- •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 |
Between 1/2" and 3/4" deflection
Fig. 7.18 Acceptable deflection of the pump belt is between V2"and 3/4".
pump one extra time before refilling the pump for use. After changing the pump oil, be sure to wipe off any oil that may have spilled on the pump, especially the motor. Oil and dirt can prevent the proper dissipation of heat, which could damage air-cooled (or fan-cooled) systems.
Any oil seen around the drive shaft is an indication that a drive shaft seal may need replacement. Unusual noise and/or heat may indicate bearings that may need to be changed or a failure of the internal lubrication system. Typically, the first section of a pump that will show wear from abrasive paniculate matter that may have entered will be the discharge valves and will be indicated with drops in pressure.
If the motor or pump mechanism requires lubrication, typically once a year is sufficient. To lubricate more often than that causes the excess grease to be forced out on the inside of the motor housing, potentially landing on the windings of the motor. This can prematurely destroy the motor.
If you have to change the belt, be sure to align the pulleys both parallel and on the same plane (see Fig. 7.17). After the belt is on, check its tightness by firmly placing your finger on the center of the belt. The deflection should be between l/{ to V{ (see Fig. 7.18). If the belt is too loose, it may slip on the pulleys; this causes friction and heat, leading to premature failure. On the other hand, if the belt is too tight, it may break, even within a week of constant use. After a day or two of operations use, recheck the tension because it is likely to have loosened.
7.3.5 Condensable Vapors
A working pump is constantly trapping condensable vapors within the pump oil. This trapping is caused both by the pump's churning action, which physically mixes the oil and gases, and by the high pressures within the pump during the compression stage of the pump cycle. During the compression stage, the gases and vapors must be brought to a pressure greater than atmospheric before they can be expelled into the atmosphere. Now, by definition, condensable vapors in a vacuum are in a gaseous state. As they are compressed within the pump, some of them may condense out and can mix into the mechanical pump oil. Then, as the veins of the pump continue past the exhaust point of the pump and back to the vacuum side, the condensed vapors can revolatilize.
Pumps 7.3 |
355 |
The revolatilization of condensable vapors will decreases vacuum potential by replenishing the vapors in the system you were just trying to remove. This can create an artificially high maximum limit on the pump's potential vacuum. In addition to the backstreaming of vapors, the pump itself is affected when condensable vapors contaminate the pump's oil. Not only will this decrease the vapor pressure of the pump oil, but the condensed vapors can cause a reduction of lubrication and sealing properties of the oil and lead to an eventual corrosion of the pump's internal parts. Other condensed liquids (such as hydrocarbons) can mix, emulsify, and/or break down the pump oil. They can also directly destroy a pump by chemical attack, or indirectly, by poor pump performance, they can cause extra wear and tear on the pump parts.
There is no one good way to prevent condensable vapors from affecting a mechanical pump. There are, however, two directions that one can take in dealing with the problem: One is to limit them from getting to the pump, the other is to prevent them from affecting the pump once they are present. Neither is the best approach, and usually it takes combinations of the two to deal effectively with the problem. An alternative approach is to constantly change the pump oil. This solution however, is neither costnor time-effective.
To prevent (or limit) condensable vapors from getting to a pump, traps [either of chilled or chemical design (see Sec. 7.4 on traps and foreline traps)], are used. Depending on the type of trap used, there are opportunities for vapors to pass on to the mechanical pump. Thus, one cannot depend fully on traps of any kind, and one must also deal with vapors at the pump itself.
To prevent (or limit) condensable vapors that reach a pump from affecting the mechanical pump oil, a gas ballast (also called a vented exhaust) is used. The gas ballast allows a small bit of atmosphere (up to 10%) into the pump during the compression stage so that the gas from the system is only part of the gas in the pump at the time of greatest compression. Thus, at the time of compression, the total percentage of condensable vapor within the pump is much less than there would be otherwise. Because the gas prior to being expelled is at a lower pressure, less of the vapor can be compressed into a liquid. Then, as the veins sweep into the vacuum side of the pump, no condensed vapor can expand back into a vapor.
Ballasting decreases the potential vacuum a pump could normally produce (about one decade of performance capability*). However, it dramatically improves its performance over the long run in the presence of condensable vapors. Plus, it helps to protect pump oils from contamination, which decreases pump breakdown possibilities and increases the longevity of the pump oils. Incidentally, running a pump with a ballast causes the pump to run a bit hotter than it otherwise would, which decreases the potential gas-carrying ability of the oil.
*If you can obtain 10"3 torr without a ballast, you may only be able to obtain 10'2 torr with a ballast. If you are not obtaining expected performance and you have a ballast, you may want to check if the ballast was inadvertently left open.
356 |
Vacuum Systems |
For maximum efficiency, a gas ballast should be open when first pumping on a vacuum system, or when pumping a system that has (or is creating) condensable vapors. Once sufficient vacuum has been achieved and the majority of condensable vapors have been removed from a system, the gas ballast can be closed, although it does not hurt (beyond ultimate pump performance) to leave a ballast open all the time.
