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UHV Techniques: Basic Concepts Lecture Contents

Lecture 1: Basics Lecture 2: Achieving Vacuum Aims: ► Levels of vacuum and ► Vacuum subsequent applications ► ● What you ought to know configurations ► Modelling flow about using vacuum ► Types of chambers and ► measurement - fittings ● What you might need atmosphere to UHV during your PhD ► Complete systems

● A quick starter for using vacuum equipment

● Cover entire range of vacuum - not just UHV

References Why know about vacuum?

● Two excellent books: ● Nearly everyone here ● Many people here will use ► “Basic Vacuum ” , A. Chambers, R.K. Fitch and needs to know a little about vacuum systems: B.S. Halliday. vacuum: ► e.g. most research groups ► “A Users Guide to Vacuum Technology” , O'Hanlon. ► vacuum used very widely use vacuum to some in modern experimental extent physics ► ● Vacuum equipment manufacturers: need to understand the ► mostly simple physical systems ► Catalogues, websites principles involved ► might need to analyse ► e.g. Leybold, Edwards, Varian, Pffeiffer, Alcatel etc. ► what is achievable with what is happening or today's technology design modifications ► the basics - so its easier to ● Local knowledge (valuable, but finite resource!): go away and find out more ► workshops ► group technical support staff Historical Perspective What is the equipment like?

BC 1657 1900 1950's 2000+ Ultra High Vacuum -10 Rough Vacuum (~10 mbar) Ancient Greeks (~10 -1 mbar) - Vacuum “inconcievable” Magdeburg Manufacture of Hemispheres Lightbulbs Experiment Ultra High Surface Vacuum treatment Sealed research to Pumps achieve XHV Hydrocarbon Sealed Pumps

Some Definitions Assume the vacuum system is a sphere...

Units:

► CHAMBER Usually use mbar (although Pa are SI, sometimes Torr) PUMP (GAS) ► 1000 mbar = 1 atm = 10 5 Pa = 760 Torr

Levels of Vacuum: Kinetic Theory: reasonably valid For at 295K: ► Low Vacuum - 1000 mbar to 1 mbar -1 -2 -1 ● Maxwell Boltzman Distribution P (mbar) n (litre ) λ J (cm s ) ► Medium Vacuum - 1 mbar to 10 -3 mbar 8kT 22 23 -3 -8 v = 1000 2.5x10 66 nm 2.9x10 ► High Vacuum - 10 mbar to 10 mbar  m 19 20 ● 1 2.5x10 66 µm 2.9x10 ► Ultra High Vacuum - 10 -12 mbar to 10 -8 mbar Mean free path 1 -3 16 17 -12 = 1x10 2.5x10 66 mm 2.9x10 ► eXtreme High Vac. - below 10 mbar 2 d 2 n -6 13 14 ● Impingement Rate 1x10 2.5x10 66 m 2.9x10 nv J = 1x10 -10 2.5x10 9 660 km 2.9x10 10 4 Applications Elementary Gas Transport

● Conductance is a Low Vacuum: Ultra high vacuum (UHV): ● Throughput, Q, is the geometric property, which volume of gas passing ► mechanical handling ► keeping surfaces clean for represents the ability of a through an area per second hours (surface science, gas to flow from a pressure at a specific temperature epitaxial growth) gradient: and pressure: Medium vacuum: -10 ► space simulation (~10 Q mbar at 1000 km) dV C= ► industrial processes Q=p P P ► dt 2 1 ► vacuum drying/packaging achieving ultra high purities (e.g. for fusion) ► vacuum ► units, e.g. mbar.l/s C Extreme high vacuum (XHV): ● p p High vacuum (HV, λ>d): Volumetric flow rate often 1 Q 2 ► storage rings called the speed, S: ► e-beams (welding, TV) dV Q ► ultra pure growth S= = ► vacuum or dt p S coating

