Since DI water quality is critical to many cleaning processes, it is important to understand its behavior. In too many cases, the quest for numbers results in high costs in the use of DI water. In short, delivering super-high resistivity water does not always have a significant impact on the resistivity of de-ionized water in the ultimate application.
DI water systems can supply water with resistance ranging from 1 megohm to greater than 18 megohms depending on the configuration of the system. Systems producing higher resistivity water are more costly to both purchase and maintain. The rating of a system is based on the water quality that will be delivered directly at the outlet of the system. The road from there is a bumpy one as the pure DI water is looking to gobble up ions wherever they can be found – and they can be found everywhere. In most cases, the quality of water delivered to the process is much more dependent on what happens as the water navigates itself to the point of use than on its quality at the outlet of the de-ionizing system. The following links to provide some insight into the actual use conditions for DI water and what water quality can be maintained in some typical applications.
Although extremely illuminating, the above are a bit lengthy so let me just summarize them quickly.
- High quality DI water (over 15 megohm resistivity) seldom exists other than within the confines of the plumbing as it exits the de-ionizing equipment. Once the water is exposed to the open environment (including external plumbing), the resistivity immediately starts to decrease.
- Maintaining anything above 1 megohm resistivity in a tank or any container open to the atmosphere is near impossible no matter what quality of water is used at the inlet. Increasing the inlet water from a purity of 1 megohm to 18 megohms will not necessarily result in higher resistivity in the process tank, at least over the long term (longer than 15 minutes).
- Testing for DI water quality other than directly at the site of use is very difficult. Once the water is collected and transported, the resistivity will likely have dropped significantly (usually by an order of magnitude). Any verification testing should be done directly at the point of use. Most DI water systems incorporate a sensor that reads resistivity within the plumbing at the outlet of the system but one can not assume that the quality of water will be maintained into the process tank.
- In most cases, the quality of DI water used in an application is considerably lower than it is measured at the source. In most rinsing applications, use of water with a resistivity above 500k ohms does not contribute to its effectiveness. A de-ionized water feed of over 2 megohm resistivity only ads cost to the process.
- De-ionized water should not be sprayed in air. Spraying only increases the surface area available for contaminants to enter the liquid.
In summary, it is important that the process engineer understand the behavior of DI water in designing a cleaning process. Similarly, the equipment supplier needs to understand what it takes to achieve and maintain high quality DI water under the intended conditions of use. Certain measures may be taken to minimize absorption of ions. They include – –
- Minimizing the length of plumbing between the DI water source and the point of use. This plumbing should also be selected specifically for DI water use.
- The surface area of the water that comes into contact with air should be minimized. Using narrow, deep tanks is better than using wide shallow tanks.
- DI water inlets should be located below the liquid level of tanks to prevent splashing as the DI water enters the tank.
– FJF –