Special Topic: Case Pressure

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Another occasional topic of discussion is overall case pressure. Before stating what is preferable, let me say that many times the only way to know what you have is to measure it directly. The factors that determine airflow through fans are too numerous to know positively what the situation is unless the intake and exhaust fans are dramatically different.

My personal view is that positive case pressure is preferable. The only reason I say this is that it provides control over air entry into the case. With positive case pressure, the vast majority of air entering the case does so through the intake fans. This allows effective filtration of air and reduces dust accumulation within the case. With negative case pressure, air will enter through every crevice it can. Depending on your particular location and case, you may end up with a thick coat of dust inside the case within weeks.

That said, filtration is both a blessing and a curse. It is a blessing in that it improves the cleanliness within the case. This will reduce operating temperatures since dust impedes convection. It may also improve component life due to those lower temperatures. In rare cases, it may prevent a catastrophic failure of components. It can be a curse because filters plug over time. A plugged filter raises flow resistance and can lower airflow drastically. Filters require maintenance to operate at satisfactory efficiency.

So how does one go about measuring case pressure? The easiest way is with a device called a manometer. This is nothing more than a u-shaped piece of tubing. One end resides inside the case (must be away from any fans or airflow pathways) while the other end sits outside the case. The bottom of the “U” must be below both ends of the tube. Pour a little water into the end of the “U” outside of the case. Now see if the water level is equal on both sides of the “U”. If it’s higher on the side outside the case, you’ve got positive case pressure. If it’s higher on the side coming from within the case, you’ve got negative case pressure. Unfortunately, the pressure differential between the inside of the case and ambient air is normally too small to measure with a water manometer. Fortunately, there are alternatives.

Probably the most effective way to determine the pressure differential tendency is with a smoke test. This involves placing a smoke source near an opening in the case. If the smoke flows away from the opening, the case pressure is higher than ambient. Vice versa if the smoke flows into the case. It’s easiest to remove an expansion slot cover and extinguish a match near the opening. The keys here are:

  1. Don’t place the smoke source near a fan intake or exhaust location
  2. Don’t burn anything during your test
  3. Don’t leave a smoke source burning indefinitely
Placing near an intake or exhaust would distort the results. If you burn anything during the test, it’s your fault not mine. Don’t leave something smoking where the smoke will get sucked into the computer. It’s as hard on electrical components as it is on human lungs.

Honestly I don’t expect many people to get a piece of tubing or a match and perform either test. The information is here only for its “educational value”. It is normally sufficient to simply select fans such that the sum of your intake fans exceeds the sum of your exhaust fans by 10-20%. Don’t forget to consider the power supply exhaust fan when you calculate your totals. If you don’t use filters on your intake fans, case pressure loses most of its significance.

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