Frequently Asked Questions

When dealing with vacuum leaks, you must first determine if the leak is a “Real” or “Virtual” leak. A real leak is one that is coming into the vacuum chamber from an outside source. A virtual leak is a gas load evolving from inside the vacuum system from outgassing. Outgassing comes from any liquid (water, solvents, etc.) that is vaporizing inside the vacuum chamber either off the chamber walls or out of a product/substance placed into the vacuum chamber for processing. A simple means of determining if you have a real or virtual leak is to do a “rate of rise test”. To begin, simply reduce the pressure in the vacuum chamber to its’ maximum obtainable vacuum level. With a calibrated/highly accurate Torr gauge, measure the rise in pressure over time in the chamber. If the pressure eventually rises to atmospheric pressure, then the leak is real. If the pressure rises and stops at some pressure below atmospheric in a 24 hour period, then the leak is virtual.

For real leaks the use of a Helium leak detector or ultrasonic leak detector maybe used depending on what size of leak you are looking for. Also, you can check for real leaks by spraying a small amount of acetone around the suspect leak areas, when the acetone enters the vacuum system there will be a spike in the pressure you will see on your vacuum gauge.

Virtual leaks can also be verified by measuring the vacuum in the system with a Mcleod gauge (partial pressure gauge). Since a Mcleod gauge can only measure permanent gases (Air, nitrogen) and not vapors (water vapor/solvents) then a low pressure reading with a Mcleod gauge and a higher pressure reading with a total pressure gauge (Torr or Thermocouple gauge) will indicate a virtual leak. Conversely, a high pressure reading on both the Mcleod & Torr/Thermocouple gauge will indicate a real leak.

Basically, one standard atmosphere at standard conditions will support a column of mercury 29.92″ high. This is where the linear measurement in vacuum comes into play. 29.92″ can be also measured in mm Hg (760mm = 29.92″) and microns (760,000 microns = 760 mm = 29.92″). Depending on what vacuum level you require, you will use a different unit of measure for the vacuum measurement. When measuring vacuum below 1 micron, we go to scientific notation (Example: 1 x 10-3 mm Hg)

SCFM stands for Standard Cubic Feet per Minute. SCFM is the air volume measured at “standard conditions” which refers to atmospheric pressure (760 m Hg), at sea level elevation and a temperature of 60 Deg F. ACFM stands for Actual Cubic Feet per Minute and represents air volume pumped at the pressure, elevation and temperature your particular system is operating at. Many vacuum pump performance curves shows ACFM to indicate the actual volume of process gas pumped at various inlet pressure to the pump.

An absolute pressure gauge is one that will measure your vacuum system without regard to and independent of local barometric pressure. Many dial (Bourdon) gauges and electronic Transducers reference local barometric pressure as their base measurement. However, since these devices are calibrated at SEA LEVEL conditions, operation of these devices above sea level will cause an erroneous reading. Either the gauge/transducer must be recalibrated for the higher elevation use or an absolute pressure gauge would need to be used.

A Torr gauge is an absolute pressure gauge and operates on the principle of an altimeter. The Gauge case is evacuated by the vacuum from the process and exerts a negative pressure on a hermetically sealed capsule. The lowering of the pressure in the gauge case causes the capsule to expand thereby causing the gauge movement to turn the pointer. The Torr gauge is highly sensitive and accurate in the lower pressure regions (0-100 mm Hg).

There is no one vacuum pump that is best for all applications. However, there are some general guidelines to remember for your selection.

Oil Lubricated Rotary Pumps are used in applications requiring fairly deep vacuum (< 1 mmHg) and pumping relatively clean gases (Air/N2). Oil lubricated pumps are available in Single stage & Dual stage depending on what vacuum level you need. Additionally, all the oil lubricated pumps are available in Belt drive or Direct drive configurations. Belt drive is preferred in applications where pump longevity and durability is desired because of the low pump rpm (<700 rpm) and their high oil holding capacity which also guards against premature wear from oil breakdown. Direct drive pumps are preferred because of their low cost, compactness and portability.

Dry Vane pumps; are used when a pump is required that does not require lubricating oil because of the objection to oil vapor discharge from the pump and filling/disposal issues with oil. Rotary vane dry pumps however are capable of only maximum vacuum of approximately 25″ Hg and can only pump clean DRY air. Any presence of moisture in the gas being pumped can lead to the pump rusting because of the absence of lube oil.

Rotary Screw Dry Pumps are used in applications where a high vacuum is required (up to 0 .010 mm Hg) and the process gas is not compatible with lubricating oil in oil sealed rotary pumps. These pumps are fairly expensive and are used where a lubricated oil sealed pump or liquid ring pump is not desired. Call U.S. Vacuum for more details.

Liquid Ring Pumps are used in applications where the process gas may contain a sizable amount of condensable vapors (water, solvents, acids, etc.) that will react negatively with the lubricating oil in Rotary Vane pumps, thereby causing pump damage. Being that a liquid ring pump is a centrifugal unit, the sealing medium can be water, oil or any other fluid compatible with the process. Liquid ring pumps are relatively inexpensive and can use any sealing fluid (water, oil, ethylene glycol, solvents, etc.) that is compatible with the process.

Dry pumps are useful for many applications, but some precautions must be taken when selecting them. Typically, dry pumps have uncoated internal parts made of cast or ductile iron. Being that these uncoated components can easily rust and oxidize in the presence of moisture or corrode in the presence of stronger reactive gases, care must be taken to protect the pump. Since dry pumps do not have a sealing fluid (oil) between it’s moving internal parts, like rotary oil sealed pumps, dry pumps must rely on very tight internal clearances (0.001″-0.003″) to obtain their low ultimate pressures. If the uncoated internal parts start to corrode/oxidize from the presence of condensable vapors the formation of rust/flaking deposits can cause the pump to lock-up (because of the pumps’ very tight operating tolerances). Proper shut-down procedures incorporating purge gases (N2) can help in removing these “wet” type of gases. Additionally, any process that will allow the incoming gas stream to accumulate on the internal rotors/screws of the pump in the form of crystals or paste can also cause clearance issues…steam cleaning would be required in these applications. For additional assistance on Dry pumps call U.S. Vacuum.

A gas ballast is a regulated in-bleed of a dry gas (usually Air/Nitrogen) into the compression portion of the pumping cycle of the vacuum pump. The gas acts as a stripping agent that will saturate with the contaminating vapors present in the pump and expelled out the discharge of the pump. Gas ballasts are usually installed as a standard component on all oil lubricated rotary vacuum pumps to aid in the removal of condensable vapors from the vacuum pump oil.