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Medical Instrument Sterilization

6 min read

Instruments are sterilised by holding them in air-free saturated steam at 121 °C, which needs about 15 psi gauge at sea level, for 15 to 30 minutes depending on the load, with the clock started only when the chamber itself reaches temperature. The step beginners miss is driving the air out first: air left in the chamber adds its own pressure to the gauge, so the gauge can read a full 15 psi while the steam is still cooler than 121 °C and the load comes out unsterile.

Part of Applications

Pressure-steam sterilisation of surgical instruments and dressings: the temperature-and-time pair, why air displacement decides the result, and how to check a cycle.

The Sterilising Condition Is A Temperature Held For A Time

Spores survive boiling, so true sterilisation needs steam hotter than 100 °C. The Pressure Gauge article tabulates 15 psi gauge as 121 °C, and the carrier article, Autoclaving and Instrument Sterilization all name 121 °C as the condition. The corpus gives the hold time in three slightly different ways:

  • The carrier: 15 minutes minimum, and 30 minutes for wrapped instrument packs.
  • Autoclaving: 15-20 minutes, up to 30 for wrapped or dense loads.
  • Instrument Sterilization: 20 minutes.

These do not conflict for a given load so much as overlap, and none is a tested schedule for a particular vessel. Where they differ, use the longer figure for the load in front of you, and 30 minutes for anything wrapped, packed densely or in a heavy tray. The pair is always 121 °C and the time together: neither half means anything alone. The time is counted from the moment the chamber, and not just the gauge, has reached 121 °C.

In pure saturated steam, pressure fixes temperature. In a mixture of steam and air, the gauge reads the total of both. Only the steam part sets the temperature. The calculation uses the corpus’s own table from the Autoclaving article, which gives 5 psi gauge as 109 °C and 10 psi as 116 °C.

Worked example (illustrative, assuming air supplies a quarter of the pressure):

  • Total pressure, absolute: 15 psi gauge + 14.7 psi atmosphere = 29.7 psia.
  • Suppose air contributes a quarter: 29.7 ÷ 4 = about 7.4 psia of air.
  • Steam pressure, absolute: 29.7 − 7.4 = 22.3 psia, which is 22.3 − 14.7 = 7.6 psi in gauge terms.
  • Interpolate between the table’s 5 psi (109 °C) and 10 psi (116 °C): 7.6 is 2.6 of the 5 psi step, so 109 + (2.6 ÷ 5) × 7 = about 112.6 °C.

The gauge reads 15 psi, and the steam is at about 113 °C: roughly 8 °C short of 121 °C. The interpolation is a straight-line approximation between two table values, and the quarter-air figure is an assumption to show the effect. The principle is not an assumption: the gauge cannot tell steam from air.

Air also gathers in pockets: in the folds of wrappers, in the bottoms of containers, in the lower part of the chamber. Air is a poor conductor of heat compared to steam, as the corpus’s Autoclaving article notes, so a pocket can stay cool even in a hot chamber.

What You Need

  • A sound vessel with all the safety devices (Safety Devices).
  • A thermometer placed low in the chamber, near the drain, where the air gathers last.
  • A vent that can be held open so steam pushes air out.
  • Perforated or loosely closed containers and wraps that steam can pass.
  • A chemical indicator strip or the sulphur check (see below).
  • Clean instruments. Blood and tissue form a coating that protects organisms from steam.

Method: A Sterilising Cycle

Step 1: Pre-clean. Scrub off all visible contamination, as in Instrument Sterilization.

Step 2: Load loosely on a rack above the water, wrapped in cloth or in perforated containers, with spaces for steam to reach every surface. Do not pack tightly.

Step 3: Seal and heat, with the vent open.

Step 4: Purge the air. Let steam flow from the vent until it comes out steadily and fully. The carrier says to vent for five minutes at atmospheric pressure before closing the vent; Autoclaving says one to two minutes of steady steam. Use the longer carrier figure as the minimum, and more for a large chamber or a dense load.

Step 5: Close the vent and bring up to 121 °C, checking the thermometer and the gauge. If the thermometer is below the table value for the gauge pressure, there is still air: open the vent and purge again.

Step 6: Start the clock when the temperature and the pressure have both reached target, and hold for the time chosen for the load.

Step 7: Cool and exhaust slowly. The carrier recommends a slow exhaust over about ten minutes, since fast venting wets the packs. Do not force-cool with water. Open only at zero pressure, standing aside.

Step 8: Keep the load sterile until use, wrapped and dry (Sterile Technique).

Checking That It Worked

  • Chemical indicator. The carrier uses a strip whose stripe changes colour at 121 °C. If it did not change, the middle of the load did not reach temperature and the load is not sterile. A colour change shows that the temperature was reached; it does not by itself show that the hold time was completed (recalled principle).
  • Sulphur in a tube, melting at 119 °C according to Autoclaving, shows the load exceeded that temperature.
  • A log entry with date, load, thermometer reading, hold start and end.
  • Biological testing with a known heat-resistant spore preparation is the gold standard where microbiology exists.

Common Mistakes

  • Closing the vent at once. Air stays, and the gauge lies.
  • Starting the clock at first steam. The load has not reached temperature.
  • Packing the chamber full. Steam cannot reach the centre.
  • Using the gauge alone. Compare it with the thermometer.
  • Forgetting altitude. Above sea level the same gauge number means a lower temperature (Altitude adjustments). Trust the thermometer.

Safety

Pressurised steam and boiling-hot liquids

Open the vessel only at zero pressure on the gauge, with the vent open and your body beside the door. Sealed bottles of liquid can erupt when disturbed straight after the cycle. Plastics that melt below 121 °C do not belong in the chamber. Dry heat (160 °C for 2 hours, metal and glass only, per Instrument Sterilization) is the alternative where no safe vessel exists.

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