak detection equipment and a thorough understanding of the residual or ambient gas interferences to identification of gases entering the vacuum system through a leak.

An introduction to the understanding of the fundamentals of leak detection, the nature of leaks and the special terminology used in leak measurement are presented. A brief introduction of variety of conventional leak detection methods is also discussed.

Leak

        We can distinguish the leaks in two types i.e. real leak and virtual leak.

Real Leak

An unintended crack, holes are porosity in the wall of a container that allows the entrance or escape of gas or fluid is called a Real Leak. Leak caused by imperfect or too-thin sections of the containing wall represent another class of leaks. For example in a plastic bottle, a particular area may leak. Microscopic examination will show a crazing effect – many minuscule “cracks” rather than one or two holes. Still another class of leak consists not of holes or cracks in the usual sense. Instead, the molecular structure of the containing wall itself is arrayed in such a way as to permit gas diffusion through the wall i.e. called permeation. The gases entering the vacuum system from external real leaks are easily identifiable by the air components, or a known test gas

Virtual leak

It is not really a leak but is the resemblance of a leak due to the evolution of gas or vapor within the system. The sources of gases evolved internally, except those originating from the pumps are identified as “outgassing” and virtual leaks. The primary sources of outgassing are desorption of atmospheric gases and contamination from surfaces and bulk materials of the vacuum system.

Small volumes of gas trapped at atmospheric pressure inside the vacuum envelope which communicates with the vacuum system by minute crevices, cracks, groves etc. Since these gases are usually trapped atmospheric gases, their mass spectra would appear to be air, thus, the name “VIRTUAL” leak. Lewin has shown the response time t for pumping out nitrogen from a tubular, capillary, idealized leak to be

Where   L = capillary length (cm)

                D = capillary diameter (cm)

        If L = 2 cm and D = 1×10-4 cm, t  is only one second, therefore, unless capillary like paths terminate in a  larger cavity or contain molecules of significant desorption activation energy, they do not produce virtual leaks of any significance.

  Figure showing; 

1.       Pressure rise in the presence of a real leak

2.       Pressure rise due to gas from the chamber walls

3.       Both effects together

 

 

 

 

 

  Sources of leaks

Leaks in newly manufactured products are most commonly caused by imperfect joints or seals by which various parts are assembled to form the finished product. Typically, these include welds, brazed joints, soldered joints, glass-to-metal seals, ceramic-to-metal seals, O’rings and other gaskets, compound sealed joints, etc. Very thin sections of the containing wall which cannot prevent gas diffusion through numerous microscopic “Cracks” is another source of leak.

  1. Characteristics of Leaks

Experimental flow of gases through leaks has been studied by Nerken [1]. Flow follows predictable relationships, which are a function of the geometry of the leak, molecular weight, temperature of the gas and a pressure difference across the leak. The Knudsen [2] equation     for    molecular   flow  through a flow

 path aperture of “zero” length is;

                         (1)                          

Where Q is the gas flow (quantity / s), R is the gas constant, T is the absolute temperature, M is molecular weight, A is the cross sectional area of flow path, P1 is upstream pressure, P2 is downstream pressure, and a is transmission probability which is geometry dependent.

When the pressure in the vacuum system P2 is much lower than P1 (usually atmospheric), P2 can be neglected, therefore QaK1P1 (K1 is a constant depending upon gas and geometry, and is for molecular flow equal to the non-pressure factors to the right of the equals sign above.) K1=11.5×10-3 m3/s.cm2 for air through an orifice at room temperature. Where viscous laminar flow exists, the Poiseuille flow equation through a circular tubular pore is given by;

Where Q is the gas flow (quantity /s), d is nominal diameter of flow path, Pa is average pressure in flow channel, h is gas viscosity, l is length of flow channel, P1 is upstream pressure, and P2 is downstream pressure. Again, when P2 (the pressure in the vacuum chamber), is much lower than P1, P2 can be neglected, then Q a K2P1 (K2 is a constant depending upon gas and geometry). If these two equations are plotted in a log-log graph as shown in Fig. 1, the slope of Q versus P for molecular flow is 1, while Q versus P for viscous flow is 2. Since flows in leaks seldom reach flow rates high enough to reach conditions of turbulent or choke flow, this area is not usually included [3].

