An extensometer, sometimes also called an extensor, is a device that measures variations in the length of an object. These variations may be so small that they are not identifiable to the naked eye, or large enough for anyone to clearly see them. Extensometry, as the practice of measuring such variations, is widely used in material testing. Stretch 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 19th century, was a contact extensometer, which requires contact with the material being measured to function. As the length of the material fluctuated, it would cause a physical reading to appear on the extensometer. This can be used in stress testing to determine how much objects and materials are deformed under pressure. But the need to be in physical contact with the object to be measured can also be a problem, as it can skew results or be impractical for certain types of testing.
As a result, Loose Extensometers were developed. These use optical extensions, typically with a laser, to measure changes from a distance. Optical extensometry can be very accurate and detailed, with the ability to detect small oscillations and provide a readout for the person administering a test or experiment. The cost of such systems varies depending on the objects they are made 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 unit is manufactured as part of the quality control, so that the unit will function as advertised when it is delivered. People also need to periodically recalibrate to confirm that the device is working properly. This can be done in a facility with a calibration kit, or by sending an extensometer to the exam in a laboratory that specializes in calibration activities.
When not in use, an extensometer needs to be protected to keep it working. Many come with guards and cases that can be used to shield the device when not in active use. The devices must also be stored in a way that protects them from shock, which can shock and shake the device, making their measurements inaccurate and causing problems for people working with extensometers. Maintenance and protection of equipment such as Extensometers is a collective effort involving equipment technicians as well as users.
- Extensometers are widely used in tensile testing.