An extensometer, sometimes also called a strain gauge, is a device which measures variations in an object's length. These variations can be so minute that they are not identifiable with the naked eye, or significant enough that someone can clearly visualize them. Extensometry, as the practice of measuring such variations is known, is used extensively in materials testing. Tensile testing usually requires an extensometer, and these devices are also used in scientific research, to make and record important observations.
The original extensometer, developed in the 1800s, was a contact extensometer, requiring contact with the material being measured to function. As the material's length fluctuated, it would cause a physical reading to appear on the extensometer. This could be used in stress testing, to find out how much objects and materials deform under strain. However, the need to be in physical contact with the object being measured could also be a problem, as it might skew results or be impractical for certain types of testing.
As a result, noncontact extensometers were developed. These use optical extensomtry, typically with a laser, to measure changes from a distance. Optical extensometry can be very precise and detailed, with the ability to detect minute fluctuations and provide a reading for the person administering a test or experiment. Cost of such systems varies, depending on the objects they are designed to measure and their precision.
Like other devices used to make measurements, an extensometer must be calibrated. Calibration is typically done at the factory where the device is produced as part of the quality control process, ensuring that the device will work ad advertised when it is delivered. People must also periodically recalibrate to confirm that the device is working properly. This can be done in a facility with a calibration kit, or by sending the extensometer out for examination in a lab which specializes in calibration activities.
When not in use, an extensometer needs to be protected to keep it working properly. Many come with protective coverings and cases which can be used to shield the device when it is not in active use. The devices also need to be stored in a way which protects them from shock, as shock and jarring movements can throw the device off, making its readings inaccurate and causing problems for people working with the extensometer. Maintenance and protection of equipment such as extensometers is a collective effort which involves equipment technicians as well as users.