When choosing among several vacuum pumps, an important factor may be how long it takes each pump to reach the needed vacuum.

In general, a small capacity pump and a large capacity pump with equal maximum vacuum capabilities will both produce the same vacuum. The smaller pump simply takes longer. How much longer depends on the capacity of the pump and the size of the system. But simply dividing system volume by open pump capacity won't produce the proper answer.

During pump-down, the higher a vacuum becomes, the fewer air molecules remain in the closed volume. Therefore, fewer molecules can be removed by each pump stroke. As a result, there is a logarithmic relationship when approaching a perfect vacuum. The time required to pump a system down to a certain vacuum level can be approximated using this formula:

t = (V×n) ÷ 4q,

where:
t is time, min
V is system volume, ft3
q is flow capacity, cfm, and
n is a constant for the application.

For exact applications, n can be determined by using a natural logarithm. For most purposes, the following will suffice:
n = 1 for vacuum to 15 in.-Hg
n = 2 for vacuum >15 but ≤ 22.5 in.-Hg., and
n = 3 for vacuum ≥ 22.5 and up to 26 in.-Hg.

One further complication: pump capacity in the equation is not the open capacity (capacity at atmospheric pressure) usually cataloged by manufacturers. Instead, it represents the average capacity of the pump as system pressure drops to the final vacuum level. This value is not readily available but can be approximated from manufacturers' pump performance curves. These curves plot pump capacity at various vacuum levels.

To mesh these curves with the equation, simply substitute values in the equation using pump capacity readings from the curve at various vacuum levels at 5-in.-Hg increments, up to the desired level. Then total these times.

Finally, note that this pump-down time is based on all system components operating at optimum levels. A 25% additional time allowance is recommended to compensate for system inefficiencies and leakage.

### Vacuum at high altitudes

Atmospheric pressure determines the maximum vacuum force that can be achieved. And standard atmospheric pressure at sea level is 29.92 in.-Hg. But what happens at locations a mile above sea level? The maximum vacuum that can be achieved in locations above sea level will be less than 29.92-in.-Hg. The force will be limited by the ambient atmospheric pressure. Vacuum pumps have maximum vacuum ratings based on sea level conditions and must be re-rated for operation at higher elevations.

First, determine the local atmospheric pressure. A rule of thumb is that for every 1000 ft. of altitude above sea level, atmospheric pressure drops by 1 in.-Hg. Using rounded-off figures, for a city at an elevation of 5,000 ft, the atmospheric pressure is about 25 in.-Hg.

To adjust a pump rating, think of that rating as a percentage of atmospheric pressure at sea level. If a pump is rated for 25 in.-Hg, it can achieve 83.4% (25 29.92) of a sea level perfect vacuum. At a 5000-ft elevation, that same pump can achieve 83.4% of 25 in.-Hg - or a vacuum of 20.85 in.-Hg.

 Percent vacuum Inches of mercury (in.-Hg) Pressure Pressure vs. vacuum 10 3.0 -1.47 psi -0.10 bar 15 4.5 -2.21 psi -0.15 bar 20 6.0 -2.94 psi -0.20 bar 25 7.5 -3.68 psi -0.25 bar 30 9.0 -4.41 psi -0.30 bar 35 10.5 -5.15 psi -0.35 bar 40 12.0 -5.88 psi -0.40 bar 45 13.5 -6.62 psi -0.45 bar 50 15.0 -7.35 psi -0.50 bar 55 16.5 -8.09 psi -0.55 bar 60 18.0 -8.82 psi -0.60 bar 65 19.5 -9.56 psi -0.65 bar 70 21.0 -10.29 psi -0.70 bar 75 22.5 -11.03 psi -0.75 bar 80 24.0 -11.76 psi -0.80 bar 85 25.5 -12.50 psi -0.85 bar 90 27.0 -13.23 psi -0.90 bar 95 28.5 -13.97 psi -0.95 bar 100 30.0 -14.70 psi -1.01 bar