Capstats are temperature adjustable ON-OFF thermostats comprising a liquid filled sensing bulb connected to an electrical switch via a capillary tube. Expansion of the liquid on rise in temperature causes the switch to open, and on cooling, it closes. Capstats may control surface temperatures such as pipes or vessels, or the sensing bulb may be positioned to sense the air temperature.They may be used to provide process temperature control in safe or Zone 1 or 2 hazardous areas by switching the heating circuits at the desired temperature and are available to suit Group LLA, LLB or LLC gases.The addition of a high limit cut-out switch (as provided with the Capstat-Dual version) is specified when the heat tracing scheme will not stabilise at a safe temperature without process control. It will operate if the process control switch fails or if an unsafe process temperature occurs.