The heat is distributed in an unstirred bath by convection and conduction, and in a stirred bath by convection, conduction and the stirring action. The heat losses from the surface of the liquid and through the sides can also cause a change in temperature.
Due to the losses and distribution of heat there are small fluctuations in temperature across the bath.
The temperature fluctuation at any one point is called the stability, and the largest temperature difference between any two points in the bath is called the uniformity.
The temperature at any point varies regularly between two limits but occasionally a larger variation is observed. The stability as stated in DIN 58966 is the temperature difference between the maximum and minimum level over 100 cycles after removing the effect of the largest 25% of readings.
The stability is determined by measuring the temperature in the center of the working volume of the bath and is stated as plus or minus one half of the measured value.
The uniformity is determined by measuring the temperature in the center and corners of the bath and is the greatest difference between the mean temperatures at any of these points. It is stated as plus or minus half this value.