Vacuum Pumping
Part of Vacuum Tubes
Principles and construction of mechanical vacuum pumps capable of evacuating glass envelopes for tube fabrication.
Why This Matters
Every vacuum tube requires the same thing: the inside of the glass envelope must be emptied of air to a pressure roughly one hundred thousand times lower than atmospheric. Without adequate vacuum, gas molecules collide with electrons in flight, scattering them and preventing the tube from functioning. Even a small residual gas pressure ruins tube performance and destroys cathode coatings within hours. The vacuum pump is the foundational tool of tube manufacture.
This sounds daunting, but mechanical vacuum pumps capable of tube-quality vacuum are buildable from metal and oil. The Sprengel pump, invented in 1865, achieves excellent vacuum using only mercury and a hand-operated water pump — no machined parts beyond basic plumbing. The rotary oil-sealed piston pump, the standard modern tool, requires more machined components but operates continuously. Understanding both gives a rebuilding community a path to vacuum capability.
The vacuum needed for tubes is what engineers call a “rough vacuum” or “medium vacuum” — pressures of 0.001 to 0.1 mbar (0.001 to 0.1 Torr). This is far from the high vacuum of research physics, but it is also far from what ordinary pumps achieve. An ordinary bicycle pump compresses; a vacuum pump must expand and seal simultaneously, which is why design matters.
Understanding Vacuum Levels
Atmospheric pressure is approximately 1,013 mbar (760 Torr). Gas behavior changes dramatically across the range from atmosphere to the vacuum needed for tubes:
At 100 mbar: the air density is 10% of atmospheric. Electrons still collide with gas molecules frequently. Not suitable for tubes.
At 1 mbar: about 0.1% of atmospheric density. The mean free path of molecules (average distance between collisions) is roughly 5 cm. Tube operation is marginal; short tubes might function.
At 0.1 mbar: mean free path around 50 cm. Electrons traverse a small tube without significant scattering. Marginally acceptable for tube operation, especially with getter assistance.
At 0.01 mbar: mean free path several meters. Excellent for tube operation. A well-made mechanical pump with proper oil sealing can achieve this.
At 0.001 mbar: very good vacuum, more than adequate for all tube types. Requires a well-maintained oil-sealed rotary pump or a diffusion pump backed by a mechanical pump.
The goal for tube manufacture is 0.01-0.001 mbar. Achieving less than 0.1 mbar requires attention to pump design and oil quality; going below 0.01 mbar requires careful sealing and often a diffusion pump second stage.
The Sprengel Pump: No Machining Required
The Sprengel pump (Herman Sprengel, 1865) achieves vacuum by entraining gas in falling drops of mercury. It is slow but can reach excellent vacuum and requires no precisely machined moving parts — an important advantage in a low-technology setting.
Construction: A glass tube, bent into a U-shape at the bottom, has its falling arm (typically 75-100 cm long) connected to the vessel to be evacuated at the top. Mercury falls through this tube from a reservoir at the top. As mercury drops fall, they trap small plugs of gas above them in the tube. These gas plugs fall with the mercury and are expelled at the bottom. Each drop cycle removes a tiny amount of gas; over thousands of cycles, the pressure drops dramatically.
The mercury falling to the bottom is collected in a flask. A second small pump (a water-powered aspirator, or a simple hand pump) lifts mercury back to the upper reservoir, maintaining continuous circulation. The entire system — reservoir, long tube, bottom collection flask, return pump — is plumbing, not precision machinery.
Performance: A well-made Sprengel pump pumping a small volume (the interior of a tube, perhaps 50-100 mL) can reach 0.001 mbar in several hours. It is impractically slow for large volumes but perfectly suited to tube evacuation. The key limitation is mercury — large quantities are required (several liters for a working system) and mercury is toxic, requiring careful handling and no spills. In a setting where mercury can be sourced from cinnabar ore (mercury sulfide, relatively common), this is achievable. Alternatively, low-vapor-pressure oils can substitute for mercury in modified designs, though with reduced performance.
Using the Sprengel pump: Connect the tube’s exhaust tubulation to the pump’s upper inlet. Begin mercury circulation slowly. Over the first 30 minutes, pressure drops from atmosphere to perhaps 10 mbar. Over the next 1-2 hours, it drops to 0.1 mbar. Below 0.01 mbar, progress slows as each mercury drop carries proportionally less gas. Monitor by watching for the tube’s cathode-heater glow to become visible and the interior gas discharge (if a test electrode is present) to change color from orange-purple (high pressure discharge) to faint blue-white (good vacuum). When the discharge disappears entirely, vacuum is adequate for tube sealing.
The Rotary Oil-Sealed Pump
The rotary oil-sealed pump is the workhorse of modern vacuum technology. It operates continuously, pumps at a useful volumetric rate, and achieves pressures in the range needed for tube manufacture (0.001-0.1 mbar) without the complications of mercury.
