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APPENDIX A: PROPORTIONAL AMBIENT SENSING CONTROL (PASC) Section 6
PASC takes advantage of the fact that the heat loss from a pipe is proportional to the
temperature difference between the pipe and the ambient air. This is true regardless of
heating cable, insulation type, or pipe size. Once the heat tracing and insulation on a
pipe has been designed to balance heat input with heat loss and maintain a particular
temperature, the main variable in controlling the pipe temperature becomes the ambient
air temperature.
The C910-485 has a control algorithm that uses the measured ambient temperature,
desired maintain temperature, minimum ambient temperature assumption used during
design, and size of the smallest pipe diameter to calculate how long the heating cable
should be on or off to maintain a near-constant pipe temperature. The power to the heat
tracing is proportioned based upon on the ambient temperature. If the ambient
temperature is at or below the “minimum design ambient plus 3°F” the heating cable will
be on 100%. If the measured ambient is at or above the “maintain temperature –3°F” the
heating cable will be on 0%. For any measured ambient between “minimum design
ambient” and “maintain temperature,” the heating cable will be on a percentage of the
time equal to (maintain temperature – measured ambient) / (maintain temperature –
minimum design temperature).
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