The mean free path is important in leak testing since it determines the type of gas flow that will occur through leaks or other passageways traversed by tracer or pressurizing gases. There are three types of gas flow considered here; viscous flow, transition flow, and molecular flow. The variables which control the types of gas flow that occurs in leaks are the molecular weight of the gas, viscosity of the flowing gas or gas mixture, pressure difference causing the flow, absolute pressure in the system, temperature of the flowing gas or gas mixture, length and radius of leak paths. The mean free path can be calculated from the pressure, temperature, and molecular properties by means of ;

Where l(mfp) is the mean free path under static pressure, n is gas viscosity (Pa s), P is absolute pressure of gas (Pa), T is absolute gas temperature (K), and M is the molecular weight of gas (g/mol).

As a convenient calculation guide, the mean free path of air at room temperature    (22°C) is given by;

                l(mfp, cm)= 6.8×10-3 / P    (Pa)                (4)

 One criterion that determines the mode of gas flow through leaks, given in terms of the mean free path length l and the orifice radius r, are as follows: (a) When the ratio l/r is less than 0.01, the gas flow is viscous; (b) when the ratio l/r has values between 0.01 and 1.00, the gas flow is transitional; (c) when the ratio l/r is greater than 1.0, the gas flow is molecular. Figure 2 shows the general relationship of flow-type to gas pressure and radius of leak channel.

Another criterion that determines the mode of gas flow is given in terms of pressure. At low pressures, where collisions between gas molecules and the wall predominate, flow is molecular.

Fig. 1:    Idealized composite gas flow vs. pressure characteristic for vacuum leak

Fig. 2:    Flow characteristics as a function of leak radius and pressure

If most collisions are between gas molecules, which is true at higher pressures, there is viscous flow. Transitional flow occurs under conditions intermediate between viscous and molecular flow.

The characteristic flow of gases depends upon molecular weight and viscosity and results in a change in flow rate if the gases are changed, i.e., when changing from an air leak to another gas the total leak rate may change significantly. The difference in flow rate of helium QHe and Qair  for a molecular leak would be determined by;

    If the leak is laminar viscous:

To be able to convert the measured helium leakage rate to an equivalent air leakage rate, the type of flow must first be identified. The following are predominate flow modes in leaks of various sizes: “Choke or turbulent ≥ 100         Pam3s-1; Viscous – Laminar for 10-2 – 100; Transitional for 10-6 – 10-2 & Molecular flow for < 10-6 Pam3s-1. If a flow rate has been identified as viscous for one gas, the viscous flow for any other gas may be determined using the expression given in the equation;

                                              (5)

Where Q1 is the flow rate for gas 1, Q2 is the flow rate for gas 2, n1 is the viscosity for gas 1, and n2 is the viscosity for gas 2.

  1. Leak Detection and its Need

The basic functions of leak detection are the location and measurement of leaks in sealed products and systems which must contain or exclude gases / fluids i.e. the test piece may be at vacuum (below atmospheric pressure) or pressure (above atmospheric pressure). These functions are carried out through the use of standard leak test techniques, which are usually selected according to the configuration of the part to be tested, the economics of the test, and the nature of the system. These techniques depend on the use of a tracer gas or liquid passing through a leak and being detected on the other side and are applicable to most of the leak detection methods.

Need

For all practical purposes, it is impossible to manufacture a sealed container or system that can be guaranteed to be leak proof without first being tested. The fundamental question in leak detection is that; what is the maximum acceptable leak rate consistent with reasonable performance life of the product? Each manufacturer of a product should know his problem and requirement for life of the product to be save.

Leak Location

It is an approach used to find the exact location or pin point of individual leaks. Leak location is very important to identify the sources of leaks in order to facilitate repair, remanufacture or even redesign in some cases.

Leakage Criteria and Philosophy

It is necessary that test criteria be established before finalizing the procedures adequate to meet the criteria. The philosophy of any leak is a flaw that must be corrected”. If the philosophy that a specific leak level which cannot be exceeded is established, the capability of the system detection equipment must be capable of reliability identifying leaks of this size or larger. Clearly specifying test criteria is a requirement to insure adequate economical, practical leak detection procedures.