Operating principle: An eccentrically mounted cylindrical rotor turns inside a cylindrical housing. One or more spring-loaded vanes extend from the rotor and contact the housing wall, dividing the crescent-shaped space between rotor and housing into chambers. As the rotor turns, each chamber’s volume first increases (drawing gas in from the inlet) then decreases (compressing the gas against the outlet valve). Oil fills the gap between rotor and housing, providing the airtight seal that makes the pump effective.
The outlet valve — a spring-loaded flap — opens when gas pressure in the compression chamber exceeds atmospheric pressure, expelling the gas. Oil is continuously carried around the rotor and replenished, maintaining the seal and lubricating the vane-to-housing contact.
Material requirements: The housing must be machined to close tolerances (gaps between vanes and housing on the order of 0.01-0.05 mm). This requires metal-turning capability — a lathe or at minimum accurate hand fitting. The rotor and housing should be hardened steel or cast iron; softer metals wear rapidly and destroy the precision clearances. Vanes are typically graphite or spring steel; graphite is self-lubricating and wears in without scratching the housing.
Oil requirements: The pump oil must have extremely low vapor pressure — if the oil itself evaporates, its vapor establishes a pressure floor that the pump cannot beat. Mineral oils with low vapor pressure (vacuum pump oils, available from technical suppliers or distilled from petroleum with careful fractionation) achieve 0.01-0.001 mbar. Engine oil is inadequate — its vapor pressure limits achievable vacuum to 1-10 mbar. Clean, properly specified oil is not optional; use the right oil.
Pump sizing: For tube evacuation, a pump displacing 50-200 mL per revolution is adequate. At modest rotation speeds (500-1,000 RPM), this gives a volumetric pumping rate of 25-200 liters per minute. The interior volume of a tube is perhaps 10-50 mL; the connecting tubing adds another 10-50 mL. This volume is pumped down to 0.1 mbar in under a minute, reaching final vacuum in 5-15 minutes with proper bakeout.
Seals, Fittings, and Connections
The vacuum system is only as good as its weakest seal. A single small leak anywhere in the connections between pump and tube will limit the achievable vacuum, typically at a pressure determined by the leak rate. Finding and fixing leaks is a recurring challenge in tube manufacture.
Connections: Glass-to-glass connections (ground glass joints, or flame-fused) are the most reliable for tube sealing. Glass-to-metal connections (through the pump body, in stopcock valves) require carefully matched expansion coefficients or flexible intermediate elements. In a simple system, rubber tubing connects glass parts to the pump; at good vacuum levels, rubber outgasses significantly and limits performance. Wax or resin seals are used for temporary connections.
Vacuum greases: Joints and stopcocks must be lubricated with a grease of very low vapor pressure. Commercial vacuum grease (silicone or hydrocarbon based) is preferred. A reasonable substitute is carefully rendered beeswax or a mixture of high-melting-point wax with a small amount of oil, worked into the joint. Apply in the thinnest effective layer; excess grease can creep into the vacuum volume and outgas.
Testing for leaks: A Tesla coil (a high-voltage, high-frequency spark device) is the traditional leak tester. When the spark probe is brought near a glass vessel under vacuum, visible sparks discharge through the glass. Near a leak, the spark pattern changes — a persistent bright spot or color change indicates where air is entering. In the absence of a Tesla coil, monitor the pressure over time: a good system should hold vacuum for hours after being isolated from the pump.
Diffusion Pumps: The Second Stage
For demanding applications (high-performance triodes, or tube types requiring better initial vacuum before getter firing), a diffusion pump in series with the mechanical pump achieves 0.0001 mbar and below.
A diffusion pump operates by boiling a low-vapor-pressure oil and directing the vapor through a nozzle at high velocity. Gas molecules from the tube are entrained in the oil vapor jet, carried downward, and expelled into the mechanical pump. The oil condenses on water-cooled walls and returns to the boiler. No moving parts — just a heater, a nozzle, and cooling water.
Building a diffusion pump requires a glass or metal body (a simple glass version can be made from laboratory glassware), a resistive heater element, and the correct pump oil. The pump must be backed by a mechanical pump — it cannot operate against atmospheric pressure. This two-stage combination (diffusion pump backed by rotary pump) achieves vacuum levels that satisfy even demanding tube types.
Vacuum Measurement
Without knowing the pressure in the tube, evacuation is guesswork. The McLeod gauge is the traditional absolute vacuum measurement tool: it traps a known volume of gas, compresses it by a known ratio, and measures the resulting pressure by mercury column height. From Boyle’s law, the original pressure is calculated. A McLeod gauge requires only glass tubing and mercury — no electricity, no calibration against known standards, just geometry and careful measurement.
Below 0.001 mbar, the McLeod gauge becomes difficult to read accurately and ionization gauges (which require electrical components) take over. For tube manufacture, McLeod gauges are adequate for measuring the final vacuum before sealing.
The investment in building a vacuum pumping system — a pump, vacuum line, gauge, and bakeout capability — returns value across all tube types. A community that builds this infrastructure once can subsequently manufacture tubes of any type, making electronic communications a sustainable rather than scavenged technology.