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Ultra high vacuum

Concepts involved

Advanced Definition

Kinetic theory of gases

Gas transport and pumping

Vacuum pumps and systems

Typical uses for ultra high vacuum

Ultra high vacuum is necessary for many surface analytic techniques such as:

X-ray photoelectron spectroscopy (XPS)

Auger electron spectroscopy (AES)

Secondary ion mass spectrometry (SIMS)

Thermal desorption spectroscopy (TPD)

Thin film growth and preparation techniques with stringent requirements for purity, such as molecular beam epitaxy (MBE) and UHV chemical vapor deposition (CVD)

Angle resolved photoemission spectroscopy (ARPES)

UHV is necessary for these applications to reduce surface contamination, by reducing the number of molecules reaching the sample over a given time period. At 0.1 mPa (106 Torr), it only takes 1 second to cover a surface with a contaminant, so much lower pressures are needed for long experiments.

UHV is also required for:

Particle accelerators

Atomic physics experiments which use cold atoms, such as ion trapping or making Bose-Einstein Condensates

Achieving ultra high vacuum

Extraordinary steps are required to reach UHV, including the following:

High pumping speed possibly multiple vacuum pumps in series and/or parallel

Minimize surface area in the chamber

High conductance tubing to pumps short and fat, without obstruction

Use low-outgassing materials such as certain stainless steels

Avoid creating pits of trapped gas behind bolts, welding voids, etc.

Electropolish all metal parts after machining or welding

Use low vapor pressure materials (ceramics, glass, metals, teflon if unbaked)

Bake the system (250 C to 400 C) to remove water or hydrocarbons adsorbed to the walls

Chill chamber walls to cryogenic temperatures during use

Avoid all traces of hydrocarbons, including skin oils in a fingerprint always use gloves

Outgassing is a significant problem for UHV systems. Outgassing can occur from two sources: surfaces and bulk materials. Outgassing from bulk materials is minimized by careful selection of materials with low vapor pressures (such as glass, stainless steel, and ceramics) for everything inside the system. Even materials which are not generally considered absorbent can outgas, including most plastics and some metals. For example, vessels lined with a highly gas-permeable material such as palladium (which is a high-capacity hydrogen sponge) create special outgassing problems.

Outgassing from surfaces is a subtler problem. At extremely low pressures, more gas molecules are adsorbed on the walls than are floating in the chamber, so the total surface area inside a chamber is more important than its volume for reaching UHV. Water is a significant source of outgassing because a thin layer of water vapor rapidly adsorbs to everything whenever the chamber is opened to air. Water evaporates from surfaces too slowly to be fully removed at room temperature, but just fast enough to present a continuous level of background contamination. Removal of water and similar gases generally requires baking the UHV system at 200 to 400 C while vacuum pumps are running. During chamber use, the walls of the chamber may be chilled using liquid nitrogen to reduce outgassing further.

Hydrogen and helium are the most common background gases in a well-designed, well-baked UHV system. Hydrogen diffuses out from the grain boundaries in stainless steel. Helium can diffuse through the steel and glass from the outside air.

Typically, there is no single vacuum pump that can operate all the way from atmospheric pressure to ultra high vacuum. Instead, a series of different pumps is used, according to the appropriate pressure range for each pump. Pumps commonly used to achieve UHV include:

Turbomolecular pumps (especially compound and/or magnetic bearing types)

Ion pumps

Titanium sublimation pumps

Non-evaporable getter (NEG) pumps

Cryopumps

UHV pressures are measured with an ion gauge, either a hot filament or an inverted magnetron type.

Finally, special seals and gaskets must be used between components in a UHV system to prevent even trace leakage. Nearly all such seals are all metal, with knife edges on both sides cutting into a soft, copper gasket. This all-metal seal can maintain pressures down to 100 pPa (~1012 Torr).

Measuring high vacuum

Main article: Pressure measurement

Measurement of high vacuum is done using a nonabsolute gauge that measures a pressure-related property of the vacuum, for example, its thermal conductivity. See, for example, Pacey. These gauges must be calibrated. The gauges capable of the measuring the lowest pressures are magnetic gauges based upon the pressure dependence of the current in a spontaneous gas discharge in intersecting electric and magnetic fields.