Modes of Gas Flow Molecular Flow

For vacuum systems: ● dominant for HV and UHV and well ● Fluid flow is described by the understood Reynolds number and the ● Flow is only turbulent at very Knudsen Number (both high and pumping ● particles flow in all directions to reach dimensionless) speeds (e.g. during initial dynamic equilibrium u D for round pipes evacuation, if unthrottled) R e= ● pumps 'wait and catch' gas particles -  ρ = density ● Viscous flow and the high vacuum pumps do not 'suck'  u = stream velocity transitional regime are Kn = D = diameter important at high pressures D λ = m.f.p. (above about 10 -3 mbar) PUMP ► gas can be 'sucked out' ● Turbulent flow if Re>2000, laminar if Re<1200. ● Molecular flow dominant below about 10 -3 mbar in ● Viscous flow if Kn<0.01, 'normal' sized chambers. molecular flow if Kn>1. Particles scattered from surface with ► gas flows through random cosine distribution to surface normal collisions with walls (Knudsen's cosine law) Molecular Conductance of an Aperture Conductance of Pipes

Net flow through an aperture ● For short pipes is convenient ● Various methods for corresponds to the rate of impingement to reciprocally add the from both sides: calculating conductance of pipes give same results (see conductances for a long pipe and equivalent aperture: P1, n 1 P2, n 2 O'Hanlon for details) Q=k T J 1J 2 A ● k T For long pipes (L>>D): C 0×CL =  p1 p2 A J A J CS= 2 m 1 2 C C D 3 2k T 0 L C L= k T 6 L m 12.4 D3 / L C 0= A ● C = l/s 2  m S 14D /3L 3 (D, L in cm) for nitrogen (air) ● C L = 12.4 D / L l/s ► conductance depends on (T/m) 1/2 at room temperature. (D, L in cm) for nitrogen (air) ► C = 11.8A l/s (A in cm 2) for nitrogen ● Accurate to about 10% 0 at room temperature. (air) at room temp. ► need to account for gas and temp.

Combining Conductances Effect of Conductance on Speed of a Pump

Conductance, C ● For conductances in series, ● For conductances in add reciprocally: parallel, add normally

► 1/C T = 1/C 1 + 1/C 2 + 1/C 3 ► CT = C 1 + C 2 + C 3

C1 CHAMBER PUMP PUMP

C1 C2 C3 C2

C3

The molecular conductance ● this assumes the volumes Effective pumping speed at of the entrance aperture are independent - need to chamber is reduced by conductance determines maximum speed of be careful of any connecting pipe any pump Viscous and Transitional Flow Conductances of Complex Shapes

Molecular Transitional Viscous ● Monte Carlo simulations ● higher pressures in pipes to required in general mechanical pumps mean transitional and viscous ● Refer to O'Hanlon flow may be important for simple 'standard' shapes ● conductance increases with pressure

● important in connections to ● Quite often, just need mechanical pumps - can approximate value use smaller connections ► can get quite a long ● accurate calculations way using simple complex approximations

● results tabulated - often in Approximate conductance is the conductance of the two apertures, mfr's catalogues added reciprocally... From BOC Edwards Catalogue

A closer look at a generic system... Quantitative Description of Pumping

Outgassing: gradual loss ● Constant volume system ● of particles adsorbed on CHAMBER governed by Ultimate pressure (dp/dt=0) walls of the chamber - Backstreaming: many pumps pressure is simply water is the major problem. can lose fluids into the vacuum dp system, causing contamination, V =Q Sp p = Q / S which can be difficult to remove. dt T T ► in the case of a leak, PUMP Real Leak: a fine ● i.e. change in gas in chamber, Q ~Q passageway to the T L Leak Initial Air air outside. d(pV)/dt is load minus gas ● Process removed by pump For initial pumpdown, Q L is not important, so we obtain Virtual Leak Backstreaming ● Solving fully requires detailed

knowledge of Q - not p = p 0 exp { -t / (v/S) } Evaporation T Virtual Leak: usually a generally available Evaporation: liquids (and greases) ► easy to calculate initial small trapped volume, will limit the vacuum until evaporated. which acts like a real ► pumpdown times using the (Clean components and WEAR evaporation and leak, but will deplete with GLOVES!) outgassing of range of pumping speed at the time. gives complex chamber behavior... QT = Q P + Q O + Q L + Q VL + Q E + Q B Limit of Pressure - Outgassing Measurement of Pressure

● Require a measurable property which changes (linearly) ● ● outgassing limits vacuum in a for UHV, must accelerate with pressure, preferably independent of gas type clean, leaktight HV/UHV degassing of water by baking chamber to ~200°c for ~24 hours ● No single principle is good between atm. and UHV.

