- •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 Gauges 7.5 |
411 |
Two -way
Stopcock
(to system)
Mercury storage bulb
Fig. 7.40 Two common configurations of the McLeod gauge.
molecular weights produce larger errors (i.e., xenon at 30% inaccurate readings).49
7.5.7 How to Read a McLeod Gauge
There are two different methods of reading a McLeod gauge: One uses a linear scale, whereas the other uses a square scale. Because the square scale is primarily used, I will limit my explanation to its use. Refer to Fig. 7.42.
Always leave the stopcock leading to the McLeod gauge closed during experimental work to limit contamination entering the gauge and mercury from entering the system. When a pressure reading is desired, open the two-way stopcock that separates the McLeod gauge from the vacuum system. After the reading is completed, return the two-way stopcock to its closed position. Once the two-way stopcock is opened, slowly and carefully open the three-way stopcock (Position 1) to let mercury rise into the upper sections of the McLeod gauge by allowing atmosphere into the lower portion of the gauge.
412 |
Vacuum Systems |
To vacuum pump
A liquid or cold trap should be placed between the McLeod gauge and the system. A manometer only requires a liquid trap.
A trap placed between the McLeod gauge and vacuum pump will prevent mercury from being accidentally sucked into the pump.
Fig. 7.41 Some traps should be placed between the McLeod gauge and the rest of the system to prevent mercury from spilling into your system.
As mercury rises within the McLeod gauge, it is drawn up into the two capillaries. At this point, it is important to keep your eye on the capillary that is open on both ends (Capillary A). When the mercury gets closer and closer to the (inside) top of the second capillary (Capillary B), place your finger on the end of the open tube of the three-way stopcock to prevent air from entering the gauge. Your finger can provide precise control of the incoming air. Once the mercury reaches the height hlt stop the air from entering and make a reading. This method may take a bit of practice, but it soon will become relatively easy.*
Once the mercury in Capillary A is the same height as the inside top of Capillary B, reading the pressure is done by simply drawing a visual line from h2 (the top of the mercury in Capillary B) over to the reading scale and reading your vacuum. In Fig. 7.42, the pressure is 0.04 |lHg, 4 x 10'5 mm Hg, or 4 x 10"5 torr.
After reading the pressure, rotate the plug of the three-way stopcock 180° to the #2 position to draw the mercury back into the storage bulb. Once the mercury is back in the storage bulb, turn the three-way stopcock 90° to a closed position. Once a vacuum reading has been made, turn the two-way stopcock connecting the McLeod gauge and the vacuum system 90° to a closed position.
Despite being used for the calibration of other gauges, McLeod gauges are not perfect. There can be errors from physical limitations such as capillary depression phenomena, sorption and desorption from the gauge walls, and gas condensation within the gauge. Operator errors also exist such as difficulties of sighting the mercury heights properly. At 10"4 torr, the accuracy of a McLeod gauge is ±3%. Assuming no operator error, at 7.5 x 10"7 torr (the upper limits of a McLeod
If you overshoot the mark, simply turn the three-way stopcock to the #2 position for a moment pulling the mercury level down. It is not necessary to bring the mercury all the way back to the storage bulb, simply bring it below the /i, level.
Vacuum Gauges 7.5
Vacuum
bulb
Mercury storage bulb
413
Two - way
stopcock (to system)
Square Linear
scale scale cal,
uH.Ci.
— .001
.02
CIA
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.05 |
1 |
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Capillary |
A |
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= 1 |
\ |
—— |
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Capillary B |
.3 |
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2 |
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.4 |
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Three-way |
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stopcock |
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3 |
I |
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t-To air |
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"-~-Jo vacuum |
.6 |
4 |
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Position 1 |
Position 2 |
Allowing air into the |
Evacuating the |
McLeod gauge. |
McLeod gauge. |
Fig. 7.42 How to use and read the McLeod gauge.
gauge), inaccuracy can be ±15%. Specially adapted McLeod gauges can have their accuracy improved up to +0.9% and ±3.5%, respectively.
7.5.8 Bringing a McLeod Gauge to Vacuum Conditions
When first starting up a vacuum system from atmospheric conditions, do not expose a McLeod gauge to a sudden vacuum. Opening up the wrong stopcock at the wrong time can cause violent outgassing of the mercury. The mercury may be inadvertently sprayed throughout your system (despite traps) with enough force to break a line.
To bring a McLeod gauge to vacuum conditions and before turning the vacuum pumps on, be sure that the two-way and three-way stopcocks are rotated to their off positions. Then, turn on the vacuum pump and also do the following:
414 |
Vacuum Systems |
1.Slowly open the two-way stopcock a small amount, allowing the mercury to slowly rise from the storage bulb up to the bottom of the second bulb, then close the stopcock.
2.Slowly open the three-way stopcock to evacuate the McLeod gauge, drawing the mercury back into the storage bulb.
3.Repeat steps 1 and 2 several more times until the mercury enters the upper bulb very slowly and evenly. The number of repetitions is directly related to the volume of your system. Thus, the larger your system, the more repetitions of steps 1 and 2 will be required.
7.5.9 Returning a McLeod Gauge to Atmospheric Conditions
If you must return a McLeod gauge to atmospheric conditions, such as would be necessary for periodic regreasing of the stopcocks, the following procedures should be observed:
1.Slowly open the two-way stopcock (to system) allowing mercury into the McLeod gauge.
2.Close the two-way stopcock.
3.Open your system to the atmosphere.
4.Partially open the three-way stopcock to the atmosphere (Position 1).
5.Slowly open the two-way stopcock, letting the mercury fall back into the storage bulb.
7.5.10 The Tipping McLeod Gauge
Figure 7.43 shows a tipping McLeod gauge. All of the same components found in a standard McLeod gauge are there (except the three-way stopcock), but in a significantly different configuration. It is much easier to use this gauge than to use a standard McLeod gauge, but it is less accurate and cannot read as low a vacuum. Depending on the design, its lowest reading is from 0.050 to 0.001 mm Hg.
The tipping McLeod gauge can be mounted directly onto a vacuum system or can sit on a metal stand and connect to a vacuum system via a flexible vacuum hose. Regardless of the mounting technique used, there should be a two-way stopcock between the gauge and vacuum system that allows the user to shut off the McLeod gauge when it is not in use. This configuration limits the amount of mercury that backstreams into the vacuum system and the amount of material from the vacuum system that drifts into the McLeod gauge.
To operate the tipping McLeod gauge (see Fig. 7.44), do the following:
1.Open the two-way stopcock separating the gauge and vacuum system (not shown in Fig. 7.42).
Vacuum Gauges 7.5 |
415 |
Support rod
Capillary |
A |
Storage bulb |
|
|
Ground joint |
Capillary |
B |
(axis of rotation) |
|
Fig. 7.43 The tipping McLeod gauge.
2.Rotate by hand the gauge from the rest position to the vacuum measurement position.
3.Read Capillary B just as in Fig. 7.42.
4.After the measurement is read, release the gauge and it will rotate back to the rest position by itself.
5.Close the two-way stopcock off.
Do not let a tipping McLeod gauge flop around too freely. Although the stopcock grease will cause some amount of drag, twisting the gauge too quickly from one position to another is likely to cause the mercury to spill into the closed capillary, where it can be difficult to remove. When the stopcock to the tipping McLeod gauge is closed, you are depending upon the static vacuum to keep the gauge attached to the line. There are metal clips that do assist this connection, and they should be used to help prevent accidental removal of the gauge from the line.
Rest position |
Vacuum measurement |
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position |
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Fig. 7.44 How to use a tipping McLeod gauge. |
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