For mixing the bath fluid a stirring propeller system is used to eliminate both horizontal and vertical gradients. The baths consist of two stain-less steel tanks. The heater and propeller are fitted at the outside stainless steel tank. The temperature controller is connected to a platinum resistance thermometer as the control sensor. The inside stainless steel tank has a depth of 60 cm and is used for thermometer calibrations. The horizontal and vertical temperature stability of the bath is±0.015 C.