SOP on operation and calibration of autoclave

What Is the SOP on Operation and Calibration of Autoclave?

An autoclave is a closed steel chamber that sterilises materials with saturated steam under pressure. The SOP is the approved document that says how to run it, how to record each cycle and how to keep its instruments accurate.

Objective: to lay down the procedure for operating, recording and calibrating the autoclave.

Scope: every steam autoclave in microbiology, QC or production. These sterilise media, glassware and garments. They also sterilise equipment parts or waste.

Responsibility: the technical operator runs the cycle and fills the logbook.
The QC analyst or production officer reviews the printout.
Engineering staff calibrate and maintain the unit.
The QA officer approves records and deviations.
The head of department (HOD) is responsible for compliance.

How Does an Autoclave Kill Microorganisms?

Saturated steam gives up heat when it condenses on a cooler surface, and that heat kills microbes. The sterilising parameter is temperature. Pressure only follows it, because saturated steam at a given temperature always has the same pressure.

Eupry’s validation guide gives the common conditions: 121 °C at about 15 psi (about 1 bar) gauge. It also gives 132–134 °C at about 2 bar gauge. These are gauge pressures, read above atmospheric pressure. Hold time depends on the load and the validated cycle. Air left in the chamber lowers the steam temperature at that spot, which is why air removal matters.

What Are the Main Parts and Which Ones Need Calibration?

Description of autoclave parts lists the following.

• Chamber: jacketed or single-wall stainless steel.
• Steam supply or generator: plant steam, pure steam or an internal heater with DM water.
• Air-removal system: a vent valve (gravity type) or a vacuum pump (pre-vacuum type).
• Door: gasket and safety interlocks.
• Safety valve: releases pressure above the set limit.
• Steam trap and drain: removes condensate.
• Controller: PLC or microprocessor with a printer.
• Sensors and gauges: temperature probes, pressure transmitter and gauge, timer.

The calibration SOP covers the chamber temperature probe (Pt100, a platinum resistance sensor), the independent recording probe, the pressure gauge or transmitter and the timer. Annex 1 clause 8.50 expects you to record each heat cycle with accurate equipment. It also expects duplex probes on independent control and monitoring systems. Those systems must be able to abort or fail a cycle that does not match the validated parameters.

Gravity Displacement or Pre-Vacuum: Which Tests Apply?

Feature	Gravity displacement	Pre-vacuum
Air removal	Steam pushes air out through a vent	Vacuum and steam pulses remove air
Typical load	Media, glassware, liquids	Porous, wrapped and hard goods
Typical cycle	121 °C, 15–30 min for lab loads	132–136 °C (134 °C nominal), 3–18 min by load
Air removal test	Not applicable	Daily
Vacuum leak test	Not applicable	Weekly

Exact values depend on the manufacturer and model; always follow your OEM manual and site SOP.

How Do You Operate an Autoclave? Step-by-Step Process Flow

1. Utility and gasket check. Read the last logbook entry, any open breakdown and the calibration label. Do not use the unit if the label is overdue. Check steam, water and compressed air supplies, the drain strainer and the door gasket. On a top-loading unit, keep the heating coil covered with DM water.
2. Air removal and leak tests. On pre-vacuum units, run the air removal test (Bowie-Dick test) before the first load. Run the vacuum leak test on the weekly day. Do not load until both pass.
3. Load per pattern chart. Follow the validated load pattern chart. Pack items dry, in wrapping that lets air out and steam in. Never sterilise sealed containers. Loosen caps on bottles.
4. Run validated cycle. Close and lock the door. Select the validated cycle and note its unique cycle identifier. On a gravity unit, vent until steam flows steadily, then close the vent valve.
5. Review printout. Check that the whole load reached temperature before the hold timer started. On pre-vacuum cycles, confirm the drain temperature record. Compare temperature, pressure and time with the cycle limits.
6. Unload and inspect load. Wait until the gauge reads zero. Never open the door under pressure. Wear heat-resistant gloves and goggles, open the door away from your face and exhaust liquids slowly. Inspect every item for dryness, damaged packs and moisture. Remove any item that fails and report it.
7. Record in logbook. Enter the cycle identifier, load, times, readings and signature (Annexure I). Mark the load as sterilised. The QA officer reviews the sterilisation record as part of batch release.

