Once a minimum suitable operating pressure has been determined, it is essential to supply the air at a constant pressure, regardless of upstream flow and pressure fluctuations. Thus, the proper regulator or pressure-reducing valve should be installed in the air line. Regulators reduce supply pressure to that required for efficient operation of downstream pneumatic equipment. A filter to protect the regulator's internal passages from damage should always be installed upstream from it.

The simplest type of regulator uses an unbalanced-poppet-style valve. This design incorporates an adjustment spring and is non-relieving. Turning an adjustment screw compresses the spring, which forces the diaphragm to move, thus pushing a poppet to uncover an orifice. As pressure rises downstream, it acts on the underside of a diaphragm, balancing against the force of the spring. The poppet throttles the orifice opening to restrict flow — and produce the desired downstream pressure. A spring under the poppet ensures that the valve closes completely when no flow exists. This is the least expensive type regulator.

Larger, more expensive regulators, Figure 5, incorporate a separate diaphragm chamber, which has an aspirator tube exposed to the output pressure. Segregating the diaphragm from the main air flow minimizes its abrasive effects and extends the life of the valve. As flow through this regulator increases, the aspirator tube creates a slightly lower pressure in the diaphragm chamber. The diaphragm deflects downward and opens the orifice without significantly reducing the output pressure. The effect is the same as increasing the adjustment setting. Thus, this style regulator has minimal droop (output pressure decay) as supply pressure varies.

Figure 6 compares how that variance occurs with a small and a large diaphragm. The larger diaphragms in these regulators improve response and sensitivity. As discharge flow through the regulator is increased over its entire range, output pressure droops. Therefore, it is important to set the regulator's desired output pressure under normal flow conditions.

Another type of regulator incorporates a balanced poppet, but otherwise has the same general construction as the separate diaphragm version. It has a significantly larger orifice to allow for greater air flow. To maintain good stability, the poppet is pressure-balanced. The effects of output pressure fluctuations cancel out, which improves sensitivity and response and reduces droop. Finally, precision regulators often employ several isolated diaphragms acting against flapper valves and nozzles in a balancing principle and are normally manufactured in limited flow capacities with smaller connection ports.

Selecting the best type of regulator for a specific application first requires a choice among these styles. Mini-regulators are commonly the direct-acting, nonrelieving type, while most standard regulators fall within the self-relieving, separate-diaphragm-chamber style. The next consideration becomes primary (unregulated supply) pressure versus desired secondary (output) pressure.

Finally, desired air flow rate must be determined. Adjusting screws are normally offered in two styles: tamper resistant, locking Tee type or push-lock, plastic knob type. The first is best when a fixed operating pressure will be set once and left alone. The adjustable knob style (quite common on modular units) is the correct choice for general use, where the operating pressure can be easily adjusted without tools.

Regulators also are defined by body size (orifice flow rating) and connection size. Although several models may appear to be acceptable for any given air flow and pressure, a larger body size regulator will produce better setting sensitivity and less droop than a smaller body model under the same set of operating conditions. An output pressure gauge is essential, although many manufacturers frequently offer it only as an option. Mounting brackets are another useful option.

Modular or combination units
Manufacturers frequently preassemble filters and regulators (and lubricators, when required) to form combination units. They are packaged together as common body sizes with common connection port sizes. Interconnections may be via threaded nipples or modular face connectors. The modular connectors allow easy removal of components for servicing or cleaning.

In addition, some manufacturers combine filters and regulators into single assemblies where the filter head becomes the regulator body. The components share common inlet and outlet connections, which makes the assembly very compact.