Refrigerated and heated laboratory circulators and baths are designed for controlling temperature.  They are used for warming or cooling liquid samples to sustain or impede reactions.  In addition to heating and cooling samples, some varieties are available with built-in shakers to agitate samples. Proper circulator and bath design includes a number of physical and mechanical features, and the electronic temperature controls. Conventional circulator and bath designs use separate heating and cooling devices immersed in the bath liquid, although other systemic styles are available. To maintain an even temperature, the bath fluid must be stirred vigorously.   However this must be accomplished without the formation of air bubbles or vortices in the fluid, which could interrupt the temperature transmission, or negatively impact the samples. The temperature control system, including the temperature sensor or thermistor, must be responsive enough to maintain uniform temperatures for long periods.