In order to accurately measure kinematic viscosity in glass capillary viscometers, the viscometers must be suspended in a constant temperature bath. The ideal bath fluid would possess low viscosity, high heat capacity, and low vapor pressure over a wide range of temperatures. In addition, the liquid should have a very high flash point and be relatively low in cost. Because the viscometer must be viewed through the bath liquid, it is important for the liquid to clear and without color. However, nowadays, no single fluid meets all these requirements. Therefore, when selecting a fluid, keep the following guidelines in mind.

Viscosity
Should be very low so that moderate stirring can effectively eliminate temperature gradients in the bath.

Heat Capacity
Temperature changes in the bath are less rapid when bath liquids with a high heat capacity (thermal conductivity) are used. With the exception of water, most choices for bath fluids will have about the same heat capacity.

Volatility
A liquid which is relatively volatile will require frequent replenishment. Furthermore, rapid evaporation at the bath surface produces a cooling effect, making control more difficult.