Common Registration Assessment, Part 1
Oxygen cylinder duration calculations
Three original questions on cylinder contents and flow time, with the working shown and the usual traps pointed out.
Published 4 October 2026. Independent revision material, not GPhC questions, not for patient care.
The volume of compressed gas in a cylinder is the gauge pressure in bar multiplied by the water capacity of the cylinder in litres. A 10 L cylinder at 137 bar holds about 1,370 L of oxygen.
Duration is simply contents divided by flow rate in litres per minute. The exam adds journey times and reserves, so keep 10 or 15 minutes spare in your answer when the stem asks for it.
The method
- Multiply the gauge pressure in bar by the cylinder's water capacity in litres to get contents in litres.
- Divide contents by the flow rate in L/min for the duration in minutes.
- Convert to hours and minutes if the question wants it.
- Subtract any stated reserve before comparing with a journey time.
- State clearly whether the supply covers the journey.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A cylinder has a water capacity of 3 L and a gauge pressure of 180 bar. Oxygen flows at 4 L/min. How long will it last?
Show the answer
Answer: 135 minutes, or 2 hours 15 minutes
Working: Contents = 180 × 3 = 540 L. 540 ÷ 4 = 135 minutes = 2 hours 15 minutes.
Question 2
A 10 L cylinder shows 137 bar. The flow is set at 6 L/min. What is the duration to the nearest minute?
Show the answer
Answer: 228 minutes, about 3 hours 48 minutes
Working: Contents = 137 × 10 = 1,370 L. 1,370 ÷ 6 = 228.3 minutes, so 228 minutes, which is 3 hours 48 minutes.
Question 3
A 2 L cylinder shows 100 bar and runs at 5 L/min. The ambulance journey is 35 minutes. Is there enough oxygen, and by how much?
Show the answer
Answer: Yes, with 5 minutes to spare
Working: Contents = 100 × 2 = 200 L. Duration = 200 ÷ 5 = 40 minutes, which covers the 35 minute journey with 5 minutes remaining.
How this is examined
Contents first, duration second. Nearly every lost mark here comes from skipping the pressure-times-capacity step and dividing the pressure by the flow directly.
Where marks are lost
- Using the pressure figure alone as the contents without multiplying by capacity.
- Leaving the answer in minutes when the question asks for hours and minutes.
- Forgetting to hold back the stated reserve when judging whether a journey is covered.
Frequently asked questions
Why multiply pressure by water capacity?
At constant temperature the gas stored is proportional to pressure times the cylinder's internal volume, so bar times litres gives the litres of gas available at atmospheric pressure.
Do I include the residual pressure?
Cylinders are treated as usable down to near-empty for these calculations unless the stem sets a minimum safe pressure or a reserve.
Sources
- Cornell Environmental Health and Safety, oxygen cylinder duration
- GPhC, Common Registration Assessment specification and permitted items for 2026
The questions above are original and use fictional drugs. PreRegExamPrep is not affiliated with or endorsed by the General Pharmaceutical Council.
Practise until the method is automatic
Try 15 free questions with worked answers. No sign-up required.
More calculation topics
- Dose by weight calculations for the GPhC assessment
- Volume to give: liquid medicine calculations
- Percentage strength calculations: w/v, w/w and ratios
- Dilution calculations with C1V1 = C2V2
- Infusion rate calculations in mL per hour
- Drip rate calculations: drops per minute
- Body surface area calculations
- Creatinine clearance (Cockcroft-Gault) calculations
- Moles and millimoles calculations
- Displacement volume calculations for reconstitution
- Quantity to supply calculations
- Using a provided formula in calculations
- Dose information from packaging and labels
- Diluting a stock to a lower strength
- Medicine cost and switch calculations
- Rounding rules in calculations
- Enteral feed rate calculations
- Electrolyte content over time
- Infusion dose per kg per hour
- Rounding doses to measurable volumes
- Relative risk and relative risk reduction
- Absolute risk reduction and number needed to treat
- Odds ratio and confidence intervals
- Relative versus absolute risk reduction
- Loading dose and volume of distribution
- Top-up doses from measured drug levels
- Salt correction factors
- Mass and molar drug concentrations
- Half-life and falling drug levels
- Steady-state concentration and infusion rate
- Bioavailability and equivalent oral doses
- Oral maintenance dose and dosing interval
- Elimination rate constant and half-life
- Accumulation towards steady state
- Ratio strengths and one in X
- Serial dilution factors
- Mixing solutions of different strengths
- Tablet quantities for reducing regimens
- Equivalent doses from provided conversion tables
- Thresholds and dose-banding algorithms
- Unit conversions for mass and volume
- Units and units per mL
- Reconstituting powders and final concentration
- Millimoles and milliequivalents
- Infusion time and volume remaining
- Days supply from pack sizes
- Percentage w/v and w/w conversions
- Ideal and adjusted body weight from formulae
- Diluting creams and ointments by weight
- Scaling a formula to a batch size
- Age-band doses from a provided table
- Tablet fractions and split doses
- Stones, pounds, inches and metric conversions
- Micrograms per minute from mg per hour
- Percentage change and mark-up
- Molar solutions and mass per volume
- Osmolarity from component concentrations
- Direct proportion and cross-multiplication
- Scientific notation and very small quantities
- Stock use and reorder quantities
- Parts per million and very dilute solutions
- Sodium chloride equivalents and isotonic solutions
- Density and specific gravity conversions
- Doses per day and hours between doses
- Finding the volume of diluent to add
- Capping a dose at a stated maximum
- Whole vials and ampoules to draw up
- Reading values from a provided graph
- Micrograms per kg per minute infusions
- Percentage composition of a compound preparation
- Suppository displacement values
- Isotonicity by freezing point depression
- Energy content of parenteral nutrition
- Syringe driver rate calculations
- Area under the curve calculations
- Renal dose adjustments using provided bands
- Insulin units and infusion calculations
- Working backwards from a stated dose
- Doses and days from multi-dose devices
- Combined additives in one infusion
- BMI calculations for the GPhC assessment
- Fewest tablets to make a dose
- Clearance from half-life and volume of distribution
- Maintenance fluid requirements by weight
- Sodium load from effervescent tablets
- Ethanol content of liquid preparations
- Urine output and fluid balance calculations
- Quantities for titration packs
- Infusion line priming and dead space
See also the formula sheet, the eight sample questions and the approved calculator page.