Infection Control Today - 07/2004: Understanding Steam Sterilizer

July 1, 2004

Understanding Steam Sterilizer Physical Parameters

Understanding Steam Sterilizer Physical Parameters

By Linda Clement andJohn Bliley

Good sterilization monitoring practicesare an essential part of any healthcare sterile processing quality assuranceprogram. Much focus has been directed toward chemical and biological monitoringof sterilization cycles; however, the first monitors that should be utilized arethe steam sterilizer physical (mechanical) monitors that are integral to thesystem itself. These include recorders, displays, digital printouts, and gaugesthat display and record time, temperature, and pressure parameters of thesystem.

These quality-assurance monitoring tools are oftenunderutilized or misunderstood. They provide a means to ensure that appropriateparameters are met during a steam sterilization cycle. They also give operatorsa record of the sterilization cycle and a method for detecting sterilizermalfunctions or sterilizer operator error.

Although physical monitors are a common part of thesterilization-monitoring process and the information provided is an importantcomponent of sterilization record keeping, many sterilizer operators do notfully understand what the data on the recording devices are telling them. It is essential that sterilizer function be closely observedand understood to ensure that the conditions inside any sterilizing chamber meetthe required sterilization parameters during its operating cycle.

Steam Sterilization Requirements

A number of factors must be considered for successful steamsterilization of instruments and medical devices to occur. Items must first bethoroughly cleaned, properly wrapped (if applicable), and positioned correctlyin the sterilizer. The sterilizer itself must be functioning correctly, thequality of steam must be acceptable, and the proper cycle type selected for theitems being processed. In addition, the appropriate time and temperature for thecycle must be selected based on the recommendations from the sterilizermanufacturer. Most importantly, the medical device manufacturers reprocessingrecommendations must be followed for each device being sterilized.

The microprocessors and computer printouts available onsterilizers today monitor and provide the operator with detailed informationregarding each cycle run. The examples in Figure 1 are based on informationavailable using this type of recording technology.

Steam Sterilizer Pressure and Exposure TemperatureRelationship

The operator must be concerned about the relationship betweenthe chamber temperature and pressure during a steam sterilization cycle, becausesterilization temperature will not be achieved without the necessary regulatedsteam pressure. Some sterilizer operators are confused by this relationship andreject loads because the sterilizer did not reach the pressure they felt wasrequired.

This situation has occurred because operators have been giventemperature/ pressure specifications that are based on published steam tablesthat have not been adjusted for the actual barometric pressure at the locationwhere the sterilizer is being used. Often, the operators are given instructionsto reject loads that have not achieved a minimum steam pressure of, for example,30 pounds per square inch gauge (psig). A single pressure level of acceptancemay be applicable to a sterilizer in one location but may not be correct for asterilizer at another location. This can lead to errors in rejecting loads thatare actually perfectly acceptable.

It is important to understand all the factors that contributeto a proper evaluation of chamber pressure: the meaning of the numbers listed inpublished steam tables, the difference between absolute pressure and gaugepressure, and the effect of local barometric pressure on gauge readings.

Most sterilizers have displays, gauges, and printouts thatindicate gauge pressure. Gauge pressure and absolute pressure are not thesame.

Gauge pressure indicates pressure in excess of current ambientatmospheric (barometric) pressure, whereas absolute pressure includes thecurrent ambient pressure. In other words, gauge pressure would equal absolutepressure minus the ambient barometric pressure.

Figure 2 shows a saturated steam temperature table. The actualrelationship between the temperature of saturated (100 percent) steam andchamber pressure has been well established by experimental means.

Steam tables always list absolute pressure, not gaugepressure. For example, at a common sterilization temperature, 270 degrees C, thepressure shown on the steam table is 41.85 psi absolute.

When steam tables are used, a common mistake is to obtain acomparable gauge pressure by simply subtracting the barometric pressure found atsea level, which is a 14.7 psi difference. This would be correct only for equipment that is located at sealevel and not for equipment at higher elevations, which have a lower barometricpressure.

