title: Diesel locomotive

text: In older locomotives, the throttle mechanism was ratcheted so that it was not possible to advance more than one power position at a time. The engine driver could not, for example, pull the throttle from notch 2 to notch 4 without stopping at notch 3. This feature was intended to prevent rough train handling due to abrupt power increases caused by rapid throttle motion ("throttle stripping", an operating rules violation on many railroads). Modern locomotives no longer have this restriction, as their control systems are able to smoothly modulate power and avoid sudden changes in train loading regardless of how the engine driver operates the controls.


title: Diesel locomotive

text: When the throttle is in the idle position, the prime mover receives minimal fuel, causing it to idle at low RPM. In addition, the traction motors are not connected to the main generator and the generator's field windings are not excited (energized) – the generator does not produce electricity without excitation. Therefore, the locomotive will be in "neutral". Conceptually, this is the same as placing an automobile's transmission into neutral while the engine is running.


title: Diesel locomotive

text: To set the locomotive in motion, the reverser control handle is placed into the correct position (forward or reverse), the brake is released and the throttle is moved to the run 1 position (the first power notch). An experienced engine driver can accomplish these steps in a coordinated fashion that will result in a nearly imperceptible start. The positioning of the reverser and movement of the throttle together is conceptually like shifting an automobile's automatic transmission into gear while the engine is idling.


title: Diesel locomotive

text: Placing the throttle into the first power position will cause the traction motors to be connected to the main generator and the latter's field coils to be excited. With excitation applied, the main generator will deliver electricity to the traction motors, resulting in motion. If the locomotive is running "light" (that is, not coupled to the rest of a train) and is not on an ascending grade, it will easily accelerate. On the other hand, if a long train is being started, the locomotive may stall as soon as some of the slack has been taken up, as the drag imposed by the train will exceed the tractive force being developed. An experienced engine driver will be able to recognize an incipient stall and will gradually advance the throttle as required to maintain the pace of acceleration.


title: Diesel locomotive

text: As the throttle is moved to higher power notches, the fuel rate to the prime mover will increase, resulting in a corresponding increase in RPM and horsepower output. At the same time, main generator field excitation will be proportionally increased to absorb the higher power. This will translate into increased electrical output to the traction motors, with a corresponding increase in tractive force. Eventually, depending on the requirements of the train's schedule, the engine driver will have moved the throttle to the position of maximum power and will maintain it there until the train has accelerated to the desired speed.


title: Diesel locomotive

text: The propulsion system is designed to produce maximum traction motor torque at start-up, which explains why modern locomotives are capable of starting trains weighing in excess of 15,000 tons, even on ascending grades.


title: Diesel locomotive

text: Current technology allows a locomotive to develop as much as 30% of its loaded driver weight in tractive force, amounting to of tractive force for a large, six-axle freight (goods) unit.


title: Diesel locomotive

text: In fact, a consist of such units can produce more than enough drawbar pull at start-up to damage or derail cars (if on a curve) or break couplers (the latter being referred to in North American railroad slang as "jerking a lung"). Therefore, it is incumbent upon the engine driver to carefully monitor the amount of power being applied at start-up to avoid damage. In particular, "jerking a lung" could be a calamitous matter if it were to occur on an ascending grade, except that the safety inherent in the correct operation of fail-safe automatic train brakes installed in wagons today, prevents runaway trains by automatically applying the wagon brakes when train line air pressure drops.


title: Diesel locomotive

text: A locomotive's control system is designed so that the main generator electrical power output is matched to any given engine speed. Given the innate characteristics of traction motors, as well as the way in which the motors are connected to the main generator, the generator will produce high current and low voltage at low locomotive speeds, gradually changing to low current and high voltage as the locomotive accelerates. Therefore, the net power produced by the locomotive will remain constant for any given throttle setting ("see power curve graph for notch 8").


title: Diesel locomotive

text: In older designs, the prime mover's governor and a companion device, the load regulator, play a central role in the control system. The governor has two external inputs: requested engine speed, determined by the engine driver's throttle setting, and actual engine speed (feedback). The governor has two external control outputs: fuel injector setting, which determines the engine fuel rate, and current regulator position, which affects main generator excitation. The governor also incorporates a separate overspeed protective mechanism that will immediately cut off the fuel supply to the injectors and sound an alarm in the cab in the event the prime mover exceeds a defined RPM. Not all of these inputs and outputs are necessarily electrical.


title: Diesel locomotive

text: As the load on the engine changes, its rotational speed will also change. This is detected by the governor through a change in the engine speed feedback signal. The net effect is to adjust both the fuel rate and the load regulator position so that engine RPM and torque (and therefore power output) will remain constant for any given throttle setting, regardless of actual road speed.


title: Diesel locomotive

text: In newer designs controlled by a "traction computer," each engine speed step is allotted an appropriate power output, or "kW reference", in software. The computer compares this value with actual main generator power output, or "kW feedback", calculated from traction motor current and main generator voltage feedback values. The computer adjusts the feedback value to match the reference value by controlling the excitation of the main generator, as described above. The governor still has control of engine speed, but the load regulator no longer plays a central role in this type of control system. However, the load regulator is retained as a "back-up" in case of engine overload. Modern locomotives fitted with electronic fuel injection (EFI) may have no mechanical governor; however, a "virtual" load regulator and governor are retained with computer modules.


title: Diesel locomotive

text: Traction motor performance is controlled either by varying the DC voltage output of the main generator, for DC motors, or by varying the frequency and voltage output of the VVVF for AC motors. With DC motors, various connection combinations are utilized to adapt the drive to varying operating conditions.


title: Diesel locomotive

text: At standstill, main generator output is initially low voltage/high current, often in excess of 1000 amperes per motor at full power. When the locomotive is at or near standstill, current flow will be limited only by the DC resistance of the motor windings and interconnecting circuitry, as well as the capacity of the main generator itself. Torque in a series-wound motor is approximately proportional to the square of the current. Hence, the traction motors will produce their highest torque, causing the locomotive to develop maximum tractive effort, enabling it to overcome the inertia of the train. This effect is analogous to what happens in an automobile automatic transmission at start-up, where it is in first gear and thereby producing maximum torque multiplication.


title: Diesel locomotive

text: As the locomotive accelerates, the now-rotating motor armatures will start to generate a counter-electromotive force (back EMF, meaning the motors are also trying to act as generators), which will oppose the output of the main generator and cause traction motor current to decrease. Main generator voltage will correspondingly increase in an attempt to maintain motor power, but will eventually reach a plateau. At this point, the locomotive will essentially cease to accelerate, unless on a downgrade. Since this plateau will usually be reached at a speed substantially less than the maximum that may be desired, something must be done to change the drive characteristics to allow continued acceleration. This change is referred to as "transition", a process that is analogous to shifting gears in an automobile.


title: Diesel locomotive

text: In older locomotives, it was necessary for the engine driver to manually execute transition by use of a separate control. As an aid to performing transition at the right time, the load meter (an indicator that shows the engine driver how much current is being drawn by the traction motors) was calibrated to indicate at which points forward or backward transition should take place. Automatic transition was subsequently developed to produce better-operating efficiency and to protect the main generator and traction motors from overloading from improper transition.


title: Diesel locomotive

text: Modern locomotives incorporate traction "inverters", AC to DC, capable of delivering 1,200 volts (earlier traction "generators", DC to DC, were capable of delivering only 600 volts). This improvement was accomplished largely through improvements in silicon diode technology. With the capability of delivering 1,200 volts to the traction motors, the need for "transition" was eliminated.


title: Diesel locomotive

text: Dynamic braking takes advantage of the fact that the traction motor armatures are always rotating when the locomotive is in motion and that a motor can be made to act as a generator by separately exciting the field winding. When dynamic braking is utilized, the traction control circuits are configured as follows:


title: Diesel locomotive

text: The aggregate effect of the above is to cause each traction motor to generate electric power and dissipate it as heat in the dynamic braking grid. A fan connected across the grid provides forced-air cooling. Consequently, the fan is powered by the output of the traction motors and will tend to run faster and produce more airflow as more energy is applied to the grid.


title: Diesel locomotive

text: Ultimately, the source of the energy dissipated in the dynamic braking grid is the motion of the locomotive as imparted to the traction motor armatures. Therefore, the traction motors impose drag and the locomotive acts as a brake. As speed decreases, the braking effect decays and usually becomes ineffective below approximately 16 km/h (10 mph), depending on the gear ratio between the traction motors and axles.


title: Diesel locomotive

text: Dynamic braking is particularly beneficial when operating in mountainous regions; where there is always the danger of a runaway due to overheated friction brakes during descent. In such cases, dynamic brakes are usually applied in conjunction with the air brakes, the combined effect being referred to as blended braking. The use of blended braking can also assist in keeping the slack in a long train stretched as it crests a grade, helping to prevent a "run-in", an abrupt bunching of train slack that can cause a derailment. Blended braking is also commonly used with commuter trains to reduce wear and tear on the mechanical brakes that is a natural result of the numerous stops such trains typically make during a run.


title: Diesel locomotive

text: These special locomotives can operate as an electric locomotive or as a diesel locomotive. The Long Island Rail Road, Metro-North Railroad and New Jersey Transit Rail Operations operate dual-mode diesel–electric/third-rail (catenary on NJTransit) locomotives between non-electrified territory and New York City because of a local law banning diesel-powered locomotives in Manhattan tunnels. For the same reason, Amtrak operates a fleet of dual-mode locomotives in the New York area. British Rail operated dual diesel–electric/electric locomotives designed to run primarily as electric locomotives with reduced power available when running on diesel power. This allowed railway yards to remain unelectrified, as the third rail power system is extremely hazardous in a yard area.


