The evolution of vacuum science which started in the 17th Century, has mirrored many other scientific achievements, including the the development of the Gas Laws and the discovery of the electron. Nevertheless, the world of vacuums still continues to excite and galvanise engineers and scientists. Indeed, ground breaking developments continue to push the boundaries of this fascinating subject.
Vacuum Physics - Basic Terms
Pressure UnitsBelow is an overview of the major pressure units and conversion of pressure units:
It is common in vacuum science to sub-divide pressure ranges into five individual regimes:
Rough (or Low) vacuum (R): Atmospheric to 1 mbar
Medium (or Fine) vacuum (MV): 1 to 10–3 mbar
High vacuum (HV): 10–3 to 10–7 mbar
Ultra-high vacuum (UHV): 10–7 to 10–12 mbar
Extreme High Vacuum (XHV): greater than 10-12 mbar.
These divisions are somewhat arbitrary, with various engineering disciplines using their own definitions, ie chemists frequently refer to their spectrum of greatest interest (100 to 1 mbar), as an “intermediate vacuum”, whilst some engineers may refer to a vacuum as “low pressure” or “negative pressure”.
Vacuum technology is usually associated with three types of flow: viscous or continuum flow; molecular flow; and a transitional range between these two known as Knudsen flow.
Viscous (or continuum) flow is found in the rough vacuum range and is determined by the close interaction of molecules. There are three sub-divisions of viscous flow: “turbulent flow” (if vortex motion appears in the streaming process); “Poiseuille flow” where layers slew over each other (which is frequently the case in vacuums); and “choked flow” which occurs when venting vacuum vessels, or where there are leaks.
Molecular flow prevails in the high and ultra-high vacuum (UHV) ranges when molecules can move freely, without any mutual interference. Molecular flow is present where a molecule’s mean free path ƛ defined as the mean distance travelled by molecules between collisions) is much larger than the diameter of the pipe.
Knudsen flow is the transitional range between viscous and molecular flow. It is prevalent in the medium vacuum range where a molecule’s free path length is similar to the diameter of the pipe.
In viscous flow, the preferential movement of gas molecules will be identical to the macroscopic direction of gas flow, since the particles are densely packed and will collide with one another far more frequently than with the boundary walls. However, in molecular flow, particles impacting with the walls, predominate.
In rough vacuums, the collision of gas particles frequently occurs, whereas in the high and ultra-high vacuums, impact of the gas particles with the container walls predominates.
All fixtures between intake of pump system and chamber will lead to a reduction of pumping speed. The pV flow through any desired piping element, i.e. pipe or hose, valves, nozzles, openings in a wall between two vessels, etc., is indicated with
Here Δp = (p1 – p2) is the differential between the pressures at the inlet and outlet ends of the piping element. The proportionality factor C is designated as the conductance value or simply “conductance”. In the molecular flow range, C is a constant which is independent of pressure; in the transitional and viscous flow range it is, by contrast, dependent on pressure. As a consequence, the calculation of C for the piping elements must be carried out separately for the individual pressure ranges.
Above equation is often referred to as the “Ohm’s law for vacuum technology”, in which qpV corresponds to current, Δp the voltage and C the electrical conductance value. Analogous to Ohm’s law in the science of electricity, the resistance to flow
has been introduced as the reciprocal value to the conductance value:
The equation thus can then be re-written as:
If components are connected in parallel, the following applies:
For components connected in series the following applies:
Pressure ranges used in vacuum technology and their characteristics
For more details about the different characteristics, click the link below download our eBook:
The volume flow rate (qV) or pumping speed (S) is the (net) volumetric flow rate or volume of gas discharged per unit time (m3/s, l/s, cfm, m3/h…). This is measured at the pump inlet and depends upon gas species, vapour etc.
The pumping capacity (throughput) for a pump is equal either to the mass flow through the pump intake port:
or to the pV flow through the pump’s intake port:
It is normally specified in mbar · l · s–1. Here p is the pressure on the intake side of the pump. If p and V are constant at the intake side of the pump, the throughput of this pump can be expressed with the simple equation
where S is the pumping speed of the pump at intake pressure of p.
