Building an atmospheric chamber

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Discussion Overview

The discussion revolves around the construction of an atmospheric chamber designed to simulate different pressure conditions, specifically transitioning from sea level to an elevation of 7000 feet. Participants explore the feasibility of using plywood for the chamber, the effectiveness of a shop vac for creating the necessary vacuum, and the calculations related to pressure differentials and power requirements for the setup.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant proposes building the chamber from 3/4" plywood and using a shop vac to create the vacuum, suggesting a method to control pressure through a vent.
  • Another participant challenges the choice of plywood, citing porosity issues that could lead to gas leakage under pressure differentials.
  • A third participant calculates the force exerted on the chamber's walls due to pressure differences, emphasizing the need for proper design rather than simple reinforcement.
  • A later reply agrees with the previous calculations and discusses the power requirements for the air pump, indicating that the shop vac's rated CFM may not hold under vacuum conditions and that a more powerful motor may be necessary.

Areas of Agreement / Disagreement

Participants express differing opinions on the materials and design considerations for the chamber, with no consensus reached on the feasibility of the proposed construction method or the adequacy of the shop vac for the task.

Contextual Notes

Participants highlight limitations in the proposed design, including potential structural weaknesses of plywood under pressure and the need for more powerful equipment than initially considered. There are also unresolved calculations regarding efficiency and power requirements.

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Building an "atmospheric chamber"

I have to build a chamber (ok, large box) to do some atmospheric pressure tests. The box will be 48 inches square. I have to simulate going from sea level (0 feet, 14.696 psia) to 7000 feet (11.34 psia). I'm thinking of building the "box" from 3/4" plywood reinforced with cross-braces (if need be). I've looked at "wet/dry shop vacs" and think one of these might be able to pull the "vacuum" needed. The box is 64 cubic feet and the shop vac can pull 170CFM.

The box will have a vent on it to make it "leaky" to control the pressure inside (I'm thinking the vacuum can suck while the vent can leak letting in air). This way I can control the pressure without resorting to flapper valves or the like. It should also prevent the shop vac getting into a "clogged inlet" and racing the motor.

Does this sound reasonable? I've been looking to find the way to calculate all this but haven't found a reference yet.

Thanks.
 
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Just my take(I'm no expert):

Plywood is not a good choice. It has porosity issues. Gases can leak through, especially during pressure differential.

The shop vac rated at 170 CFM draw assumes relatively non-vacuum conditions. That is, the CFM for a store-bought shop vac will substantially reduce VERY quickly in drawing air out of a sealed container.

Just some basic thoughts...
 


Forget trying to make this from wood.

Supppose your 64 cu ft box is a 4 ft cube. The area of each face is 16 sq ft = 2300 sq in.
A pressure difference of 14.7 - 11.3 = 3.4 psi will give you a total force on each face of 2300 x 3.4 = 7800 lb which is about 3.5 tons.

If you really want to build this, you need to do some proper design work on it, not something "reinforced by cross braces if need be".
 


I agree with the calculations of AlephZero.

Plus, concerning air pump requirement, the power needed is the airflow times the pressure differential.

With the appropriate unit conversion:

Power = 170 CFM X 3.4 psi X 3.254 = 1880 W = 2.5 hp

This is without accounting for inefficiencies (motor, pump, duct restrictions, etc.). That power is related to the concept of "http://en.wikipedia.org/wiki/Airwatt" ". You will need a motor a lot more powerful than that.

You will have to consider that power when selecting your controlled «leakage».
 
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