title: Diesel locomotive

text: Diesel–hydraulic locomotives use one or more torque converters, in combination with fixed ratio gears. Drive shafts and gears form the final drive to convey the power from the torque converters to the wheels, and to effect reverse. The difference between hydraulic and mechanical systems is where the speed and torque is adjusted. In the mechanical transmission system that has multiple ratios such as in a gear box, if there is a hydraulic section, it is only to allow the engine to run when the train is too slow or stopped. In the hydraulic system, hydraulics are the primary system for adapting engine speed and torque to the train's situation, with gear selection for only limited use, such as reverse gear.


title: Diesel locomotive

text: Hydraulic drive systems using a hydrostatic hydraulic drive system have been applied to rail use. Modern examples included shunting locomotives by Cockerill (Belgium), 4 to 12 tonne narrow gauge industrial locomotives by Atlas Copco subsidiary GIA. Hydrostatic drives are also utilised in railway maintenance machines (tampers, rail grinders).


title: Diesel locomotive

text: Application of hydrostatic transmissions is generally limited to small shunting locomotives and rail maintenance equipment, as well as being used for non-tractive applications in diesel engines such as drives for traction motor fans.


title: Diesel locomotive

text: Hydrokinetic transmission (also called hydrodynamic transmission) uses a torque converter. A torque converter consists of three main parts, two of which rotate, and one (the stator) that has a lock preventing backwards rotation and adding output torque by redirecting the oil flow at low output RPM. All three main parts are sealed in an oil-filled housing. To match engine speed to load speed over the entire speed range of a locomotive some additional method is required to give sufficient range. One method is to follow the torque converter with a mechanical gearbox which switches ratios automatically, similar to an automatic transmission in an automobile. Another method is to provide several torque converters each with a range of variability covering part of the total required; all the torque converters are mechanically connected all the time, and the appropriate one for the speed range required is selected by filling it with oil and draining the others. The filling and draining is carried out with the transmission under load, and results in very smooth range changes with no break in the transmitted power.


title: Diesel locomotive

text: Diesel-hydraulic locomotives are less efficient than diesel–electrics. The first-generation BR diesel hydraulics were significantly less efficient (c. 65%) than diesel electrics (c. 80%), Moreover, initial versions were found in many countries to be mechanically more complicated and more likely to break down. Hydraulic transmission for locomotives was developed in Germany. There is still debate over the relative merits of hydraulic vs. electrical transmission systems: advantages claimed for hydraulic systems include lower weight, high reliability, and lower capital cost.


title: Diesel locomotive

text: By the 21st century, for diesel locomotive traction worldwide the majority of countries used diesel–electric designs, with diesel-hydraulic designs not found in use outside Germany and Japan, and some neighbouring states, where it is used in designs for freight work.


title: Diesel locomotive

text: In Germany and Finland, diesel–hydraulic systems have achieved high reliability in operation. In the UK the diesel–hydraulic principle gained a poor reputation due to the poor durability and reliability of the Maybach Mekydro hydraulic transmission. Argument continues over the relative reliability of hydraulic systems, with questions over whether data has been manipulated to favour local suppliers over non-German ones.


title: Diesel locomotive

text: Diesel–hydraulic drive is common in multiple units, with various transmission designs used including Voith torque converters, and fluid couplings in combination with mechanical gearing.


title: Diesel locomotive

text: The majority of British Rail's second generation passenger DMU stock used hydraulic transmission. In the 21st century, designs using hydraulic transmission include Bombardier Turbostar, Talent, RegioSwinger families; diesel engined versions of the Siemens Desiro platform, and the Stadler Regio-Shuttle.


title: Diesel locomotive

text: Diesel–hydraulic locomotives have a smaller market share than those with diesel–electric transmission – the main worldwide user of main-line hydraulic transmissions was the Federal Republic of Germany, with designs including the 1950s DB class V 200, and the 1960 and 1970s DB Class V 160 family. British Rail introduced a number of diesel-hydraulic designs during its 1955 Modernisation Plan, initially license-built versions of German designs (see ). In Spain, Renfe used high power to weight ratio twin-engine German designs to haul high speed trains from the 1960s to 1990s. (See Renfe Classes 340, 350, 352, 353, 354)


title: Diesel locomotive

text: Other main-line locomotives of the post-war period included the 1950s GMD GMDH-1 experimental locomotives; the Henschel & Son built South African Class 61-000; in the 1960s Southern Pacific bought 18 Krauss-Maffei KM ML-4000 diesel–hydraulic locomotives. The Denver & Rio Grande Western Railroad also bought three, all of which were later sold to SP.


title: Diesel locomotive

text: In Finland, over 200 Finnish-built VR class Dv12 and Dr14 diesel–hydraulics with Voith transmissions have been continuously used since the early 1960s. All units of Dr14 class and most units of Dv12 class are still in service. VR has abandoned some weak-conditioned units of 2700 series Dv12s.


title: Diesel locomotive

text: In the 21st century series production standard gauge diesel–hydraulic designs include the Voith Gravita, ordered by Deutsche Bahn, and the Vossloh G2000 BB, G1206 and G1700 designs, all manufactured in Germany for freight use.


title: Diesel locomotive

text: Steam-diesel hybrid locomotives can use steam generated from a boiler or diesel to power a piston engine. The "Cristiani Compressed Steam System" used a diesel engine to power a compressor to drive and recirculate steam produced by a boiler; effectively using steam as the power transmission medium, with the diesel engine being the prime mover


title: Diesel locomotive

text: The diesel-pneumatic locomotive was of interest in the 1930s because it offered the possibility of converting existing steam locomotives to diesel operation. The frame and cylinders of the steam locomotive would be retained and the boiler would be replaced by a diesel engine driving an air compressor. The problem was low thermal efficiency because of the large amount of energy wasted as heat in the air compressor. Attempts were made to compensate for this by using the diesel exhaust to re-heat the compressed air but these had limited success. A German proposal of 1929 did result in a prototype but a similar British proposal of 1932, to use an LNER Class R1 locomotive, never got beyond the design stage.


title: Diesel locomotive

text: Most diesel locomotives are capable of multiple-unit operation (MU) as a means of increasing horsepower and tractive effort when hauling heavy trains. All North American locomotives, including export models, use a standardized AAR electrical control system interconnected by a 27-pin MU cable between the units. For UK-built locomotives, a number of incompatible control systems are used, but the most common is the Blue Star system, which is electro-pneumatic and fitted to most early diesel classes. A small number of types, typically higher-powered locomotives intended for passenger only work, do not have multiple control systems. In all cases, the electrical control connections made common to all units in a consist are referred to as trainlines.


title: Diesel locomotive

text: The result is that all locomotives in a consist behave as one in response to the engine driver's control movements.


title: Diesel locomotive

text: The ability to couple diesel–electric locomotives in an MU fashion was first introduced in the EMC EA/EB of 1937. Electrical interconnections were made so one engine driver could operate the entire consist from the head-end unit.


title: Diesel locomotive

text: In mountainous regions, it is common to interpose helper locomotives in the middle of the train, both to provide the extra power needed to ascend a grade and to limit the amount of stress applied to the draft gear of the car coupled to the head-end power. The helper units in such distributed power configurations are controlled from the lead unit's cab through coded radio signals. Although this is technically not an MU configuration, the behaviour is the same as with physically interconnected units.


title: Diesel locomotive

text: Cab arrangements vary by builder and operator. Practice in the U.S. has traditionally been for a cab at one end of the locomotive with limited visibility if the locomotive is not operated cab forward. This is not usually a problem as U.S. locomotives are usually operated in pairs, or threes, and arranged so that a cab is at each end of each set. European practice is usually for a cab at each end of the locomotive as trains are usually light enough to operate with one locomotive. Early U.S. practice was to add power units without cabs (booster or B units) and the arrangement was often A-B, A-A, A-B-A, A-B-B, or A-B-B-A where A was a unit with a cab. Center cabs were sometimes used for switch locomotives.


title: Diesel locomotive

text: In North American railroading, a cow-calf set is a pair of switcher-type locomotives: one (the cow) equipped with a driving cab, the other (the calf) without a cab, and controlled from the cow through cables. Cow-calf sets are used in heavy switching and hump yard service. Some are radio controlled without an operating engineer present in the cab. This arrangement is also known as master-slave. Where two connected units were present, EMD called these TR-2s (approximately ); where three units, TR-3s (approximately ).


title: Diesel locomotive

text: Cow-calves have largely disappeared as these engine combinations exceeded their economic lifetimes many years ago.


title: Diesel locomotive

text: Present North American practice is to pair two 3,000hp GP40-2 or SD40-2 road switchers, often nearly worn-out and very soon ready for rebuilding or scrapping, and to utilize these for so-called "transfer" uses, for which the TR-2, TR-3 and TR-4 engines were originally intended, hence the designation TR, for "transfer".


title: Diesel locomotive

text: Occasionally, the second unit may have its prime-mover and traction alternator removed and replaced by concrete or steel ballast and the power for traction obtained from the master unit. As a 16-cylinder prime-mover generally weighs in the range, and a 3,000hp traction alternator generally weighs in the range, this would mean that would be needed for ballast.


title: Diesel locomotive

text: A pair of fully capable "Dash 2" units would be rated . A "Dash 2" pair where only one had a prime-mover/alternator would be rated 3,000hp, with all power provided by master, but the combination benefits from the tractive effort provided by the slave as engines in transfer service are seldom called upon to provide 3,000hp much less 6,000hp on a continuous basis.


title: Diesel locomotive

text: A standard diesel locomotive presents a very low fire risk but "flame proofing" can reduce the risk even further. This involves fitting a water-filled box to the exhaust pipe to quench any red-hot carbon particles that may be emitted. Other precautions may include a fully insulated electrical system (neither side earthed to the frame) and all electric wiring enclosed in conduit.


title: Diesel locomotive

text: The flameproof diesel locomotive has replaced the fireless steam locomotive in areas of high fire risk such as oil refineries and ammunition dumps. Preserved examples of flameproof diesel locomotives include:


title: Diesel locomotive

text: Latest development of the "Flameproof Diesel Vehicle Applied New Exhaust Gas Dry Type Treatment System" does not need the water supply.


