- •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
Vacuum Systems
7.1 How to Destroy a Vacuum System
Carpenters have a saying "measure twice, cut once." This saying implies that any extra time spent in preparation saves time and materials that may otherwise be wasted. Likewise, any time spent preparing an experiment or equipment (including general maintenance) saves time and materials and, potentially, may also save a life.
This section lists fundamental potential pitfalls when working with a vacuum system. If you read no other section in this book, read this one. By following the rules and guidelines that you are directed to within this section, many hours, and perhaps weeks, of problems will be avoided. It is better to avoid the situation of "not having enough time to do it right, but plenty of time to repair the wrong." This list is not meant to be comprehensive—it cannot be. It is, however, a collection of the more common disasters that occur on a laboratory vacuum system.
1.Blowing up a vacuum line by freezing air in a trap (see Sec. 7.4.3)
2.Sudden bursts of pressure in McLeod gauges that can cause mercury to spray throughout a line (problem) or break a line (big problem) (see Sees. 7.5.6-7.5.9)
3.Breaking stopcocks off vacuum lines (see Sec. 7.7.1, point 8)
4.Breaking off glass hose connections when pulling flexible tubing off (see Sec. 7.3.17)
5.Destroying the oil in a diffusion pump (see Sec. 7.3.1)
6.Destroying the oil in a mechanical pump (see Sec. 7.4.1)
7.Destroying a mechanical pump (see Sec. 7.3.4)
8.Wasting time re-evacuating vacuum lines (see Sec. 7.3.14)
9.Causing virtual leaks in cold traps (see Sec. 7.4.3)
10.Frothing the oil of a two-stage pump (see Sec. 7.3.3)
321
322 |
Vacuum Systems |
11.Breaking a cold trap off a vacuum line by not venting it to the atmosphere before removing the bottom (see Sec. 7.4.4)
12.Achieving a poor-quality vacuum when starting a vacuum system up for the first time (see Sec. 7.6.3)
13.Dissolving O-rings during vacuum leak detection (see Sec. 7.6.6).
14.Punching holes in glass with a Tesla coil (see Sec. 7.6.7)
15.Burning the filament of an ion gauge (see Sec. 7.5.20)
16.Imploding glass items on a vacuum system (see Sec. 7.7.1, point 2)
17.Placing items on a vacuum rack so that they fall into the rack (see Sec. 7.7.1, point 3)
18.Breaking vacuum system tubing when tightening two and/or three finger clamps on a vacuum rack (see Sec. 7.7.1, point 4)
19.Sucking mechanical pump oil into a vacuum line (see Sec. 7.3.4)
7.2 An Overview of Vacuum Science
and Technology
7.2.1 Preface
In this book I have intentionally avoided equations whenever possible. The reason is simple: Most people do not need, or use, equations when using equipment. Despite that, this introductory section on vacuum technology contains equations. They are presented so the reader may better understand the relationship between the various forces in vacuum systems. None are derived, and none are used beyond presenting some basic points. If you are interested in the derivation of any formula, see the recommended books at the end of this chapter.
It is not necessary to know the material in this introductory section to run a vacuum system. However, it does explain terms and ideas that are used throughout the rest of this chapter. Because of the basic information contained within, I recommend that you read this section.
Vacuum systems are used in the lab for such purposes as: preventing unwanted reactions (with oxygen and/or other reactive gases); distilling or fractionally distilling compounds (a vacuum can lower the boiling point of a compound); or transferring materials from one part of a system to another (using cryogenic transfer). A vacuum system can also be used for more sophisticated processes such as thin film deposition, electron microscopy, and enough other processes that just to list them could fill a book.
An Overview of Vacuum Science and Technology 7.2 |
323 |
The goal in creating a vacuum is to get rid of, or bind up, a significant amount of the gases and vapors (mostly air and water vapor) within a vacuum system. Regardless of the approach, the goal is a net reduction of pressure in the system.
A perfect vacuum is a lack of everything, and we must accept that we cannot achieve this state in the laboratory. Table 7.1 illustrates that by decreasing a vacuum to 10~3 torr (a good-quality vacuum in a standard lab) we can remove over 99.99% of the particles* that are present at room pressure. At 10"6 torr, a high vacuum, there is still quite a considerable number of particles remaining (2.45 x 1017/ m3). Despite the (seemingly) large quantity of particles remaining, this vacuum is still sufficient to successfully limit the chance of oxidation and/or unwanted reactions for many laboratory requirements. More than 95% of all vacuum studies and technique can be successfully achieved within the vacuum ranges of 10"2 to 10"6 torr. The other 5% (studies in the ultrahigh-vacuum range) consist of surface and material studies and space simulation, which require as little contamination as possible.
