A dilatometer is a scientific device used for measuring changes in volume that occur in fluids or solids. The types of volume changes that are usually measured using dilatometry include volume changes due to temperature fluctuation, chemical reactions, absorption of fluids, or physical stress such as pressure on a solid substance. Many dilatometers are highly complex scientific instruments, but there are also some simple types of dilatometers. One example of a simple device is a mercury thermometer. When mercury gets warmer it expands, and it is this increase in volume that is measured by the scale on a mercury thermometer.
Some different types of dilatometers include the flat plate dilatometer, the connecting rod dilatometer, and the more advanced modern laser and optical dilatometers. Flat plate dilatometers, also sometimes called capacity dilatometers, are characterized by having two plates, one of which is stationary and another that can move. This type of tool is often used for measuring changes in volume of solid materials. A long thin piece of the material being measured is inserted into the device. When it expands, for example, if the temperature is raised, then the expansion pushes the movable plate. The movable plate may be connected to a computer that can measure even tiny movements.
Connecting rod dilatometers, also sometimes called push-rod dilatometers, are typically used to measure the expansion of a material when it is heated. In this type of device, the change in volume of the test specimen is measured relative to the rod inside the instrument. This rod is a control specimen with a known expansion coefficient, and comparison of the test specimen with the rod allows the user to calculate the expansion of the specimen undergoing the dilatometer test.
Dilatometer testing with modern laser or optical devices can be extremely sensitive, and measure tiny changes in volume. Some laser dilatometers, for example, work by passing a laser beam repeatedly over the test item, and a laser receptor measures minute changes in the diameter of the resulting shadow. An optical dilatometer will typically utilize a sensitive digital camera to record tiny changes in the size of an object or substance. This has the added benefit of being a non-contact system that can be used for items or substances that need to be measured without being physically manipulated.