UHV manipulator

A UHV manipulator allows an object which is inside a vacuum chamber and under vacuum to be mechanically positioned. It may provide rotary motion, linear motion, or a combination of both. The most complex devices give motion in three axes and rotations around two of those axes. To generate the mechanical movement inside the chamber, two basic mechanisms are commonly employed: a mechanical coupling through the vacuum wall (using a vacuum-tight seal around the coupling), or a magnetic coupling that transfers motion from air-side to vacuum-side. Various forms of motion control are available for manipulators, such as knobs, handwheels, motors, stepping motors, piezoelectric drives, and pneumatics.

The manipulator or sample holder may include features which allow additional control and testing of a sample, such as the ability to apply heat, cooling, voltage, or a magnetic field. Sample heating can be accomplished by electron bombardment or thermal radiation. For electron bombardment, the sample holder is equipped with a filament which emits electrons when biased at a high negative potential. The impact of the electrons bombarding the sample at high energy causes it to heat. For thermal radiation, a filament is mounted close to the sample and resistively heated to high temperature. The infra-red energy from the filament heats the sample.

References and notes

^ DJ Pacey (W. Boyes, editor) (2003). Measurement of vacuum; Chapter 10 in Instrumentation Reference Book (Third Edition ed.). Boston: Butterworth-Heinemann. p. 144. ISBN 0750671238. http://books.google.com/books?id=sarHIbCVOUAC&pg=PA144&lpg=PA144&dq="measurement+of+vacuum"&source=web&ots=aWbFsOCsvf&sig=NcVTpfYXN5X-8STfeOIIKeCG0EU#PPA144,M1.

^ LM Rozanov & Hablanian, MH (2002). Vacuum technique. London; New York: Taylor & Francis. p. 112. ISBN 041527351X. http://books.google.com/books?id=8yEGJCtS2XgC&pg=PA79&lpg=PA79&dq="measurement+of+vacuum"&source=web&ots=RmEYhDyVOY&sig=Muaslx8-GZW6zWBCa403si5cuRM#PPA112,M1.

^ LM Rozanov & Hablanian, MH. p. 95. ISBN 041527351X. http://books.google.com/books?id=8yEGJCtS2XgC&pg=PA79&lpg=PA79&dq="measurement+of+vacuum"&source=web&ots=RmEYhDyVOY&sig=Muaslx8-GZW6zWBCa403si5cuRM#PPA95,M1.

How to deep fry turkey outdoors with a propane cooker

Equipment Needed:

*15-20lb. turkey
*28-Qt. or larger turkey fryer pot
*Propane burner stove
*Oil thermometer made to withstand over 550 Fahrenheit 300 Celsius
*LP propane tank
*Thick work gloves(flame retardant cooking mitts works best), long and heavy sleeves(such as flannel)
*BC or ABC fire extinguisher, never use water to contain oil or grease fires

Preparations:

Completely thaw the turkey. This is extremely important for even cooking, measurable cooking time, and safety. Allow 1 day thawing time per 5lbs. of turkey in the refrigerator. Thawing at room temperature can breed and spread bacteria and also spoil the turkey. Once thawed, remove giblets and neck. Trim excess fat and skin. Make sure the opening at the neck cavity is wide and clear. Trim the opening larger if needed. Make a 1-inch cut at the leg joints to allow oil to drain after it's finished cooking.

Next, measure the oil level needed for this bird. Mount the turkey on the poultry rack and insert into the fryer pot. Fill the pot with water until the turkey is fully submerged. Make a mark about a 3/4 inch below the waterline because oil will expand when heated. Pots made for turkey frying will usually have a maximum waterline, do not fill past this line. If there is not a maximum line, always leave at least 3 inches from the top of the pot. If the turkey is too large for the pot, the turkey will need to be trimmed down. It is very important to keep in mind that the water used to measure with will be contaminated. Thoroughly clean after draining the water. Completely dry the pot before adding oil.

