- •Contents
- •Foreword
- •Preface
- •1 Materials in the Lab
- •2 Measurement
- •3 Joints, Stopcocks, and Glass Tubing
- •4 Cleaning Glassware
- •5 Compressed Gases
- •6 High and Low Temperature
- •7 Vacuum Systems
- •8 The Gas-Oxygen Torch
- •APPENDIX
- •Appendix A Preparing Drawings for a Technician
- •Index
- •Foreword
- •Preface
- •For the Second Edition
- •Please note:
- •1 Materials in the Lab
- •1.1 Glass
- •1.1.1 Introduction
- •1.1.2 Structural Properties of Glass
- •1.1.3 Phase Separation
- •1.1.4 Devitrification
- •1.1.5 Different Types of Glass Used in the Lab
- •1.1.6 Grading Glass and Graded Seals
- •1.1.7 Separating Glass by Type
- •1.1.9 Stress in Glass
- •1.1.11 Tempered Glass
- •1.1.13 Limiting Broken Glass in the Lab
- •1.1.14 Storing Glass
- •1.1.15 Marking Glass
- •1.1.16 Consumer's Guide to Purchasing Laboratory Glassware
- •1.2 Flexible Tubing
- •1.2.1 Introduction
- •1.2.2 Physical Properties of Flexible Tubing
- •1.3 Corks, Rubber Stoppers, and Enclosures
- •1.3.1 Corks
- •1.3.2 Rubber Stoppers
- •1.3.3 Preholed Stoppers
- •1.3.4 Inserting Glass Tubing into Stoppers
- •1.3.5 Removing Glass from Stoppers and Flexible Tubing
- •1.3.6 Film Enclosures
- •1.4 O-Rings
- •1.4.2 Chemical Resistance of O-Ring Material
- •1.4.3 O-Ring Sizes
- •2 Measurement
- •2.1 Measurement: The Basics
- •2.1.1 Uniformity, Reliability, and Accuracy
- •2.1.2 History of the Metric System
- •2.1.3 The Base Units
- •2.1.4 The Use of Prefixes in the Metric System
- •2.1.5 Measurement Rules
- •2.2 Length
- •2.2.1 The Ruler
- •2.2.2 How to Measure Length
- •2.2.3 The Caliper
- •2.2.4 The Micrometer
- •2.3 Volume
- •2.3.1 The Concepts of Volume Measurement
- •2.3.2 Background of Volume Standards
- •2.3.4 Materials of Volumetric Construction #1 Plastic
- •2.3.5 Materials of Volumetric Construction #2 Glass
- •2.3.6 Reading Volumetric Ware
- •2.3.7 General Practices of Volumetric Ware Use
- •2.3.8 Calibrations, Calibration, and Accuracy
- •2.3.9 Correcting Volumetric Readings
- •2.3.10 Volumetric Flasks
- •2.3.11 Graduated Cylinders
- •2.3.12 Pipettes
- •2.3.13 Burettes
- •2.3.14 Types of Burettes
- •2.3.15 Care and Use of Burettes
- •2.4 Weight and Mass
- •2.4.1 Tools for Weighing
- •2.4.2 Weight Versus Mass Versus Density
- •2.4.3 Air Buoyancy
- •2.4.5 Balance Location
- •2.4.6 Balance Reading
- •2.4.7 The Spring Balance
- •2.4.8 The Lever Arm Balance
- •2.4.9 Beam Balances
- •2.4.10 Analytical Balances
- •2.4.11 The Top-Loading Balance
- •2.4.12 Balance Verification
- •2.4.13 Calibration Weights
- •2.5 Temperature
- •2.5.1 TheNature of Temperature Measurement
- •2.5.2 The Physics of Temperature-Taking
- •2.5.3 Expansion-Based Thermometers
- •2.5.4 Linear Expansion Thermometers
- •2.5.5 Volumetric Expansion Thermometers
- •2.5.7 Thermometer Calibration
- •2.5.8 Thermometer Lag
- •2.5.9 Air Bubbles in Liquid Columns
- •2.5.10 Pressure Expansion Thermometers
- •2.5.11 Thermocouples
- •2.5.12 Resistance Thermometers
- •3.1 Joints and Connections
- •3.1.1 Standard Taper Joints
- •3.1.2 Ball-and-Socket Joints
- •3.1.3 The O-Ring Joint
- •3.1.4 Hybrids and Alternative Joints
- •3.1.5 Special Connectors
- •3.2 Stopcocks and Valves
- •3.2.1 Glass Stopcocks
- •3.2.2 Teflon Stopcocks
- •3.2.3 Rotary Valves
- •3.2.4 Stopcock Design Variations
- •3.3.1 Storage and Use of Stopcocks and Joints
- •3.3.2 Preparation for Use
- •3.3.3 Types of Greases
- •3.3.4 The Teflon Sleeve
- •3.3.5 Applying Grease to Stopcocks and Joints
- •3.3.6 Preventing Glass Stopcocks and Joints from Sticking or Breaking on a Working System
- •3.3.7 Unsticking Joints and Stopcocks
- •3.3.8 Leaking Stopcocks and Joints
- •3.3.9 What to Do About Leaks in Stopcocks and Joints
- •3.3.10 General Tips
- •3.4 Glass Tubing
- •3.4.1 The Basics of Glass Tubing
- •3.4.2 Calculating the Inside Diameter (I.D.)
