I'm sure other will provide more correct definitions.
Pressure control can be opening a valve and filling a vessel or column until it reaches a set pressure. The set pressure can be controlled by a pressure regulator and/or flow controller, and the device setting is independent of the downstream vessel or column, so bigger vessels just need more.
That's the normal method used to fill a car tyre, or provide gas pressures to an analytical instrument. To achieve improved control, more stages can be added, so you effectively have two or more regulators in series.
Technically, these are forward pressure regulators, because the regulator will keep allowing fluid/gas to flow until the set pressure is reached. Because they are the most common form, they are known just as "regulators".
If you want to ensure the pressure in a vessel stays constant for various flows, you add pressure-control after the vessel, and the controller opens and vents once the set pressure is reached. The most common form of this is a dreschel bottle with a head of water and the outlet tube going into the water. When the pressure exceeds the back pressure exerted by the water, bubbles start passing through the water. This is known as a back pressure regulator.
In LC and GC, the column also acts as a restrictor,which can be considered as a very large number of regulators in series, and creates a "back pressure" according to the fluid/gas viscosity and flow through the gaps.
With regard to the peak distribution. consider a large school of children running through a large, full car park to the other side for the first time. Some will be lucky and have shorter distances to go around cars, most will have similar distances to travel, and some will have longer distances to go around the cars.
They may all see the same number of cars, but just have different distances to negotiate because their pathways aren't narrow, straight lanes, but voids between cars partially filled with other schoolchildren also heading in the same direction. They don't all arrive simultaneously at the other side.
Bruce Hamilton