Annex 1 clauses 8.45 and 8.56–8.58 support these records and checks. Tape or a colour-change sticker shows the process occurred, nothing more.

Key Operating Parameters

Parameter	Typical value	Note
Temperature, liquids and glassware	121 °C minimum, hold 15–30 min	Validated per load
Temperature, porous load	132–136 °C, hold 3–18 min	134 °C nominal
Chamber pressure	About 1.0 bar (15 psi) gauge at 121 °C; about 2.0 bar (30 psi) gauge at 134 °C	Gauge pressure
Leak test	Pressure rise of 1.3 mbar/min or less	Eupry value; manufacturer limit governs
F0	Equivalent minutes at 121 °C, with z = 10 °C	Overkill cycles are commonly set at 12 min or more

How Do You Calibrate an Autoclave?

Calibration means comparing an instrument with a traceable reference and recording the error. Use reference standards traceable to national standards, ideally from an ISO/IEC 17025 accredited laboratory.

Instrument	Method	Points
Chamber temperature probe	Compare with a reference probe placed beside it	Working temperatures, such as 121 °C and 134 °C
Independent recorder probe	Compare with the reference and with the controller	Same points
Pressure gauge or transmitter	Compare with a reference gauge, in gauge pressure	Working pressures
Timer	Compare with a calibrated stopwatch over a hold period	Hold time

1. The QA officer approves the calibration plan. Engineering staff isolate the empty autoclave.
2. Record the as-found readings before any adjustment.
3. Adjust only if a reading is outside the tolerance in the approved SOP. Record the as-left readings.
4. Attach a label with the calibration date, due date and initials.
5. Complete Annexure IV. The QA officer approves it.

Many plants calibrate every 6–12 months and after major repair. Regulations set no fixed interval. 21 CFR 211.68(a) asks for routine calibration, inspection or checking under a written program.

If an instrument is out of tolerance, open a deviation. Then assess every cycle run since the last good calibration. For example, if the chamber probe read higher than the reference, the real chamber temperature was lower than the printout showed. Those loads need review and possible re-sterilisation.

Preventive Maintenance

Check the door gasket and drain strainer every day. Test the vacuum weekly. Service the steam trap, safety valve and vent filter on the manufacturer’s schedule, and replace the gasket when it hardens or cracks. Trend cycle data and probe drift, because slow drift often shows before a failure.

Data Integrity of Autoclave Records

Keep the printout with the logbook entry. On PLC units, restrict user access and keep the audit trail on. Record corrections with date, reason and signature, never by overwriting.

Annexures

Annexure I: Daily Usage Logbook

Date	Cycle identifier	Load	Start/end time	Hold temp/pressure	Printout attached	Operator	Reviewed by

Annexure II: Air Removal Test (Bowie-Dick Type) Record

Date	Test pack lot	Cycle temp/time	Result (uniform/not)	Operator	QA officer

Annexure III: Weekly Leak Test Record

Date	Hold time	Pressure rise	Limit	Pass/fail	Operator

Annexure IV: Calibration Record

Instrument	Reference ID/certificate	Set point	As-found	As-left	Tolerance	Pass/fail	Next due

Annexure V: Cycle Printout Review

Cycle identifier	Whole load at temperature before hold	Hold time met	Drain temp recorded	Load dry and packs intact	Reviewer

Annexure VI: Breakdown/Deviation Record

Date	Description	Load status	Immediate action	Root cause	Impact on earlier cycles	QA approval

Annexure VII: Load Pattern Chart

Load type	Container/pack	Maximum quantity	Position in chamber	Cycle	Revalidation date

Frequently Asked Questions

References

1. EU GMP Annex 1: Manufacture of Sterile Medicinal Products, European Commission (2022)
2. Validation, Cleaning Validation, PIC/S, 2007
3. Plant SOP (library)