Understanding Purge Calculations
Understand PacWizz's pressure-dilution oxygen estimate, the five-cycle manual example, and why the result is not an oxygen measurement.
Why repeat pressurizing and venting?
Air initially contains about 21% oxygen. Adding oxygen-free purge gas dilutes the oxygen in the tank's gas space. Venting removes some of that mixture; repeating the process reduces the oxygen fraction further.
PacWizz automates this dilution method. The selected purge, vent, and finish pressures, plus the number of cycles, determine the calculated result.
The displayed percentage is a calculation, not an oxygen-sensor reading. It does not measure dissolved oxygen in your beverage. Mixing, gas purity, leaks, and the actual installation can affect the real outcome; use your facility's verification process where a measured oxygen result is required.
The pressures in the calculation
| Symbol | Meaning | Manual's example |
|---|---|---|
| Ap | Atmospheric pressure | 14.7 psi, approximately 1.01325 bar |
| Pp | Purge pressure | 15 psi |
| Vp | Vent pressure | 3 psi |
| Fp | Finish pressure | 9.25 psi |
The device uses local atmospheric pressure in its calculations. The example atmosphere above is a reference, not a value that applies at every altitude.
A plain-language view of the maths
Dilution ratios use absolute pressure: add atmospheric pressure to a gauge-pressure value. For an ideal, well-mixed tank, adding oxygen-free gas reduces the oxygen fraction by the ratio of the pressure before filling to the pressure after filling.
For the first fill from atmospheric pressure:
Oxygen after first fill (%) = 21 × Ap / (Ap + Pp)
Venting an ideally mixed gas does not itself change its oxygen percentage. Each refill from vent pressure to purge pressure multiplies the current oxygen percentage by:
(Ap + Vp) / (Ap + Pp)
After the last vent, filling to the finish pressure applies:
(Ap + Vp) / (Ap + Fp)
You do not need to calculate these values by hand to operate PacWizz: review the device's Purge Summary before confirming a run.
The manual's five-cycle example
With 15 psi purge, 3 psi vent, 9.25 psi finish, and five cycles, the supplied manual shows:
| Display stage | Calculated oxygen shown in the manual |
|---|---|
| Purge 1 | 10.29% |
| Purge 2 | 6.09% |
| Purge 3 | 3.60% |
| Purge 4 | 2.13% |
| Purge 5 | 1.26% |
| Finish | 0.99% |
These are the manual's illustrative display values, not guaranteed values for every unit or installation. Applying the ideal formulas above with exactly 14.7 psi atmospheric pressure gives a final estimate of approximately 0.97%; the manual's rounded display example shows 0.99%. Both illustrate a result just below 1%, but do not treat either as a measured result or a margin of safety for your process.
The manual describes below 1% as a typical gas-space oxygen target for fermented products. Use your own product/process requirements when deciding whether a purge is sufficient.
What changing a setting does
- More cycles generally lower the calculated residual oxygen but take more time and gas.
- Higher purge pressure increases dilution per fill, but must stay within equipment limits.
- Lower vent pressure leaves less mixed gas before the next refill.
- Finish pressure affects the final dilution step and the pressure held after completion.
Review the calculated result after any change. Never change pressure settings beyond the limits in Safety & Site Requirements.
Return to Running a Tank Purge, or plan your run with Purge Times & Gas Supply.