- •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
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Vacuum Systems |
Real leak
Outgassing
Time
Fig. 7.55 A chart of a real leak versus outgassing as a matter of time.
vention of this is to have a sufficiently powerful forepump and/or very good fore traps.
The maintenance of baffles and traps can be expensive both in materials and time. There are several dry pumps (see page 347) that have no grease or liquids in the pumping region.
Uncontrollable backstreaming can occur if there is a total system power failure that controls your vacuum system. If power in your building should go out and all mechanical pumping ceases, the efficiency of the traps and baffles to maintain a barrier in a static system will be tested. If someone is around at the time, close all stopcocks between each pump from any other and the stopcocks between the pumping system and the vacuum system.
7.6.5 Isolation to Find Leaks
When a vacuum system is suspected of having a leak, one of the first tests is to determine whether the leak may be caused by outgassing. An easy way to determine this is to chart the rate of pressure loss versus time. To chart this rate, obtain the lowest vacuum you can in a reasonable amount of time, then close the section in question* from the pumping section by a stopcock or valve. Next, periodically over a few minutes, or an hour or two (or three), note the pressure and elapsed time. As seen in Fig. 7.55, a real leak will indicate a constant rate of pressure rise over time while an outgassing problem will indicate a decreasing rate of pressure rise over time.
Once you have determined that you have a leak (as opposed to outgassing), you must then decide if the leak is virtual or real. If you know your system (and its history), you should be able to review your own operations, procedures, and activities to make this determination. On the other hand, if this system is new (to you) or there are a variety of people who work on the same system, then you may have to assume that there is a real leak and prove that it does or does not exist. Once you have proved that there is no real leak, and all other indications lead you to believe that a leak exists, you can assume that you have a virtual leak.
Regardless of whether a leak is real or virtual, the first step after a leak is verified is to isolate the area or section of the vacuum system that is leaking. This iso-
* A thermocouple gauge, Pirani gauge, or manometer must have access to the section being tested.
Leak Detection and Location 7.6 |
443 |
Vacuum
gauge
?•—2>. Upper section
Upper branch
Lower branch
Fig. 7.56 A stylized vacuum system.
lation decreases the time involved in locating the specific leak location. It's easier to play "hide and seek" if you know to look in a single room rather than the entire house. To find a leak's location, first examine major sections followed by smaller and smaller sections until you have the section in question. A stylized version of a vacuum system is shown in Fig. 7.56.
One of the best vacuum gauges to use when looking for a leak is a thermocouple or Pirani gauge. McLeod gauges, although very accurate, take too long to cycle through a reading. The ion gauges can only be used at good to high vacuums, which make their use irrelevant at the low vacuum levels that exist with large leaks.
In the stylized vacuum system shown in Fig. 7.56, one would go through the following process to locate a leak:
1.Close Stopcocks 5 and 2, and open Stopcocks 1, 3, and 4.
2.Once a vacuum is showing on the vacuum gauge, close Stopcock 4 and verify that the gauge is not leaking (this procedure may seem silly, but it caught me off-guard once). If the gauge leaks, you will be unable to check the rest of the system with any validity (that is why this step is important). If there is no leak, open Stopcock 4 and leave it open.
3.Close Stopcock 1. If the vacuum gauge shows a drop in the vacuum, the leak is somewhere within this area. If there is no leak, continue.
4.Open Stopcock 1 and then rotate Stopcock 5 so that the upper section is connected to the vacuum. Stopcock 6 should be closed. Wait until a vacuum is obtained and close Stopcock 1. If there is no leak, continue.
5.Open Stopcock 1 and then rotate Stopcock 6 so that the upper branch is connected to the vacuum. Wait until a vacuum is obtained and close Stopcock 1. If there is no leak, continue.
6.Open Stopcock 1 and then rotate Stopcock 5 so that the lower section is connected to the vacuum. Stopcock 7 should be closed. Wait until a vacuum is obtained and close Stopcock 1. If there is a leak, you must locate
444 |
Vacuum Systems |
and repair the leak before you can continue. It is important to continue, because there may still be a leak in the lower branch.
