Capstats are temperature adjustable ON-OFF thermostats comprising a liquid filled sensing bulb connected to an electrical contactor switch, via a capillary tube. Expansion of the liquid on rise in temperature causes the contactor switch to open, removing power from the heating circuit. As a liquid cools, the switch closes, energising the heater(s). Capstats may control surface temperatures such as pipes or vessels. Alternatively, the sensing bulb may be positioned to sense the air temperature. Capstats may be used to provide process temperature control in safe areas, by switching the heating circuits at the desired temperature. 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. The high limit cut-out switch will operate if the process control switch fails, or if an unsafe process temperature occurs.