To understand how atmospheric (barometric) pressure willaffect the sterilizer in different locations, we will use two examples. For a hospital at sea level on the Florida coast, thebarometric pressure would be about 14.7 psi. According to the steam tables, toachieve 270 degrees F (132 degrees C) requires 41.85 psi absolute pressure. Tosee what the gauge on the sterilizer would read at 270 degrees F (132 degreesC), subtract the barometric pressure from the absolute pressure (41.85 minus14.7) and you get 27.15 psi gauge pressure. Note that most sterilizers operateat a slightly higher temperature than set point (about 2 to 3 degrees F above)which requires approximately one or two psi more steam, resulting in a gaugereading of about 29 psi.

The next example will be for a sterilizer in Denver, which isapproximately a mile above sea level and has a barometric pressure of about 11psi. To reach 270 degrees F (132 degrees C) still requires 41.85 psi absolute,but since the barometric pressure is lower, the sterilizer gauge would read adifferent number (41.85 minus 11, or 30.85). Again the sterilizer runs the same2 to 3 degrees F above set point, and adding the same couple of psi as in theprevious example results in a gauge pressure of about 33 psi. This is almostfour psi higher than the same sterilizer operating in Florida.

Note that barometric pressure differences are not the onlyfactor that affects the pressure required to achieve sterilization temperature. The steam tables are based on steam that is saturated, or 100percent quality. Lesser steam quality, which is more typical, will require aneven higher chamber pressure. You must have 50 psig to 80 psig of dynamic steampressure coming from the steam source which is then regulated for yoursterilization needs at the sterilizer.

Because of the variances described above and theimpracticality of expecting operators to interpret barometric pressures andsteam quality, a more practical way to understand what is typical for yourspecific sterilizer is to review your sterilizer recording devices, establish anaverage pressure and temperature for a particular sterilizer, and use that as astandard. If an operator observes pressures that vary by more than plus or minus2 psi, it may be an indication that the unit calibration should be checked.

Steam pressure must be increased by a half pound for every1,000 feet above sea level to achieve sterilization temperature; therefore, if Denver is 5,000 feet above sea level, thesterilizer may have to be adjusted by a service technician to achieve the higheroperating pressure.

As you can see, atmospheric pressure alone will have adramatic effect on a sterilizers required steam pressure, as will differencesin steam quality. Therefore, a standardized published listing of 30 psi as thecorrect pressure needed for a 270 degrees F (132 degrees C) cycle is notaccurate or useful to sterilizer operators.

Understanding Prevacuum (Prevac) Sterilizer Readings

In vacuum-assisted steam sterilizers there are three phases ofthe cycle with which operators should be familiar: the conditioning,sterilizing, and exhaust (drying) phases.

* Conditioning phase: During theconditioning phase (a C prints on the paper tape) the unit goes throughalternating pressure and vacuum pulses (prevac cycle). The pressure pulse will always be the same 26 psig. Thisis controlled by a pressure switch setting. The vacuum pulse must reach aminimum of 10 inches of mercury (Hg), and when fully loaded, that is often themaximum level that will be achieved. Subsequent vacuum pulses (there are four) will go deeper sothe operator will normally observe approximately 10 inches, then 12 inches, then16 inches, then 18 inches. These numbers are not absolute but they give the operator anindication that the vacuum gets deeper. Smaller loads will allow the unit topull an even deeper vacuum. The sterilizer operator should watch for vacuum levels thatremain only between 10 inches or 11 inches. This would indicate that either thesterilizer is overloaded or that the vacuum system is struggling. If this isobserved, loading of the sterilizer should be evaluated and/or a service callshould be initiated to have the sterilizer checked.

