GE CTR 80 User Manual

8 General Operation8.1 Bath FluidMany fluids work with the CTR –80 bath. Choosing a fluid requires consider-ation of many important characteristics of the fluid. Among these are tempera-ture range, viscosity, specific heat, thermal conductivity, thermal expansion,electrical resistivity, fluid lifetime, safety, and cost. If the viscosity becomes toogreat, the stirrer may not function.8.1.1 Temperature RangeOne of the most important characteristics to consider is the temperature rangeof the fluid. Few fluids work well throughout the complete temperature rangeof the bath. The temperature at which the bath is operated must always bewithin the safe and useful temperature range of the fluid. Set the cutout to meetthe temperature limits of the selected fluid. The lower temperature range of thefluid is determined by the freeze point of the fluid or the temperature at whichthe viscosity becomes too great. The upper temperature is usually limited byvaporization, flammability, or chemical breakdown of the fluid. Vaporization ofthe fluid at higher temperatures may affect temperature stability because ofcool condensed fluid dripping into the bath from the lid.8.1.2 ViscosityViscosity is a measure of the thickness of a fluid, how easily it can be pouredand mixed. Viscosity affects the temperature stability of the bath. With low vis-cosity, fluid mixing is better which creates a more uniform temperaturethroughout the bath. This improves the bath response time which allows it tomaintain a more constant temperature. For good control the viscosity should beless than ten centistokes. Twenty centistokes is about the upper limit of allow-able viscosity. Viscosities greater than this cause very poor control stability andmay also overheat or damage the stirring motor. With oils viscosity may varygreatly with temperature.When using fluids with higher viscosities the controller proportional band mayneed to be increased to compensate for the reduced response time (see Sec-tion9.6). Otherwise the temperature may begin to oscillate.8.1.3 Specific HeatSpecific heat is the measure of the heat storage ability of the fluid. Specificheat, to a small degree, affects the control stability. It also affects the heatingand cooling rates. Generally, a lower specific heat means quicker heating andcooling. The proportional band may require some adjustment depending on thespecific heat of the fluid.278 General Operation