Resistance temperature detector (RTD) probes operate on the principle that an electrical resistance of a metal increases as its temperature increases. The RTD sensing element consists of pure metal (frequently platinum) and shows a small positive, linear change in resistance per degree of temperature change. RTDs are more accurate and stable than thermocouples, but cannot be used to measure extremely high temperatures. RTDs offer the best stability and repeatability, but are less sensitive than thermistors. They usually can be purchased to higher calibrated accuracy than thermistors for equivalent cost.