Invar tapes are available in lengths of 20, 30 and 100 metres. Invar tapes are used when high degree of accuracy and precision in linear measurements is required such as measurement of base lines. Invar tapes are made of alloys of nickel and steel and have very low coefficient of thermal expansion. Invar tapes are more expensive as compared to other tapes. Main disadvantage of this tape is that it's length and coefficient of thermal expansion is not constant. It keeps changing with time. Therefore it is suggested to determine the length and coefficient of thermal expansion time to time.