Do You Really Need Distilled Water in Your Autoclave?
Water quality is important to proper function of your sterilizer.
Why? You'll find out why, because this month in Practice Tips, we’ll look at the principles of autoclave sterilization.
The sterilizer is one of the most important pieces of equipment in the office. If you don’t have sterile instruments, you can’t work. But, how do these vital pieces of machinery function?
First, we’ll review a little history.
Most dental offices have what are classified as “table top” sterilizers as they are quite compact and small – small enough to fit on a table top (or counter top, more often).
Table top sterilizers were originally designed as “flash” sterilizers for use in medical operating rooms. They were intended for quick “flash” sterilization of instruments whose sterility was somehow compromised in the operatory (most commonly by being dropped). Their (comparatively) short cycle times and small size makes these sterilizers ideal for the dental office as well.
Now that we know a little background, we can move on to matters of function:
Taken at the most basic level, an autoclave is really just a boiling tank. You have water in a reservoir which is drained (or pumped) into the chamber at the beginning of the cycle. The chamber heats up causing the water to boil. The chamber continues to heat, super-heating the steam which then builds up pressure so that the steam permeates all items in the chamber. According to CDC guidelines, most items require 3-7 minutes of exposure to this superheated pressurized steam to be rendered sterile. The specific time will vary depending on the nature of the item and how it is packaged.
Once sterilization has been achieved, the pressurized steam is vented from the chamber and the heating elements turn off allowing the chamber and contents to cool. The vented steam is sometimes exhausted out a drain, or sometimes vented back into the reservoir where there is a condensation coil to facilitate cooling of the steam returning it to a liquid. Finally, the contents of the chamber are allowed to dry (the specific method of drying will vary widely from one make and model of autoclave to another).
As you can see, water (or steam created from this water) is integral to the function of an autoclave. For this reason, it is very important that you only use distilled water or water that has been treated for use in a sterilizer (typically, de-ionized). Properly treated water will have a very low Total Dissolved Solids (TDS) level- 5ppm or less. Distillation or de-ionization are normally the only way to achieve such low TDS levels. Other methods of water treatment (for example, reverse osmosis) are not normally adequate to achieve such low TDS levels.
The water and steam flows through a variety of small pipes and valves many of which can become clogged from the various minerals and additives that may be in untreated (i.e. tap) water. Poor or low water flow can lead to overheating (damaging expensive instruments), failure to achieve pressure (which is required for sterilization to be achieved), as well as damaging components of your autoclave. Naturally, damage to components of the autoclave can lead to malfunction, costly downtime, and expensive repairs.
As an additional safeguard, autoclaves have filters on the water line to catch any debris before the water enters the chamber. Check your filters regularly as part of your routine maintenance as well as your routine sterilizer maintenance. Two examples of fill line filters are pictured below:
This sintered bronze filter is used on Pelton & Crane Sterilizers:
A mesh filter is used on Tuttnauer and Midmark sterilizers:
Many sterilizers will also have a filter in the reservoir- be sure to check it frequently as well. If you notice debris in the reservoir of your autoclave, be sure to clean it out thoroughly using your suction to remove any debris or dirty water.
Water quality is so important that some autoclaves (e.g. the Statim cassette sterilizers) have a built-in water quality sensor that will prevent function if the water is not of suitable purity. Of course, sometimes the sensor can be damaged or get dirty, so make certain to keep it clean & inspect it as part of your regular maintenance.
The water quality sensor also brings to mind a common problem with self-distilled water. Most counter top water distillers are manufactured and marketed for home use so they include carbon filters. The carbon filter is included to improve the taste of the water for drinking. However, they will actually introduce carbon into the water, so remove the carbon filter if distilling water for your sterilizer. The carbon from these filters will often trigger the water quality sensor of the Statim and could impede the function of this and other sterilizers.
As you can see, water quality can play a significant role in the daily use of your autoclave so make certain to only use appropriately prepared water in yours to keep it working well and maintain the vital sterility of your instruments.