Although gas ballasts can be found on both singleand double-vane pumps, as well as piston design pumps, they are more likely to be found on double-stage pumps than on single-stage pumps. It is interesting to note that a vane pump tends to run noisier with the gas ballast open, whereas a piston pump tends to run quieter with the gas ballast open.
When work is over, there is a tendency to turn everything off. Normally this approach is proper. However, it is better to leave a mechanical pump on (ideally pumping against a dry nitrogen purge) after regular use. The purge should be held at about 300(x for as long as a day to help "flush out" any condensate from the pump oil. This extended pumping will not help pump oil already broken down, but it will help decrease any more pump oil from further destruction by expelling any remaining contaminating material. It also helps to remove water vapor from the pump oil. When letting your pump run for such an extended period of time, always let the pump pull against a load. In other words, never let a pump run with the inlet open to atmospheric pressure because the pump oil will froth and lose its protective characteristics—which can ruin the pump.
You can make a dry nitrogen leak fairly easily. Take a copper tube with fittings to go between a compressed nitrogen tank and the vacuum system, and smash about two inches of the tube flat with a hammer. You can check the quality of your "leak" by attaching this tube to the nitrogen tank, and open the main valve. Place the other end in a container of water and observe the gas bubble formation as you increase the delivery pressure. Sufficient pressure to produce a bubble every 10 seconds or so will provide an adequate leak for a good flushing. Certainly not high-tech, but it seems to work quite well. This can be used not only to help flush out condensable gases from a mechanical pump, but also to flush out a vacuum line of contaminants (provided that the vapor pressure of the contaminants can be achieved with the extra pressure of the dry nitrogen leak purging into the system).
There is an alternate approach to preventing condensable vapors from entering mechanical pump oils—that is, maintaining the pump at relatively high temperatures. The high heat prevents the vapors from condensing within the pump despite the high pressure. However, due to the inherent dangers of this type of pump, as well as the problem that pump oils begin to deteriorate after being maintained at high temperatures, this approach is seldom used.
7.3.6 Traps for Pumps
All pumps should be protected from materials within the system, the system should be protected from all pump oils, and a diffusion pump (if any) should be
Pumps 7.3 |
357 |
protected from mechanical pump oils in the foreline.* These protections can be achieved by using traps. More information on traps is presented in Sec. 7.4, but the following information is important for general operation.
Any water or hydrocarbon solvents left from previous operations should be removed from traps before beginning any new vacuum operations. Otherwise, any materials in a trap when first beginning operation will be drawn directly into the pumps, which is what the traps are trying to prevent. Having no material in the traps to begin with removes this possibility.
A buildup of water or hydrocarbon solvents during operation can decrease, or cut off, throughput of gas through a trap. If this situation occurs, close off the trap from the rest of the vacuum line and pump. Once isolated, vent the trap to the atmosphere^ let the trap come to room temperature, remove the lower section, and empty the trap. In some cases it may be necessary to remove the trap before it has thawed and place it within a fume hood to defrost. It is a good idea to have some extra lower sections of cold traps to exchange with one that is being cleaned to limit your downtime. If you expect to remove frozen trap bottoms often on a glass system, it may facilitate operations if O-ring joints are installed on the system rather than standard taper joints because O-ring joints are easy to separate, even if the trap is cold (see page 398).
Cold traps must be used if mercury is used in your system (such as manometers, diffusion pumps, bubblers, or McLeod gauges) and if your mechanical pump has cast aluminum parts. Mercury will amalgamate with aluminum and destroy a pump. Even if your mechanical pump does not have aluminum parts, the mercury may form a reservoir in the bottom of the mechanical pump, which may cause a noticeable decrease in pumping speed and effectiveness. Aside from a cold trap between the McLeod gauge and the system, place a film of low vapor pressure oil in the McLeod gauge storage bulb. This oil will limit the amount of mercury vapor entering the system that makes its way to the mechanical pump.17 In addition, an oil layer should be placed on the mercury surface in bubblers and other mercuryfilled components.
When first starting up a vacuum system, let the pumps evacuate the system (if starting up the system for the first time) or the traps (if they have been vented to the atmosphere) for a few minutes* before setting the traps into liquid nitrogen. Otherwise you are likely to condense oxygen in the traps and create a potentially dangerous situation when the pumps are turned off (see Sec. 7.4.3).
*The foreline is the section of the vacuum system between the high-vacuum pump (i.e., diffusion pump) and the fore pump (i.e., mechanical pump).
+ Traps should always have stopcocks or valves on both sides and should also have a third stopcock or valve to vent the trap to the atmosphere to allow removal of the trap. This allows the trap to be disassembled without exposing the system or the pump to the atmosphere.
*When a pump is working against a no-load situation, it is often louder than when it is pumping against a vacuum. You can use the volume change as a guide as to when it is safe to begin using liquid nitrogen.