It is an application of dye penetrant inspection. Examples of this technology include eddy current, ultrasonic, dye penetrant, and X-ray inspections. There are many types of dye used in penetrant inspections. Common inspection methods for steel castings are "magnetic particle testing" and "liquid penetrant testing". This process also includes the use of a penetrant chemical known as a hardener. The penetrant is usually a brilliant coloured mobile fluid with high wetting capability. Permeability is affected by the penetrant size. This is because of the nature of the fluorescent penetrant that is applied. The method is called "krypton gas penetrant imaging". It can be congenital or caused by a penetrant injury. The process of waiting for the penetrant to seep into flaws is called Dwell Time. Having removed excess penetrant, a contrasting developer may be applied to the surface. Where M is the molecular weight of the penetrant gas (g/mol). Various chemicals can be used for such a process and vary by specific penetrant types. The removal method is controlled by the type of penetrant used.