Vacuum Technology Glossary
The essential terms around vacuum pumps, pressure and leak tightness, each explained briefly and linked to the page that covers it in depth.
Pressure and vacuum ranges
Absolute pressure
Also: psia, mbar(a)
Pressure measured from a perfect vacuum (zero) upwards, so it is never negative. Standard atmospheric pressure at sea level is 1013.25 mbar absolute (14.7 psia). Vacuum technology works almost entirely with absolute pressure, because it does not change with weather or altitude.
More: Pressure units explained
Gauge pressure (vacuum)
Also: relative pressure, psig, inches of vacuum
Pressure measured relative to the surrounding atmosphere. Vacuum then appears as a negative value (for example −0.8 bar) or as inches of mercury of vacuum (inHg). The deepest possible reading is about −1013 mbar at sea level and less at altitude, which is why gauge values are unsuitable for specifying deep vacuum.
Micron and Torr
Also: microns, millitorr, mTorr, Torr
One Torr is 1/760 of a standard atmosphere, which is 133.3 Pa or 1.333 mbar. One micron (of mercury) is a thousandth of a Torr, also called a millitorr, which is about 0.00133 mbar. Microns are the everyday unit for HVAC evacuation in the US, where 500 microns is a common target.
More: Microns to Torr
Partial pressure
The share of the total pressure contributed by one gas in a mixture; the partial pressures of all gases add up to the total (Dalton’s law). In vacuum systems, partial pressures matter more than the total when water vapour, oxygen or a process gas is the real concern. Vacuum furnaces, for example, often run at a controlled partial pressure of argon or nitrogen so that alloying elements do not evaporate.
More: Vacuum furnaces
Triple point
The single combination of pressure and temperature at which a substance exists as solid, liquid and vapour at the same time. For water it lies at 6.1 mbar (611.7 Pa) and 0.01 °C. Below this pressure, liquid water cannot exist and ice turns directly into vapour, which is the principle behind freeze drying.
More: Freeze drying
Vacuum ranges
Also: rough vacuum, medium vacuum, fine vacuum, high vacuum, ultra-high vacuum, UHV
Vacuum is divided by absolute pressure into rough vacuum (atmosphere to 1 mbar), medium or fine vacuum (1 to 10⁻³ mbar), high vacuum (10⁻³ to 10⁻⁸ mbar) and ultra-high vacuum (below 10⁻⁸ mbar), following ISO 3529-1. Each range needs different pumps and gauges, because the way gas behaves changes from range to range.
More: Vacuum levels and ranges
Vapor pressure
Also: vapour pressure, saturation vapor pressure
The pressure at which a liquid or solid is in balance with its own vapour at a given temperature. If the surrounding pressure drops below it, the substance boils or sublimes; water, for example, has a vapour pressure of about 23 mbar at 20 °C. This is why vacuum lowers boiling points and why moisture limits how quickly a system can be evacuated.
Pump performance
Backing pump
Also: roughing pump, fore pump, critical backing pressure, fore-vacuum
A pump that takes the exhaust of a high-vacuum pump, such as a turbomolecular or diffusion pump, and discharges it to atmosphere; it usually also pumps the chamber down first. The high-vacuum pump only works while its outlet stays below its critical backing pressure (fore-vacuum tolerance). Rotary vane, scroll, screw and diaphragm pumps are typical backing pumps.
Compression ratio
The ratio of a pump’s outlet pressure to its inlet pressure for a particular gas when no gas flows through it. It decides how low a pump can go for a given backing pressure. Turbomolecular pumps compress heavy gases such as nitrogen very strongly but light gases such as hydrogen and helium far less, so these dominate the residual gas.
Effective pumping speed
The pumping speed that actually reaches the chamber once the connecting pipework is taken into account. It follows from 1/S_eff = 1/S + 1/C, where S is the pump’s speed and C the conductance of the line. A narrow or long line can cut the effective speed to a fraction of the pump’s rating, especially in molecular flow.
