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An air source can be made from 1 or more vacuum motors mounted in a box. Air sources need to be able to supply air flow in both directions for most flow benches. The following will show you some air source ideas. No dimensions are given, this is because typically, the larger the box, the more efficient the air source will be.

This diagram shows a 2 X 6 vacuum motor air source. A typical air source has a divider panel that separates the vacuum motor input from the output, and provides a surface to mount the vacuum motors.
A vacuum motor can be mounted to a panel using 3 course thread screws and some bent fender washers. Not too tight, just snug so that you do not distort the housing.
Check Valves
When multiple vacuum motors are mounted in a box, it is always very convenient to be able to turn on only the motors required to achieve the desired test pressure. But motors that are not turned on will be air leaks, and they will spin backwards as air is pulled through them by the other motors. A vacuum motor that is spinning backwards, besides being an air leak for the air source, may be damaged when turned on. For these reasons, it is a good idea to install check valves on the vacuum motors. Check valves will seal-off vacuum motors that are not turned on.

Irrigation check vales can be used for air source vacuum motors. You should specify 2" swing gate check vales, not spring loaded valves. These valves will not restrict air flow volume much, but will drop a little test pressure.
Stock 2" swing gate check valves will drop about 4.2"wc of test pressure at full unrestricted flow through a vacuum motor
By cutting off the inlet end of a 2" swing gate check valve and putting a radius on the inlet, and removing some restrictive areas on the outlet end, the valve will now drop only 2.5"wc of vacuum pressure through an unrestricted vacuum motor at full flow.
You can easily make some check valves from some 3" ID plastic pipe, and some sheet rubber and plastic. A 10" chop-saw can cut 3" pipe at a 45 degree angle. The 45 degree angle gives a larger opening in the end plate and keeps the valve door closed using gravity. 3" ABS or PVC pipe works well. The end plate can be made from 1/4" styrene or foam PVC (Tap Plastics). The flapper door is easily made with 1/16" styrene attached to 1/16" sheet rubber using mounting tape. Mounting tape is a foam strip with adhesive on both sides (available most anywhere). You can expect less than 1"wc pressure drop from this design.
Bi-Directional Air Source Design
Most flow bench designs require air to flow in both directions. The following shows how this can be accomplished using a sliding valve plate to control air flow to a plenum. The flow bench air can be taken from the plenum, and the test pressure can also be controlled at the plenum through a bleed valve.
This directional valve design takes advantage of a sliding valve plate by using square holes for the ports. Square holes allow maximum port area for more efficiency.
Air Flow Directional Valving
An air flow directional valve can be made by sandwiching a sliding valve plate between two plates. 3/4" melamine is a good material to use for your valve plates.
Fig A shows the top of a typical air source box. As in Fig B, mark off the green, red and blue lines as shown. These are imaginary lines, and should be the same thickness as the box material (3/4"). Dimension B is half of A. Dimension D is half of C. Square X is the size of all openings you will use for all 3 plates. The intake side of the box most not be bigger than the exhaust side for this design.
Make 2 identical plates, the lower valve plate and upper valve plate.
You need 2 plates, an upper directional valve plate, and a lower directional valve plate. The lower plate will be attached to the air box, while the upper plate will be attached to the top piece and plenum. The sliding valve plate will go between the upper and lower plates. Fig C shows where to mark your openings for these 2 plates. Fig D shows what the plates will look like when done.
Make a sliding valve plate.
You need to make a sliding valve plate to go between the upper and lower directional valve plates. You may want to make this plate a bit longer than the upper and lower plate, so that you can attach a push/pull handle. Fig E shows where to cut the openings in the sliding plate. Fig F shows how the plate will look when finished.
Top view of upper plate with plenum.
Fig G shows a top view of the upper plate with ports and plenum. The red squares are exhaust ports, while blue squares are intake. The ports that are not inside the plenum are referred to as vents. When the vacuum motors are creating a negative pressure to the plenum (vacuum) the red vent will be exhausting the air from the motors. When the vacuum motors are applying a positive pressure to the plenum, the blue vent will be supplying the vacuum motors with air. The sliding valve plate will open and close the ports and vents to allow the changes in air direction.
Directional valve assembly in action
Fig H shows the sliding plate fully inserted, providing the plenum with intake pressure. Fig I shows the sliding plate pulled out providing the plenum with exhaust pressure. The gray boxes are not in use.
A top piece can be mounted to the air source to locate the directional valve assembly and plenum. Using springs, the top piece can float on top of the air source box to provide tension to the valve assembly but not too much tension to restrict movement of the sliding valve plate. The area of the top piece next to the plenum should be covered with aluminum window screen to keep debris from falling into the intake port or to restrict sparks or debris from being blown out the exhaust port.
Design Notes
When building a valve assembly, the upper and lower valve plates should not have screws inserted into the sides of the plates as shown in Fig B. This might make the material expand so that they will no longer be flat. Screws should only be used through the plate material and counter sunk so that the heads are below the surface as shown in Fig A.
Typical thu-motor vacuum motors
Thru-motor vacuum motors vent the air that they draw in through the motor for cooling. This heats the exhaust air considerably. The Ametek motor is available from Grainger for less than $100.00 USD. Both motor perform about the same at 28"wc of pressure, about 100 cfm. The Ametek will produce more pressure.
High efficiency dual plenum bi-directional air source design
This air source design has many unique features. The dual plenums give you more flexibility with your installation and final design. It uses a square shaft running through a tube to turn the circular valve plates of each plenum. A piece of rounded corner square steel tubing (red) will often fit nicely inside 1" schedual 40 PVC pipe (blue). The PVC pipe acts as a seal inside the main chamber, and short sections of the PVC act as bushing's at the ends of the plenums. Springs seal the plenums by pushing the valve disks (green) against the valve plates (brown) and the sealing disks against the plenum walls. The 2 valve disks are 180 degrees opposed to each other venting through large semi-circular vents (grey).  An unused plenum can have some walls removed for more efficiency.
Simple Pivoting Pipe Air Source
This design may be for you if you have limited woodworking skills. It can be used with a very simple vacuum motor box design. The pipe connecting your flow bench to your air supply is pivoted from intake to exhaust port by simply pushing or pulling the assembly from one port to the other.. The pivot is provided by a male threaded coupling threaded into a female threaded coupling. This makes a very sturdy, leak resistant pivot point for your pipe.

Using hose clamps allows fine adjustments in height and length of the assembly for precise alignment.

This design does not work well with manual bleed test pressure adjustment unless the valve has a remote control.
This top view of the air supply shows the pipe connected to the air supply intake port while the exhaust port is freely venting. By using a port plate on top of the air source, the ports can be lined-up with the toilet flange-pipe, or replaced for design changes or modifications to air supply box. Cleats help secure the toilet flange when in place.
This top view shows how the vacuum motors are positioned within the air supply box in relation to the ports.
This side view of a locating cleat shows the use of rubber tubing to locate the fender washer. The rubber tubing helps with adjusting the height and locking force of the fender washer by tightening or loosening the screw. A bevel applied to the edge of the flange on the toilet flange helps the flange slip under the fender washer. The intake port does not need to be tight because the vacuum pressure tends to pull the pipe against the port plate.
Air leaks in your air source and the plumbing leading to the flow rate measuring device will not affect flow rate readings. Only between your test piece and the measuring device will air leaks affect the flow rate readings. Air leaks from your air source or connecting plumbing will lower maximum test pressure and flow rate of your flow bench.