title: Diesel locomotive

text: The lights fitted to diesel locomotives vary from country to country. North American locomotives are fitted with two headlights (for safety in case one malfunctions) and a pair of ditch lights. The latter are fitted low down at the front and are designed to make the locomotive easily visible as it approaches a grade crossing. Older locomotives may be fitted with a Gyralite or Mars Light instead of the ditch lights.


title: Diesel locomotive

text: Although diesel locomotives generally emit less sulphur dioxide, a major pollutant to the environment, and greenhouse gases than steam locomotives, they are not completely clean in that respect. Furthermore, like other diesel powered vehicles, they emit nitrogen oxides and fine particles, which are a risk to public health. In fact, in this last respect diesel locomotives may perform worse than steam locomotives.


title: Diesel locomotive

text: For years, it was thought by American government scientists who measure air pollution that diesel locomotive engines were relatively clean and emitted far less health-threatening emissions than those of diesel trucks or other vehicles; however, the scientists discovered that because they used faulty estimates of the amount of fuel consumed by diesel locomotives, they grossly understated the amount of pollution generated annually. After revising their calculations, they concluded that the annual emissions of nitrogen oxide, a major ingredient in smog and acid rain, and soot would be by 2030 nearly twice what they originally assumed. In Europe, where most major railways have been electrified, there is less concern.


title: Diesel locomotive

text: This would mean that in the USA diesel locomotives would be releasing more than 800,000tons of nitrogen oxide and 25,000tons of soot every year within a quarter of a century, in contrast to the EPA's previous projections of 480,000tons of nitrogen dioxide and 12,000tons of soot. Since this was discovered, to reduce the effects of the diesel locomotive on humans (who are breathing the noxious emissions) and on plants and animals, it is considered practical to install traps in the diesel engines to reduce pollution levels and other methods of pollution control (e.g., use of biodiesel).


title: Diesel locomotive

text: Diesel locomotive pollution has been of particular concern in the city of Chicago. The "Chicago Tribune" reported levels of diesel soot inside locomotives leaving Chicago at levels hundreds of times above what is normally found on streets outside. Residents of several neighborhoods are most likely exposed to diesel emissions at levels several times higher than the national average for urban areas.


title: Diesel locomotive

text: In 2008, the United States Environmental Protection Agency (EPA) mandated regulations requiring all new or refurbished diesel locomotives to meet Tier II pollution standards that slash the amount of allowable soot by 90% and require an 80% reduction in nitrogen oxide emissions. "See" List of low emissions locomotives.


title: Diesel locomotive

text: Other technologies that are being deployed to reduce locomotive emissions and fuel consumption include "Genset" switching locomotives and hybrid Green Goat designs. Genset locomotives use multiple smaller high-speed diesel engines and generators (generator sets), rather than a single medium-speed diesel engine and a single generator. Because of the cost of developing clean engines, these smaller high-speed engines are based on already developed truck engines. Green Goats are a type of hybrid switching locomotive utilizing a small diesel engine and a large bank of rechargeable batteries. Switching locomotives are of particular concern as they typically operate in a limited area, often in or near urban centers, and spend much of their time idling. Both designs reduce pollution below EPA Tier II standards and cut or eliminate emissions during idle.


title: Diesel locomotive

text: As diesel locomotives advanced, the cost of manufacturing and operating them dropped, and they became cheaper to own and operate than steam locomotives. In North America, steam locomotives were custom-made for specific railway routes, so economies of scale were difficult to achieve. Though more complex to produce with exacting manufacturing tolerances ( for diesel, compared with for steam), diesel locomotive parts were easier to mass-produce. Baldwin Locomotive Works offered almost 500 steam models in its heyday, while EMD offered fewer than ten diesel varieties. In the United Kingdom, British Railways built steam locomotives to standard designs from 1951 onwards. These included standard, interchangeable parts, making them cheaper to produce than the diesel locomotives then available. The capital cost per drawbar horse power was £13 6s (steam), £65 (diesel), £69 7s (turbine) and £17 13s (electric).


title: Diesel locomotive

text: Diesel locomotives offer significant operating advantages over steam locomotives. They can safely be operated by one person, making them ideal for switching/shunting duties in yards (although for safety reasons many main-line diesel locomotives continue to have two-person crews: an engineer and a conductor/switchman) and the operating environment is much more attractive, being quieter, fully weatherproof and without the dirt and heat that is an inevitable part of operating a steam locomotive. Diesel locomotives can be worked in multiple with a single crew controlling multiple locomotives in a single train – something not practical with steam locomotives. This brought greater efficiencies to the operator, as individual locomotives could be relatively low-powered for use as a single unit on light duties but marshaled together to provide the power needed on a heavy train. With steam traction, a single very powerful and expensive locomotive was required for the heaviest trains or the operator resorted to double heading with multiple locomotives and crews, a method which was also expensive and brought with it its own operating difficulties.


title: Diesel locomotive

text: Diesel engines can be started and stopped almost instantly, meaning that a diesel locomotive has the potential to incur no fuel costs when not being used. However, it is still the practice of large North American railroads to use straight water as a coolant in diesel engines instead of coolants that incorporate anti-freezing properties; this results in diesel locomotives being left idling when parked in cold climates instead of being completely shut down. A diesel engine can be left idling unattended for hours or even days, especially since practically every diesel engine used in locomotives has systems that automatically shut the engine down if problems such as a loss of oil pressure or coolant loss occur. Automatic start/stop systems are available which monitor coolant and engine temperatures. When the unit is close to having its coolant freeze, the system restarts the diesel engine to warm the coolant and other systems.


title: Diesel locomotive

text: Steam locomotives require intensive maintenance, lubrication, and cleaning before, during, and after use. Preparing and firing a steam locomotive for use from cold can take many hours. They can be kept in readiness between uses with a low fire, but this requires regular stoking and frequent attention to maintain the level of water in the boiler. This may be necessary to prevent the water in the boiler freezing in cold climates, so long as the water supply is not frozen. After use a steam locomotive requires a lengthy disposal operation to perform cleaning, inspection, maintenance and refilling with water and fuel before it is ready for its next duty. By contrast, as early as 1939 EMD was promoting its FT Series locomotive as needing no maintenance between 30-day inspections beyond refuelling and basic fluid level and safety checks which could be performed with the prime mover still running. Railways converting from steam to diesel operation in the 1940s and 1950s found that for a given period diesel locomotives were available for, on average, three or four times more revenue-earning hours than equivalent steam locomotives, allowing locomotive fleets to be cut drastically in size while maintaining operational capacity.


title: Diesel locomotive

text: The maintenance and operational costs of steam locomotives were much higher than diesels. Annual maintenance costs for steam locomotives accounted for 25% of the initial purchase price. Spare parts were cast from wooden masters for specific locomotives. The sheer number of unique steam locomotives meant that there was no feasible way for spare-part inventories to be maintained. With diesel locomotives spare parts could be mass-produced and held in stock ready for use and many parts and sub-assemblies could be standardized across an operator's fleet using different models of locomotive from the same builder. Modern diesel locomotive engines are designed to allow the power assemblies (systems of working parts and their block interfaces) to be replaced while keeping the main block in the locomotive, which greatly reduces the time that a locomotive is out of revenue-generating service when it requires maintenance.


title: Diesel locomotive

text: Steam engines required large quantities of coal and water, which were expensive variable operating costs. Further, the thermal efficiency of steam was considerably less than that of diesel engines. Diesel's theoretical studies demonstrated potential thermal efficiencies for a compression ignition engine of 36% (compared with 6–10% for steam), and an 1897 one-cylinder prototype operated at a remarkable 26% efficiency.


title: Diesel locomotive

text: However, one study published in 1959 suggested that many of the comparisons between diesel and steam locomotives were made unfairly, mostly because diesels were a newer technology. After painstaking analysis of financial records and technological progress, the author found that if research had continued on steam technology instead of diesel, there would be negligible financial benefit in converting to diesel locomotion.


title: Diesel locomotive

text: By the mid-1960s, diesel locomotives had effectively replaced steam locomotives where electric traction was not in use. Attempts to develop advanced steam technology continue in the 21st century, but have not had a significant effect.


title: Hip replacement

text: Hip replacement is a surgical procedure in which the hip joint is replaced by a prosthetic implant, that is, a hip prosthesis. Hip replacement surgery can be performed as a total replacement or a hemi (half) replacement. Such joint replacement orthopaedic surgery is generally conducted to relieve arthritis pain or in some hip fractures. A total hip replacement (total hip arthroplasty or THA) consists of replacing both the acetabulum and the femoral head while hemiarthroplasty generally only replaces the femoral head. Hip replacement is one of the most common orthopaedic operations, though patient satisfaction varies widely. Approximately 58% of total hip replacements are estimated to last 25 years. The average cost of a total hip replacement in 2012 was $40,364 in the United States, and about $7,700 to $12,000 in most European countries.


title: Hip replacement

text: Total hip replacement is most commonly used to treat joint failure caused by osteoarthritis. Other indications include rheumatoid arthritis, avascular necrosis, traumatic arthritis, protrusio acetabuli, certain hip fractures, benign and malignant bone tumors, arthritis associated with Paget's disease, ankylosing spondylitis and juvenile rheumatoid arthritis. The aims of the procedure are pain relief and improvement in hip function. Hip replacement is usually considered only after other therapies, such as physical therapy and pain medications, have failed.


title: Hip replacement

text: Risks and complications in hip replacement are similar to those associated with all joint replacements. They can include infection, dislocation, limb length inequality, loosening, impingement, osteolysis, metal sensitivity, nerve palsy, chronic pain and death. Weight loss surgery before a hip replacement does not appear to change outcomes.


title: Hip replacement

text: Dislocation (the ball coming out of the socket) is the most common complication. The most common causes vary by the duration since the surgery.


title: Hip replacement

text: Hip prosthesis dislocation mostly occurs in the first three months after insertion, mainly because of incomplete scar formation and relaxed soft tissues. It takes eight to twelve weeks for the soft tissues injured or cut during surgery to heal. The chance of this is diminished if less tissue is cut, if the cut tissue is repaired and if large diameter head balls are used.