  1. Role of Gas Characteristics in various Leak Detection Methods

        Some phenomena associated with gases entering into a chamber through a leak path are useful in detecting and measuring leaks. Some of the important property based leak detection methods are given below:

  1. Mass spectra of tracer gas
  2. Thermal conductivity
  3. Ionization probability
  4. Glow discharge
  5. Radioactive Isotopes
    1. Mass Spectra of Tracer Gases

To find the existence of a leak and its location, the mass spectra of a gas is most useful. The mass spectra of a typical vacuum system possessing an air leak (Fig. 3a), produced by a mass spectrometer indicates the presence of several principal components at mass to charge the air leak (Fig. 3b) shows significant changes in ratios (m/e) of O2, N2, CO, H2O, HO etc.

An m/e spectra from this same vacuum system without indicating the reduction in partial gas pressures in the vacuum of the air components oxygen, nitrogen and carbon dioxide. When Nitrogen is added to the vacuum system through the same leak (Fig.3c results) shows the increase in m/e = 28 and 14 but the absence of an increase in m/e = 32 indicating the difference in Spectra which might be expected if an atmospheric leak were exposed to a nitrogen probe gas. The simplistic comparison of spectra noted above is indicative of the use of such spectra in leak detection.

    Fig. 3: Typical mass spectra – leak effects

A specially constructed mass spectrometer which is sensitive to helium is the familiar Helium mass spectrometer leak detector.

  1. Thermal Conductivity

Thermal conductivity of gases can be used to sense change in gas concentration leak by another gas. The thermal conductivity of gas is an approximately linear function of pressure at pressures between 0.13 Pa and 133 Pa. the thermal conductivity of the gas is largely independent of pressure above approximately 2.7×103 Pa.

If a sensitive, heated element is exposed to vacuum environment at pressures between 7×10-2 Pa and 2.7×103 Pa the collision of gas molecules with the heated element permits the gas molecules to carry away significant amounts of heat. An equilibrium condition develops which then causes an equilibrium temperature of the heated element if the pressure remains constant. The average gaseous thermal conductivity changes if a gas of thermal conductivity different from that of air is introduced to an air leak path. The resulting change in concentration of residual gas components alters the average thermal conductivity of the gas which alters the equilibrium temperature of the heated element. If the temperature of the heated element can be measured, a method of sensing leaks is available. The familiar thermocouple and Pirani Vacuum Gauges are examples which can be used to detect leaks. Fig-4 shows the response of various gases thermal conductivity.

Fig. 4:    Typical calibration curves for Pirani and thermocouple vacuum gauges. (From vacuum System Design, N.T.M. Dennis and T.A. Heppell, Chapman & Hall Ltd, London, 1968)

  1. Ionization Probability

In this method molecular specie is impacted with a stimulus which causes ionization. From ion-electrons pairs ions are collected which give response directly proportional to Residual gas used in vacuum system. Efficiency of ionization for various gases compared with that of air is given as;

Oxygen                               =  0.8

Helium                                =  0.14  –  0.21

Hydrogen          =  0.4  –   0.5

Carbon Monoxide        = 1.03  –  1.06

Carbon Dioxide             = 1.3  –  1.6

Water vapor                      = 1.1 –  2.0

Argon                                  = 1.2  – 1.5

Air                                        = 1.0

If an air leak is replaced by He, the magnitude of the response of the ionization gauge would drop to 14% of the initial indication for air due to ionization probability. If the leak was molecular flow controlled the helium would flow faster by a factor of about 2.7 thus the ion gauge would show a net current of about 0.38 of that due to air. Familiar Alphatron can be used as leak detection device as it ionizes residual gases by Alpha particles from radioactive materials in vacuum systems.

  1. Glow Discharge/ Tesla Coil

When an electric field is applied to a gas or mixture of gases it yields a glow discharge color, which is dependent upon the composition and pressure of the gas within certain limits, for example, usually between 5×10-2 and 1.5×101 mbar. The glow discharge may be produced by any mechanism which excites the gas to exit visible light. High voltage and radio frequency electric fields are two methods of producing the glow discharge. The familiar Tesla Coil may be used to produce visible glow in glass vacuum systems to pinpoint leak paths through the walls of container as well as excites the residual gas. The current from the Tesla Coil must be controlled to prevent puncturing the glass wall.

