Yield strength, like tensile strength, is measured in pascals (Pa) or megapascals (MPa). Mild steel has a yield strength of about 250 MPa. Hot-rolled carbon steel has a yield strength of about 350 MPa. There are two main types of pressure testing equipment: hydraulic and mechanical. Hydraulic testing machines use oil or water as the operating fluid that is applied to the specimen using a piston system. Mechanical testing machines use clamps or jacks to apply force to the sample. Which type of machine is best for which application depends on factors such as cost, accuracy requirements, test speed, etc. The amount of force applied to the sample during the pressure test is called the load. The maximum load that can be applied without causing permanent deformation of the material is called the yield point. Below this load, the sample will return to its original shape when released; above this load, it will remain deformed. Pressure testing is used to determine if a product will fail a quality control test called "yields" at an expected stress level. For example, a manufacturer might want to know how much force it takes to break welds on pipe being manufactured.