- •3.4.3 Sample Volume Calculations
- •4 Cleaning Glassware
- •4.1 The Clean Laboratory
- •4.1.1 Basic Cleaning Concepts
- •4.1.2 Safety
- •4.1.3 Removing Stopcock Grease
- •4.1.4 Soap and Water
- •4.1.5 Ultrasonic Cleaners
- •4.1.6 Organic Solvents
- •4.1.7 The Base Bath
- •4.1.8 Acids and Oxidizers
- •4.1.9 Chromic Acid
- •4.1.10 Hydrofluoric Acid
- •4.1.11 Extra Cleaning Tips
- •4.1.12 Additional Cleaning Problems and Solutions
- •4.1.13 Last Resort Cleaning Solutions
- •5 Compressed Gases
- •5.1 Compressed GasTanks
- •5.1.1 Types of Gases
- •5.1.2 The Dangers of Compressed Gas
- •5.1.3 CGA Fittings
- •5.1.4 Safety Aspects of Compressed Gas Tanks
- •5.1.5 Safety Practices Using Compressed Gases
- •5.1.6 In Case of Emergency
- •5.1.7 Gas Compatibility with Various Materials
- •5.2 The Regulator
- •5.2.1 The Parts of the Regulator
- •5.2.2 House Air Pressure System
- •5.2.4 How to Use Regulators Safely
- •5.2.6 How to Purchase a Regulator
- •6 High and Low Temperature
- •6.1 High Temperature
- •6.1.1 TheDynamics of Heat in the Lab
- •6.1.2 General Safety Precautions
- •6.1.3 Open Flames
- •6.1.4 Steam
- •6.1.5 Thermal Radiation
- •6.1.6 Transfer of Energy
- •6.1.7 Hot Air Guns
- •6.1.8 Electrical Resistance Heating
- •6.1.9 Alternatives to Heat
- •6.2 Low Temperature
- •6.2.1 TheDynamics of Cold in the Lab
- •6.2.2 Room Temperature Tap Water (=20°C)
- •6.2.8 Safety with Slush Baths
- •6.2.9 Containment of Cold Materials
- •6.2.10 Liquid (Cryogenic) Gas Tanks
- •7 Vacuum Systems
- •7.1 How to Destroy a Vacuum System
- •7.2.1 Preface
- •7.2.2 How to Use a Vacuum System
- •7.2.4 Pressure, Vacuum, and Force
- •7.2.5 Gases, Vapors, and the Gas Laws
- •7.2.6 Vapor Pressure
- •7.2.7 How to Make (and Maintain) a Vacuum
- •7.2.8 Gas Flow
- •7.2.9 Throughput and Pumping Speed
- •7.3 Pumps
- •7.3.1 The Purpose of Pumps
- •7.3.2 The Aspirator
- •7.3.3 Types and Features of Mechanical Pumps
- •7.3.4 Connection, Use, Maintenance, and Safety
- •7.3.5 Condensable Vapors
- •7.3.6 Traps for Pumps
- •7.3.7 Mechanical Pump Oils
- •7.3.8 The Various Mechanical Pump Oils
- •7.3.9 Storing Mechanical Pumps
- •7.3.11 Ultra-High Vacuum Levels Without Ultra-High
- •7.3.12 Diffusion Pumps
- •7.3.13 Attaching a Diffusion Pump to a Vacuum System
- •7.3.14 How to Use a Diffusion Pump
- •7.3.15 Diffusion Pump Limitations
- •7.3.17 Diffusion Pump Maintenance
- •7.3.18 Toepler Pumps
- •7.4 Traps
- •7.4.1 The Purpose and Functions of Traps
- •7.4.2 Types of Traps
- •7.4.3 Proper Use of Cold Traps
- •7.4.4 Maintenance of Cold Traps
- •7.4.5 Separation Traps
- •7.4.6 Liquid Traps
- •7.5 Vacuum Gauges
- •7.5.2 The Mechanical Gauge Family
- •7.5.4 The Liquid Gauge Family
- •7.5.5 The Manometer