  1. Radio-Active Isotopes

Radio-active isotopes may be used as test gas in leak detection. These gases are either naturally radioactive or may be made by exposure to nuclear reaction e.g. Kr85 and Ar41. These gases are normally used in such systems where residual gases already contain the radioactive gases like them. Hermetically sealed relays and transistors are used as vacuum equipment in this method.

Apart from the abovementioned methods, some other techniques are also used for leak detection in vacuum technology as given below.

  1. Acoustical method (Inside-out testing)

Here sonic (or ultrasonic) energy generated by expansion of gas through orifice is noted.

Anyone referring to a large leak as a “whistler” is noting the acoustical energy produced. Leaks of high pressure lines and ductwork are detected upto ~10-3 stdccs-1. by this method.

Fig. 5:   Ultrasonic leak test setup

  1. Pressure decay / Vacuum decay

It requires only a pump or compressor and a pressure gauge. System is given a limited time after pressurizing. Here sensitivity is directly proportional to wait time and range ~10-4    stdccs-1. Major drawback of this method is inaccuracy due to temperature fluctuations and virtual leaks.

  • Halogen Leak Detection

In this method system is pressurized with a gas containing an organic halide such as Freon. Then suspected leak is pointed out with a sniffer probe sensitive to test gas. Halogen leak detectors may be used for vacuum systems and locate leaks upto 10-7 std cc s-1.

Fig. 6:  Halogen leak detector

  1. Dye Penetration

 In this method low viscosity fluid is painted on one side of suspected leak site of welds or cracks and detected on the far side after some time. Another technique is that spray the penetrant fluid on the suspected area and then clean that area after a suitable time with appropriate cleaner and then paint another fluid called developer which will extract the penetrant on the surface if there is any crack or hole which show the exact location of the fault. Sensitivity may be as high as 10-6 std cc s-1.

   Advantages

  • Conceptual simplicity
  • Low cost
  • Inherent record keeping

  Disadvantages

  • Limited applicability
  • Limited time response
  • Part can not be washed clean enough for use.

Fig. 7:    Dye penetrant testing

  1. Bubble Testing

This method consists of pressurizing a system, then either dipping it in to water and looking for stream of bubbles from leak or applying a soap solution to suspected point.

Advantages

  • Irrelevant leaks are ignored
  • Minimum cast and complexity
  • Modest worker’s skill
  • Increasable sensitivity (~10-4 std cc s-1) can be achieved by using proper light, dark background and fluid of low surface tension

  Disadvantages

  • Total leakage cannot be measured
  • Requires continuous attention of operator
  • Wetting of system requires drying before use

 

Fig.  8:   Bubble testing simplified

  1. Physical Properties of Gas for Leak Detection

Some characteristics of gas which can be use for leak detection process are (1) Molecular Velocity (2) Vapor pressure and (3) Molecular diameter. The average velocity of the gas molecules can show the time required for the test gas to reach at a sensing location. Average velocity Va is

           

Where R = gas constant; T = absolute temperature (°K) M = Molecular weight (in gms).

Some typical values of average molecular velocity at room temperature (20°C).

Hydrogen                              1.75×103 m/s

         Helium                                     1.24×103 m/s

Air                                           4.64×102 m/s

Nitrogen                                4.71×102 m/s

        Oxygen                                    4.40×102 m/s

        Argon                                     3.94×102 m/s

From above it can be seen that the Helium appears in an evacuated chamber is very rapid. If helium introduced into an evacuated chamber having size 100 cm in diameter from a leak is transported very rapidly to all points in the chamber in 8.0×10-4 s if molecular flow conditions exist. The vapor pressure, temperature and geometrical location of a material in a vacuum chamber determine the rate with which the material evaporates.

