The trade-offs necessary to choose a fluid involve a consideration of application requirements such as health, safety, and environmental effects, and fluid properties such as viscosity, stability, compressibility, gas solubility, and lubricity.

Viscosity measures how a fluid resists flow. It is the single most important property of a hydraulic fluid. A hydraulic fluid that is too viscous usually causes high-pressure drop, sluggish operation, low-mechanical efficiency, and high-power consumption. High-pressure, high-precision systems are particularly sensitive to viscosity at low temperatures. They can be penalized harshly by pump cavitation and sluggish response of critical actuators.

Low-viscosity fluids permit efficient low-drag operation, but tend to increase wear, reduce volumetric efficiency, and promote leakage. In theory, this leakage can be predicted on the basis of fluid viscosity and streamline flow through a known gap. But the clearance of some pump leakage paths depends on operating pressure and temperature, so leakage may show considerable deviation from the theoretical viscosity-flow curves.

Viscosity index measures how viscosity changes with temperature. Ideally, the fluid should have the same viscosity at very low temperatures as at high temperatures. In reality, this goal is unattainable. Fluids that come close to achieving the goal have high viscosity indexes; low indexes, on the other hand, indicate wide fluctuations of viscosity with temperature. Typical viscosity indexes for petroleum oils range from 90 to 105, and those for polyglycols from 160 to 200.

A high viscosity index is most important in applications subjected to a wide temperature range: mobile hydraulic systems used outdoors, industrial systems that are stopped and started during the winter in an unheated plant, and the like. These systems require a high viscosity-index fluid. An industrial system in a heated plant, on the other hand, could get by with a low viscosity-index fluid.

Some fluids have fairly high viscosity indexes to begin with. Others commonly have the viscosity index bolstered through the use of special additives. The additives are expensive, and tend to lose effectiveness under high shear rates during long service. But if monitored carefully, oils with viscosity-index additives perform well in industrial service.

Pour point is the lowest temperature at which oil flows when chilled under specified test conditions. It is important if the system is regularly exposed to low ambient temperatures, but relatively insignificant if the system is to be used inside a heated plant. Pour point of the oil should be about 20°F below the lowest expected temperature.

Compressibility is the degree to which a fluid undergoes a reduction in volume under pressure. As a rule of thumb, compressibility is about 0.5% for each 1,000-psi pressure increase up to 4,000 psi.

Compressibility, or bulk modulus, has its greatest effect on performance in servo applications. It determines system static rigidity and strongly influences system gain, or amplification. Because compressibility increases with pressure and temperature, it is important to systems with high-pressure pumps and motors.

In positive-displacement pumps, the effect of bulk modulus shows up as a loss in volume. This loss represents a power loss, because few actuators recover the compressive energy in the fluid.

Stability is the most important property for longevity of service. Ideally, the properties of hydraulic fluids should not change with use. But certain factors can adversely affect fluid performance.

Mechanical stress from flow and cavitation can shear polymer chains, breaking down viscosity improvers and reducing viscosity. Oxidation and hydrolysis tend to cause chemical changes and the formation of volatile components, insoluble materials, and corrosive products.

Heat can also destroy a fluid, and the higher the temperature, the shorter the life of the fluid. A hydraulic system should operate below 250°F, ideally between 100 and 150°F. An old rule of thumb states that for every 10°C or 18°F of temperature rise, the rate of oxidation doubles. Thus, a fluid used at 110°C has about half the life as at 100°C.

When a system operates at high temperatures, additives must be selected carefully. Some additives, while doing an excellent job at normal operating temperatures, have limited thermal stability. Chemical breakdown occurs, and the additive, instead of being a benefit, begins to harm the system.

Contaminant particle sizes that can be tolerated in a hydraulic system are the same whether a fluid is synthetic or petroleum based, emphasizing the need and importance of good filtration and diligent maintenance practices. Insoluble contaminants can be hard particles or sludges and gums. Hard particles can accelerate wear of closely mated moving parts such as pumps, motors, and valves. Sludges and gums can slow valve action, particularly if these agents harden into varnish films. Furthermore, silting caused by all forms of solid materials will slow servovalves. Excessive production of these solids can clog even large filters.