- •7.5.6 The McLeod Gauge
- •7.5.7 How to Read a McLeod Gauge
- •7.5.8 Bringing a McLeod Gauge to Vacuum Conditions
- •7.5.10 The Tipping McLeod Gauge
- •7.5.11 Condensable Vapors and the McLeod Gauge
- •7.5.12 Mercury Contamination from McLeod Gauges
- •7.5.13 Cleaning a McLeod Gauge
- •7.5.14 Thermocouple and Pirani Gauges
- •7.5.15 The Pirani Gauge
- •7.5.16 Cleaning Pirani Gauges
- •7.5.17 The Thermocouple Gauge
- •7.5.18 Cleaning Thermocouple Gauges
- •7.5.19 The lonization Gauge Family
- •7.5.20 The Hot-Cathode Ion Gauge
- •7.5.21 Cleaning Hot-Cathode Ion Gauges
- •7.5.24 The Momentum Transfer Gauge (MTG)
- •7.6 Leak Detection and Location
- •7.6.1 AllAbout Leaks
- •7.6.3 False Leaks
- •7.6.4 Real Leaks
- •7.6.5 Isolation to Find Leaks
- •7.6.6 Probe Gases and Liquids
- •7.6.7 The Tesla Coil
- •7.6.8 Soap Bubbles
- •7.6.9 Pirani or Thermocouple Gauges
- •7.6.10 Helium Leak Detection
- •7.6.11 Helium Leak Detection Techniques
- •7.6.13 Repairing Leaks
- •7.7 More Vacuum System Information
- •7.7.1 The Designs of Things
- •8 The Gas-Oxygen Torch
- •8.1.2 How to Light a Gas-Oxygen Torch
- •8.1.3 How to Prevent a Premix Torch from Popping
- •8.2.2 How to Tip-Off a Sample
- •8.2.3 How to Fire-Polish the End of a Glass Tube
- •8.2.4 Brazing and Silver Soldering
- •Appendix
- •A.2 Suggestions for Glassware Requests
- •B.1 Introduction
- •B.2 Polyolefins
- •B.3 Engineering Resins
- •B.4 Fluorocarbons
- •B.5 Chemical Resistance Chart
- •C.1 Chapter 1
- •C.4 Chapter 4
- •C.5 Chapter 5 & Chapter 6
- •C.6 Chapter 7
- •C.7 Chapter 8
- •D.1 Laboratory Safety
- •D.2 Chemical Safety
- •D.3 Chapter 1
- •D.4 Chapter 2
- •D.5 Chapter 3
- •D.6 Chapter 4
- •D.7 Chapter 5 and the Second Half of Chapter 6
- •D.8 Chapter 7
- •D.9 Chapter 8
- •Index
Leak Detection and Location 7.6 |
431 |
levitated bymagnetism. The levitated ball is rotated at speeds of up to 100,000rps and then allowed to coast. The only mechanism used to slow down theball is the friction of airon its surface. The less airinthe system, theless friction onthe ball. Currently, MTGs read the vacuum ranges 10"2to 10"8 torr and, with modern electronics, have demonstrated remarkable accuracy and sensitivity (±1% accu-
racy in the 10~2- to 10"5-torr range).64
Because the MTG neither heats nor ionizes the gases within a vacuum system, it is unique in theinferred gauge category. Bynotaltering or changing thecomposition of the gases within a closed system, it offers special opportunities. However, MTGs require extensive andexpensive electronic controls, measuring equipment, and special vibrationless platforms. Thus, very few research labs have the needs that can justify theexpense and demands of these gauges.
