Queendom Tools

Molecular Flow Aperture Conductance

Enter values below to calculate the conductance (C) for a thin orifice in the molecular flow regime using C = A · √((R·T) / (2πM)).

— L/s
— m³/s
C = A · √((R·T) / (2πM))
Results are preliminary estimates only, not a substitute for review by a qualified/licensed engineer.

Introduction

For Ultra-High Vacuum (UHV) engineers, accurately determining the conductance of system components is critical for designing efficient pumping lines. This calculator provides a first-pass estimation of the molecular flow conductance for a thin orifice, allowing engineers to quickly assess whether the orifice or the downstream plumbing will dominate the overall effective pumping speed. It uses the fundamental formula: C = A * sqrt((R*T) / (2*pi*M)) which applies for a thin orifice in the molecular flow regime (Kn > 1).

The tool calculates the conductance live as you input the aperture area, gas molar mass, and temperature, displaying results in both L/s and m³/s. It also includes an optional check for wall thickness against the equivalent circular diameter to warn when the 'thin orifice' assumption might be stretched.

How to Use This Calculator

  1. Enter the aperture area in the designated field and select its unit (mm², cm², or m²).
  2. Choose a gas from the molar mass presets dropdown, or manually type a custom molar mass in g/mol.
  3. Set the temperature. The default is 293 K, but you can switch to °C if preferred.
  4. Optionally, enter the wall thickness in mm. This helps evaluate if the orifice is truly "thin" according to the formula's assumptions.
  5. The conductance in L/s and m³/s will update automatically as you type, along with the substituted formula.

FAQ

Q: What conductance does a 1 cm² orifice have for nitrogen at 293 K?

A: With A = 1×10⁻⁴ m², M = 0.028014 kg/mol and T = 293 K, C = A·√(RT/2πM) works out to about 0.0118 m³/s, i.e. roughly 11.8 L/s.

Q: How much more conductance does hydrogen give than nitrogen through the same aperture?

A: C scales as 1/√M, so H₂ at 2.016 g/mol gives √(28.014 / 2.016) ≈ 3.7× the nitrogen value at the same temperature.

Q: What does the "wall thickness > 0.1 × diameter" warning mean?

A: The formula assumes a zero-length thin orifice; the tool compares the wall thickness you enter against 0.1 × the equivalent circular diameter d = 2√(A/π) derived from your area and flags the result when that limit is exceeded.

Q: Can this conductance be used at any pressure?

A: Only in the molecular flow regime where Kn > 1; the tool takes no pressure input, so the regime has to be confirmed separately before the number is applied.