TABLE TOP STERILIZERS: OVERVIEW & SERVICE TIPS

By Phil Goldstein and Andy Sandelski
RPI Product Development Department

An RPI document.

TABLE TOP STERILIZERS: MANUAL VS AUTOMATIC

The newer automatic type sterilizers basically work on the same principle as the older manual machines. The following are some of the similarities between the two types:

• Sterilization cycles are between 250 °F (121 °C) and 270 °F (132 °C).
• Water is stored in a reservoir and a condensation coil takes steam after each cycle and condenses it back into water.
• Both types of sterilizers utilize a triac to supply the high current to the heaters.
• A mechanical or electrical (solenoid valve) air bellows is used to maintain the saturated steam inside the chamber.
• Output devices such as gauges (manual) or LED displays (automatic) are used to display temperature and pressure.
• Mechanical timers (manual) or programmable timers (automatic) are used to monitor the length of the sterilization cycle.
• Safety devices for pressure (safety relief valves) and temperature (bi-metallic or snap action switches) are incorporated on both types of sterilizers.
• Door gaskets are used to seal the chamber of the sterilizer.
• Mechanical valves (manual) or solenoid valves (automatic) are used to fill the chamber of the sterilizer.
• Filters to filter the water to the chamber from the reservoir and from the chamber back into the reservoir.

Some of the added features an automatic sterilizer has that are not found on the manual units:

• Self-diagnostic software.
• Display codes to assist you with troubleshooting the unit when it will not sterilize properly.
• Heater circuit is microprocessor controlled and automatically changes from FILL to STERILIZE to DRYING cycle and powers down at the end of the drying cycle.
• Timing circuits that actuate solenoid valves to allow a premeasured volume of water for each cycle. Thus eliminating operator errors in filling the chamber.
• Additional electronic surface temperature monitoring to prevent damage to the chamber because of low water conditions.
• Dams or water guards that prevent condensate from dripping on counter tops.
• Additional solenoid valve ports to expedite the venting of the sterilizer.
• Automatic door release feature (pulse solenoid latch) to prevent wet packs from occurring because the door was not cracked open.

TABLE TOP STERILIZERS: MANUAL

We will reference the "Omni-Clave" line of manually operated sterilizers by the Pelton-Crane company for our "Theory of Operation" for all non-electronically controlled sterilizers. The "Omni-clave" line was introduced for the doctors' office in the early 1960s. Three chamber sizes were available; 6, 8 and 10 inch to accommodate everything from handpieces to wrapped instruments.

Simplicity in design. Ease of Operation.

The success of the Omni-clave line is partially due to their simple mechanical construction. There aren't any "whistles and bells" on these sterilizers. Everything is straightforward. A stainless steel water reservoir is used to store the distilled water required for steam. The water reservoir is connected via 5/16 copper tubing to the main valve. The main valve controls the direction of the distilled water entering the sterilizing chamber prior to the sterilization cycle, and vents the steam back into the water reservoir after the sterilization cycle is complete. In addition to controlling the direction of water and steam, the main valve is directly connected to the Power and Function switches that control the electrical characteristics of the machine. All performed by simply turning the control knob on the front panel.

Theory of Operation.

The operator fills the water reservoir with distilled water and replaces the cover. The items to be sterilized are placed into the holding trays. The control knob on the front panel is turned to the FILL position. This does two things. It applies line voltage across the three heating elements and allows the chamber to pre-heat. At the same time, water enters the main chamber through the main valve to a predetermined level.

Once this level is reached, the control knob is turned to the STERILIZE mode and the door is closed and bolted shut. When the main valve is in the STERILIZE mode, the valve closes off the FILLING port to the main chamber and at the same time applies total line voltage to the center heating element only. The two outside heating elements are not energized in the STERILIZE mode. A mechanical timer is set by the operator for the duration of the STERILIZE cycle (30 minutes). Since the timer is not a switching mechanism, it only signifies when the set time is up. It is the responsibility of the operator to go back to the machine and turn the control knob to the VENTING position. This in turn opens the VENT side of the main valve and allows steam to vent back into the water reservoir. Once again the line voltage is divided equally by the three heating elements. The operator opens the door just slightly, resets the timer and allows the instruments to dry. When the DRYING cycle is complete, the operator turns the control knob to the OFF position, which shuts off all power and closes the main valve.

Components.

The thermostat control system consists of a 1 meg ohm thermistor and a solid state pc board which is triac controlled. The thermistor operates under the negative heat coefficiency property. The higher the temperature rises the lower the resistance of the thermistor. At room temperature the thermistor should have a range between 1.3 meg and 900k. At peak sterilizing temperature (270 °F) the thermistor resistance drops to approximately 17-18k.

There are two safety devices on the machine. One is a bimetallic over-temperature switch located on the center heating element. This is an adjustable thermostat used to open the circuit to the heating elements when the temperature inside the sterilizing chamber rises above the set temperature. In conjunction with the over-temperature switch, an over-pressure safety valve is plumbed into the sterilizing chamber and will open to release pressure between 33 to 35 psig.

Another area of importance is the bellows and bellows housing assembly. If you remember during the FILL cycle the main valve is dumping water into the sterilizing chamber while the three heating elements are warming up the chamber. As the chamber warms up, cold air is pushed out through the bellows housing. When the air gets hot enough, the bellows expands and closes off the valve and allows the system to pressurize. During the VENTING cycle everything operates in reverse. The main valve is opened, the sterilizing chamber vents through the condensation coils in the water reservoir. When the pressure is released and the air cools, the bellows contracts to its original size.

HOW MICROPROCESSOR CONTROLLED STERILIZERS REGULATE FOR PROPER STERILIZATION

The automatic microprocessor controlled sterilizers basically monitor the steam temperature and the inside chamber pressure. These two parameters are measured with solid state devices that are mounted in the chamber for steam and on a PCB (Printed Circuit Board) for pressure.

The steam sensor is a hybrid IC that is an active device that utilizes a 5V DC supply and gives an output of 10 mV / °C that is sent to the MPU (Microprocessor Unit). A pressure transducer receives the steam pressure via a tube connected from the chamber and sends a millivolt signal to the MPU. Most microprocessor type sterilizers usually regulate by the pressure parameter simply because some pressure devices can regulate within 3-5 KPA ( ~ 1 PSI) . Sometimes a third parameter is utilized to monitor the surface temperature of the chamber to prevent damage to the chamber or heater in case of low water conditions. This device is usually a solid state on/off switch or a RTD that sends a signal to the MPU. All gains are preset to match the requirements of the MPU.

When any of these three signals do not correspond with the information the sterilizer (MPU) is programmed to see the unit will alarm and will abort the sterilization cycle. If all three parameters are within the programmed range of the sterilizer (MPU) the unit will successfully complete a sterilization cycle.