Aside from earthbound technological approaches to achieve a vacuum, the further away from the earth's surface you go, the less atmosphere there is and therefore the greater the vacuum (relative to atmospheric pressure on the earth's surface) that can be achieved. In fact, on earth, someone standing on top of Mt. McKinley experiences vacuum greater than can be created with a standard vacuum cleaner at sea level. Table 7.1 shows the approximate miles above earth to obtain various conditions of vacuum. As this table shows, outer space offers wonderful opportunities to produce vacuum conditions for experimental or industrial
Table 7.1 Pressure, Particles", and the Mean Free Path3'4
Pressure |
Quality |
Miles Above |
# of Particles |
Mean Free |
|
Earth* |
|||||
(torr) |
of Vacuum |
(per m"3) |
Path (m) |
||
(= miles) |
|||||
|
|
|
|
||
760 |
None |
0 |
2.48 x 1O25 |
6.5 x 10"8 |
|
0.75 |
Medium |
5 |
2.45 x 1022 |
6.5 x 105 |
|
7.5 x 10'3 |
High |
35 |
2.45 x 1020 |
6.5 x 10'3 |
|
7.5 x 10"6 |
Veryhigh |
50 |
2.45 x 1017 |
6.64 |
|
7.5 x 10"8 |
Ultrahigh |
90 |
2.45 x 1015 |
664 |
|
7.5 x 10"10 |
Extreme ultrahigh |
290 |
2.45 x 10° |
6.6 x 104 |
" This table is based on dry air because of the daily variations of water vapor.
* These approximate values are provided to allow the reader to appreciate the value, and difficulties, of space travel.
For this discussion, water vapor has been ignored because its percent concentration varies on a day- to-day basis.
324 Vacuum Systems
work. Outer space can provide an infinite vacuum system, an essentially contami- nation-free environment, and blip free (no pressure surge) conditions. The possibilities of ultravacuum research in space is discussed by Naumann in his paper on the SURF (Space Ultravacuum Research Facility) system.2 Unfortunately, space is an opportunity that is as far from the expected uses of this book (no pun intended) as one can get. Back on earth we are still limited to using pumps, traps, oils, gauges, glass, metal-support clamps, elastomers, and other implements of laboratory vacuum systems.
Although vacuum systems are not as expensive as space travel, they are not inexpensive, and the greater the vacuum desired, the more expensive it will be to achieve. If anyone is concerned about the bottom line, it is important to consider your needs before you begin designing your system. Not only is there an increase in the cost of pumping equipment to cover the range from poorto good-quality vacuums (mechanical to diffusion to turbomolecular to cryogenic pumps), but there are also the correspondingly increasing costs of support equipment (power supplies, thermocouple and ion gauges, and their controllers), peripheral equipment (mass spectrometry and/or He leak detectors), peripheral supplies (cooling water, Dewars, and liquid nitrogen), and support staff (technicians to run and/or maintain the equipment). These requirements can all add up.
Sometimes, you will have no alternative but to use comparatively expensive equipment. For example, a diffusion pump must be used in conjunction with a reasonably powerful (and therefore relatively more expensive) mechanical pump: If you were to use the diffusion pump with a small classroom demonstration mechanical pump, the diffusion pump would not work. On the other hand, do not assume that price is necessarily indicative of the best choice in equipment. You need to match components by their capabilities. That is, if your system is capable of only a low vacuum, you are wasting money purchasing a gauge designed with high-vacuum capabilities (which can be significantly more expensive). The requirement of matching components is equally important whether you are purchasing vacuum equipment from scratch or adding to components you already own.
Because a perfect vacuum cannot be achieved, and the next best thing (outer space) is for all intents and purposes out of reach, there are definable limits to the ultimate pressure of any system you are planning. These limits are based on:
1.How much you can spend
2.How much maintenance you are willing/needing to do
3.What supplies and materials you have (or can have) available
4.Your vacuum experience and knowledge
5.What technical support you have available
It is therefore important that you know what the demands and needs of your experimental work are. It is neither economical nor practical to have an elaborate vacuum system for simple vacuum needs.