steam quality

How to Validate an Autoclave: Steam Quality Testing

The quality of the steam feeding an autoclave is an important factor in steam sterilization. Like time, temperature, and pressure, steam is a critical variable in the success and repeatability of the sterilization process. As such, steam quality should be part of the validation of any steam sterilizer. In previous articles other phases of validation were discussed (IQ, OQ, and PQ); the focus of this blog is on Steam Quality testing as part of validating an autoclave.

Steam quality is defined as the measurable physical aspects of steam used for sterilization. These physical aspects include temperature (superheat), dryness (liquid water content), and non-condensable gas content. (Steam quality is not a measure of the impurity content of the steam.) Deviations from established ranges of these aspects of the steam can result in the following issues:

  • Wet loads
  • Damaged loads
  • Unsterile loads
  • Sterilization (biological and chemical) indicator failures
  • Staining and corrosion of instruments and containers

Each of these issues has a specific cause or causes and can usually be remedied.

What to Know About Steam Quality

Almost every sterilizer manufacturer recommends “97% pure steam”. In general, this is not defined, rarely measured, and, if discussed at all, is relegated to the mythology of sterilizer arcana. The good news is that essentially all laboratory autoclaves on the market today can provide sterile, dry, and intact sterilization loads if provided good quality steam from the steam supply. The bad news is that any steam autoclave can experience the above problems, and the cause is not always something that can be predicted.

With careful design, following well-established principles, and proper maintenance, the system (steam supply and sterilizer) can be engineered to provide a large margin of security against steam quality noncompliance. For a production or GMP environment, steam quality testing should be part of annual preventative maintenance and qualification testing.

Steam Quality (SQ) Testing Methods and Acceptance Criteria

When steam quality testing is performed, three parameters are measured:
Steam Dryness The amount of the steam by weight that is steam and not liquid water
Non-condensable gases The amount of the steam by volume that is not steam or water, but is air or other gas that does not contribute meaningfully to sterility of the load
Superheat The temperature of the steam above the temperature of saturated steam for a given moisture content

EN 285, the European Large Steam Sterilizer standard [1], is the world’s baseline authority for steam quality acceptance criteria. It is referenced in most national standards and in ISO 17665 [2]. With the release of EN 285:2015, the bar has been raised. The acceptance criteria are shown in the following table.

Steam Dryness Non-condensable gases Superheat
>0.95 w/w* ≤3.5% v/v ≤25K

*For laboratory autoclaves, >0.90 w/w is considered acceptable.

Steam Dryness

 Steam dryness is calculated by measuring the temperature change in a known amount of water in relation to the mass of steam that is required to cause that temperature change. Ideally, the temperature rise is exactly proportional to the amount of steam delivered to the water to heat it, resulting in a dryness value of 1.0 (i.e. perfectly dry steam with no liquid water content.) Normally, the dryness value is less than 1.0, as there are thermal losses in any piping system even if it is well insulated. Because the dryness value of the steam at the chamber entry point can be quite a bit lower than the dryness value in the sterilizer, measurements of steam dryness should be made at both locations.

The acceptance criterion for steam dryness (the fraction of steam relative to water – 1.0 = all steam, no water) is at least 0.95, or 95% by weight. A dryness level down to 90% is considered acceptable for laboratory autoclaves, however, steam below this value is considered to be wet steam.

Wet steam does not deliver as much energy to the load as >90% saturated steam and can cause what is known as “wet packs”.  If the steam is wet, or if the saturation level has decreased since the last validation, the expected Sterility Assurance Level is probably not being achieved. This is especially important for bioburden-based validations, since overkill cycles have more of a safety margin by their very nature.

Non-condensable gases

 Non-condensable gases are generally air and air is a poor sterilant compared to steam. As an example, a typical dry-heat sterilization exposure phase lasts upwards of two hours at a temperature of at least 160°C/320°F. Steam sterilization typically is done with exposure phases of 15 minutes at 121°C/250°F or 3.5 minutes at 134°C. The efficacy difference is notable. For a comparison, consider a contact lens manufacturer that must sterilizer contact lens blisters to a 10-6 sterility assurance level. Sterilization can be performed using a “steam/air mix” cycle that runs at 122°C/252°F for 45 minutes with a steam/air mix of approximately 48% steam (using absolute pressures for the calculation). The same result can be achieved in 15 minutes with saturated steam alone.

