Calculations

Core Sterilization and Sterile Barrier Formulas, Worked Step by Step

The five formulas that decide load size, dose, aeration, seal integrity, and material yield, each solved with real units and worked numbers.

Five calculations carry most of the daily decisions in sterile barrier work: cycle capacity, ethylene oxide residual decay, radiation dose, seal peel strength, and pouch yield per roll. Each has a defined input source. Cycle capacity comes from chamber drawings and case cube. EO residual comes from headspace GC data and ISO 10993-7 limits. Dose comes from bioburden and a chosen D10 value. Seal strength comes from ASTM F88 tensile data. Yield comes from roll length and pouch pitch. Get the inputs right and the arithmetic is straightforward. The Sterilization Cycle Capacity and Gamma Dose Cost tools handle the heavier setups, but you should be able to check every number by hand.

Start with load capacity. A 20 m3 EO chamber rarely loads to 100 percent; usable volume after gas circulation clearance runs about 65 to 70 percent, so call it 13.5 m3. If a pallet footprint is 1.2 m by 1.0 m by 1.5 m tall, that is 1.8 m3, and you fit 7 pallets. Each pallet holds 40 shipper cases, each case 45 devices, giving 40 times 45 times 7, or 12,600 devices per cycle. Divide annual demand of 3,000,000 units by 12,600 and you need roughly 238 cycles per year. The Sterilization Cycle Capacity calculator formalizes the packing density term most people forget.

EO residual aeration follows first-order decay: R(t) equals R0 times e raised to minus k times t. Suppose desorption testing gives an initial residual R0 of 38 mg per device and a rate constant k of 0.045 per hour. The ISO 10993-7 limited-exposure limit is 4 mg per device. Solve 4 equals 38 times e to the minus 0.045 t. Divide to get 0.105, take the natural log to get minus 2.25, divide by minus 0.045, and t equals 50 hours of aeration. Warehouse dwell at 20 degrees C is slower than a 43 degree heated aeration room, so k is temperature dependent. The EO Residual Hold Time tool lets you fit k from your own timepoints.

Radiation dose for sterility uses D equals D10 times the log reduction required. D10 is the dose that kills 90 percent of the population, typically 2.0 to 3.0 kGy for mixed bioburden. If average bioburden N0 is 1,000 CFU per device and the target sterility assurance level is 10 to the minus 6, the log span is log 1,000 minus log 10^-6, or 3 minus negative 6, which equals 9. With D10 of 2.5 kGy, the sterilizing dose is 22.5 kGy. Most programs default to the VDmax25 verification at 25 kGy for bioburden under 1,000, which is why 25 kGy is the common gamma set point.

Seal strength converts a peel test into a normalized value. ASTM F88 reports peak or average force across the seal width. If a 15 mm wide pouch strip peels at a peak force of 27 N, seal strength is 27 divided by 15, or 1.8 N per mm, equivalent to about 10.3 N per 15 mm reported directly. To compare against a 1.0 lbf per inch floor, note 1.0 lbf per inch equals 0.175 N per mm, so 1.8 N per mm is well above minimum. The Sterile Barrier Seal Strength tool converts between N per 15 mm, N per mm, and lbf per inch so specification limits line up.

Pouch material yield sets how much film and Tyvek you buy. Usable pouches per roll equals roll length divided by pouch pitch, times web efficiency. A 500 m roll at a 0.20 m pitch gives 2,500 pouch positions. Apply 96 percent web efficiency after splice loss and edge trim and you get 2,400 good pouches. If each pouch uses 0.20 m by 0.30 m of film, that is 0.06 m2, so a roll converts to 144 m2 of finished barrier. The Pouch Material Yield calculator layers in setup scrap so your material requisition matches the shop floor draw, not the theoretical maximum.

Two supporting numbers close the loop. Bioburden test load under ISO 11737-1 uses a sample item portion, or SIP: if you test one third of a device, the SIP is 0.33 and recovered counts are divided by 0.33 to estimate the full device. A recovery of 120 CFU on a 0.33 SIP implies about 364 CFU per device. The Bioburden Test Load tool sizes the 10-unit sample sets. For scheduling, dose rate ties time to dose: at a gamma dose rate of 8 kGy per hour, reaching 25 kGy needs 3.1 hours of exposure, before conveyor indexing and dose mapping overhead.

Published 2026-07-02.