Bath liquids. Use of the appropriate liquids for heat transfer listed here offers many advantages: Safe to use; non toxic; and generally odourless formulations. Resistance to ageing and viscosity changes with low susceptibility to corrosion. Synthetic heat transfer fluids (Synth): Usually formulated from a hydrocarbon base with low viscosity within the recommended working temperature range. Viscosity: For optimal temperature stability the bath fluid must be of low viscosity. Fire point: Fluids can spontaneously ignite above a certain temperature. EN 61010 stipulates that the maximum working temperature of the liquid should be 25 °C below its' fire point. Heating up range: Prolonged use is not permitted as the self-heating effect within these liquids may cause the overtemperature protection system of the pump unit to operate and shut down the pump. Operating temperature range: Thermostats may only be operated for an extended time in this range under certain conditions. Viscosity must not exceed 30 mPas. Otherwise pumps will not achieve their stated performance figures. Working temperature range: The long-term use range for peak thermostatic bath performance. Maximum viscosity level is approx. 5 mPas.