In short, non-condensable gases decrease sterilization efficacy. As with wet steam, the Sterility Assurance Level will be less than expected if non-condensable gas content has increased since product sterility validation. The percentage of non-condensable gases in the steam should be less than or equal to 3.5% by volume.


The steam is sampled in free expansion into ambient air. The maximum temperature measured at a precise location in the jet is the temperature upon which the superheat analysis is based.  When the temperature and moisture content do not match up, two things can happen: 1) If the moisture content is higher than saturation for the temperature, wet loads occur, as discussed previously. 2) When the moisture content is lower than saturation for the temperature, the condition is called superheat. In superheat, the steam is too dry and its energy content is too high. When the steam condenses on the load, the energy released is enough to melt plastic packaging and actually char paper packaging. Neither is a good outcome.

The amount of superheat present in the steam should be no more than 25 degrees Kelvin (~25 degrees Celsius) above the temperature in free expansion into atmosphere at the current atmospheric pressure. For all intents and purposes, this corresponds to an upper limit of 125°C in the measurement.

What causes non-compliant results?

 Wet steam

  • Inadequate insulation around the sterilizer or steam piping, allowing energy loss and condensation
  • Poorly controlled steam boiler chemistry (especially a deficiency of sulfites)
  • Low sections of piping between the boiler and the sterilizer, allowing condensate to pool and be carried over with the steam entering the chamber
  • Too great a pressure drop across a regulator or between the jacket and chamber, which causes the “extra” water in the steam at the higher pressure to fall out as condensate
  • No/clogged steam filters, either letting condensate pass if no filter, or causing a pressure drop that causes condensate to fall out
  • No/clogged steam traps/separators, in either case, condensate in the steam line is not removed
  • Steam trap/filter too far from the sterilizer, allowing condensate to be generated between the trap or filter and the sterilizer
  • Inadequate number of steam traps for the distance that steam must travel from its source to the sterilizer
  • Bad steam system design (vertical drops of steam direct to the sterilizer, no traps, no header, etc.)
  • Load too dense/too cold when placed in sterilizer
  • Foaming of the water in the boiler

Non-condensable gases

These are brought into the steam primarily via two sources:

  • Leaks/cracks in the steam plumbing, filters, separators, etc.
  • Inadequate deaeration of the boiler feed water


Superheat can result from the following sources:

  • Jacket temperature/pressure too high
  • Steam pressure/temperature too high entering the sterilizer
  • Steam flowing through a small orifice or tight-radiused direction change between its source and the chamber causing a large pressure reduction/steam velocity increase

The temperature shown on the sterilizer controls generally will not show superheat values, even if superheat is present.  Since the temperature is measured in the drain of the sterilizer chamber, superheat will have been dissipated into the load, chamber wall, door and backhead long before it reaches that sensor.


 Each of the steam quality parameters can be measured and, if issues arise, addressed.  The first step is to measure, even if there are no problems. This should be done on a regular basis — at initial installation, and after preventative maintenance to establish a baseline for the system. Measurements made when there are no problems can also provide an indication if the sterilizer is close to having a problem. Measurements should also be made when changes are made to supply plumbing.

Persons experienced in steam quality analysis can usually make cost-effective suggestions to fix the problems, and of course measure to see if the problem is, in fact, fixed.

Contact Consolidated if you are interested in having your steam quality tested.

(Written by: Jonathan A. Wilder, Ph.D, Stericert div. of H & W Technology, LLC)

[1] EN 285, “Sterilization. Steam sterilizers. Large sterilizers”, CEN, national Standards Making Bodies.

[2] ISO 17665-1, “, Sterilization of health care products – Moist heat – Part1: Requirements for the development, validation, and routine control of a sterilization process for medical devices,” ISO, Geneva, Switzerland, 2006.

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