Free Neon Manifold Vacuum Pump Evacuation Time Calculator: Pump-Down to 5 to 15 Microns

Vacuum pump evacuation time is the time required to reduce the pressure inside the neon sign manifold system from atmospheric pressure (760 torr) to the target vacuum pressure needed for gas fills (typically 5 to 15 microns for US neon work). It matters because the bombarding step that cleans the interior glass surfaces and activates the electrodes cannot begin until this target vacuum is achieved. Starting bombarding before the manifold has reached the correct pressure leaves residual air and water vapor in the system. When bombarding current is applied, this residual gas is ionized and can become embedded in the glass walls or react with the fill gas during the subsequent gas fill step, producing tubes that glow with the wrong color, develop dark electrode deposits early in service, or fail to strike reliably in cold weather. Getting the pump-down right is the prerequisite for every other step in neon tube processing.

The pump-down time formula derives from the ideal gas law applied to isothermal pumping: t equals V times the natural logarithm of P_initial divided by P_final, all divided by S_effective. In notation: t_sec = V_L x ln(P0_torr / Pf_torr) / S_eff_Ls. V_L is the total system volume in liters (manifold plus all tube volumes). S_eff is the effective pumping speed in liters per second, which equals the pump’s rated speed converted from cfm to L/s (multiply by 0.47195) times a system efficiency factor that accounts for manifold conductance losses, pump speed reduction at low pressures, and gas load from surface outgassing. The natural logarithm factor is approximately 11.24 for a 10-micron neon fill target (ln of 760 divided by 0.010), and changes only slightly across the normal neon shop target range of 5 to 20 microns.

The US neon industry standard for neon gas (Ne) fills is 5 to 10 microns (millitorr). The minimum acceptable vacuum for neon fills that will produce commercially acceptable tube life is generally considered 10 microns by US trade practice. Shops that target 5 microns consistently produce tubes with longer service life because the lower residual gas pressure means fewer impurity molecules remain in the tube to contaminate the fill gas. The target must be verified with a thermocouple vacuum gauge (also called a Pirani gauge or TC gauge) calibrated to measure in the 0 to 1,000 micron range. Standard Bourdon tube analog vacuum gauges cannot measure in this range and cannot be used for neon fill verification. The reading must also be stable (not continuing to drop) at the target pressure for at least 30 seconds before bombarding begins, confirming that the pump has overcome the outgassing load and the system has genuinely reached the target, not just transiently passed through it.

The US neon industry standard for argon (Ar) and argon-mercury (Ar-Hg) gas fills is 8 to 15 microns. The argon target is slightly less stringent than the neon target in terms of absolute pressure, primarily because argon fill pressure is higher than neon fill pressure, making the residual gas fraction smaller as a proportion of the total fill. However, argon-mercury tubes (which are used to produce phosphor-excited colors) are much more sensitive to water vapor contamination than pure neon tubes. Water vapor reacts with mercury in the tube to produce hydrogen gas, which causes premature tube blackening and dramatically reduces service life. For argon-mercury tubes, professional US shops aim for 5 to 10 microns rather than the 15-micron minimum, even though argon fills technically allow the higher target. Getting to the low end of the target range costs only marginally more pump-down time (a few additional minutes) but can double or triple the service life of the finished tube.

The standard vacuum pump type used in US neon sign shops is the oil-sealed rotary vane pump (also called a roughing pump or mechanical vacuum pump). These pumps consist of a cylindrical stator housing with an off-center rotor that carries spring-loaded vanes. As the rotor turns, the vanes sweep against the stator wall, trapping gas on the inlet side and compressing it to the exhaust. Oil sealed between the vanes and the stator wall prevents gas from bypassing the vanes and acts as a lubricant and sealing medium. Typical rotary vane pumps used in neon shops are two-stage designs, meaning two pumping stages are arranged in series, which allows them to achieve ultimate vacuum pressures of 0.5 to 5 microns, well below the 10-micron neon fill target. Single-stage pumps achieve about 50 to 100 microns ultimate vacuum and are not suitable for neon sign work where 10-micron targets are required. The American Vacuum Society (AVS) provides educational resources on vacuum pump technology and maintenance for US professionals.

The total system volume is the sum of the tube volumes and the manifold hardware volume. Tube volume for each tube is calculated as pi divided by 4, times the internal diameter in meters squared, times the electrode-to-electrode length in meters, times 1,000 to convert to liters. For 15mm ID (0.015m) tube of 6 feet (1.829m): V = pi/4 x (0.015)^2 x 1.829 x 1000 = 0.323 liters per tube. For the manifold hardware volume (headers, valves, gauge ports, pump-to-manifold tubing), estimate 0.5 liters for a very small compact setup, 1.0 to 2.0 liters for a standard 2 to 4 tube US neon shop manifold, and 3 to 5 liters for a larger production manifold. The manifold volume can be estimated more precisely by disconnecting the tubes, plugging the tube connections, pumping down the manifold hardware alone, then slowly admitting a known volume of air and measuring the pressure rise. This water displacement method gives the exact dead volume of your manifold hardware.