7.6Leak Detection and Location
7.6.1AllAbout Leaks
There areseven sources of leaks in vacuum systems. All of these leak sourcescan make the job of maintaining a vacuum more difficult than obtaining a vacuum. An illustration of these unwanted gas sources within a vacuum system canbe seen in Fig. 7.50. Perhaps one of themost subtle sources of leaks into an ultrahigh-vac- uum system is permeation of the glass by helium, present in the atmosphere at 5.24 ppm(seeTable 7.3). Helium permeation* of glass can be useful for a standard leak (used with a He leak detector, see Sees. 7.6.10 - 7.6.12), but can be debilitating when trying to obtain extremely ultrahigh vacuum with glass components. This canessentially eliminated if aluminosilicate glass is used in the construction in the region where such is not desired. For more information on aluminosilicate glass, see page 14.
Evaporation (outgassing) is therelease of adsorbed molecules at room temperature and pressure, while desorption (degassing) is theforced evaporation of molecules by either applying greater heat ordecreasing theambient pressure.
Diffusion is the movement of atoms or molecules through solids, liquids, or gases. The rate oftransport is always governed both byconcentration variations in the material and by the size ratio of the diffusing material and the material it's passing through. Once thematerial hasdiffused to thesurface, it canbe desorbed into thesystem.
*Permeation (and absorption) areboth conditions that only apply when thepenetrating molecule is much smaller than the molecules of the wall material. A molecule of any size can be involved in adsorption.
432 |
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Vacuum Systems |
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Internal |
External |
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leakage |
leakage |
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(virtual |
(real |
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leak) |
leak) |
Diffusion |
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Evaporation |
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Permeation |
-£> |
Desorption |
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Backflow
Fig. 7.50 Gas sources in a vacuum system. From An Elementary Introduction to Vacuum Technique, G. Lewin, Fig. 19, ©1987 by The American Vacuum Society, American Institute of Physics, New York, reprinted (abstracted) with permission
Permeation is a three part process. The material is first absorbed into the material it is about to pass through. Then, the contaminant diffuses through the material. Finally, the contaminant desorbs into the system.
The second gas law states that there cannot be unequally distributed partial pressures in any system. Thus, the greater the vacuum, the greater the rate of desorption, diffusion, and permeation to overcome the differences in concentration as "nature abhorring a vacuum" tries to equalize the system.
Backflow (also called back diffusion or back migration) occurs when the pressures at the outlet and inlet have established a constant ratio (this is analogous to the compression ratio found in mechanical pumps). At that point, gases can drift either way in the vacuum system. Proper trapping or baffling is an easy mechanism to prevent this problem.
False (virtual) leaks are discussed in Sec. 7.6.3 and external (real) leaks are discussed in Sec. 7.6.4.
Water adsorbs into the walls of a glass container, but that adsorption is the extent of its adhesion. Some adsorbed molecules react chemically with some types of containers in a process called chemical adsorption (chemisorption. For example, carbon monoxide chemisorbs with palladium, but not with gold). The bonds resulting from chemisorption can hold molecules to the surface with far greater force than would exist with only physical attraction. It is also possible for a molecule (that normally would not chemisorb with the container wall) to break up when hitting the wall's surface. At that point the molecule's constituent parts chemisorb with the container walls. When an adsorbed gas reacts with the materials of a container, it is called reconstruction (for example, the reconstruction of iron with oxygen is rust).
Leak Detection and Location 7.6 |
433 |
7.6.2 IsPoor Vacuum aLeak oraPoor Vacuum?
Looking for a leak is only fruitful when you know there is a leak. Just because you have a poor vacuum does notmean that you have a leak. In theart andscience of leak detection, you must first verify whether a problem really exists before youtry to do something about the (perceived) problem. Only after youhave established that there is a problem can you begin to locate thesource of the problem.
It is impossible to obtain a perfect vacuum; there is noway to remove allmolecules from a given area. With that idea in mind, thebest vacuum of any vacuum system is limited by thequality anddesign of your vacuum pumps, thecomposition of material in your vacuum system, andthedesign of your vacuum system. Ultimately, a leak-tight vacuum system* will belimited by leaks from the outgassing of materials from your system, diffusion of gases from thewalls, permeation through the walls, evaporation anddesorption from wall surfaces, and backflow from the pumping systems.
