The measured value assigned to a standard or measurement instrument has the essential qualification that the assigned value was valid at the specific time and under the specific conditions of the calibration. No assurance is made regarding the future value or the value under different conditions. If the device is physically stable with time, and if a sufficient history is compiled under environmental conditions that are compatible with the previous calibration, the confidence level of the assigned value increases. If the specific conditions of measurements are not repeated when the device is again calibrated, any variables introduced can create substantial uncertainty. With this fact in mind, it is obvious that there is a need for knowing and maintaining the environmental factors associated with the various types of measurement standards and instruments. Aside from the necessity of securing repeatable measurements, environmental controls help to reduce the number of tedious corrections necessary in making measurements that are affected by adverse environments.