The effective pumping speed is the actual rate at which the pump removes gas from the manifold system, accounting for three factors that reduce performance below the pump’s rated nameplate speed. First, manifold conductance: every pipe, fitting, valve, and elbow in the path from the tube being processed to the pump inlet presents resistance to gas flow, reducing the rate at which gas can reach the pump. Second, pump speed reduction at low pressures: rotary vane pumps have their rated speed at relatively high pressure, and their actual pumping speed decreases significantly below about 100 millitorr as back-leakage through the vane seals becomes a larger fraction of throughput. Third, outgassing gas load: glass and rubber surfaces release adsorbed moisture and other volatiles under vacuum, continuously adding gas molecules that the pump must remove before the pressure can fall further. The effective speed is the rated speed multiplied by a system efficiency factor (eta) that combines all three effects. For a standard US neon shop, eta is approximately 0.25, meaning the effective speed is about 25 percent of the pump’s rated speed.

Glass tube outgassing is one of the most significant practical factors slowing pump-down in neon sign shops. When glass tube is bent and the electrodes are attached, the interior surface adsorbs a layer of atmospheric moisture and organic molecules. Under vacuum, these adsorbed molecules desorb and enter the gas phase, continuously adding gas load that the pump must remove. The rate of outgassing depends on the tube temperature (higher temperature means faster outgassing, which is why bombarding at 400 F greatly accelerates the cleaning of the glass wall), the surface condition of the glass (freshly bent tubes outgas more than tubes that have been previously processed and sealed), and the humidity of the shop environment when the tube was assembled. For freshly bent, unbombarded glass tubes at room temperature, the outgassing contribution to gas load can be the dominant factor in pump-down time, effectively reducing the system efficiency factor (eta) to 0.10 to 0.15. This is why the pump-down to 10 microns before the first bombarding of a new tube may take 3 to 5 times longer than the pump-down of a previously processed tube of the same size.

OSHA 29 CFR 1910.132 requires employers to assess the workplace for hazards requiring personal protective equipment and to provide appropriate PPE at no cost. For vacuum pump operation in neon sign shops, the relevant hazards include: electrical hazards from the pump motor (standard electrical safety equipment); hot surfaces on the pump body and exhaust ports; exposure to pump oil vapors if the pump exhaust is not properly vented; and, for shops that process argon-mercury tubes, potential mercury vapor exposure from pump oil contamination. OSHA 29 CFR 1910.252 applies to torch and hot work operations adjacent to the manifold station. The vacuum pump exhaust should be vented to the outside or through appropriate filtration if mercury vapor may be present from processing Hg-Ar tubes. Contact your state OSHA office or an industrial hygienist if your shop processes more than occasional argon-mercury tubes to determine whether mercury exposure monitoring is required.

Vacuum leaks are identified when the pump-down time is significantly longer than expected, when the gauge reading stabilizes above the target pressure despite adequate pump-down time, or when the reading slowly climbs after the pump is isolated from the manifold (indicating gas entering the isolated system through a leak). The standard US neon shop technique for locating leaks uses a high-frequency spark tester (Tesla coil leak detector): pump the system down to about 500 microns, then slowly pass the probe along all joints, O-ring seals, electrode boots, and glass connections. A spark discharge to the surface at the leak point indicates the location. Common leak locations include: manifold compression fittings that are slightly under-tightened, O-ring seals with dried or cracked rubber, electrode boots that have stiffened from age or heat, and hairline cracks in glass sections that may have developed during bending or annealing. Fixing leaks: tighten fittings, replace O-rings (use Viton O-rings for better chemical resistance), replace electrode boots, and reprocess or discard cracked glass sections.

The pump-down vacuum level directly affects tube service life through its effect on the purity of the fill gas. Neon sign tubes are filled with ultra-high purity fill gas (99.99% or better purity). The small amount of residual gas remaining in the tube at 10 microns (approximately 1.3 percent of atmospheric density) must come entirely from the residual gas in the manifold after evacuation, not from atmospheric air ingress (which would be pure nitrogen and oxygen). If the pump-down only reaches 50 microns instead of 10 microns, the residual gas density in the tube before fill is 5 times higher. This residual gas, which is primarily nitrogen, oxygen, and water vapor from the atmosphere, dilutes the fill gas and introduces contaminants. Nitrogen and oxygen in a neon sign cause the discharge to produce a blue-white or pink-purple tint instead of the correct red-orange neon color. Water vapor causes black deposits near the electrodes within months. Reaching 10 microns or better is therefore not a theoretical ideal but a practical requirement for producing tubes that perform correctly and last for their expected 10 to 50 year service life.