● made up of general 'grot', ► desorption is activated greases, water vapour etc. with Boltzmann factor Pressure (mbar) 10 3 10 0 10 -3 10 -6 10 -9 10 -12 ► rule of thumb: rate doubles Capsule gauge for every extra 10°c Diaphragm Pirani (thermal cond.) ► can get to UHV in days Capacitance manometer instead of years Spinning rotor Penning Bayard-Alpert (Ionis.) ● what is the ultimate limit? Inverted Magnetron log(pressure) ► diffusion of H through Extractor Ion 2 RGA Mass Spec. log(time) chamber walls

Vacuum Gauges Mechanical Total Pressure Gauges

● In practise, integrated ● Sense pressure by systems now widely available mechanical deformation ► almost 'plug and play' ● Measure total pressure - independent of gas type ► connect to PC for logging ● ► Capsule gauge: can link to pump Capsule Gauge controllers ► simple mechanical lever from expanding capsule to dial ● Composite gauge heads can Annular Diaphragm measure over wide ranges ► good for 1-1000 mbar electrode

► ● Diaphragm gauges combined pirani & ion Disc gauge ► electrode ● e.g. integrated ion gauge sense by mechanical pressure under ► measure from atm. to UHV and controller deflection measurement ► ► Still need care in operation sense by change in Chemical - e.g. responses different capacitance, very accurate and good to 10 -5 mbar, but for different gases expensive Capacitance Manometer Pirani Gauge - Low to Medium Vacuum Bayard Alpert Ion Gauge - HV and UHV

● measures thermal ● hot filament emits electrons,

conductivity from hot wire to RF RC which are attracted to grid and surroundings through vacuum spiral around

● typically set up in bridge V ● electron impact ionised arrangement with a residual gas inside grid compensating filament, and ● calibrated at HV positive gas ions reach collector and are detected by 30 V 180 V ● 'standard' backing line gauge, electrometer cost few £100 ● usable between about 10 -3 and ● needs calibrating to atm. and 10 -11 mbar, cost ~£1000 high vacuum ● limited by x-rays from grid Gas Typical Correction Factor Response of gauge depends on: He 0.16 N2 1.0 ► geometry of gauge CO 2 1.4

Heat Loss Heat ► emission current Xe 2.4 C H 5.8 Pirani gauge and controller 6 6 10 -4 1 P (mbar) ► ionisation probability of gas

Penning Gauge Inverted Magnetron Gauge

● similar principle, but works ● cold cathode ionisation gauge down to ~10 -10 mbar - no filaments to blow ● single initial ionisation event ● single ion initially created +2kV gives electron few spontaneously (e.g. cosmic kV B ray) ● electrons perform cycloid motion in crossed E and B ● electrons attracted to anode, fields, ionising gas within but crossed B and E fields B cause long spiraling paths ● produces stable discharge ● response non-linear, so not similar to Penning gauge ● provides alternative to ion ● electrons cause further regarded as so accurate gauge ionisation to maintain a stable ● some pumping effect ● some pumping effect discharge ► no filaments to blow ● considerable sputtering at ● robust, widely used gauge ● ion current given by i = kp n -5 ► no light, no heating (industrially) for range 10 -3 to high pressures (avoid >10 where k and n are constants, 10 -7 mbar mbar for long periods - ► but, slow to start at low and 1.1

● most gauges are only accurate to ~20% at best ● wide variety of pumps used at all pressure levels (often worse) ● concentrate on main types of pumps used in research ● a few 'precision' gauges are All Vacuum Pumps available ► capacitance manometer (limited to ~10 -5 mbar) Gas Transfer ► spinning rotor gauge Positive Kinetic Entrapment (limited to ~10 -7 mbar) Displacement

● precision gauges are Moving components Momentum imparted to Gas particles react expensive, and limited range displace and eject a individual gas particles, chemically and trapped, but can be used to calibrate volume of gas. driving them to exhaust or ionised, accelerated and embedded in pump ion gauges etc. when e.g. reciprocating piston e.g. rotor in a turbo- walls. pressure critical Spinning Rotor Gauge or rotating vanes molecular pump

Vacuum Pumps Rotary Vane Pump

Viscous Flow Molecular Flow ● 'standard' mechanical pump, used to achieve rough vacuum

● sliding vanes rotate, Pressure (mbar) compressing and ejecting gas to atmosphere 10 3 10 0 10 -3 10 -6 10 -9 10 -12 ● sealed with oil