Peanut oil is the most commonly used to deep fry turkey. It has a high flash point and is cholesterol free. Any good host should be mindful of peanut allergies of their guests. Any vegetable oil can be used for a lighter taste or due to allergies.

Once the waterline is measured, you may season the turkey. Dry seasoning works best on the turkey's surfaces. Liquid marinade and or herbs may be injected into the the meaty parts. Mixing orange juice, beer, or wine with dry seasonings works wonders when injected.

Placing the Fryer:

One of the most important safety measure is properly placing the burner.The fryer needs at least 10 feet clearance on all sides from any buildings or flammable structures and should never be under any type of overhang or ceiling. Common dangerous mistakes include placing it on wooden decks, under the garage door, or loose uneven grass and dirt. Concrete or brick are the best surfaces to place the fryer. If using on the lawn, clear the area of grass and make sure the dirt is packed, firm, and even.

Wind and weather can be dangerous factors. Never use if there's a chance for rain. Never use in medium or stronger winds. Always position the propane tank upwind and at least 2 feet from the burner. There needs to be extra clearance downwind than 10feet depending on how strong the breeze is. If winds are too strong, do not attempt to make a wind breaker or blocker. It's too dangerous to use a gas burner in strong winds.

Time to Cook: NEVER leave the cooker unattended!

Light the burner before placing the pot on it for better sight and chance of knocking over the pot. Once lit, never leave the cooker unattended. The most important thing for a great fried turkey and to prevent any accidents is to always have at least one adult watching the cooker at all times. Add the oil to the measured waterline before placing the pot on the lit burner. Placing an empty pot on the burner will damage the pot.

Monitoring the oil temperature is important for deep frying anything. If the temperature reaches too high, there's risk of fire. If the temperature drops too low, the food will stop "steaming" and oil will soak in causing it to become greasy a soggy. 350 Fahrenheit is the target cooking temperature for a turkey with peanut oil. Slight less, around 335 Fahrenheit is target for lighter oils such as canola. 400 Fahrenheit is a good critical limit. This means once 400 Fahrenheit is reached, shut off the gas. It will take more heat to flash the oil on fire than 400 Fahrenheit, but there's no need to go this high and it will hurt the estimated cooking time. Temperatures under 400 Fahrenheit that are higher than target, should be controlled by lowering the heat on the burner. Turn off the gas if at any point the oil starts to smoke. Keep in mind that reusing oil will lower it's flash point temperature.

Once the target temperature is reached, about 350 Fahrenheit, it's time to drop the bird. Reduce the heat of the burner. Wearing thick gloves, long sleeves and thick shoes or boots, very slowly lower the turkey rack into the pot using a long grab hook. Lower the turkey so slow that it'll take over a minute to completely do. Without gloves or long sleeves, oil splatter could cause "flinching" and might result in knocking over the pot filled with boiling oil.

After the turkey is completely inserted, increase the heat to raise the oil temperature back to target, this could take several minutes. Once the target temperature is reached, adjust the heat to maintain it. Be mindful of the critical limit, once 400 Fahrenheit is reached, don't attempt anymore adjustments and turn off the gas. A turkey takes about 3 to 3 and a half minutes per pound to deep fry, about one hour for an 18lb turkey.

When the turkey is golden brown and done cooking, turn off the gas. Place newspapers or paper towels close by on a flat surface. Slowly remove the turkey rack from the pot using the same safety clothing as when it was lowered. Make sure the rack is high and clear of the pot before attempting to move to reduce chance of knocking over the pot still filled with boiling oil. Place the rack on the newspapers or paper towels to cool and drain. For several minutes.

Do not leave the fryer pot and burner unattended until it has cooled down which will take quite some time. Do not attempt to move the pot and burner until the oil has dropped down past 110 Fahrenheit. It's very important to keep in mind that the oil is still boiling after the cooking is done and to keep children away from it.

Congratulations on your beautiful bird. Only problem is that you won't ever be able to go back to dried oven cooked turkeys again.

About the Author

Stephen French, owner and user of http://www.turkeyfryercamp.com
Selling Bayou Classic turkey fryers and other indoor and outdoor cookware.