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Types of Sensing Control
Ambient Sensing Control
Strict ambient-sensing control utilizes a thermostat or a simple electronic controller which senses the ambient tem-
perature via an RTD, Thermocouple or Bulb & Capillary sensor. This is the simplest type of control as the heating
circuit is energized only when the ambient temperature drops below the setpoint of the controlling device. This is
also known as On/Off control. A relatively large temperature variation around the setpoint is anticipated. Mechani-
cal thermostats are often employed due to their low cost and acceptable accuracy. Multiple heater circuits are
controlled by a singular ambient sensed temperature. This is the least efficient type of control and it is typically
limited to freeze-protection applications.
PASC - Proportional Ambient Sensing Control
Proportional ambient sensing control (PASC) utilizes an electronic controller which continuously compares ambient
temperature and the rate of the change of the ambient temperature to the desired temperature setpoint and regulates
the heater output power accordingly. The result is a smaller temperature variation around the desired setpoint and
a much smoother response to changing weather conditions. Like ambient sensing control, multiple heater circuits
are controlled by a singular ambient sensed temperature. PASC control is therefore significantly more energy ef-
ficient than ambient sensing control. PASC control can be employed in a variety of heat trace temperature control
applications but is typically used for freeze protection.
Line Sensing Control
Line sensing control is where each heated section of pipe (or heated surface area) is controlled independently of
other sections of pipe and therefore must have its own controller. Multiple sections of pipe may also be independently
controlled by an electronic multiple circuit control system. The control method may be either proportional or On/
Off control. Each section may have different setpoints and different allowances around the setpoint temperature.
Line sensing control is typically found in process temperature maintenance applications. These applications are
typically more critical and thus demand tighter temperature control. Electronic control is highly recommended over
thermostat control. Like PASC, the pipe surface temperature and the rate at which it is changing is continuously
compared to the setpoint temperature. The controller responds to these variations and regulates the heater output
power accordingly. Mission critical applications should only be managed by controllers which monitor all parameters
and present alarms both locally and remotely via communications or supervisory control means.