**Concepts**
**Remember - there is no such force as suction**
It is a common mistake to think a pump can suck gas from a chamber. The reality is that to create a vacuum, molecules of gas are removed by pumping air from a vacuum chamber.
The remaining molecules diffuse, in their random nature such that they fill the chamber equally. As air has been removed there are fewer molecules present to push on the chamber walls so the pressure is reduced.
**Pumping Speed**
Pumping speed is the primary information given about any vacuum pump and is always given as volume per unit time.
Pumping speed is a volumetric flowrate and is pressure dependent (for most pumps) i.e. the amount of gas pumped is dependent on the pressure of.
It is usually measured in m3 hr-1 or lpm (liters per minute) for low vacuum and l.s-1 (liters per second) for high vacuum.
**Pumping Speed Calculation**
P_{1}V_{1}T_{1}^{-1} = P_{2}V_{2}T_{2}^{-1}
P_{1} = Gas inlet pressure (usually atmospheric pressure)
V_{1} = Gas inlet flowrate (volume per unit time m^{3}hr^{-1})
T_{1} = Gas temperature (ambient Temperature)
P_{2} = pump inlet pressure
V_{2} = pump Speed (volume per unit time m^{3}hr^{-1})
T_{2} = pump temperature
Pump speed is calculated at the pump inlet hence rearranging for V_{2}
V_{2} (pump speed S, m^{3}hr^{-1}) = P^{1}V^{1}T_{2}P_{2}^{-1}T1^{-1}
__Note:__* V1 is a volume flow rate not a volume*
**Throughput**
Pumping speed alone does not relate to an amount of gas, but if the pressure of the gas is multiplied by the pump speed we arrive at a figure that is throughput.
Throughput (Q) = Pressure (P) x Volume (V) / time (t)
Where Volume/time = pump speed (S)
i.e., Q = PVt^{-1} or Q = PS
Throughput is an important characteristic of a pump. It delivers the amount of gas (in pressure/volume units) going through a pump per unit time. Throughput is a mass flowrate in the sense we cannot have a throughput without a mass flow (although throughput alone does not tell us what the mass flow is).
For example, the quantity of gas in a 1-liter container at 500 mbar is less than a quantity in the same container at 1000 mbar. There is in fact twice as much gas in the 1000 mbar container although they are both 1 liter in volume. Throughput allows us to include pressure as well as volume in an equation.
**Mass Flowrate**
A standard volume is the volume that a gas would occupy at standard temperature and pressure, which are usually 1013 mbar and 0 degrees C.
Mass flowrate can be derived from throughput by converting into a standard volume and can be expressed as volume per unit time at standard conditions using the Combined Gas Laws.
**Ultimate Vacuum**
Ultimate vacuum is the lowest pressure that a vacuum pump can produce.
Typically
Single stage rotary vane vacuum pump - 5x10^{-2} mbar
Two stage rotary vane vacuum pump - 10^{-3} mbar
Roots with two stage rotary vane vacuum pump - 10^{-4} mbar
## Summary
**Pump speed** - the actual volume of gas a pump can handle per unit time at a given pressure.
**Throughput** - this is the pump speed multiplied by the pressure of the gas pumped giving you the actual amount of gas pumped.
**Mass flow rate** – this is the throughput converted into standard into the volume it would occupy at standard pressure and temperature.
**Ultimate vacuum** – lowest achievable pump pressure.
## Tips
You cannot have negative pressure
There is no such force as suction
Lower pressure means higher vacuum and vice versa.
For real engineering calculations we must use absolute pressure not gauge pressure.
Gauge pressure assumes atmospheric pressure (1013 mbar) is 0 |