title: Hip replacement

text: Dislocations occurring between three months and five years after insertion usually occur due to malposition of the components, or dysfunction of nearby muscles.


title: Hip replacement

text: Surgeons who perform more operations tend to have fewer dislocations. An anterior approach seems to lower dislocation rates when small diameter heads are used, but that benefit has not been shown when compared to modern posterior incisions with the use of larger diameter heads. The use of larger diameter head size in itself decreases dislocation risk, even though this correlation is only found in head sizes up to 28 mm: larger heads do not additional decreases in dislocation rate. Keeping the leg out of certain positions during the first few months after surgery further reduces risk.


title: Hip replacement

text: Infection is one of the most common causes for revision of a total hip replacement. The incidence of infection in primary hip replacement is 1% or less in the United States. Risk factors for infection include obesity, diabetes, smoking, immunosuppressive medications or diseases, and history of infection.


title: Hip replacement

text: Most adults have a limb length inequality of 0–2 cm which causes no deficits. It is common for people to sense a larger limb length inequality after total hip replacement. Sometimes the leg seems long immediately after surgery when in fact both are equal length. An arthritic hip can develop contractures that make the leg behave as if it is short. When these are relieved with replacement surgery and normal motion and function are restored, the body feels that the limb is now longer than it was. This feeling usually subsides by six months after surgery as the body adjusts to the new hip joint. The cause of this feeling is variable, and usually related to abductor muscle weakness, pelvic obliquity, and minor lengthening of the hip during surgery (<1 cm) to achieve stability and restore the joint to pre-arthritic mechanics. If the limb length difference remains bothersome to the patient more than six months after surgery, a shoe lift can be used. Only in extreme cases is surgery required for correction. The perceived difference in limb length for a patient after surgery is a common cause for lawsuits against the healthcare provider.


title: Hip replacement

text: Intraoperative fractures may occur. After surgery, bones with internal fixation devices in situ are at risk of periprosthetic fractures at the end of the implant, an area of relative mechanical stress. Post-operative femoral fractures are graded by the Vancouver classification.


title: Hip replacement

text: Venous thrombosis such as deep vein thrombosis and pulmonary embolism are relatively common following hip replacement surgery. Standard treatment with anticoagulants is for 7–10 days; however, treatment for 21+ days may be superior. Extended-duration anticoagulants (up to 35 days following surgery) may prevent VTE in people undergoing hip replacement surgery. Other research suggested that anticoagulants in otherwise healthy patients undergoing a so-called fast track protocol with hospital stays under five days, might only be necessary while in the hospital. Emerging evidence supports the use of aspirin for venous thromboembolism prophylaxis. Large randomised control trials suggested that aspirin is not inferior to low-molecular weight heparins and rivaroxaban. However, aspirin may not be appropriate in all cases, especially for patients who have additional risk factors for venous thromboembolisms or may have an inadequate response to aspirin.


title: Hip replacement

text: Some physicians and patients may consider having an ultrasonography for deep vein thrombosis after hip replacement. However, this kind of screening should only be done when indicated because to perform it routinely would be unnecessary health care.


title: Hip replacement

text: Intermittent pneumatic compression (IPC) devices are sometimes used for prevention of blood clots following total hip replacement.


title: Hip replacement

text: Many long-term problems with hip replacements are the result of osteolysis. This is the loss of bone caused by the body's reaction to polyethylene wear debris, fine bits of plastic that wear off the cup liner over time. An inflammatory process causes bone resorption that may lead to subsequent loosening of the hip implants and even fractures in the bone around the implants. Ceramic bearing surfaces may eliminate the generation of wear particles. Metal cup liners joined with metal heads (metal-on-metal hip arthroplasty) were developed for similar reasons. In the lab these show excellent wear characteristics and benefit from a different mode of lubrication.


title: Hip replacement

text: Highly cross-linked polyethylene plastic liners experience significantly reduced plastic wear debris. The newer ceramic and metal prostheses may not have long-term performance records. Ceramic piece breakage can lead to catastrophic failure. This occurs in about 2% of implants. They may also cause an audible, high pitched squeaking noise with activity. Metal-on-metal arthroplasty can release metal debris into the body. Highly cross linked polyethylene is not as strong as regular polyethylene. These plastic liners can crack or break free of the metal shell that holds them.


title: Hip replacement

text: On radiography, it is normal to see thin radiolucent areas of less than 2 mm around hip prosthesis components, or between a cement mantle and bone. These may indicate loosening of the prosthesis if they are new or changing, while areas greater than 2 mm may be harmless if they are stable. The most important prognostic factors of cemented cups are absence of radiolucent lines in DeLee and Charnley zone I, as well as adequate cement mantle thickness. In the first year after insertion of uncemented femoral stems, it is normal to have mild subsidence (less than 10 mm). The direct anterior approach has been shown to itself be a risk factor for early femoral component loosening.


title: Hip replacement

text: Concerns were raised in the early 2000s regarding metal sensitivity and the potential dangers of metal particulate debris from hip prostheses, including the development of "pseudotumors", soft tissue masses containing necrotic tissue, around the hip joint. It appears these masses were more common in women, and these patients showed a higher level of iron in the blood. The cause was then unknown, and was probably multifactorial. There may have been a toxic reaction to an excess of particulate metal wear debris or a hypersensitivity reaction to a "normal" amount of metal debris.


title: Hip replacement

text: Metal hypersensitivity is a well-established phenomenon and is not uncommon, affecting about 10–15% of the population. Skin contact with certain metals can cause immune reactions such as hives, eczema, redness and itching. Although little is known about the short- and long-term pharmacodynamics and bioavailability of circulating metal degradation products "in vivo", there have been many reports of immunologic-type responses temporally associated with implantation of metal components. Individual case reports link immune hypersensitivity reactions with adverse performance of metallic cardiovascular, orthopedic and plastic surgical and dental implants.


title: Hip replacement

text: Most hip replacements consist of cobalt and chromium alloys, or titanium. Stainless steel is no longer used. Any metal implant releases its constituent ions into the blood. Typically, these are excreted in the urine, but in certain individuals the ions can accumulate in the body. In implants which involve metal-on-metal contact, microscopic fragments of cobalt and chromium can be absorbed into the person's bloodstream. There are reports of cobalt toxicity with hip replacement, particularly metal-on-metal hip replacements, which are no longer in use.


title: Hip replacement

text: Use of metal-on-metal hip replacements from the 1970s was discontinued in the 1980s and 1990s, particularly after the discovery of aseptic lymphocyte-dominant vasculitis-associated lesions (ALVAL). However, the FDA's 510k approval process allowed companies to have new and "improved" metal-on-metal hips approved without much clinical testing. Some people with these prostheses experienced similar reactions to the metal debris as occurred in the 20th century; some devices were recalled.


title: Hip replacement

text: Post operative sciatic nerve palsy is another possible complication. The frequency of this complication is low. Femoral nerve palsy is another, but much rarer, complication. Both of these will typically resolve over time, but the healing process is slow. Patients with pre-existing nerve injury are at greater risk of experiencing this complication and are also slower to recover.


title: Hip replacement

text: A few patients who have had a hip replacement suffer chronic pain after the surgery. Groin pain can develop if the muscle that raises the hip (iliopsoas) rubs against the edge of the acetabular cup. Bursitis can develop at the trochanter where a surgical scar crosses the bone, or if the femoral component used pushes the leg out to the side too far. Also some patients can experience pain in cold or damp weather. Incision made in the front of the hip (anterior approach) can cut a nerve running down the thigh leading to numbness in the thigh and occasionally chronic pain at the point where the nerve was cut (a neuroma).


title: Hip replacement

text: By 2010, reports in the orthopaedic literature increasingly cited the problem of early failure of metal-on-metal prostheses in a small percentage of patients. Failures may have related to the release of minute metallic particles or metal ions from wear on the implants, causing pain and disability severe enough to require revision surgery in 1–3% of patients. Design deficits of some prothesis models, especially with heat-treated alloys and a lack of specialized surgical experience, accounted for most of the failures. In 2010, surgeons at medical centers such as the Mayo Clinic reported curtailing their use of metal-on-metal implants by 80 percent over the previous year, in favor of those made from other materials, such as combinations of metal and plastic. The cause of these failures remains controversial, and may include both design factors, operative technique factors, and factors related to patient immune response. In the United Kingdom, the Medicines and Healthcare products Regulatory Agency commenced an annual monitoring regime for metal-on-metal hip replacement patients from May 2010. Data which are shown in The Australian Orthopaedic Association's 2008 National Joint replacement registry, a record of nearly every hip implanted in that country over the previous 10 years, tracked 6,773 BHR (Birmingham Hip Resurfacing) hips and found that less than 0.33% may have been revised due to the patient's reaction to the metal component. Other, similar, metal-on-metal designs have not fared as well, with some reports showing that 76–100% of people with these metal-on-metal implants with aseptic implant failures and needing revision surgery also had histological evidence of inflammation, accompanied by extensive lymphocyte infiltrates characteristic of delayed-type hypersensitivity reactions. It is not clear to what extent this phenomenon negatively affects orthopedic implant patients. However, for patients presenting with signs of allergic reaction, testing for sensitivity should be conducted. Removal of the device should be considered, since removal may alleviate the symptoms. Patients who have allergic reactions to alloy jewelry are more likely to have reactions to orthopedic implants. There is increasing awareness of the phenomenon of metal sensitivity, and many surgeons now take this into account when planning which implant is optimal for each patient.


title: Hip replacement

text: On March 12, 2012, "The Lancet" published a study, based on data from the National Joint Registry of England and Wales, finding that metal-on-metal hip implants failed at much higher rates than other types of hip implants, and calling for a ban on all metal-on-metal hip prostheses. The analysis of 402,051 hip replacements showed that 6.2% of metal-on-metal hip implants had failed within five years, compared to 1.7% of metal-on-plastic and 2.3% of ceramic-on-ceramic hip implants. Each increase in head size of metal-on-metal hip implants was associated with a 2% increase in failure rate. Surgeons of the British Hip Society recommended that large head metal-on-metal implants should no longer be implanted.