More: Line sizing calculator
Gas ballast
Also: water vapor tolerance, water vapour tolerance
A small, controlled bleed of air into the compression stage of an oil-sealed pump. It lets the exhaust valve open before vapours such as water reach their condensation pressure, so they leave the pump instead of mixing with the oil. The pump’s water vapour tolerance states how much water vapour it can handle this way; the price is a slightly higher ultimate pressure.
Gas load
The total flow of gas entering a vacuum system that the pumps have to remove: leaks, outgassing, permeation through seals and any process gas. In steady state the pressure settles where the effective pumping speed balances the gas load (p = Q/S_eff). Reducing the gas load is often cheaper than buying a bigger pump.
Pump-down time
Also: evacuation time
The time needed to evacuate a chamber from a starting pressure to a target pressure. In rough vacuum it follows t = (V/S)·ln(p₀/p₁), with chamber volume V and effective pumping speed S. Near the target, outgassing and leaks slow the process down, so real pump-down times are usually longer than this estimate.
Pumping speed
Also: volume flow rate, displacement, CFM, m³/h, l/s
The volume of gas a pump removes per unit of time, measured at its inlet pressure, in m³/h, l/s or CFM (1 l/s = 3.6 m³/h; 1 CFM ≈ 1.7 m³/h). The rated value applies over the pump’s main working range and falls towards its ultimate pressure. Together with the chamber volume, it sets how quickly a system can be pumped down.
Throughput (pV flow)
Also: gas throughput, pV flow, mass flow
The quantity of gas flowing per unit of time, expressed as pressure times volume flow (Q = p·S), usually in mbar·l/s or Pa·m³/s. Unlike pumping speed, throughput is the same at every point along a pipe in steady state. Leak rates and gas loads use the same unit, which makes them directly comparable.
Ultimate pressure
Also: base pressure, ultimate vacuum
The lowest pressure a pump can reach with its inlet blanked off and no gas load. Real systems settle above it because of leaks, outgassing and the conductance of the pipework. Typical values are about 10⁻³ mbar for a two-stage rotary vane pump and 10⁻⁹ to 10⁻¹⁰ mbar for a turbomolecular pump.
Gas flow
Conductance
Also: flow conductance
A measure of how easily gas flows through a pipe, valve or opening, in l/s; it is the reciprocal of flow resistance. In viscous flow it rises with pressure, while in molecular flow it is independent of pressure and, for a long tube, grows with the cube of the diameter. Undersized pipework is one of the most common reasons a pump underperforms.
Knudsen number and flow regimes
Also: Knudsen number, viscous flow, molecular flow, transitional flow, Knudsen flow
The Knudsen number Kn is the mean free path divided by a characteristic size such as the pipe diameter. Below Kn = 0.01 the flow is viscous: molecules mainly hit each other and move as a stream. Above 0.5 it is molecular: molecules mainly hit the walls and move independently, and in between lies the transitional (Knudsen) regime. Conductance and pump behaviour change fundamentally between these regimes.
Mean free path
The average distance a gas molecule travels before it collides with another molecule. It is inversely proportional to pressure: for air at room temperature it is about 0.07 mm at 1 mbar and about 6.6 cm at 10⁻³ mbar. Comparing it with the size of a pipe or chamber shows which flow regime applies.
Leaks and gas sources
Bake-out
Also: bakeout, baking
Heating a vacuum chamber and its components while pumping, typically to 150–250 °C, to drive adsorbed water and other gases off the surfaces. It shortens the time to reach high vacuum and is essential for ultra-high vacuum. All components, seals and gauges must be rated for the bake-out temperature.
More: Vacuum levels and ranges
Helium leak detection
Also: helium leak detector, sniffing, sniffer probe, MSLD
The standard method for finding and measuring small leaks, using helium as a tracer gas and a mass spectrometer tuned to it. In the vacuum method, helium is sprayed onto the outside of the evacuated part and the detector measures what gets in; in the sniffing method, the part is filled with helium and a probe scans the outside. The vacuum method is far more sensitive, and both give the leak rate directly.