The average molecular size is of interest when considering the leak path size in the vacuum envelope. The concept of a rigid spherical molecule is used, however, in considering gas viscosity and mean free path. It has been shown that

          

Where d = molecular diameter in cm

       r  =  gas density in gm/cm3

      Va = average molecular velocity in   cm/sec

        n  =  number of molecules per  cubic centimeter

         h = gas viscosity in poise

  1. Helium Mass Spectrometer Leak Detection

Helium mass spectrometer leak detection is the method most often discussed when the topic of leak measurement comes up. It can be applied to both techniques i.e. detector- probe mode and tracer probe mode. This method is good to10-12  mbar L s-1 and is capable of finding leaks as large as 1 mbar L s-1. This method is the most versatile of industrial and laboratory leak detection testing methods. Use of Mass Spectrometer Leak Detector (MSLD) requires definition of a number of characteristics to describe the functioning and capability of the leak detector. While several of these terms may be applicable to any measurement system, of which is the leak detector is but one, the definitions will be specifically related to a typical mass spectrometer leak detector. Several of the characteristics can be applied to test “setups” as well as to the leak detector itself

  • Standard leak
  • Calibrated leak
  • Scale divisions
  • Background or residual signal
  • Sensitivity
  • Linearity
  • Minimum detectable leak
  • Response time
  • Cleanup time
  • Inlet pump speed

Standard Leak

A standard leak is a precision, calibrated leak provided with its own source of tracer gas for pressurization of the leak. It is standardized by a precision flow measurement system which can be traced to national standards through traceable geometry, thereby justifying the term standard leak. The actual leak may be a precision capillary porous plug, or a permeable membrane of glass, ceramic or metal. All leaks are temperature sensitive and will require replacement or recalibration as the supply of gas is depleted in the sealed volume. Permeation type leaks exhibit the greatest temperature sensitivity.

Calibrated Leak

A calibrated leak is a stable leak which has been calibrated by reference to a standard leak or a precision flow measurement system. Calibrated leaks usually are constructed such that the pressurizing tracer gas can be supplied from an independent controllable source. The data supplied for calibrated leaks should include leak rate vs. pressure and leak rate vs. temperature information. Since calibrated leaks usually are open-ended, it is possible to plug the flow path by dirt, liquids and condensable vapors.

Scale division

The collector amplifier visual indicator is a meter or chart recorded associated with the leak detector which is used to indicate the flow rate of a leak measured by the mass spectrometer leak detector. The increments of indication on this meter or chart paper may scribed as 0 to 100% with hundred divisions either scribed on the meter face or paper or implied by scale factors based upon 100 divisions.

Background or Residual Signal

The response of the collector amplifier indicator, when no external source of tracer gas is present to which the leak detector is tuned is defined as background. The background is composed of several independent sources, the most common of which are (a) real tracer gas residual signal left in the mass spectrometer from previous tracer gas exposure or which back diffuse into the mass spectrometer from the vacuum pump exhaust system, (b) spurious electronic noise usually rapidly varying in nature (less than 5 seconds), and (c) drift in the indictor “zero” from a present zero indication usually long-term in nature (greater than 5 minutes)

Sensitivity

The sensitivity of the leak detector is defined as a leak rate per scale division of collector amplifier visual indicator response. It is determined by admitting a known flow rate of tracer gas to the mass spectrometer. The response of the leak detector is the net number of scale division (indicated tracer response minus any background or residual response in scale division). The known tracer gas flow rate is then divided by the net scale division response to give the sensitivity in flow rate units per scale division of leak detector indicator response.

      

 SL  =  Known flow rate from standard leak in  atm.cc/sec

LIR  = Leak indicator response in scale division

BG  = Background leak indicator response in scale division

    S = Leak detector sensitivity in Padm3/sec-scale division

Linearity

The response of a leak detector to varying leaks can be plotted graphically to show the leak detector indication vs. leak rate. If the leak rate is repeatedly doubled producing a corresponding doubling of leak detector indication at each doubling, the leak detector can be said to linear. If the leak detector indication per unit of leak rate changes, the leak detector is said to be non linear. The sensitivity of a linear responding leak detector is constant; the sensitivity of a non-linear detector varies. Most leak detectors’ linearity varies as a function of absolute mass spectrometer pressure, source pressure or both. At minimum detectable leak rate, the linearity is affected because of the background; at some maximum leak rate the linearity is affected because of saturation of electronic systems or mass spectrometer ion-residual gas interference. Some leak detectors have been purposely designed to be usable leak indication or improve the sensitivity at low leak rates.

