Measuring Vacuum - V2 - Trapped Balloon

Proven

A simple gauge to detect when the vacuum bag reaches about 80% of atmospheric pressure (roughly a 20% vacuum).

It relies on Boyle’s law, which states:

\[ P_1 V_1 = P_2 V_2 \]

As the external pressure drops slightly, the trapped air expands inside a balloon placed in a rigid plastic container.
When the balloon expands to a known volume, it gives a simple visual cue for partial vacuum.

Example Calculation:

Parameter	Symbol	Value	Notes
Ambient pressure	\(P_1\)	1.0 atm	Normal atmospheric pressure
Balloon volume at rest	\(V_1\)	27 mL	Container (35 mL) minus occupied volume (8 mL)
Target pressure (80 % of atmospheric)	\(P_2\)	0.8 atm	Corresponds to 20 % vacuum
Expanded volume	\(V_2 = \dfrac{P_1 V_1}{P_2}\)	33.75 mL	Theoretical balloon volume at 0.8 atm
Container volume	—	35 mL	Balloon almost fills container
Vacuum indication	—	~20 % vacuum	Balloon just touching container walls

When the balloon almost fills the container (but does not press firmly against it),
the pressure has dropped to roughly 80 % of atmospheric pressure, or ≈0.8 atm absolute.
That corresponds to a modest vacuum, enough to confirm that the vacuum bag is sealed and pulling correctly.

Goal

Detect when we've hit 80% of atmospheric pressure.

Reference Images

Container with holes	Measure Displaced Volume

Trapped Balloon at atmospheric pressure	Trapped Balloon at 80% atmospheric pressure

Time needed

Type	Hours
Implementation	0.5 h
Waiting	0 h

Bill of Materials

Material	Quantity	Unit Cost	Line Cost
Small Plastic Container (35ml) Consumable A very small, transparent, plastic container	1	Inexpensive
Balloon Consumable A simple balloon, small preferred	1	Inexpensive

Tools Required

Tool	Purpose
Syringe (10ml)	Measure the container volume
Power Drill	Drill holes into the plastic container

Instructions (step-by-step)

1. Prepare the container
   Choose a small, clear plastic container with a tight-fitting lid.
   Drill four small holes near the corners of the top lip to allow air to pass.
   Drill a larger central hole to fit the syringe head snugly.
   This will be used for measuring water volume and later for the air path.

2. Measure the container volume
   Use the syringe to fill the container with water, keeping track of how many milliliters it takes until full.
   Example: the container holds about 35 mL of water.

3. Inflate the balloon
   Partially inflate a small balloon until it fills roughly 80% of the container volume.
   You want it to just start touching the sides when placed inside.
   Seal the balloon tightly with a knot or clip.

4. Measure the balloon volume
   Place the sealed balloon inside the container.
   Close the lid and use the syringe to inject water through the central hole until the container is full again.
   The difference in water volume tells you how much air is inside the balloon.
   Example: the container with balloon accepts 8 mL of water, meaning the balloon’s volume is
   $$ V_{\text{balloon}} = 35 - 8 = 27\,\text{mL} $$

5. Using the gauge
   Place the container face down onto the breather cloth inside your vacuum bag.
   The small holes let air escape freely while allowing the balloon to expand as the pressure drops.
   When the balloon nearly fills the container, the bag has reached about 80% of atmospheric pressure (0.8 atm).

---

Limitations

• Inflating the balloon to exactly 80% of the container volume is difficult, so accuracy varies.
• The balloon adds its own resistance to expansion, not accounted for by Boyle’s law.
• The gauge provides only an approximate visual indication of pressure.