The "Piston Effect" in a Still Air Box: Separating Fact from Fiction

The "Piston Effect" in a Still Air Box: Separating Fact from Fiction

For growing mushrooms, you have an option to use either a traditional still air box, which is a plastic tote with armholes cut out, or a still air box with a self-sealing arm port or attached gloves. However, members of the mushroom cultivation community are worried that a sealed still air box, such as the Bella Bora Still Air Box, will produce a "piston effect" that will cause air currents and stir up contaminants into the air.


What is the "Piston Effect"?

The "piston effect" refers to the phenomenon that occurs in internal combustion engines during the intake stroke, where the upward movement of the piston creates a low-pressure area in the cylinder. This low-pressure area acts as a suction, pulling air into the engine from the air intake system. A similar example of the "piston effect" occurs in syringe needles, when you are drawing up liquid culture and inoculating your grain bags, where the change in volume creates negative pressure to draw up liquid culture, and positive pressure to expunge liquid culture out of the syringe.


The Myth

The term "piston effect" when applied to sealed still air boxes, refers to the concern that when a user is operating within a sealed still air box, their arm movements will cause the box cover and/or self-sealing arm ports to move back and forth, and draw in air from the outside environment.

Why the Piston Effect Does Not Apply to Sealed Still Air Boxes

The truth is that the piston effect, as described, is based on a misunderstanding of the physics of air intake in a sealed still air box, and this "piston effect" does not actually occur within the Bella Bora Still Air Box for the following reasons. 


Reason 1: No change in total volume does not generate negative pressure

The "piston effect" assumes that there is significant change in chamber volume, which rapidly generates negative pressure that will draw in air from the positive-pressured external environment. However, the Bella Bora Still Air Box does not demonstrate this dramatic change in volume and inside air pressure. Instead, the Bella Bora Still Air Box utilizes flexible walls to stabilize the pressure when you are moving your hands inside the box. This is different from the rigid design of an engine piston. When you are operating the Bella Bora Still Air Box, all the walls of the box move in unison, following the ebb and flow of your hands. This behavior is more similar to a closed system. Think water balloon. You apply force to one side, the opposite side gives to accommodate for the force without changing total volume of the closed system.


Reason 2: Force transmission between air molecules does not generate current

Further, in a closed system, when force is applied to one side, the space inside the closed system acts as a fluid, where there are actually no air currents generated. Instead, the air particles push against each other to transmit the force from one end of the closed system to the opposite end. The force is transmitted between air molecules, and not by moving air particles. A prime example of this phenomena is the transmission of whale songs underwater over 10 thousand miles. In this case, the water molecules do not physically travel over those 10 thousand miles, but rather they vibrate and transmit the sound from one molecule to the other. Therefore, no actual current is generate. 


Reason 3: Sealed arm holes dampen force transmission

Further, the form-fitting arm holes creates sufficient seal and resistance to air flow from the outside environment. Any abrupt movement of your arms while in the arm ports will be dampened by the self-sealing arm holes, and will transmit that force throughout the still air box via the plastic covering, to the PVC frame, and into the table. This force is not transferred to the air inside the box because the transmission coefficient of air is relatively low and insignificant in comparison to solid physical connections. Therefore, the force being transferred to the air itself is negligible.


Experimental Evidence

We have tested the effectiveness of the Bella Bora Still Air Box, and compared it to the traditional plastic tote SAB with arm holes cut out. We found that the Bella Bora Still Air Box significantly reduces contamination rates to near 0%. In contrast, traditional SABs have a 5-10% contamination rate even with exceptional sterile technique performed by an experienced mushroom cultivator. Many users have also confirmed these percentages with us. 



The term "piston effect" is often used as a counter argument to as why the sealed Bella Bora Still Air Box would lead to contamination. Unfortunately, despite being a commonly cited concept, the piston effect is largely a myth and fails to accurately describe the dynamics of physics.

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