Oil Compressibility

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The increasing stringency of hydraulic system performance requirements means that more attention must be paid to all fluid properties. Unfortunately, there are some important properties that are often overlooked by designers. Compressibility, or bulk modulus, is one of these. Considering that bulk modulus is a property which can affect overall system performance, it is not a factor that should be ignored.

What Bulk Modulus Means

Bulk modulus refers to the fact that most substances experience a reduction in volume when a uniform pressure is applied externally. The compressibility of any fluid can be plotted on a graph, with a flat slope indicating low bulk modulus or a moderately compressible fluid, and a steep slope indicative of fluid having only slight compressibility or a high bulk modulus.

The Importance Of Bulk Modulus

The bulk modulus of a fluid can seriously affect system performance where stability, response time, power level and position are concerned. The entrained air content, as well as fluid temperature, will strongly influence the bulk modulus of any fluid.

How Bulk Modulus Is Calculated

When looking at a graph of fluid compressibility, one has two options when trying to define bulk modulus. The slope of a line between any two points of a graph’s curve can be calculated. This is also known as the average or ‘secant’ bulk modulus because it appears as a curve of an arc when drawn between two points.

The second option is to find the true or tangent bulk modulus. This can be accomplished by placing a straight edge tangent to the curve at a specific point.

Air Air will affect the bulk modulus of hydraulic fluid. Many of the published bulk modulus values that are available were derived from laboratory methods which involved the de-gassing of fluid prior to trapping and compression. Any air which becomes trapped in hydraulic fluid will therefore reduce its bulk modulus.


The temperature of a fluid will also affect its bulk modulus. This property is very important, because the temperature will rise as the fluid is compressed. When temperature rises, the fluid will expand, causing additional pressure to be created. Temperature can be factored into bulk modulus in two ways:

1) By compressing fluid rapidly and measuring pressure, and;

2) By compressing fluid very slowly to allow for the dissipation of heat.

Pump And Actuator Output

The bulk modulus of fluid will determine the positional relationship of master and slave actuators as well as pump output. For pumps, a loss of horsepower will indicate volume loss in the output. A loss of stroke of the slave will indicate volume loss in master-slave actuators. Overall, less volume reduction will be seen in fluids with higher bulk modulus values.

Ability To Move A Load

Before a system cylinder’s piston can move any load, the actuator fluid must undergo compression. However, this process means system downtime because of useful work is not performed during the compression. Work cannot resume until an amount of fluid equal to the compressed volume has been added.

Ability To Stop Load Already In Motion

When a cylinder moves a load with a constant flow going to it, the momentum of that load will need to be absorbed by the fluid and the system when the sudden closure of valve flow occurs. A rise in fluid pressure will result when the valve is located downstream. The bulk modulus of the fluid will determine the peak fluid pressure reached when this occurs.

Considering all the ways in which bulk modulus can affect hydraulic systems, as well as the increasing demand for better response times and horsepower, paying attention to this property is more important than ever before.

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