Industrial PRTs can have uncertainties of around 0.01 °C at 0 °C, with relatively low cost and are more rugged than Standard Platinum Resistance Thermometers. When lower uncertainties are needed Standard Platinum Resistance Thermometers can be used. Primary SPRTs are constructed in such a way that the platinum wire coil is free to expand and contract without strain. The flipside of the performance benefit is that the SPRTs are very fragile and require very careful handling. Primary SPRTs need the most care, whilst secondary or working SPRTs still need careful handling they are less prone to mechanical shock and can still be calibrated to low uncertainties, typically 0.001 °C at 0 °C and 0.006 °C at 660 °C.