More: Leak rate converter
Leak rate
The quantity of gas passing through a leak per unit of time, usually in mbar·l/s, Pa·m³/s, Torr·l/s or sccm. In a sealed chamber it can be found from the pressure rise: leak rate = volume × pressure rise ÷ time. Acceptable values range from about 10⁻² mbar·l/s for rough vacuum work to 10⁻¹⁰ mbar·l/s and below for UHV systems.
More: Leak rate converter
Outgassing
Also: desorption
The release of gas from the surfaces and materials inside a vacuum system: adsorbed water, gas dissolved in metals, and vapours from plastics and elastomers. In unbaked systems water vapour dominates and sets how quickly high vacuum is reached. Clean surfaces, suitable materials and bake-out reduce it.
More: Vacuum levels and ranges
Rate-of-rise test
Also: leak-up rate, pressure rise test, decay test
A simple tightness check: the evacuated chamber is isolated from the pump and the pressure rise is recorded over time. A rise that levels off points to outgassing or evaporating moisture, while a steady, linear rise points to a leak. Multiplied by the chamber volume, the rate of rise gives the total leak rate; HVAC technicians call the same check a decay test.
Virtual leak
Gas trapped in a pocket inside the vacuum system that escapes slowly and behaves like a leak, although there is no path to the outside. Typical causes are blind tapped holes closed by a screw, double welds and gaps behind seals. A leak detector finds nothing outside, so virtual leaks are avoided by design, for example with vented screws.
Equipment and gauges
Capacitance diaphragm gauge
Also: CDG, capacitance manometer, Baratron
A gauge in which the pressure deflects a thin diaphragm and the deflection is measured as a change in electrical capacitance. Because it measures force per area directly, the reading is independent of the gas type and accurate to a fraction of a percent. Each head covers about four decades, so different heads are used between atmosphere and about 10⁻⁵ mbar.
More: Vacuum gauges
Cold trap
Also: condenser, vapor trap
A cooled surface between chamber and pump on which vapours such as water or solvents condense before they reach the pump. Cooling is by refrigeration or liquid nitrogen (−196 °C). It protects the pump oil, increases the effective pumping speed for vapours and, in a freeze dryer, removes almost all of the water.
More: Freeze drying
Ionization gauge
Also: ion gauge, hot cathode, cold cathode, Bayard-Alpert, Penning gauge, inverted magnetron
The standard gauge for high and ultra-high vacuum: it ionizes gas molecules and measures the resulting ion current, which is proportional to the gas density. Hot-cathode types (Bayard-Alpert) use a heated filament as electron source and reach about 10⁻¹⁰ mbar; cold-cathode types (Penning, inverted magnetron) use a discharge in a magnetic field and have no filament to burn out. The reading depends on the gas and is normally calibrated for nitrogen.
More: Vacuum gauges
Oil mist separator
Also: exhaust filter, oil mist filter
A filter on the exhaust of an oil-sealed pump, such as a rotary vane pump, that captures the fine oil mist carried out with the gas. The collected oil drains back to the pump or into a reservoir, which keeps the workspace clean and reduces oil consumption. A clogged element raises the exhaust pressure and should be replaced at the intervals the manufacturer gives.
Pirani gauge
Also: thermal conductivity gauge, micron gauge
A thermal conductivity gauge: a heated wire loses heat to the surrounding gas, and the lower the pressure, the less heat it loses. It covers roughly atmosphere to 10⁻⁴ mbar at low cost, making it the most common gauge for rough and medium vacuum. Because gases conduct heat differently, the reading depends on the gas and is usually calibrated for nitrogen or air.
More: Vacuum gauges
Roots pump
Also: Roots blower, Roots booster, booster pump, mechanical booster
A dry pump with two figure-eight lobes that rotate in opposite directions without touching each other or the housing. It has no internal compression and cannot discharge to atmosphere, so it always works with a backing pump. In the range of roughly 1 to 10⁻³ mbar it multiplies the pumping speed of the backing pump, which is why it is common on freeze dryers, furnaces and coating plants.