Because no vacuum system can be truly leak-free, it is important to determine whether or notyouhave a leak of consequence. In other words, does anysystem leak actively affect your work? For example, a common rubber balloon holds water better than it holds air, andit holds airbetter than it holds helium. If your needs are tocontain water, a standard rubber balloon is sufficient. Similarly, if you want to contain helium for a limited time, again a rubber balloon is sufficient. However, if you want a helium balloon to stay up for several days, then a rubber balloon is insufficient andyou must spend themoney for a Mylar balloon, which can contain helium much better than a rubber balloon.
In addition to knowing the intended use for a vacuum line, you also need to know itshistory. Byknowing thevacuum line's history, you canrule outthe possible reasons for vacuum failure andproceed to locate thecause. Forexample, if yesterday a vacuum line wasachieving a 10"6 torr vacuum, buttoday it canonly achieve a 10~2 torr vacuum, something dramatic hasobviously occurred, which is more likely to be a leak. Ontheother hand, if the same drop in vacuum occurred gradually over a period of several weeks, it is very unlikely the cause is a leak and is more likely caused bygradual component failure.
7.6.3 False Leaks
You should notassume that you have a leak just because thevacuum in your system is poor. Reasons for inadequate vacuum can be carelessness (stopcock left open), anxiousness (insufficient pump-down time), orneglect (the diffusion pump was never turned on). You can save a lot of time by eliminating thevarious reasons why your vacuum is notperforming up to expectations before youlook for leaks. Factors that can prevent a system from reaching a desired low pressure include:
*If such a thing existed, this vacuum system would have noimperfections (holes) in thewalls to the atmosphere.
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Vacuum Systems |
1.Pumps that are incapable of pulling a greater vacuum because they are too small (or slow) for the given system or need maintenance.
2.System components that cannot be baked (to facilitate outgassing).
3.Inaccurate, broken, or defective gauges.
4.Outgassing of high vapor pressure materials from the system.
Poor Pump Performance. This problem is a common one in labs. It is caused either by purchasing too small a pump for the desired task or, more likely, by insufficient maintenance which causes a perfectly good pump to run poorly. Insufficient trapping can cause shortor long-term damage to pump oils. Running a mechanical pump too long against a "no-load" condition will cause pump oils to froth and take more time for outgassing. Probably the greatest cause for poor pump performance is not changing pump oils on a regular or timely basis.
Baking Out a System. Generally, the need to bake out a system implies that very high or ultrahigh vacuums are required. A system must be baked at about 150°C to remove surface water vapor, and it must be baked at greater temperatures (250°C to 450°C) to remove enough water required to obtain very low pressures (<108 torr)65 for ultrahigh vacuums. The materials of the vacuum system's construction must be considered when the tasks of the vacuum system are established. If a vacuum system is constructed with the wrong types of components, proper baking may be impossible, eliminating the possibility of achieving very high or ultrahigh vacuums. Because it is not possible to heat glass stopcocks this high (and still expect them to function), a glass system is not recommended for vacuum systems requiring bake-outs.
Inaccurate Vacuum Gauges. Discrepancies can be caused by such reasons as a gauge that is either inaccurate, used beyond its range, or is in need of calibration. More complex reasons for vacuum gauge errors could be that the gauge is tuned to a different gas species than what is in the vacuum system, the gauge is poorly placed within the vacuum system, something within the vacuum system has damaged or affected the gauge (mercury can cause a thermocouple gauge to have drifting readings), or the gauge is being affected by external interference such as a magnet. The resolutions to these problems can include calibrating the gauge for the specific gas being read, obtaining a gauge which is accurate for the pressure being read, and/or properly locating the gauge within the system (and in the proper alignment). You may need to obtain calibration equipment for your gauge (some controllers have calibration units built into the electronics).
Outgassing. This problem is typically one of user impatience, poor cleaning, or poor choice of materials within the system. Many a beginning vacuum user expects a brand new vacuum system to get to 10"7 torr within 15 minutes of the system's first use. A new system with the usual amounts of atmospheric water vapor within can typically require overnight pumping to reach its lower limits. The same will be required of any system left open to the atmosphere for too long. The longer a system is left open to the atmosphere, the longer it will take for it to