Purpose of Indicators
Clogging indicators are warning devices that signal visually and/or electrically that the filter element is filled with contaminants and should be changed or cleaned. These devices activate (trip) when the flow of fluid causes a pressure drop across the filter that exceeds the indicator settings. In filters that incorporate bypass valves, contaminated fluid will bypass the element if the operator does not respond to the indicator warning signal within a reasonable time. In non-bypass filters, if the indicator warning is not heeded, the pressure across the filter will build up to the point where system performance is degraded or the element fails.

The indicator is set to trip well before the element becomes fully clogged, thereby giving the operator sufficient time to take corrective action. The indicator warning may be a visual signal at the filter site (pop-up button, light, etc.); or, some form of signal at a remote location (trouble light, sound alarm, etc.). In some critical applications, where contamination is intolerable, the signal from the indicator may be used to shut down the system so as to compel immediate servicing.

Some users install filters without indicators, preferring instead to change and/or clean elements according to a fixed schedule - or based on number of hours of operation. There is some risk in utilizing this approach. It may be difficult to establish a reliable schedule for installing new elements because the rate of dirt ingression is not known, and, in fact, may vary from time-to-time and from machine-to-machine. Use of a clogging indicator has two main benefits: first, it eliminates the need to guess when the element will clog; second, it avoids the unnecessary cost of replacing elements too soon.