The volume of buret is not completely consistent with its marked volume, so it must be calibrated in analytical work requiring high accuracy. Because glass has the characteristics of thermal expansion and cold contraction, the volume of buret is different at different temperatures. During calibration, a common temperature value must be specified, which is the standard temperature. The international standard temperature of glass containers is 20 ℃, that is, the volume of glass containers is calibrated to the actual volume at 20 ℃ during calibration.