The throughput value is important in determining the size of the backing pump in relationship to the size of a high vacuum pump with which it is connected in series in order to ensure that the backing pump will be able to “take off” the gas moved by the high vacuum pump.
Ultimate pressure pult is the lowest pressure of a blank-flanged vacuum pump under defined conditions without gas inlet. At ultimate pressure, the usable pumping speed will be zero. It is a theoretical value.
The lowest pressure which can be achieved in a vacuum vessel will be determined by
Vapor pressure of lubricants
Degassing of solved gases in lubricants
Desorption of gases from internal surfaces of vessel
Leak tightness of system itself
Diffusion of gas though vacuum wall or seals
Compression of vacuum pump system
The compression ratio CR (or k) is the ratio of the exhaust/outlet pressure (pout) to the inlet pressure (pin)
The maximum compression k0 (Outlet pressure / Inlet pressure) assumes no flow conditions (zero pumping speed) and is a theoretical value.
The correct choice and usage of a vacuum pump is essential. Vacuum pumps can be divided into two main types: “Gas Transfer” or “Capture/Entrapment”. Gas transfer pumps naturally divide into two sub-divisions: “positive displacement” or “kinetic” pumps and can typically be categorised into “wet” (where oil is used as sealant / lubricant in the pumps’ suction chamber) or “dry” pump types (no oil used as sealant within the pumps suction chamber).
Positive displacement pumps operate in the R and MV ranges (and are either pumps that ‘reciprocate’, including diaphragm and piston, or those that ‘rotate’, including vane, screw, roots and scroll). Reciprocal pumps employ an oscillating piston or surface within a barrel/space, whilst rotating pumps employ an object rotating around an axis within a vessel.
Kinetic pumps, which operate in the HV and partly UHV ranges (ie turbo-molecular, diffusion and vapour booster), employ spinning blades or supersonic vapour jets impacting the gas molecules. Kinetic pumps require a supporting primary pump since they are unable to exhaust to atmospheric pressure.
“Capture (or Entrapment) pumps” include: cryogenic, ion-getter, non-evaporate getter and sublimation pumps, operate in the UHV and XHV ranges. Capture pumps immobilize gas molecules on special surfaces within the vacuum system. They do not need a continuously supporting ‘primary’ pump but operation requires the establishment of a suitable low pressure.
More information about the pump principles can be found in our eBook here.
In order to ensure a proper pump selection for a vacuum application several points need to be considered, such as:
Pump fit for the vacuum system (performance, installation, operation, control,...)
Potential impact from vacuum process/application on pump
Potential impact from vacuum pump on vacuum process/application
Economic considerations such as investment costs, cost of ownership, maintenance demand etc.
Safety and regulatory requirements
Click here to find out more about the criteria you should consider to select the right pump in our eBook.
In our next eBook, we will explore the potential impact of the vacuum pump on the vacuum process/application and the potential impact of the vacuum process/application on the pump in more detail.
The physical properties of gases change with pressure. For example, the thermal conductivity and the internal friction of gases in the MV range are highly sensitive to pressure. However, in the HV range, these two properties are virtually independent of pressure. Therefore, different vacuum gauges will be required to accommodate different vacuum ranges with their usage dependent upon a wide range of factors including: the pressure range; what gases are involved (which will determine any correction factors, media compatibility and potential for chemical reactions); the accuracy required; the operating conditions (dirty vs. clean, vibrations, temperature, shock (possibly due to venting), radiation and magnetic fields); installation positions of the gauges; and how the pressure is to be read (or recorded).
There are two methods of pressure measurement in vacuum science: the “direct” method where pressure = force per area, which is only suitable for rough to Medium Vacuum. The “indirect” method is depending on the density of the gas being used, as this impacts upon heat conductance, viscosity, and ionisation probability, and thus are dependent upon gas types. Indirect measurement is best suited to Medium to Ultra high vacuum.
Absolute pressure measurement has vacuum as reference point; relative pressure measurement has ambient pressure as reference point.
