The flow hood is a sterile air blower. The particles of the air are separated by a pre-filter and a HEPA filter. This results in almost pure air, separated at 99.97% from particles greater than 0.3 µm. This clean air exerts a positive pressure on the work plan, allowing to easily produce axenic cultures.

The first prototype met the expectations, but several details deserved to be improved. The plans, drawings and the budget of the second version are available below.

Structure & aesthetics:

• The fan replaces the legs and reduces the total height.
• The pre-filters are placed facing the air entry of the fan.
• The pre-filters are more accessible for cleaning.
• The mounting depth of the pre-filters is doubled.
• The cover of the pre-filters and the lamp cover are removed.
• The depth of the air chamber is reduced.
• The internal shelves are made of glass.
• The structure requires less wood (lighter, more compact, less expensive).

Electronic Components:

• Lighting with a prefabricated LED strip. More uniform, requires less wiring.
• Transformer 12V 3A, instead of 12V 1A.
• Grouped switches with a more intuitive utilization.
• The magetic stirrers use two magnets rather than one (magnetic fields stronger).
• The stirrers engines are replaced by 200rpm 12V 0.5A DC motors. They are stronger (can shake 2.8 L), and no longer need dimmer.

Plywood

Choose select quality plywood. On one side, the holes left by the nodes are filled and wood is sanded. For the interior of the hood body, especially in the work area, cleaning will be easier. This prevents the accumulation of particles in the cracks.

Primer & Paint

Before cutting, paint both sides of the plywood. First clean the dust with a damp cloth, and when the wood is dry, apply a primer. A primer before the first coat of paint provides better grip and faster drying, preventing it from entering the wood.

Then give two to three coats of white epoxy paint. Although rather toxic, it is durable and easy to clean. Use gloves to protect your skin and apply the paint in a well ventilated area. Use a grease to clean your skin if necessary. Between layers, wrap the paint brush and the roller into plastic wrap and store them in a cool place.

Centrifugal fan (“squirrel cage”)

The chosen model is the most powerful of Fasco in this category. If you choose another fan, make sure that its strength allows enough airflow despite the strong resistance exerted by the static pressure of the HEPA filter. You will probably have to do weld a custom bracket¹⁾.

Assembly

• When possible, plan a 1/8“ space between each parts to compensate for the width of the saw cut.
• (optional) Pre-assemble all pieces with a air nailer.
• To avoid cracking of the plywood, pre-drill the screw holes with a bit of a smaller diameter.
• Seal well all the internal parts of the hood by drawing a silicone seal. The junction between the two parts (filter and engine), and the periphery of the prefilters are sealed with packing tape, much easier to remove than silicone. Each pre-filter is individually sealed to the support.

Tools needed

• Painting set (tarp, tray, roller, paint brush)
• Circular saw
• Jigsaw
• Drill & bits (3/32 ”, 5/8“, 1 ½ ”)
• Caulking gun
• Glue gun
• Soldering iron
• Measuring tape, screwdriver, locking pliers (4), square, pencil
• (optional) Pliers, wire stripper, hot air gun, crimping tool, air nailer

Commercial magnetic stirrers are large, expensive, and often stronger than necessary. To stir one to two liters, you can build one simply by sticking two hard drive magnets²⁾ on a coupler mounted on a DC motor. The result, very compact, is easy to integrate directly below the work area.

• Align SF to the center of DF and tighten with clamps. Screw the two pieces together and remove the clamps. As shown, align the two SFa on DF, place the clamps, screw them, then remove the clamps. SF, DF and SFa 1-2 are now linked in a single piece.
• Drill 1 1/2“ holes through the thickness of SF and 5/8” holes through the thickness of DF+SFa+SFa.
• Add the legs SFb and SFc, then prepare the stirrers;

• Pick up the magnets from an old hard drive. You will probably need a special screwdriver (torx). Each magnet is mounted on a metal plate. To recover it, bend the plate in the opposite direction to the magnet.
• Join two magnets together, then preheat the magnets and the coupler (with a hot air gun or a bain-marie…). Meanwhile, preheat a hot glue gun.
• Put some glue on the coupler and attach it to the magnets, well centered. Leave to dry.
• Place the assembly on a motor, then secure the coupler by tightening the screw against the shaft.

• Place the motors into the previously drilled holes.
• Solder the wires to the poles of the motor, as shown in the wiring diagram.
• In order to keep a space of 1/16“ between the magnets and the surface of SF, place a cardboard piece of 1/16” thick on a straight board, and push it against the magnets.
• Place a piece of electrical tape onto the wire end of the motors and secure it with hot glue.
• Solder the wires to the 6-pin female plug and insulate the connections.

Cleaning

Before each use, the internal surfaces of the work area should be cleaned with 70% isopropyl alcohol. The “floor” must be particularly clean. Pre-filters are cleaned with a vacuum cleaner, if needed.

Lubrication

The motor should be oiled once a year with a suitable lubricant. With a screwdriver, remove the cap in the center of the fan and apply a few drops of sewing machine oil.

Each plan represent a sheet of plywood 4 'x 8' x 3/4 “.

The hood is assembled in two modules. The gray cuts are for the engine part, while those in green are for the filter part.

The LED strip and the motors are connected in parallel, as shown. A switch can turn off the AC/DC transformer, and five secondary switches provide independant use for the lighting and each magnetic stirrers. A connector is used to facilitate the disassembly of the hood into two parts, in the event that it should be moved. Since only one thread links all the negative poles, it must bear the intensity of all modules simultaneously (2.6 A). To avoid overheating, the wire must have a minimum quality of 16 AWG (3.7 A). It was hard to find a generic connector with both the right amount of pins and a sufficient wire gauge, so in this case 16 AWG wires were manually soldered to a connector³⁾. The end of each wire has been insulated with an heat-shrink tubing to avoid short-circuit.

The three-position switch allow fan speed selection. It may be replaced by a two-position switch, set to the highest speed.

Most parts are easily found in hardware stores. The electronics come from Ebay Canada, the HEPA filter from Fungi Perfecti and the 6-pin connector from Sparkfun.

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