Excessively long conditioning phases could also be anindication of a potential problem. The sterilizer control system will allow asignificant amount of time before it will sound an alarm or indicate that acycle has been in the conditioning phase too long. If this occurs, operatorsshould examine their typical cycle printout tapes and graphs, and establish anaverage time for their particular sterilizers and loads. This should runapproximately 20 minutes. A time variation of a couple of minutes is not aproblem, but a variation of 10 to 15 minutes for identical size loads couldindicate a problem. If longer than average conditioning phases are observed,then a service call should be initiated.

Sterilizing phase: During thesterilizing phase (an S prints to mark the phase) in a normal 270 degreesF (132 degrees C) cycle, the first print on the recording device will indicate270 degrees F (132 degrees C), while subsequent recordings during this phasewill be at a higher temperature. Some sterilizer models will overdrive toapproximately 273 degrees F for the remaining sterilize phase (S) prints,while other units will only reach 271.5 degrees F. The important thing is toensure that the subsequent prints during the sterilization phase are actuallyhigher than the first S print. If the subsequent S prints are justbarely above 270 degrees F (for example at 270.2 degrees F), then there may bean adjustment problem with the pressure regulator. This fault would be morecommon on older sterilizer units because the regulator (Hi-Lo valve) isaccessible to operators and may have been incorrectly set. In some casesall the operator may need to do is turn the Hi-Lo valve handle a little moreclockwise. Newer sterilizers do not have adjustable Hi-Lo valves and thissituation would not apply.

Exhaust (drying) phase: There willonly be two recorder printings during the exhaust phase, one at the beginningand one at the end. The most important thing to observe is the vacuum levelachieved at the end of the exhaust phase. The vacuum should be a minimum of 25(although elevation will also come into play here in areas of higheraltitude such as Denver, you may only see 20 inches). Again, you should establish an average for the unit, and callfor service if the vacuum level varies by two to three inches for cycles withcomparable loads and drying times.

Gravity Displacement Steam Sterilizers

In gravity displacement steam sterilizers the same threephases of the cycle, the conditioning, sterilizing, and exhaust phases, shouldbe carefully observed.

Conditioning phase: Unlike pre-vacuumsterilization, there is not much to look at during this phase other than chargetimes. This is where an average charge time should be established and thenvariances in these times should be monitored by the operator. An indication thata sterilizer has taken too long to charge is usually caused by a defectivechamber steam trap. This would also be applicable in pre-vacuum units.

Sterilizing phase: There is nodifference between pre-vacuum cycles and gravity cycles during the sterilizingphase.

Exhaust (drying) phase: The exhaustphase on gravity displacement sterilizers varies from model to model. On somegravity sterilizers, the exhaust phase is the same as it is in a pre-vacuumsterilizer. On those units the information described for a pre-vacuum sterilizerwill apply. However some older gravity sterilizers use a different type ofdrying phase that does not pull deep vacuums because it relies on the flowing ofcool air over the load. The sterilizer turns on the vacuum system and opens theair inlet valve to allow air to enter the chamber. In these units there will bevery little vacuum in the chamber (two inches or three inches) when runningproperly. If the air inlet valve is malfunctioning, or the air filter is wet,the operator would observe deeper vacuums, which will result in poor dryingrather than better drying.


A central service departments quality assurance programdepends on many elements, one of which is system monitoring information that isunderstood and used appropriately by personnel. In order to assure sterilization and the proper operation oftheir sterilization systems, sterilizer operators must thoroughly review thephysical monitor recordings at the end of each steam sterilization cycle andassure that all appropriate parameters have been met for that particular cyclebefore removing the load from the sterilizer.

By knowing what the appropriate sterilization parameters andvalues should be for that system at that geographic location, and by monitoringfor inconsistencies from cycle to cycle, operators will easily detect variationsin normal operation. This can serve as a detector for sterilizer malfunction,and will ensure that record-keeping practices are consistent.

Linda Clement is consulting service manager and John Bliley ismanager of service engineering, both for STERIS Corporation.