So you can see there is a variety of methods with which to measure vacuum, so knowing your environment is key to picking the right gauge.
Vacuum Gauge Types
Table of Technologies and their standard measuring ranges
Piezo - Works on the principle that under a mechanical load, Semiconducting materials experience a change in their resistivity. Pressure differences result in a voltage change.
Capacitance - A more sophisticated variant of the diaphragm principle in that the deflection of a diaphragm is measured electrically rather than mechanically. The diaphragm is part of a capacitor and the pressure change results in a capacity change which is measured electronically. This is more complicated but offers much greater accuracy and stability. The diaphragm is commonly made of either Ceramic or metal and designed for extremely long life under harsh conditions.
Heat conductivity - In its most common form (Filament Pirani) it uses a heated filament (usually around 100°C above ambient) as part of a balanced bridge. This filament is then kept at a fixed temperature, and the voltage needed to keep it constant during a pressure change can then be converted to a pressure. MEMS is a new type of sensor using the “micro-electro-mechanical systems” with the temperature for the filament only 30-40°C above ambient, making it more suitable for reactive gasses.
Hot Cathode ('hot ion gauge') - Gas particles are ionized by energized electrons. These ionizing electrons originate from a Hot Filament cathode. They are then accelerated to a grid-shaped anode. The ions are accelerated to a collector and the current can be then converted into a pressure reading. Several variants of this method exist, commonly used is the Bayard-Alpert method and the Extractor is used in UHV applications, due to its ability to measure into the -12 mbar region.
Cold Cathode - For this gas particles are ionized by electrons emitted from a cathode at room temperature. Applying a large voltage difference between the Cathode and anode a self supporting discharge can be created, provided that the path between the cathode can be made sufficiently long. This is achieved by applying a strong magnetic field. Its most common form is the Inverted Magnetron, although Penning type gauges are also widely used in some applications
Active vs Passive
Generally, vacuum gauges fall into two categories: active and passive.
“Active gauges” contain measurement electronics in their head. The reading output can be S-shaped, a linearized analog voltage or follow digital communication protocols with characteristic curves that can be modified by software.
“Passive gauges” contain only a sensor, which is part of a circuit with external control (ie a Wheatstone bridge), whilst the ‘signal’ is processed separately. Although interference along the cable can lead to pressure errors, these gauges are less sensitive to radiation.
However, vacuum measurement is not only about choosing the right gauge; correct maintenance (and cleanliness) of the gauges is also important. For example, dirt in a compression vacuum gauge will cause an incorrect and uncontrollable pressure indication. Contaminated Pirani sensors will show a pressure which is too high in the lower measurement range since the surface of the hot wire has changed. In Cold Cathode vacuum gauges, contamination will induce pressure readings which are too low, since the discharge current will become smaller. To this end, most vacuum gauges have easily replaceable or cleanable components so that you can rely on the pressure readings during your process.
What is a leak? A leak is a small hole in the technical system that allows the undesirable entry or exit of fabrics into or out of the system. The leak rate describes the size of the leak in terms of the gas amount that passes through the leak per time at a given pressure difference.
No vacuum device or system can ever be absolutely vacuum-tight and it does not actually need to be. The simple essential is that the leak rate be low enough that the required operating pressure, gas balance and ultimate pressure in the vacuum container are not influenced. It follows that the requirements in regard to the gas-tightness of an apparatus are the more stringent the lower the required pressure level is.
When searching for leaks one will generally have to distinguish between two tasks:
- Locating leaks and
- Measuring the leak rate
In addition, we distinguish, based on the direction of flow for the fluid, between the a. vacuum method (sometimes known as an “outside-in leak”), where the direction of flow is into the test specimen (pressure inside the specimen being less than ambient pressure), and the b. positive pressure method (often referred to as the “inside-out leak”), where the fluid passes from inside the test specimen outward (pressure inside the specimen being greater than ambient pressure). The specimens should wherever possible be examined in a configuration corresponding to their later application – components for vacuum applications using the vacuum method and using the positive pressure method for parts which will be pressurised on the inside. When measuring leak rates we differentiate between registering
- individual leaks (local measurement)
- the total of all leaks in the test specimen (integral measurement).