7.Open Stopcock 1 and rotate Stopcock 7 so that the lower branch is connected to the vacuum. Wait until a vacuum is obtained and close Stopcock 1. If there is no leak, go back to work—the system is checked and repaired.
As you can see, this process is aided or thwarted by vacuum system design. Temporary isolation of the various parts of any vacuum system may be impossible by bad design. Therefore, be forewarned: If you are designing a vacuum system, provide a stopcock at every branch* of the system to aid in leak detection (section isolation not only helps in leak detection, but creates a more robust vacuum system allowing greater control and protection).
Once you have isolated the part of the system in question, examine, re-grease, or tighten (if necessary) the stopcocks, joints, and other connections. In a glass system, look for cracks that may have developed on the tubing. You should look especially where the tubing has been worked, such as places where the glass is joined to another piece, or has been bent.
In searching for leaks, you must always look first for larger leaks followed by progressively smaller leaks. It makes sense: If you have some very large leaks (sometimes called "hissers"), there is no way a small leak will show up. However, if you have found and repaired some large leaks, do not assume that you are finished. You still may have leaks that can affect your work. Therefore, after repairing a leak, recheck your system for leaks. Just because you found one leak does not mean you've found them all.
7.6.6 Probe Gases and Liquids
Probe gases and liquids are often used in a variety of leak detection techniques. Probe gases and liquids are materials not normally found within the vacuum system, or at least not in the quantity created when they enter through a leak. Turnbull72 defines four characteristics of the probe gas, vacuum system, and leak detector that affect the speed and effectiveness of leak detection:
1.The viscosity of the search gas, which governs the rate at which gas enters the leak
2.The speed at which the search gas is removed from the system by the pumping system
3.The sensitivity of the leak-detecting element to the particular search gas used
4.The volume of the system
*On the other hand, prevent unnecessary constrictions because they can limit gas flow and thereby slow system pumping speed. Therefore, do not use small stopcocks (such as 2 or 4 mm) along main lines, and do not place extra stopcocks along a tube for no purpose.
Leak Detection and Location 7.6 |
445 |
Table 7.14 Liquids Used in Leak Detection
Rated Best (Top) to Worst (Bottom)74
Acetone
Ether (diethyl)
Methanol
Pentane
Benzene
Toluene
Ethyl alcohol
Carbon tetrachloride
Xylene
Isopropyl alcohol
Butyl alcohol
Although each of these factors can be considered individually, their effects on the speed of effective leak detection are cumulative. Materials that are less viscous will enter a given leak faster than those with greater viscosity. Materials that can be removed from the system faster will allow for faster verification. Materials that are easy for the detector to notice require less hesitation during detection. Finally, the smaller a system is, the less time that is needed for the probe gas to fill all areas.
When selecting probe gases (or liquids) and techniques to locate leaks, consider how they may affect your leak detection. Probe liquids are easy to see, easy to handle, and can be used with greater control, thereby providing an accuracy that is typically unobtainable with gases. Fiszdon73 analyzed 12 different liquids that are commonly used in leak detection and developed the following list (in Table 7.14) of their merits for leak detection analysis. Note that the vapor pressures of the various liquids are irrelevant. Rather, low molecular weight and low viscosity are more important.
Probe liquids have their own peculiar problems. For example, although water will pass right through a large hole, it may effectively plug up a small, thin tunnel, giving the illusion that the leak is gone. In addition, a liquid may fill the entire surface of a crack causing a very slow removal (or "cleanup") of the indicator. Baking a system can reopen blocked holes and facilitate cleanup, but not all systems can be baked.
On the other hand, gases require special, cumbersome handling and often require you to enclose a given section of a vacuum system in some sort of bag. Bagging can facilitate localizing the area of a leak, but cannot help in locating the exact location of the leak.
Do not use liquids for leak detection if you are considering using in a mass spectrometer further on in your experimentation. Liquids tend to have slow cleanup times and can severely slow down, or confound, future experimentation. Thus some rules for the use of probe gases and liquids are as follows: