8 General Operation
8.1.4Thermal Conductivity
Thermal conductivity measures how easily heat flows through the fluid. Ther- mal conductivity of the fluid affects the control stability, temperature unifor- mity, and probe temperature settling time. Fluids with higher conductivity distribute heat more quickly and evenly improving bath performance.
8.1.5Thermal Expansion
Thermal expansion describes how the volume of the fluid changes with temper- ature. Thermal expansion of the fluid used must be considered since the in- crease in fluid volume as the bath temperature changes may cause overflow. Excessive thermal expansion may also be undesirable in applications where constant liquid level is important. Many fluids including oils have significant thermal expansion.
8.1.6Electrical Resistivity
Electrical resistivity describes how well the fluid insulates against the flow of electric current. In some applications, such as measuring the resistance of bare temperature sensors, it may be important that little or no electrical leakage oc- cur through the fluid. In such conditions choose a fluid with very high resistivity.
8.1.7Fluid Lifetime
Many fluids degrade over time because of evaporation, water absorption, gel- ling, or chemical breakdown. Often the degradation becomes significant near the upper temperature limit of the fluid, substantially reducing the fluid’s lifetime.
8.1.8Safety
When choosing a fluid always consider the safety issues associated. Obviously where there are conditions of extreme hot or cold there can be danger to people and equipment. Fluids may also be hazardous for other reasons. Some fluids may be considered toxic. Contact with eyes, skin, or inhalation of vapors may cause injury. A proper fume hood must be used if hazardous or bothersome va- pors are produced.
Warning: Fluids at high temperatures may pose danger from BURNS, FIRE, and TOXIC FUMES. Use appropriate caution and safety equip- ment.
Fluids may be flammable and require special fire safety equipment and proce- dures. An important characteristic of the fluid to consider is the flash point. The
28