Integral Testing: Sample under pressure
Integral Testing: Sample under vacuum
Local Testing: Sample under pressure
Local Testing: Sample under vacuum
The detection of leaks in both pressured and vacuum systems, as well as their elimination, management and/or accountability, are important. As gas is compressible, the pressure (or vacuum) influences the amount of the leak, so they are quoted in mbar litre/sec, with the ‘leak rate’ being the amount of gas that flows through a leak at a given pressure differential per time.
There are several ways of measuring leaks, each depends on the lowest detectable leak rate applicable:
Differential pressure measurement
Pressure decay tests / Pressure rise tests
Helium Sniffer Mode / Helium Vacuum Mode
Of these, the most interesting are the helium “sniffer” and the helium “vacuum” tests. These procedures are run with a helium leak detector using a sector field mass spectrometer. These units are the most sensitive and also provide the greatest degree of certainty. Here “certain” is intended to mean that there is no other method with which one can, with greater reliability and better stability, locate leaks and measure them quantitatively. For this reason helium leak detectors, even though the purchase price is relatively high, are often far more economical in the long run since much less time is required for the leak detection procedure itself.
A helium leak detector comprises basically two sub-systems in portable units and three in stationary units. These are:
- the mass spectrometer
- the high vacuum pump and
- the auxiliary roughing pump system in stationary units.
In simple terms, the helium “sniffer” test involves a sniffer probe being passed around the unit under observation. The sniffer test has the advantage that it shows where leaks actually occur. However, helium concentrations of 5ppm in air, limits the minimum detectable rate, and furthermore ambient background signals can impact the possible detection of minimal leaks.
The helium “vacuum” test is usually employed on units subjected to UHVs. In simple terms, the unit is placed inside a vessel and pressurised with helium. Gas within the vessel is then subjected to a mass spectrometer test, and any helium detected will indicate a leak. The major disadvantage—though not the only one—is that the unit needs to be placed within a vessel of a suitable size.
The analysis of gases at low pressures is not only useful when analysing residual gases from a vacuum pump, leak testing at a flange connection, but also for vacuum supply lines. They are also essential in the broader fields of vacuum technology applications and processes. The equipment used for the qualitative and/or quantitative analyses of gases includes specially developed mass spectrometers with small dimensions which can be connected directly to the vacuum system.
Partial pressure is that exerted by a particular gas within a mixture of gases. The total of all partial pressures gases present, gives the total. The distinction among the various types of gases is essentially on the basis of their molar masses. The primary purpose of analysis is therefore to register qualitatively the proportions of gas within a system and determine (quantitatively) the amount of each gas.
An RGA consists of the sensor, the interface box and the control unit.
The sensor must be exposed to vacuum as it requires low pressures to start working.
The measurement cell is a hot cathode ionization vacuum gauge. Electrons are emitted from the hot filaments and accelerated toward the source via an electrical bias. The fast moving electrons collide with the gas molecules, dislodging electrons, thus ionizing them. Positive gas ions provide their current to the negative loaded ion collector. Ion current is proportional to pressure: The more ions, the higher the ion current.
The quadrupole array is composed of four stainless steel rods. A direct current and high frequency voltage is applied to all rods: -DC on one pair (two opposed rods) and +DC on the other pair (two opposed rods). This creates a complex magnetic field and by varying the voltage the magnetic field is controlled. The resulting vibration only allows ions of a specific mass to “fly” through it and reach the detection part:
Ions with an appropriate mass to charge ratio (m/e), will oscillate in a stable three dimensional trajectory through the poles
Ions of incorrect m/e will oscillate out of control and collide with the poles
RGAs are basically partial pressure gauges:
RGAs show the composition of gas by using this principle. Typical applications for RGAs are:
Leak Detection and Identification
Find and Identify Contaminants
Verify Gas Purity
Control recovery process after venting
Product/Process Quality Assurance
Process & Equipment Diagnostics and Control
Optimise Process Performance and Yield