Minimum Detectable Leak

The minimum detectable leak is that magnitude of leak indication which can be reliably observed on the leak detector. The minimum detectable leak may be the same as the leak detector sensitivity or greater depending upon the magnitude of any background. The criteria for determining the minimum detectable leak is arbitrary. Commonly accepted practice is to establish a signal-to-noise ratio of one (1) as that required for statement of minimum detectable leak.  If the leak detector produces one (1) division of background (the sum of noise and drift), and indication of leak of one (1) division must be developed to produce a signal-to-noise ratio of one (1) or two divisions of indication (or 2% of full scale). The minimum detectable leak is then two divisions of signals multiplied by the sensitivity. If the sensitivity of a leak detector is 1×10-8 Pa.dm3/sec.

Response Time

The response time of a leak detector is the time required for a known leak to be indicated on the leak detector from zero to 63% of its maximum equilibrium level. It is dependent upon the volume of the leak detector vacuum plumbing and the pump speed of the vacuum system at the sensing point where the leak indication is measured, i.e. the mass spectrometer. The response time (RT) for 63% of maximum indication can be shown to be:

           V = Volume of vacuum system in liters

           Sp= Speed of vacuum system liters per second (assumed to be constant over the pressure range for which the time constant period is observed)

Response time can be specified for the leak detector itself or the leak detection test setup which includes the object to be tested. If the volume of the vacuum system or component under test is large, or the pumping speed at the component under test low the response time must be carefully evaluated in the leak test procedure in order to insure that the leak test was valid.

Clean Up Time

The cleanup time of a leak detector is the time required after removal of a tracer gas form a leak for the initial leak indication to decay 63% of the indication at the time the leak was removed. IN general, the same physical processes occurs as those involved in determining response time. Frequently, however, the cleanup time is longer than response time since desorption of tracer gas may be slower than experience during the increase in tracer gas concentration use for response time measurements. The same equation is used of cleanup time (CT) and response time (RT), i.e.

Cleanup time can be specified for the leak detector itself or the leak detection setup which includes the object to be tested. It is affected in the manner as response time noted above.

Inlet pump speed

The inlet pump speed is the pumping speed available at the leak detector inlet. It is necessary that these characteristics be known in order to analyze and design a leak test system of optimum response time and system sensitivity in the event that auxiliary pumping systems are used. The speed must be known for the tracer test gas as well as for air or other ambient gas in the event that gases. Other than air are to be evacuated from the component to be leak tested.

References

  1. Nerken, Vacuum Symposium Transactions (Pergamon, Newyork, 1956), P-1
  2. Knudsen, Physics (1909; (1911)
  3. G. Wilson And L. C. Beavis, Hand Book of Vacuum Leak Detection, AVS Monograph Series 1979
  4. Vacuum Technology (Second, Revised Edition) By A. Roth, North-Holland
  5. Frank Scuotto, Journal of Vac. Sci. & Tech. A. Second Series, Vol. 4, No.5, Sep/Oct.1986
  6. Charles D. Ehrlich, Journal Of Vac. Sci. & Tech. A. Second Series, Vol. No.5, Sep/Oct.1986
  7. Richard W, J Of Vac. Sci. & Tech. A, Vol.4, Number-3, Part1, May / June 1986
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“Engineering Horizons” is the first & leading technical magazine of Pakistan covering Process, Mechanical, Metallurgical, Mining, Electrical & Electronics field under a single cover. We also feel pleasure in saying that this is the only magazine of its own kind & style, which is widely circulated in all Engineering Sectors of Pakistan.

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As you know, monthly “Engineering Horizons” is the first & Leading Technical Magazine of Pakistan covering Process, Mechanical, Metallurgical, Mining, Electrical & Electronics fields under a single cover. We also feel pleasure in saying that this is the only magazine of its own kind & style, which is widely circulated in all Engineering Sectors of Pakistan.