Corrosive agents usually form because of thermal or oxidative decomposition, or in the course of hydrolysis. These corrosives are usually acidic, but not all acidic materials are corrosive. In most instances, corrosion increases leakage by opening up tolerances of close-fitting parts. But where pitting occurs, there can be substantial localized loss of strength.

Lubricating ability is an important quality factor in hydraulic fluids, since the fluid must lubricate moving parts of the system to minimize wear. Most petroleum fluids satisfy the lubrication requirement in pumps, motors, and valves. However, certain types of hydraulic pumps and motors place severe load-carrying requirements on the oil; fluids for these applications should be fortified by antiwear additives. Where boundary lubrication conditions prevail, glycol-based fluids are satisfactory. They are stable in hydraulic service and their viscosities are unaffected by high rates of shear. They also resist the formation of varnish and sludge, and resist viscosity increases caused by soluble oxidation products.

Volatility is rarely the cause of pump cavitation. The reason: Vapor pressures of most hydraulic fluids are too low to cause boiling at the pump inlet. In most systems, the cause of cavitation is dissolved air.

The amount of gas dissolved in hydraulic fluid depends on the partial pressure of the gas in contact with the fluid. Although gas solubility increases slightly with temperature, pressure has a far greater effect.

Volumes shown in the accompanying graph were calculated under the assumption that fluids were saturated with nitrogen at 30 psia, that gas phase follows the gas laws, that gas evolving from solution is saturated with vapor of the liquid, and that equilibrium is established. For pumps with short inlet lines and laminar flow within the line, the actual volume evolved may be less, because the fluid does not reach equilibrium. However, turbulence agitates the fluid and speeds gas evolution.

To prevent cavitation, some reservoirs -- particularly in aircraft systems -- are pressurized. Systems pressurized by pumping air into the reservoir require higher reservoir and inlet pressure than those pressurized mechanically. The reason is that air pressurization automatically dissolves more gas in the fluid.

Aeration and foaming resistance is another important indicator of fluid quality. Fluid in a hydraulic system always contains air as entrained bubbles, as well as in solution. The entrained air tends to increase compressibility of the fluid, making the system elastic, noisy, and erratic. Compression of this air generates heat and can increase oxidation.

Volumetric efficiency of the pump is reduced because air bubbles in the oil on the inlet side expand as the oil enters the pump. When the bubbles collapse on the discharge side, they can produce damage similar to cavitation erosion. Many high-quality fluids contain antifoaming additives to release air readily.

Corrosion prevention capabilities of an oil are important because moisture is always present in hydraulic systems. Since most components have ferrous metal surfaces, subject to rusting, corrosion prevention is essential. It is normally provided through an additive called a rust inhibitor in the oil. The inhibitor plates out on the metal surfaces to form a protective film.

Materials compatibility enters the picture as a final arbiter of which seals and components can be used with a certain fluid. For example, natural rubber is not oil resistant and should not be used in hydraulic systems using petroleum oils.

Synthetic rubbers vary widely in their behavior when exposed to different fluids. In contact with a given fluid, some are unaffected, but others swell, shrink, or otherwise deteriorate.

Most seals in contact with oil are made from nitrile rubbers. Other materials suitable for oil systems include Neoprene, silicones, and fluorocarbon rubbers. Most tubing and fitting manufacturers present extensive tables of materials compatible with their product specifications.

Some antirust additives used in hydraulic systems may attack zinc. Therefore, galvanized and other zinc-coated surfaces should be avoided. Zinc is also objectionable in a hydraulic system because products of oil oxidation can react with it to form metallic soaps.

Other materials susceptible to corrosion are magnesium-based alloys and lead. Magnesium alloys generally suffer heavy corrosion when water is present. Lead is attacked by products of oxidation.

Copper should also be avoided in hydraulic systems, but for a different reason. It is an effective catalyst in promoting oxidation of all types of oils. So heat exchangers should be of steel, rather than copper or brass.