Traditionally, filters in hydraulic systems or in lubrication applications are equipped with a device to indicate that the filter element inside needs to be changed. There is a variation of methods ranging from a simple pressure gauge on tank top return line filters to clogging indicators with analogue signal output; some provide both, a visual plus an electrical signal at the same time. Physically any signal is based on the fact that the pressure loss across the filter element is increasing while the element is catching particulates from the oil stream. This increase is caused by the reduction of the elements free surface caused by the accumulation of dirt particulates. In tank top return line filters the pressure loss is increasing the backpressure upstream the element, while in in-line filters the differential pressure from the up-stream to the down- stream side is increasing.

 

This principle is disturbed by the fact that oil is changing its viscosity with the temperature. Cold oil passing the filter element is also creating a higher pressure loss across the element, but the clogging indicator cannot distinguish if it is cold oil or the exhausted dirt holding capacity responsible for the increase! The effect is that numbers of filter elements are changed with no need along with all the losses due to down time for the exchange. The Bühler VSA series of capacity monitors eliminate the influence of cold oil to the readings and provide always two signal points, one alert alarm if about 75% of the dirt holding capacity are absorbed and one at 100% as the final alarm. All signals are indicated visual and electrical as self locking outputs.

The VSA series are an important tool to reduce filter changes and operational cost.

 

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