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Measuring Vacuum - V1 - Syringe Gauge

Legacy

A simple gauge to monitor mild vacuum (~81 kPa absolute, −20 kPa gauge) directly inside a vacuum bag. It uses Boyle’s law: trapped air expands as pressure drops, moving a rubber plunger seal you can read through the bag.

Goal

Monitor vacuum level inside a bag using plunger movement.

Time needed

Type Hours
Implementation 0.5 h
Waiting 0 h

Bill of Materials

Material Quantity Unit Cost Line Cost
10 mL syringe
Consumable Thin barrel syringe for plunger travel		 1 Inexpensive
Luer-lock tip cap
Consumable Single cap to seal the syringe tip		 1 Inexpensive
Candle wax shavings
Consumable Small amount to lubricate the rubber seal		 0.05 block Inexpensive

Tools Required

Tool Purpose
Knife or scissors Trim tubing and syringe components to length
Permanent marker Mark graduations and reference points on the gauge

Instructions (step-by-step)

  1. Modify plunger

    • Remove plunger from syringe.
    • Cut off the plastic rod, leaving only the rubber seal.

  2. Prepare barrel

    • Clean inside of syringe.
    • Lightly rub candle wax on the rubber seal for low friction.

  3. Seal the syringe

    • Insert rubber seal back into barrel at the start volume:
      • 5 mL mark for a 10 mL syringe
      • 10 mL mark for a 20 mL syringe
    • Cap the tip airtight with a Luer-lock or push-on tip cap.

  4. Mark the scale

    • Draw an ambient mark at the initial plunger position.
    • Mark your target 80% atmospheric (~81 kPa abs):
      • 10 mL syringe → +1.25 mL (plunger moves ~7.6 mm on standard barrel, ~20 mm on thin insulin barrel)
      • 20 mL syringe → +2.5 mL (plunger moves ~8.9 mm)

Data to Record / Results

Record plunger position at ambient and at target pressure.

Calibration Reference

Target positions relative to start volume:

Absolute Pressure % of Atmosphere 10 mL Syringe (start 5 mL) 20 mL Syringe (start 10 mL)
101 kPa 100% (ambient) 5.0 mL 10.0 mL
95 kPa 94% 5.33 mL (Δ0.33) 10.67 mL (Δ0.67)
90 kPa 89% 5.63 mL (Δ0.63) 11.26 mL (Δ1.26)
85 kPa 84% 5.96 mL (Δ0.96) 11.92 mL (Δ1.92)
81 kPa 80% target 6.25 mL (Δ1.25) 12.5 mL (Δ2.5)
75 kPa 74% 6.75 mL (Δ1.75) 13.5 mL (Δ3.5)

Δ = plunger movement from ambient

Limitations

  • The friction between the plunger and the barrel limits the precision greatly