► needs replacing periodically

Mechanical (Rotary) Pump ► need foreline trap to keep oil from backstreaming into inlet lines Single stage rotary vane pump Diffusion Pump ► special oils available for pumping oxygen/aggresives - else BANG! Rotary Vane Pump Rotary Vane Pump

Flexible pipeline ● 2 stage pump gives better to chamber Foreline trap ultimate pressure (catches oil in ► adsorbant beads - 2 stages in series regen. periodically) ● difficult to expel condensables Inlet Connection (to chamber) ► e.g. water vapour Exhaust conn. ► gas ballast helps (leak air in (always exhaust to roof/outside - between stages) SAFETY ISSUE) -3 ● typically get down to 2x10 Oil sight mbar with a good rotary pump Motor glass Two stage rotary vane pump ● pumping speeds between about 0.5 and 80 m 3/hr

3 Pump housing ● cost ~£1000 for few m /hr (oil casing)

'Dry' Pumps Other mechanical pumps

● Other mechanical pumps are 15cm dia. Roots Piston ● can avoid contamination ● Dry pumps generally: available, e.g.: inlet flange pump pump altogether with dry pumps, ► lower pumping speed for ► piston pumps if necessary same size or cost ► roots pump ► e.g. silicon wafer ► poorer ultimate pressures ● Usually designed for high processing ► more expensive pumping speeds needed in ● several mechanisms industry available, e.g. diaphragm, ► noisier PTFE sealed pistons

Principle of a roots pump

Large roots and piston pumps used on a beam source Getting to High Vacuum Diffusion Pump

● boils highly refined, high ● Need different type of pump molecular weight fluid to get below about 10 -3 mbar Chamber ● vapor jets impart downward momentum to gas entering ● Usually means using either: pump

► diffusion pump <10 -6 mbar ● oil condenses on water cooled body & recycled ► turbo pump HV Pump ● Discharges to rough vacuum

► ● High vacuum pumps can't ~10 -2 mbar 'critical backing pressure' ~0.5 mbar (depends on discharge to atmospheric Backing model) pressure Pump Vent to ● reliable 'standard', gets down ► roof -7 -11 permanently need a rotary 1000 mbar to between ~10 and ~10 pump as a 'support' or mbar depending on setup 'backing' pump

Diffusion Pump Diffusion Pump: Chevron Baffles

● Ultimate pressure depends ● all diff pumps backstream on quality of fluid used Liquid N 2 Inlet pump fluid when hot reservoir ► cheap fluid (e.g. Corning baffle ● DC704) ~10 -6 mbar cold, optically dense baffle catches oil, but reduces the ► good fluid (e.g. Edwards pumping speed L9) ~ 10 -9 mbar To rest of system ● need liquid nitrogen temp. ► best fluid (Santovac 5) -10 to reduce vapour pressures ~10 mbar (with baffle) Thermal Sensor to UHV level ● Different fluids have ► different safety issues Water once nitrogen reservoirs Optically dense cooling filled, need to be kept filled chevron baffle, ● connected to Need chevron baffle to ► internal water cold reservoir catch backstreaming pump condensation can re- To diffusion pump fluid and achieve best freeze in cracks and cause pressures Heater leaks Diffusion Pump Turbomolecular Pump

● fast moving rotors and stators impart momentum to gas molecules Advantages: Disadvantages: ● frequencies of up to ~1000 Hz ● fairly cheap (start ~£1000) ● slow to start and stop ● high precision greased/oiled or magnetic bearings ● reliable - little to go wrong ► ~½ hr to warm up ► heater is replaceable ► ~1 hr to cool down before venting ► cooling coils can be descaled ● expensive to run large pumps (£1000s per year) ► can take to bits and scrub out inside ● require liquid nitrogen ● often found 'lying around' baffles for true UHV lab (require daily filling!) ● Dependant on cooling water – MUST BE INTERLOCKED

Turbomolecular Pump Turbomolecular Pump

Advantages: ● pumping speed varies with Disadvantages gas and pressure ● quick to start and stop ● much more expensive to buy ● Getter compression varies with ● and service (typically around Chamber Pump low electrical costs molecular mass £10 000) ● clean – completely so for a ► typically 10 9 for N ● 2 mag-lev - no baffles required occasional catastrophic failure ► typically ~10 4 for He ► something dropped in top Turbo ● can mount in more 3 Pump orientations ► bearing seizes ► typically ~10 for H 2 ► magnetic controller fails ● difficult to remove lighter ● water cooling much less gases Turbo critical ● need to be well secured Pump (SAFETY RISK) ► can add extra pump in series ► pump should turn off if too to improve compression hot, usually after >1/2 hr ► e.g. Leybold T1600 has to ► add chemical pump in Backing ► air cooling options available be secured to withstand parallel to pump reactive Pump an impact torque of 20 000 Vent to ● mag-lev turbos good for species (e.g. H ) roof Nm 2 aggressive gases Getter Pump