title: Hip replacement

text: On February 10, 2011, the U.S. FDA issued an advisory on metal-on-metal hip implants, stating it was continuing to gather and review all available information about metal-on-metal hip systems. On June 27–28, 2012, an advisory panel met to decide whether to impose new standards, taking into account findings of the study in "The Lancet". No new standards, such as routine checking of blood levels of metal ions, were set, but guidance was updated. The U.S. FDA does not require hip implants to be tested in clinical trials before they can be sold in the U.S. Instead, companies making new hip implants only need to prove that they are "substantially equivalent" to other hip implants already on the market. The exception is metal-on-metal implants, which were not tested in clinical trials, but, due to the high revision rate of metal-on-metal hips, the FDA has stated that, in the future, clinical trials will be required for approval, and that post-market studies will be required to keep metal-on-metal hip implants on the market.


title: Hip replacement

text: The modern artificial joint owes much to the 1962 work of Sir John Charnley at Wrightington Hospital. His work in the field of tribology resulted in a design that almost completely replaced the other designs by the 1970s. Charnley's design consisted of three parts:


title: Hip replacement

text: The replacement joint, which was known as the Low Friction Arthroplasty, was lubricated with synovial fluid. The small femoral head () was chosen for Charnley's belief that it would have lower friction against the acetabular component and thus wear out the acetabulum more slowly. Unfortunately, the smaller head dislocated more easily. Alternative designs with larger heads such as the Mueller prosthesis were proposed. Stability was improved, but acetabular wear and subsequent failure rates were increased with these designs. The Teflon acetabular components of Charnley's early designs failed within a year or two of implantation. This prompted a search for a more suitable material. A German salesman showed a polyethylene gear sample to Charnley's machinist, sparking the idea to use this material for the acetabular component. The UHMWPE acetabular component was introduced in 1962. Charnley's other major contribution was to use polymethylmethacrylate (PMMA) bone cement to attach the two components to the bone. For over two decades, the Charnley Low Friction Arthroplasty, and derivative designs were the most used systems in the world. It formed the basis for all modern hip implants. An example can be seen at the Science Museum, London.


title: Hip replacement

text: The Exeter hip stem was developed in the United Kingdom during the same time as the Charnley device. Its development occurred following a collaboration between Orthopaedic Surgeon Robin Ling and University of Exeter engineer Clive Lee and it was first implanted at the Princess Elizabeth Orthopaedic Hospital in Exeter in 1970. The Exeter Hip is a cemented device, but with a slightly different stem geometry. Both designs have shown excellent long-term durability when properly placed and are still widely used in slightly modified versions.


title: Hip replacement

text: Early implant designs had the potential to loosen from their attachment to the bones, typically becoming painful ten to twelve years after placement. In addition, erosion of the bone around the implant was seen on x-rays. Initially, surgeons believed this was caused by an abnormal reaction to the cement holding the implant in place. That belief prompted a search for an alternative method to attach the implants. The Austin Moore device had a small hole in the stem into which bone graft was placed before implanting the stem. It was hoped bone would then grow through the window over time and hold the stem in position. Success was unpredictable and the fixation not very robust. In the early 1980s, surgeons in the United States applied a coating of small beads to the Austin Moore device and implanted it without cement. The beads were constructed so that gaps between beads matched the size of the pores in native bone. Over time, bone cells from the patient would grow into these spaces and fix the stem in position. The stem was modified slightly to fit more tightly into the femoral canal, resulting in the Anatomic Medullary Locking (AML) stem design. With time, other forms of stem surface treatment and stem geometry have been developed and improved.


title: Hip replacement

text: Initial hip designs were made of a one-piece femoral component and a one-piece acetabular component. Current designs have a femoral stem and separate head piece. Using an independent head allows the surgeon to adjust leg length (some heads seat more or less onto the stem) and to select from various materials from which the head is formed. A modern acetabulum component is also made up of two parts: a metal shell with a coating for bone attachment and a separate liner. First the shell is placed. Its position can be adjusted, unlike the original cemented cup design which are fixed in place once the cement sets. When proper positioning of the metal shell is obtained, the surgeon may select a liner made from various materials.


title: Hip replacement

text: To combat loosening caused by polyethylene wear debris, hip manufacturers developed improved and novel materials for the acetabular liners. Ceramic heads mated with regular polyethylene liners or a ceramic liner were the first significant alternative. Metal liners to mate with a metal head were also developed. At the same time these designs were being developed, the problems that caused polyethylene wear were determined and manufacturing of this material improved. Highly crosslinked UHMWPE was introduced in the late 1990s. The most recent data comparing the various bearing surfaces has shown no clinically significant differences in their performance. Potential early problems with each material are discussed below. Performance data after 20 or 30 years may be needed to demonstrate significant differences in the devices. All newer materials allow use of larger diameter femoral heads. Use of larger heads significantly decreases the chance of the hip dislocating, which remains the greatest complication of the surgery.


title: Hip replacement

text: When available implants are used, cemented stems tend to have a better longevity than uncemented stems. No significant difference is observed in the clinical performance of the various methods of surface treatment of uncemented devices. Uncemented stems are selected for patients with good quality bone that can resist the forces needed to drive the stem in tightly. Cemented devices are typically selected for patients with poor quality bone who are at risk of fracture during stem insertion. Cemented stems are less expensive due to lower manufacturing cost, but require good surgical technique to place them correctly. Uncemented stems can cause pain with activity in up to 20% of patients during the first year after placement as the bone adapts to the device. This is rarely seen with cemented stems.


title: Hip replacement

text: There are several incisions, defined by their relation to the gluteus medius. The approaches are posterior (Moore), lateral (Hardinge or Liverpool), antero-lateral (Watson-Jones), anterior (Smith-Petersen) and greater trochanter osteotomy. There is no compelling evidence in the literature for any particular approach.


title: Hip replacement

text: The "posterior" ("Moore" or "Southern") "approach" accesses the joint and capsule through the back, taking piriformis muscle and the short external rotators of the femur. This approach gives excellent access to the acetabulum and femur and preserves the hip abductors and thus minimizes the risk of abductor dysfunction post operatively. It has the advantage of becoming a more extensile approach if needed. Critics cite a higher dislocation rate, although repair of the capsule, piriformis and the short external rotators along with use of modern large diameter head balls reduces this risk. Limited evidence suggests that the posterior approach may cause less nerve damage.


title: Hip replacement

text: The "lateral approach" is also commonly used for hip replacement. The approach requires elevation of the hip abductors (gluteus medius and gluteus minimus) to access the joint. The abductors may be lifted up by osteotomy of the greater trochanter and reapplying it afterwards using wires (as per Charnley), or may be divided at their tendinous portion, or through the functional tendon (as per Hardinge) and repaired using sutures. Although this approach has a lower dislocation risk than the posterior approach, critics note that occasionally the abductor muscles do not heal back on, leading to pain and weakness which is often very difficult to treat.


title: Hip replacement

text: The "anterolateral approach" develops the interval between the tensor fasciae latae and the gluteus medius. The Gluteus medius, gluteus minimus and hip capsule are detached from the anterior (front) for the greater trochanter and femoral neck and then repaired with heavy suture after the replacement of the joint.


title: Hip replacement

text: The "anterior approach" uses an interval between the sartorius muscle and tensor fasciae latae. Dr. Joel Matta and Dr. Bert Thomas have adapted this approach, which was commonly used for pelvic fracture repair surgery, for use when performing hip replacement. When used with older hip implant systems that had a small diameter head, dislocation rates were reduced compared to surgery performed through a posterior approach. With modern implant designs, dislocation rates are similar between the anterior and posterior approaches. The anterior approach has been shown in studies to variably improve early functional recovery, with possible complications of femoral component loosening and early revision compared to other approaches.


title: Hip replacement

text: The dual incision approach and other minimally invasive surgery seeks to reduce soft tissue damage through reducing the size of the incision. However, component positioning accuracy and visualization of the bone structures can be significantly impaired as the approaches get smaller. This can result in unintended fractures and soft tissue injury. The majority of current orthopedic surgeons use a "minimally invasive" approach compared to traditional approaches which were quite large comparatively.


title: Hip replacement

text: Computer-assisted surgery and robotic surgery techniques are also available to guide the surgeon to provide enhanced component accuracy. Several commercial CAS and robotic systems are available for use worldwide. Improved patient outcomes and reduced complications have not been demonstrated when these systems are used when compared to standard techniques.


title: Hip replacement

text: The prosthetic implant used in hip replacement consists of three parts: the acetabular cup, the femoral component, and the articular interface. Options exist for different people and indications. The evidence for a number of newer devices is not very good, including: ceramic-on-ceramic bearings, modular femoral necks, and uncemented monoblock cups. Correct selection of the prosthesis is important.


title: Hip replacement

text: The acetabular cup is the component which is placed into the acetabulum (hip socket). Cartilage and bone are removed from the acetabulum and the acetabular cup is attached using friction or cement. Some acetabular cups are one piece, while others are modular. One-piece (monobloc) shells are either ultra-high-molecular-weight polyethylene (UHMWPE) or metal, they have their articular surface machined on the inside surface of the cup and do not rely on a locking mechanism to hold a liner in place. A monobloc polyethylene cup is cemented in place while a metal cup is held in place by a metal coating on the outside of the cup. Modular cups consist of two pieces, a shell and liner. The shell is made of metal; the outside has a porous coating while the inside contains a locking mechanism designed to accept a liner. Two types of porous coating used to form a friction fit are sintered beads and a foam metal design to mimic the trabeculae of cancellous bone and initial stability is influenced by under-reaming and insertion force. Permanent fixation is achieved as bone grows onto or into the porous coating. Screws can be used to lag the shell to the bone providing even more fixation. Polyethylene liners are placed into the shell and connected by a rim locking mechanism; ceramic and metal liners are attached with a Morse taper.