The American Vacuum Society (AVS) is the US professional organization for vacuum science and technology, founded in 1953 and headquartered in New York. It serves researchers, engineers, and technicians who work with vacuum systems across industries including semiconductor manufacturing, scientific research, glass coating, and specialty gas processing including neon sign manufacturing. The AVS publishes the Journal of Vacuum Science and Technology (JVST), maintains technical standards for vacuum measurement and pump performance, and operates a certification program for vacuum technology professionals. For US neon sign shops, the AVS is the authoritative technical reference for vacuum gauge selection and calibration, pump maintenance standards, and vacuum system design. AVS student and professional memberships provide access to technical symposia and publications that inform best practices in vacuum pump operation and maintenance.

Pump oil change frequency in a US neon sign shop depends on production volume and the types of tubes being processed. As a general rule, oil should be changed when it turns tan to brown in color rather than on a fixed calendar schedule. For a high-production shop processing multiple tubes daily, this may be monthly or more frequently. For a low-production studio doing occasional processing, oil may remain light-colored for 3 to 6 months. The key indicator is the oil appearance in the sight glass: light amber to golden means the oil is good; tan to brown means change soon; dark brown or black means change immediately. In addition to color, water-contaminated oil may appear milky white or foamy, particularly after the pump has been running for a few minutes. Water contamination comes from pumping systems with air leaks or from processing glass that was exposed to high humidity. Water in the oil dramatically degrades the pump’s ability to achieve micron-level vacuum. Some US neon shops run a 15-minute blank pump-down (pump without any tubes connected) each morning to drive water from the oil before beginning production.

A single-stage rotary vane pump has one pumping chamber and can typically achieve an ultimate vacuum of about 50 to 100 microns. A two-stage pump has two pumping chambers arranged in series: the exhaust of the first stage feeds the inlet of the second stage, allowing the second stage to pump against a much lower absolute pressure than the first stage. Two-stage pumps achieve ultimate vacuum of 0.5 to 5 microns. For neon sign work where targets of 5 to 15 microns are required, only two-stage pumps are appropriate. A single-stage pump cannot reliably reach 10 microns and would not be able to reach the 5-micron target at all under typical shop conditions. When purchasing a vacuum pump for a US neon sign shop, always verify the pump is a two-stage design and that the manufacturer’s ultimate vacuum specification is at least 1 to 2 microns (to provide an adequate margin below the 5 to 10 micron working target). Common two-stage pumps used in US neon shops include the Welch 1402 and 1405 series, the Alcatel 2004 and 2010 series, and equivalent models from Edwards and Pfeiffer.

Manifold conductance describes how easily gas can flow through the manifold plumbing from the glass tubes to the pump inlet. Low conductance (narrow tubing, many bends, long runs) restricts gas flow and means that even a fast pump cannot achieve its rated speed at the tube end of the system. The conductance of a tube is proportional to the fourth power of its diameter (in the viscous flow regime) and to the diameter cubed divided by length (in molecular flow). For a neon shop manifold, the practical guidance is: use the largest practical bore tubing (minimum 1/2 inch OD, 3/8 inch ID; preferably 3/4 inch OD for multi-tube manifolds), keep connections short and direct, minimize the number of elbows and right-angle fittings (each elbow is equivalent to several inches of straight tubing in conductance terms), use full-bore valves rather than needle valves or globe valves for the main vacuum line, and keep the pump-to-manifold connection as short as practical. A well-designed compact manifold with short metal connections and a pump located close to the manifold will consistently outperform a sprawling manifold with long rubber hose connections even when both use the same pump.

Fresh glass tubes that have never been bombarded or processed have a significant outgassing burden on their interior surfaces. The glass was exposed to atmospheric air, humidity, and trace organic compounds during bending, electrode attachment, and storage. These molecules adsorb onto the glass surface and are released slowly under vacuum at room temperature. The outgassing rate from fresh glass at room temperature can be high enough to maintain the manifold pressure at 20 to 50 microns for minutes even with a correctly functioning pump of adequate size. This is why the pump-down time for fresh tubes before the first bombarding is significantly longer than subsequent pump-downs after the tubes have been bombarded. Bombarding at 400 F drives off most surface contamination rapidly, so tubes that have been bombarded, sealed, and then returned to the manifold for any reason (such as a second processing run) will pump down much faster than fresh tubes. Some US neon shops deal with this by pre-heating the manifold station and tubes with an infrared heat lamp for 15 to 30 minutes before pump-down when working with fresh glass, which accelerates outgassing and reduces total pump-down time.