● ● type of entrapment pump, chemical 'getter' pump, reacts ● operating like a series of with and contains reactive use strip along entire Penning gauges elements length of particle accelerator Ion pump cell beamlines to achieve XHV ● ● electrons spiral in strong various types of getter ● magnetic field, ionising gas once activated, pumps without ► sublimated coatings (TSP power - portable! species on impact uses Ti/Mo alloy)

● ions are accelerated to and ► blocks of reactive, sintered get buried in surface of anode material (NEG) ● particularly good at pumping H , unlike many other pumps +kV 2 ● clean pumping to UHV, but requires large magnet ● limited capacity - has to be -kV B used at very low pressures, ● needs additional pump to get e.g. in a load-locked chamber down to about 10 -4 mbar before ion pump will start ● cannot pump inert/rare gases

Choice of Pumps Low to High Vacuum Fittings

● Choice of pump depends ● careful consideration and High vacuum fittings: on risk assessment is vital ● elastomer seals ► level of vacuum ● number of big mfrs.: ► size of chamber ● usually use KF (Klein (outgassing) ► never pay list price for new Flange) fittings with o-rings vacuum equipment ► gas throughput of process ► discounts of up to ~50% ► pumpdown time are routine for academia ► type of gas (nasty?) ► level of cleanliness required ► size and positioning ► finances available (!) ► ... Ultra High Vacuum Fittings Vacuum Feedthroughs

To achieve UHV: metal flange ● Need electrical, fluid and ● not difficult, just need to do mechanical connections to the job properly and cleanly experiment

● require materials which are ► commercial feedthroughs vacuum side not gassy ceramic insulator ► all metal fittings, plus ● Electrical connections Electrical feedthrough ceramics, glass(fibre), PTFE ► ► ceramic to metal seals no , adhesives, metal bellows to allow solder, brass ► delicate - avoid bending pins! motion in vacuum ► http://outgassing.nasa.gov

● usually use CF (Conflat) knife ● Mechanical connections edge flanges and copper ► o-ring sealed for HV ► differentially pumped or metal ► can seal with elastomer bellows sealed for UHV gaskets temporarily (indicate Push-pull linear feedthrough by using few bolts) Knife Edge

Vacuum Valves Putting it all together

Diaphragm valve HV 90° valve UHV 90° valve (typically on backing lines)

Butterfly valve Gate valve (high conductance) (high conductance + UHV) Standard Vacuum Symbols Another Simple System

Manual Valve , Adsorption General Pump Getter UHV Variable Pump Chamber Leak Valve

Rotary Vane Electromag. Roughing Line Cryopump Valve Pump UHV Gate Valve All Metal Exhaust Diffusion Sputter-Ion Valve to roof Turbo Pump Pump Pump Sorption Backing Line Trap Turbomolecular Getter Pump Pump Vacuum Leak Test Exhaust Gauge to roof

Reaching the Ultimate Pressure Cleaning How-To

Normal procedure: Then, if you have ● Not very exciting, but very A generic procedure: problems... important to achieving good ● Design system carefully high vacuum performance 1. Start with 'mechanical ● Try to identify source of cleaning' (scrub with contamination ● Specialist cleaning possible detergent) ● Avoid contamination for certain materials ► degrease and clean all 2. 'Buzz' in detergent in ● ► ceramics, glass components Check system processes ultrasonic cleaner for 5-15 ► see textbooks minutes. ► assemble with clean gloves ● Check system design and ● Some components difficult to 3. Rinse in clean tap water clean: construction 4. Rinse in deionised water ● Locate and address leaks ► a single unsuitable ► clean as well as possible by component will limit the hand 5. Dry carefully pressure achieved ► then use ultrasonic cleaner 6. Rinse in solvent ● Bake the system to achieve ► avoid gassy materials UHV Leaks Virtual Leaks

● Often present - is it important? ● Trapped volumes of air (or ● Need to work out the source time real leak other gas / vapour) constitute of the gas to fix the leak a virtual leak

