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The viscosity of a fluid is a measure of its resistance to flow. There are several different types of viscosity that are referred to in the literature. One is the dynamic viscosity, η, which is the ratio of the force needed to maintain a flow of unit velocity. The basic cgs unit for this quantity is the poise (P) and it represents one dyne-sec per cm2. The SI unit for the dynamic viscosity is the Pascal-second (Pa-s). The conversion factor between these two units is that 1 P = 10 Pa-s. Another type of viscosity is termed the kinematic viscosity, v, and its basic unit is the Stokes (St). Its metric units are one Stokes is equal to 10-4 square meters per second. The relationship between the dynamic and kinematic viscosity is the density, ρ, of the fluid at the same temperature, the equation is η = ρv. Dynamic and kinematic viscosities are usually reported at 25C.

Another type of viscosity measurement is the Saybolt Universal second (SUS) and it refers to the time of flow to empty a cup of the fluid though an orifice at a given temperature. The dimensions of the cup and the size and shape of the orifice are precisely defined for the procedure. The Saybolt Universal second value can be calculated from the kinematic viscosity using the tables in the ASTM D 2161 method. Saybolt Universal seconds are usually reported at 100F.

The viscosity of a fluid is a function of the temperature of the fluid and usually decreases with increasing temperature. There also is a strong dependence on the nature and molecular structure of the fluid. The approximate temperature relationship is given by the following formula:

Log η = (A/T)+B; where T is in degrees Kelvin and A and B are constants for a particular fluid


Kinematic viscosities are usually determined using glass capillary viscometers in which a fixed volume of the fluid is allowed to flow under gravity through a calibrated capillary tube. The time for the fluid to flow between two calibration marks is measured while the entire viscometer is kept at a constant temperature. The entire details of the procedure for determining the kinematic viscosity and the subsequent conversion to dynamic viscosity or Saybolt Universal seconds is covered in several ASTM methods. ASTM method D 445 covers the determination of the kinematic viscosity and its conversion to the dynamic viscosity. ASTM method D 446 covers the standard specifications and operating instructions for glass capillary kinematic viscometers. ASTM method D 2161 covers the conversion of kinematic viscosity to Saybolt Universal seconds. The various types of viscometers are calibrated against a standard fluid, usually water, and calibration constants are determined at various temperatures.


One of the primary purposes of an insulating fluid in a transformer is to act as a coolant. The fluid has to absorb the heat being generated by the unit and then transport it to a radiator where the heat can be given up to another media such as the air or to a water cooled device. The transport of the fluid can be either by convection or forced circulation. The viscosity of the fluid will have a direct bearing on the efficiency of this process. The type of fluid flow that exists within the unit will also be dependent on the viscosity. All other factors being the same, at high values of viscosity the fluid flow will be laminar (low Reynold's number) while at lower values of viscosity the flow will be turbulent (high Reynold's number). Heat transfer is more efficient with turbulent flow than with laminar flow.

The viscosity is strongly dependent on molecular composition and increases with molecular weight, size, and flexibility. The change in viscosity with temperature is dependent on the type of insulating fluid. Among the hydrocarbon fluids, the aromatics have the highest rate of change and the paraffinics the least with the naphthenics being intermediate. The temperature dependence of Askarels is related to their chlorine content. The higher the chlorine content, the steeper the temperature dependence. The silicones have the least temperature dependence of all the insulating fluids.

The viscosity of an insulating fluid is usually determined as part of the acceptance standard and as such it is not usually monitored during the use of a unit unless problems arise that might be replated either directly of indirectly to the viscosity or its rate of change with temperature. Some typical values are given below for several types of insulating fluids:

Type of Fluid Viscosity
Mineral Oil 12.0 cSt Max. @ 40C
Silicones 47.5 to 52.5 cSt @ 25C
Askarels 31 to 92 SUS @ 100F depending on the chlorine content

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