title: Hip replacement

text: The femoral component is the component that fits in the femur (thigh bone). Bone is removed and the femur is shaped to accept the femoral stem with attached prosthetic femoral head (ball). There are two types of fixation: cemented and uncemented. Cemented stems use acrylic bone cement to form a mantle between the stem and to the bone. Uncemented stems use friction, shape and surface coatings to stimulate bone to remodel and bond to the implant. Stems are made of multiple materials (titanium, cobalt chromium, stainless steel, and polymer composites) and they can be monolithic or modular. Modular components consist of different head dimensions and/or modular neck orientations; these attach via a taper similar to a Morse taper. These options allow for variability in leg length, offset and version. Femoral heads are made of metal or ceramic material. Metal heads, made of cobalt chromium for hardness, are machined to size and then polished to reduce wear of the socket liner. Ceramic heads are more smooth than polished metal heads, have a lower coefficient of friction than a cobalt chrome head, and in theory will wear down the socket liner more slowly. As of early 2011, follow-up studies in patients have not demonstrated significant reductions in wear rates between the various types of femoral heads on the market. Ceramic implants are more brittle and may break after being implanted.


title: Hip replacement

text: The articular interface is not part of either implant, rather it is the area between the acetabular cup and femoral component. The articular interface of the hip is a simple ball and socket joint. Size, material properties and machining tolerances at the articular interface can be selected based on patient demand to optimise implant function and longevity whilst mitigating associated risks. The interface size is measured by the outside diameter of the head or the inside diameter of the socket. Common sizes of femoral heads are , and . While was common in the first modern prostheses, now even larger sizes are available from 38 to over 54 mm. Larger-diameter heads lead to increased stability and range of motion whilst lowering the risk of dislocation. At the same time they are also subject to higher stresses such as friction and inertia. Different combinations of materials have different physical properties which can be coupled to reduce the amount of wear debris generated by friction. Typical pairings of materials include metal on polyethylene (MOP), metal on crosslinked polyethylene (MOXP), ceramic on ceramic (COC), ceramic on crosslinked polyethylene (COXP), and metal on metal (MOM). Each combination has different advantages and disadvantages.


title: Hip replacement

text: Post-operative projectional radiography is routinely performed to ensure proper configuration of hip prostheses.


title: Hip replacement

text: The direction of the acetabular cup influences the range of motion of the leg, and also affects the risk of dislocation. For this purpose, the "acetabular inclination" and the "acetabular anteversion" are measurements of cup angulation in the coronal plane and the sagittal plane, respectively.


title: Hip replacement

text: The first line approach as an alternative to hip replacement is conservative management which involves a multimodal approach of oral medication, injections, activity modification and physical therapy. Conservative management can prevent or delay the need for hip replacement.


title: Hip replacement

text: Preoperative education is an important part of patient care. Some evidence indicates that it may slightly reduce anxiety before hip or knee replacement, with low risk of negative effects.


title: Hip replacement

text: Hemiarthroplasty is a surgical procedure that replaces one half of the joint with an artificial surface and leaves the other part unchanged. This class of procedure is most commonly performed on the hip after an intracapsular fracture of the femur neck (hip fracture). The procedure is performed by removing the head of the femur and replacing it with a metal or composite prosthesis. The most commonly used prosthesis designs are the Austin Moore and Thompson prostheses. A composite of metal and HDPE that forms two interphases (bipolar prosthesis) can be used. The monopolar prosthesis has not been shown to offer any advantage over bipolar designs. The procedure is recommended only for elderly/frail patients, due to their lower life expectancy and activity level. This is because over time the prosthesis tends to loosen or to erode the acetabulum. Independently mobile older adults with hip fractures may benefit from a total hip replacement instead of hemiarthroplasty.


title: Hip replacement

text: Hip resurfacing is an alternative to hip replacement surgery. It has been used in Europe since 1998 and became a common procedure. Health-related quality of life measures are markedly improved and patient satisfaction is favorable after hip resurfacing arthroplasty.


title: Hip replacement

text: Viscosupplementation is the injection of artificial lubricants into the joint. Use of these medications in the hip is off label. The cost of treatment is typically not covered by health insurance.


title: Hip replacement

text: Some authorities claim that the future of osteoarthritis treatment is bioengineering, targeting the growth and/or repair of the damaged, arthritic joint. Centeno et al. reported on the partial regeneration of an arthritic human hip joint using mesenchymal stem cells. It is yet to be shown that this result will apply to a large group of patients and result in significant benefits. The FDA stated that this procedure does not conform to regulations, but Centeno claims that it is exempt from FDA regulation. It has not been shown in controlled clinical trials to be effective.


title: Hip replacement

text: Total hip replacement incidence varies in developed countries between 30 (Romania) and 290 (Germany) procedures per 100,000 population per year. Approximately 0.8% of Americans have undergone the procedure.


title: Hip replacement

text: According to the International Federation of Healthcare Plans, the average cost of a total hip replacement in 2012 was $40,364 in the United States, $11,889 in the United Kingdom, $10,987 in France, $9,574 in Switzerland, and $7,731 in Spain. In the United States, the average cost of a total hip replacement varies widely by geographic region, ranging from $11,327 (Birmingham, Alabama) to $73,927 (Boston, Massachusetts).


title: Hip replacement

text: The earliest recorded attempts at hip replacement were carried out in Germany in 1891 by Themistocles Gluck (1853–1942), who used ivory to replace the femoral head (the ball on the femur), attaching it with nickel-plated screws. Subsequently, he used a cement made from plaster of Paris, powdered pumice and glue.


title: Hip replacement

text: Molded-glass implants were introduced in the 1920s by Smith-Peterson in the USA. Although these showed good bio-compatibility, they were mechanically fragile so he started experiments with metallic prostheses in the 1930s. In 1938, Philip Wiles of Middlesex General Hospital, UK carried out a total hip replacement using a stainless-steel prosthesis attached by bolts. In 1940, Dr. Austin T. Moore (1899–1963) at Columbia Hospital in Columbia, South Carolina performed a hip replacement using a prototype prosthesis made of the cobalt-chrome alloy Vitallium; it was inserted into the medullary canal and "fenestrated" to promote bone regrowth. A commercial version known as the "Austin Moore Prosthesis" was introduced in 1952; it is still in use today, typically for femoral neck fractures in the elderly. Following the lead of Wiles, several UK general hospitals including Norwich, Wrightington, Stanmore, Redhill and Exeter developed metal-based prostheses during the 1950s and 1960s.


title: Hip replacement

text: Metal/Acrylic prosthesis were tried in the 1950s but were found to be susceptible to wear.  In the 1960s, John Charnley at Wrightington General Hospital combined a metal prosthesis with a PTFE acetabular cup before settling on a metal/polyethylene design. Ceramic bearings were developed in the late 1970s.


title: Hip replacement

text: The means of attachment also has diversified.  Early prostheses were attached by screws (e.g. Gluck, Wiles) with later developments using dental or bone cements (e.g. Charnley, Thompson) or cementless systems which relied on bone regrowth (Austin-Moore, Ring). The choice of alloy, bearing material, attachment and detailed geometry has led to the wide variety of prosthesis designs available today.


title: Hip replacement

text: The London Science Museum has a collection of hip prostheses which reflect developments in the US, UK and elsewhere. These show the use of different materials and different designs for different circumstances (e.g. cemented and uncemented arthroplasty.)  Some are on display in the museum's "Medicine: The Wellcome Galleries".


title: Jody Hill

text: Jody Hill (born October 15, 1976) is an American director, producer, screenwriter, and actor. In 2006, he directed, co-wrote, co-produced and co-starred in his first film "The Foot Fist Way", which he based on his experience as a Taekwondo black belt and instructor. His follow-up film, "Observe and Report", stars Seth Rogen and was released on April 10, 2009. Hill is also co-creator, director and executive producer of the HBO series "Eastbound & Down", "Vice Principals", and "The Righteous Gemstones".


title: Jody Hill

text: Hill's first film, "The Foot Fist Way," was seen by Will Ferrell and Adam McKay who bought the distributing rights to the film. Subsequently, Hill was invited to the set of "Knocked Up" where he met future collaborator, Seth Rogen. Hill was asked to do a cameo appearance with writing partner Ben Best in Rogen's next film, "Superbad," as Tut Long John Silver. Hill later cast Rogen as the leading man in his film, "Observe and Report," which opened at number 4 at the box office. The role of Ronnie was written specifically for Rogen.


title: Jody Hill

text: In 2009, Hill directed The Avett Brothers music video "Slight Figure of Speech," which featured Wolfe and Andy Daly. The following year he directed the parody music video "Swagger Wagon" as part of ad agency Saatchi & Saatchi's campaign for the Sienna SE minivan from Toyota.


title: Jody Hill

text: In 2009, Hill co-created the HBO show "Eastbound & Down", on which he acted as Executive Producer and directed a total of 16 episodes. The show finished its fourth and last season in 2013.


title: Jody Hill

text: Hill's next film, "L.A.P.I.," was announced as an action comedy about a hardboiled private investigator, played by friend and frequent collaborator Danny McBride. It is the first film Hill would direct but not write. It is also the first film produced by Hill's, McBride's and David Gordon Green's production company, Rough House Pictures. In 2014, Hill and McBride created and began preparing the comedy series "Vice Principals" for HBO.


title: Jody Hill

text: Since 2019, Hill has directed 8 episodes of "The Righteous Gemstones". He also acts in the series as Levi.


title: Jody Hill

text: Hill is a graduate of the University of North Carolina School of the Arts. As of 2009, Hill was in a long term relationship, from at least 2008, with actress Collette Wolfe, who appeared in his movies "Observe and Report" and "The Foot Fist Way". The couple eventually married in 2012 in Cabo San Lucas, but have since divorced.