► seal off system, monitor pressure ● Difficult to 'degas' - can take a pressure long time ► leak test ● Avoid creating virtual leaks by ► analyse gases in chamber time virtual leak careful design in the first place: ● Fix real leaks: pressure ► use internal welding ► re-seal leaking flanges

► re-weld crack in welding ► vent trapped volumes (avoid leak-sealant!) directly time both ► leak-seal cracks in ► drill screw holes right feedthrough connections through pressure

Helium Leak Testing Helium Leaktest How-To

● Almost VITAL to have some 1. First try to detect the leak with 5. Use as little helium as leak testing facilities - acetone possible especially at UHV level Chamber 2. Connect the leaktester to the 6. Ventilate the area ● Two common approaches system (~4 helium leak 7. Work from the top down ► apply acetone + look for ANY Helium testers around Cavendish) pressure change 1. use a leak test port, or 8. Isolate sections of vacuum ► use mass-spec helium HV system in bags leaktester Pump 2. replace one of the backing pumps 9. If gas load is small, divert entire backing load through 3. Apply a fine jet of helium from leaktester, to maximise ● Helium leak-tester is a simple a leak test cylinder (borrow response. mass-spec designed to detect from liquifier facility) helium Helium Leak Tester Backing 4. Follow response of leaktester ► connect up and spray Helium Pump over period of 10's of seconds 10.Once leak identified, locate around potential leaks (depending on pumping path) its position precisely to help ► versatile technique, used fix it widely outside 'vacuum field' Bakeout Bakeout How-To

Bake On Bake Off 1. Check that the components in 2. Enclose chamber in custom ● Generally necessary to made oven -8 the system are OK for baking achieve p < 10 mbar without & decide on max. temp. waiting too long 6. Do final check on bake zone 2. Disconnect all non bakeable 7. Slowly increase temperature ● Bake for as hot and as long connections/fittings and store to e.g. 200°c for 24 hours as possible! 3. Wrap sensitive components (heat up over a period of a ● Max. temperature determined (feedthroughs, windows) in few hours ) by components foil. 8. Monitor temperatures and ensure an even heat - avoid ► Viton seals limit ~150°c 4. Position thermocouples at various characteristic points of cold spots ► PTFE limit ~200°c the system (test them now!). 9. Hold at temperature until ► All metal systems are OK to 5. Either: pressure sufficiently low higher temperatures 1. Wrap chamber in heating 10.Cool down over a few hours Baking allow UHV to be reached in tape and several loose layer days, rather than months or years 11.Degas filaments while of foil chamber still warm

RGA Mass Spectrometer RGA of a High Vacuum System

18 - water, chamber needs baking Residual mass ion ioniser Gases filter counter 4 (He), from process, or diffusing through elastomer gaskets 28, 32 - N 2 (or CO) and O , suggest a control 2 electronics leak to atmosphere

only 2 (H 2), 28 (CO)

and 44 (CO 2) should ● Provides lowest pressure ● allows analytical be present in measurements - down to measurements and clean, leaktight UHV -15 Forest of peaks pp. of ~10 mbar diagnosis around 40-70 suggests pump oil ● provides pressure ● modern systems are run contamination (fit baffle or trap to breakdown by mass from a PC pump) number ● start at ~£5000 Cracking Patterns Good Working Practice for HV / UHV

Do: Don't: ● different species give different mass patterns ● keep a log of the behavior ● vent HV / UHV systems ► cracking, isotopic abundances etc. of system - it can help with air ● allows individual components to be identified identify problems ● vent system while ► understanding a process ● record reference RGA components are cold (e.g. ► diagnosing contaminants spectra of the chamber, if nitrogen traps, cryopumps)

● possible cracking patterns tabulated by RGA mfrs. and in O'Hanlon ● handle any components ● vent system to dry nitrogen without gloves Species Mass Abund. Mass Abund. Mass Abund. 2 100 1 2.7 3 0.31 ● open smallest flanges ● get any fluid/oil on your skin Helium 4 100 2 0.12 possible (keep under slight ● burn yourself on diff. Methane 16 100 15 83 14 15 overpressure of nitrogen?) Water 18 100 17 27 16 3.1 pumps(!) Argon 40 100 20 5.0 36 0.36 ● ensure regular Acetone 43 100 58 27 27 8.0 maintenance on pumps (if possible!) - no one else will!