Apart from sluggish performance to avoid oscillations,
another problem with proportional-only control is that power application is
always in direct proportion to the error. In the example above we assumed that
the set temperature could be maintained with 50% power. What happens if the
furnace is required in a different application where a higher set temperature
will require 80% power to maintain it? If the gain was finally set to a 50° PB,
then 80% power will not be applied unless the furnace is 15° below set point,
so for this other application the operators will have to remember always to set
the set point temperature 15° higher than actually needed. This 15° figure is
not completely constant either: it will depend on the surrounding ambient
temperature, as well as other factors that affect heat loss from or absorption
within the furnace.