title: Monica Bedi

text: Monica Bedi (born 18 January 1975) is an Indian actress and television presenter. She debuted in Hindi films in the mid-1990s and her notable work includes "Pyaar Ishq Aur Mohabbat" and "Jodi No.1". She is known for participating in "Bigg Boss 2" and "Jhalak Dikhhla Jaa 2" and playing Gumaan Kaur Vyas in Star Plus's show "Saraswatichandra".


title: Monica Bedi

text: Bedi is a Punjabi and was born to Prem Kumar Bedi and Shakuntala Bedi in the village Chabbewal, Hoshiarpur district, Punjab. Her parents moved to Drammen, Norway in 1979. She completed her education from Delhi University in 1995.


title: Monica Bedi

text: In September 2002, Bedi and Abu Salem, an Indian gangster were arrested and later served prison sentences in Portugal for entering the country on forged documents. In 2006, an Indian Court convicted Bedi for procuring a passport on a fictitious name. In November 2010, the Supreme Court of India upheld her conviction but reduced the prison sentence to the period that she had already served. Various newspaper including magazines had reported that Abu used his criminal power to get roles for Monica Bedi in Bollywood.


title: Monica Bedi

text: Bedi got her first role in Telugu language film "Taj Mahal" (1995), produced by D. Ramanaidu. Ramanaidu also cast her in "Sivayya" and "Speed Dancer". She debuted in Bollywood in 1995 with "Surakshaa".


title: Monica Bedi

text: Bedi was a participant in "Bigg Boss" Season 2, a television reality show. She was one of the contestants of the reality shows "Jhalak Dikhhla Jaa 3" and "Desi Girl".


title: Unicode subscripts and superscripts

text: Unicode has subscripted and superscripted versions of a number of characters including a full set of Arabic numerals. These characters allow any polynomial, chemical and certain other equations to be represented in plain text without using any form of markup like HTML or TeX.


title: Unicode subscripts and superscripts

text: The World Wide Web Consortium and the Unicode Consortium have made recommendations on the choice between using markup and using superscript and subscript characters:


title: Unicode subscripts and superscripts

text: When used in mathematical context (MathML) it is recommended to consistently use style markup for superscripts and subscripts... However, when super and sub-scripts are to reflect semantic distinctions, it is easier to work with these meanings encoded in text rather than markup, for example, in phonetic or phonemic transcription.


title: Unicode subscripts and superscripts

text: The "intended" use when these characters were added to Unicode was to allow chemical and algebra formulas and phonetics to be written without markup, but produce true superscripts and subscripts. Thus "H₂O" (using a subscript character) is "supposed" to be identical to "HO" (with subscript markup).


title: Unicode subscripts and superscripts

text: In reality most fonts that include these characters ignore the Unicode definition, and design the digits for mathematical numerator and denominator glyphs, which are smaller than normal characters but are aligned with the cap line and the baseline, respectively. When used with the solidus, these glyphs are useful for making arbitrary diagonal fractions (similar to the ½ glyph). Making fractions using existing software super/subscripts requires many characters and does not look like the rendered fraction (example: /), so font designers provided this alternative. This also makes the superscript letters useful for ordinal indicators, more closely matching the ª and º characters. However it makes them incorrect for normal super and subscripts, and formulas are rendered correctly by using markup rather than these characters.


title: Unicode subscripts and superscripts

text: Unicode intended to produce diagonal fractions through a different mechanism but it is very poorly supported. The fraction slash U+2044 is visually similar to the solidus, but when used with the ordinary digits (not the superscripts and subscripts) is intended to tell a layout system that a fraction such as ¾ should be rendered using automatic glyph substitution for the digits. Some browsers support this but not in all fonts. A selection of fonts is shown in the below table.


title: Unicode subscripts and superscripts

text: The most common superscript digits (1, 2, and 3) were in ISO-8859-1 and were therefore carried over into those positions in the Latin-1 range of Unicode. The rest were placed in a dedicated section of Unicode at to U+209F. The two tables below show these characters. Each superscript or subscript character is preceded by a normal "x" to show the subscripting/superscripting. The table on the left contains the actual Unicode characters; the one on the right contains the equivalents using HTML markup for the subscript or superscript.


title: Unicode subscripts and superscripts

text: Unicode version 15.0 also includes subscript and superscript characters that are intended for semantic usage, in the following blocks:


title: Unicode subscripts and superscripts

text: Consolidated, the Unicode standard contains superscript and subscript versions of a subset of Latin, Greek and Cyrillic letters. Here they are arranged in alphabetical order for comparison (or for copy and paste convenience). Since these characters appear in different Unicode ranges, they may not appear to be the same size or position due to font substitution in the browser. Shaded cells mark small capitals that are not very distinct from minuscules, and Greek letters that are indistinguishable from Latin, and so would not be expected to be supported by Unicode.


title: Unicode subscripts and superscripts

text: Many of these characters were added to Unicode 15, in the Cyrillic Extended-D block, and published in 2022.


title: Unicode subscripts and superscripts

text: Primarily for compatibility with earlier character sets, Unicode contains a number of characters that compose super- and subscripts with other symbols. In most fonts these render much better than attempts to construct these symbols from the above characters or by using markup.


title: Osama bin Laden's compound in Abbottabad

text: Osama bin Laden's compound, known locally as the Waziristan Haveli (), was a large, upper-class house within a walled compound used as a safe house for militant Islamist Osama bin Laden, who was shot and killed there by U.S. forces on 2 May 2011. The compound was located at the end of a dirt road southwest of the Pakistan Military Academy in Bilal Town, Abbottabad, Khyber Pakhtunkhwa, Pakistan, a suburb housing many retired military officers. Bin Laden was reported to have evaded capture by living in a section of the house for at least five years, having no Internet or phone connection, and hiding away from the public, who were unaware of his presence.


text: Completed in 2005, the main buildings in the compound lay on a plot of land, much larger than those of nearby houses. Around its perimeter ran concrete walls topped with barbed wire, and there were two security gates. The compound had very few windows. Little more than five years old, the compound's ramshackle buildings were badly in need of repainting. The grounds contained a well-kept vegetable garden, rabbits, some 100 chickens and a cow. The house itself did not stand out architecturally from others in the neighbourhood, except for its size and exaggerated security measures; for example, the third-floor balcony had a privacy wall. Photographs inside the house showed excessive clutter and modest furnishings. After the American mission, there was extensive interest in and reporting about the compound and its design. To date, the Pakistani government has not responded to any allegations as to who had built the structure.


text: After the September 11 attacks in 2001, the U.S. searched for bin Laden for nearly 10 years. By tracking his courier Abu Ahmed al-Kuwaiti to the compound, U.S. officials surmised that bin Laden was hiding there. During a raid on 2 May 2011, 24 members of the United States Naval Special Warfare Development Group arrived by helicopter, breached a wall using explosives, and entered the compound in search of bin Laden. After the operation was completed and bin Laden was killed, Pakistan demolished the structure in February 2012.


text: In the urban setting, the architecture of the bin Laden hideout was called by an architect as "surprisingly permanent – and surprisingly urban" and "sure to join Saddam Hussein's last known address among the most notorious examples of hideout architecture in recent memory". The compound was fortified with many safeguard features intended to confuse would-be invaders, and U.S. officials described the compound as "extraordinarily unique". Associated Press identified the owner as Abu Ahmed al-Kuwaiti, who purchased the vacant land for the complex in 2004 and four adjoining lots between 2004 and 2005 for the equivalent of US$48,000.


text: Constructed between 2003 and 2005, the three-story structure was located on a dirt road northeast of the city centre of Abbottābad. The local architect for the project said it was only built and planned for a two-story structure and that the third floor (where bin Laden lived) was built afterwards in an illegal construction. While the compound was assessed by U.S. officials at a value of 1 million, local real estate agents assess the property value at 250,000. Intelligence reports indicated that bin Laden may have moved into the complex on 6 January 2006.


text: On a plot of land much larger than those of nearby houses, it was surrounded by concrete walls topped with barbed wire. Apart from its size, it did not stand out from others in the neighborhood and it was difficult to see from a distance. The compound walls were higher than usual in the neighbourhood, although nearly all houses in Bilal Town have barbed wire. There were no phones or Internet wires running into the compound. Security cameras were found, and aerial photographs showed several satellite dishes. There were two security gates and the third-floor balcony had a privacy wall. The compound measured in size, and had relatively few windows.


text: The compound was known as Waziristan Haveli () by the local residents. The compound's casual name referred to Waziristan, a region in Pakistan, and a "haveli", which means "mansion". It was owned by a transporter from Waziristan; bin Laden previously spent time in the Waziristan area of Pakistan.


text: The house where the bin Laden family lived on the two upper floors was large and modestly furnished. It had "cheap foam mattresses, no air conditioning (but central heating) and old televisions." Several of the bedrooms had an attached kitchen and a bathroom. One of the first floor rooms was furnished with a whiteboard, markers and textbooks, to serve as a classroom for the children in the house, who were home-schooled in Arabic.


text: The self-described brothers of the house known to the neighbours would frequently visit the local shops. They would buy enough food to feed ten people, and purchased "the best brands—Nestle milk, the good-quality soaps and shampoos", Pepsi and Coca-Cola. The food found at the house by the Pakistani authorities was basic, such as dates, nuts, eggs, olive oil and dried meat. The brothers would visit Rasheed's corner store, about a minute's walk from the house, with young children for whom they bought sweets and soft drinks. They also purchased bread from a local bakery.


text: Rabbits, 100 chickens and a cow were reared on the compound grounds. A vegetable garden at the back of the house was well-kept, and Shamraiz, a neighbouring farmer, was paid to plant vegetables about twice a year. Days before the May 2011 raid, Shamraiz was called to plough additional ground in the compound using a tractor. He never went inside the house itself.


text: The compound had an adjacent grazing area that hosted cows and a buffalo as well as a deep water well, possibly allowing it a water supply separate from the local municipality. There was a small garden on the north side of the house that included poplar trees. A farmer's field growing cabbages and potatoes surrounded the compound on three sides, and wild cannabis plants grew up to the side of the compound.


