sentences:  The use of this technology survived until the early 1990s at which point the technology was all but replaced by power MOSFET and IGBT solid state equipment.However, there are still many valve oscillators still in existence, and at extreme frequencies of 5 MHz and above they are often the only viable approach and are still produced.Mains frequency induction heaters are still widely used throughout manufacturing industry due to their relatively low cost and thermal efficiency compared to radiant heating where piece parts or steel containers need to be heated as part of a batch process line.