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The Importance of Differential Pressure

The difference between the inlet and outlet pressure of a filter housing is called differential pressure and is represented by the symbol
ΔP or Delta P. When fuel encounters resistance the pressure will drop – the measurement of the drop is ΔP. The high pressure reading is on the inlet side and the low pressure reading the outlet side. It is essential to measure ΔP as it is the primary indicator of the filter’s condition.

There are a few major factors that affect ΔP:

  • A filter will increase ΔP. However, a clean filter regardless of type should read no more than 2 or 3 pounds of ΔP when initially installed.

  • Water contamination will slow the flow and increase ΔP.

  • Particulate contamination increases ΔP.

  • Microbial contamination increases ΔP

  • A bypassed filter (i.e. hole in filter or filter failure) will cause a drop in ΔP. It should be remembered filters NEVER get better over time. The ΔP will always rise with use.

  • Higher flows can create higher pressure drops.

Differential pressure should rise slowly and steadily over time until it reaches its limit. The standard ΔP limit of a filter is 25 pounds. As the ΔP moves closer to the limit, it can quickly rise. This is due to contaminants clogging the filter pores and creating pressure. Sudden premature rises in ΔP should be investigated. An inline contamination monitor can help to identify issues and problems associated with ΔP.