text: In November 2017, the CIA publicly released the contents of a cache of nearly 470,000 computer files discovered on 183 separate devices during the Abbottabad Compound raid, giving the world a glimpse into the home life of Bin Laden, his family, and his closest allies.


text: Among the files discovered were a diary of Bin Laden's movements and thoughts regarding the state of Al Qaeda, several videos of beheadings, a 19-page report about Al Qaeda's links to Iran, and a video of Osama Bin Laden's son, Hamza, at his wedding (the first images of Hamza since his childhood). There were a number of US-produced documentaries about Bin Laden, including Morgan Spurlock's "Where in the World is Osama Bin Laden?" and CNN's "In the Footsteps of bin Laden".


text: But also found in the cache was a great deal of more typical internet-browsing and pirated content, including home videos, cat videos, clip art, a video called "HORSE_DANCE," wildlife documentaries, a copy of Charlie Bit My Finger, the animated films "" and "Antz", episodes of "Jackie Chan Adventures", "Tom and Jerry", and "Case Closed", and copies of the video games "Final Fantasy VII", "Devil May Cry", "Counter-Strike", "Half-Life", and "Resident Evil 2". There were also, reportedly, several pornographic video games and videos, although the CIA has not released specifics and withheld certain titles, citing copyright concerns.


text: "Gulf News" reported that it had previously been used as a safe house by Inter-Services Intelligence, but was no longer being used for this purpose. ISI alleged that this compound was raided in 2003 while under construction as Abu Faraj al-Libbi was suspected of living there. However, this account was disputed by American officials who said that satellite photos show that in 2004 the site was an empty field. The compound was believed to be built around the summer of 2005 to late 2006, based on local accounts, most likely completed in late 2005 as intelligence reports indicate Bin Laden may have moved into the house on 6 January 2006.


text: American intelligence officials discovered bin Laden's whereabouts by tracking one of his couriers, Abu Ahmed al-Kuwaiti. Information was collected from Guantánamo Bay detainees who gave intelligence officers al-Kuwaiti's pseudonym and said that he was a protégé of Khalid Sheikh Mohammed. In 2007, U.S. officials discovered the courier's real name, and, in 2009, that he lived in Abbottābad. Using satellite photos and intelligence reports, the CIA surveilled the inhabitants of the compound. In September, the CIA concluded that the compound was "custom built to hide someone of significance" and that it was very likely that bin Laden was residing there. Officials surmised that he was living there with his youngest wife. U.S. Intelligence estimates that bin Laden lived in the compound for five or six years. Bin Laden's wife confirmed to the Pakistani authorities that they had lived in the compound for five years. Prior to moving to the compound, they lived in the village of Chak Shah Muhammad, in the nearby Haripur District, for nearly two and a half years.


text: Osama bin Laden was killed in Waziristan Haveli on 2 May 2011, shortly after 01:00 local time, by a United States special forces military unit.


text: Encounters between the SEALs and the residents took place in the guest house, in the main building on the first floor where two adult males lived, and on the second and third floors where bin Laden lived with his family.


text: The operation, code-named "Operation Neptune Spear", was ordered by United States President Barack Obama and carried out in a U.S. Central Intelligence Agency (CIA) operation by a team of United States Navy SEALs from the United States Naval Special Warfare Development Group (informally known as DEVGRU or by its former name SEAL Team Six) of the Joint Special Operations Command in conjunction with CIA officers. The raid on the compound was launched from Afghanistan. After the raid, U.S. forces took bin Laden's body to Afghanistan for identification, then buried it at sea within 24 hours of his death.


text: Following the raid, the former hideout was placed under the security control of the Pakistan Police. Days after the raid, police allowed reporters and locals to approach the walls of the compound, but kept the doors sealed shut. There was intense media interest in the architecture of the compound. The construction included highly fortified walls made of concrete blocks with three gates, separating the building from the large courtyard and a garden planted with immature fruit trees in front of a collapsed wall.


text: Pakistan security agencies demolished the compound in February 2012 to prevent mujahideen from memorialising it. In February 2013, Pakistan announced plans to build a R265 million ($2.7m) amusement park in the area, including the property of the former hideout.


text: Locals disclosed details about their interactions with the residents of the compound to an AP journalist in Pakistan. A woman who distributed polio vaccines to the compound said she saw expensive SUVs parked inside. The men received the vaccine and instructed her to leave. A woman in her 70s said one of the men from the hideaway gave her a ride to the market in rainy weather. Her grandchildren played with the children living in the house, and received rabbits as presents. One farmer said, "People were skeptical in this neighbourhood about this place and these guys. They used to gossip, say they were smugglers or drug dealers. People would complain that even with such a big house they didn't invite the poor or distribute charity." Present at some neighbourhood funerals, two men from the compound were "tall, fair skinned and bearded" and self-identified as cousins from elsewhere in the region. Neighbors said that if a child's ball went over the fence, the men in the compound did not return that ball; instead they paid the child 100–150 rupees (about US$0.60–$0.90), many times its value.


title: Sulfur hexafluoride

text: Sulfur hexafluoride or sulphur hexafluoride (British spelling) is an inorganic compound with the formula SF. It is a colorless, odorless, non-flammable, and non-toxic gas. has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom. It is a hypervalent molecule.


title: Sulfur hexafluoride

text: Typical for a nonpolar gas, is poorly soluble in water but quite soluble in nonpolar organic solvents. It has a density of 6.12 g/L at sea level conditions, considerably higher than the density of air (1.225 g/L). It is generally transported as a liquefied compressed gas.


title: Sulfur hexafluoride

text: Sulfur hexafluoride on Earth exists primarily as a man-made industrial gas, but has also been found to occur naturally.


title: Sulfur hexafluoride

text: Alternatively, utilizing bromine, sulfur hexafluoride can be synthesized from SF and CoF at lower temperatures (e.g. 100 °C), as follows:


title: Sulfur hexafluoride

text: There is virtually no reaction chemistry for . A main contribution to the inertness of SF is the steric hindrance of the sulfur atom, whereas its heavier group 16 counterparts, such as SeF are more reactive than SF as a result of less steric hindrance (See hydrolysis example). It does not react with molten sodium below its boiling point, but reacts exothermically with lithium.


title: Sulfur hexafluoride

text: The electrical power industry used about 80% of the sulfur hexafluoride produced in 2000, mostly as a gaseous dielectric medium. Other main uses as of 2015 included a silicon etchant for semiconductor manufacturing, and an inert gas for the casting of magnesium.


title: Sulfur hexafluoride

text: is used in the electrical industry as a gaseous dielectric medium for high-voltage sulfur hexafluoride circuit breakers, switchgear, and other electrical equipment, often replacing oil-filled circuit breakers (OCBs) that can contain harmful polychlorinated biphenyls (PCBs). gas under pressure is used as an insulator in gas insulated switchgear (GIS) because it has a much higher dielectric strength than air or dry nitrogen. The high dielectric strength is a result of the gas's high electronegativity and density. This property makes it possible to significantly reduce the size of electrical gear. This makes GIS more suitable for certain purposes such as indoor placement, as opposed to air-insulated electrical gear, which takes up considerably more room.


title: Sulfur hexafluoride

text: Gas-insulated electrical gear is also more resistant to the effects of pollution and climate, as well as being more reliable in long-term operation because of its controlled operating environment. Exposure to an arc chemically breaks down though most of the decomposition products tend to quickly re-form , a process termed "self-healing". Arcing or corona can produce disulfur decafluoride (), a highly toxic gas, with toxicity similar to phosgene. was considered a potential chemical warfare agent in World War II because it does not produce lacrimation or skin irritation, thus providing little warning of exposure.


title: Sulfur hexafluoride

text: Alternatives to as a dielectric gas include several fluoroketones. Compact GIS technology that combines vacuum switching with clean air insulation has been introduced for a subset of applications up to 420 kV.


title: Sulfur hexafluoride

text: is used to provide a tamponade or plug of a retinal hole in retinal detachment repair operations in the form of a gas bubble. It is inert in the vitreous chamber. The bubble initially doubles its volume in 36 hours due to oxygen and nitrogen entering it, before being absorbed in the blood in 10–14 days.


title: Sulfur hexafluoride

text: Sulfur hexafluoride was the tracer gas used in the first roadway air dispersion model calibration; this research program was sponsored by the U.S. Environmental Protection Agency and conducted in Sunnyvale, California on U.S. Highway 101. Gaseous is used as a tracer gas in short-term experiments of ventilation efficiency in buildings and indoor enclosures, and for determining infiltration rates. Two major factors recommend its use: its concentration can be measured with satisfactory accuracy at very low concentrations, and the Earth's atmosphere has a negligible concentration of .


title: Sulfur hexafluoride

text: Sulfur hexafluoride was used as a non-toxic test gas in an experiment at St John's Wood tube station in London, United Kingdom on 25 March 2007. The gas was released throughout the station, and monitored as it drifted around. The purpose of the experiment, which had been announced earlier in March by the Secretary of State for Transport Douglas Alexander, was to investigate how toxic gas might spread throughout London Underground stations and buildings during a terrorist attack.


title: Sulfur hexafluoride

text: Sulfur hexafluoride is also routinely used as a tracer gas in laboratory fume hood containment testing. The gas is used in the final stage of ASHRAE 110 fume hood qualification. A plume of gas is generated inside of the fume hood and a battery of tests are performed while a gas analyzer arranged outside of the hood samples for SF to verify the containment properties of the fume hood.


title: Sulfur hexafluoride

text: It has been used successfully as a tracer in oceanography to study diapycnal mixing and air-sea gas exchange.


title: Sulfur hexafluoride

text: According to the Intergovernmental Panel on Climate Change, is the most potent greenhouse gas. Its global warming potential of 23,900 times that of when compared over a 100-year period. Sulfur hexafluoride is inert in the troposphere and stratosphere and is extremely long-lived, with an estimated atmospheric lifetime of 800–3,200 years.