# Delta P: Filter Differential Pressure and its Connection with Fuel Quality Control

Dec. 20, 2017

Clean, dry, on-specification fuel to aircraft is a “safety of flight” issue; the way to ensure this is always achieved is through proper filtration. Interestingly enough, aviation fuel is typically filtered to 1 micron and dried to less than 15 parts per million (ppm) of water at the point the fuel is delivered to an aircraft.

Many times in training sessions we are asked, “What exactly does that mean, what is the meaning of 1 Micron and how much is 15 ppm?”

A micron is one-millionth of a meter; to put that into perspective, a human hair is approximately 70 micron. We can commonly see to somewhere in the range of 40 micron. A red blood cell is 8 micron and aviation fuel is filtered to 1 micron.

When contemplating the maximum allowable water content, only 15 gallons is permitted in every 1 million gallons of aviation fuel. So, how do we obtain this level of cleanliness and extreme dryness? Proper filtration.

You may be thinking, “How can we determine how well the filter is working?”

The simple answer is differential pressure, sometimes referred to as Delta P.

Differential pressure is the measurement of the difference between the inlet and the outlet of the filter vessel. As the filter does its’ job of removing particulates, an increase in the difference between a pressure reading at the inlet of the filter vessel and the reading at the outlet of the filter vessel will change. An increase in this pressure difference indicates that the filter elements are becoming filled with particulate and will need to be changed at a set value (15 psi/pounds per square inch at rated flow of the vessel).

In many installations, the gauges used to measure Differential Pressure have a 0-30 psi range when a 0-15 psi scale would be much easier to determine changes. To make this change possible, a change of internal spring and external scale can be installed.

Another common situation is that the DP gauge is located in a place that the gauge cannot be observed during the actual fueling process. The remedy for this is to incorporate a “Peak Hold” feature; this causes the gauge to remain at the highest differential pressure experienced during the fueling operation.

We researched the marketplace and found a gauge that solves these two issues. For further information about differential pressure contact Aviation Training Academy at 757-348-5862, or visit aviationta.aero.