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4.2.1: Control Method: Proportional
Proportional Control can operate with either Solid
State Relay (SSR) or Mechanical Relay (MR). The
heat trace will be set to operate at 100% power
(continuously on) at the minimum ambient for APC
or APC-M. It will operate at the maintain temperature
for Line Sensing Control, reducing power delivered
as the RTD temperature rises above the maintain
temperature, at which point the power delivered
will be 0%. This “Proportional Control” is achieved
by cycling the power to the heat trace “on” and
“off” proportionally to the difference between the
“Maintain” and RTD temperatures. (Note: Before
switching off, the minimum power delivery is 8%, so
that the electrical current and earth leakage current
levels can be accurately measured.)
The switching on/off cycle for mechanical relays is
fixed at 20 minutes. The time remaining before the
heater will be re-energized is displayed under “Circuit
Configuration” as the next heat on cycle.
Example of Ambient Proportional Control (APC):
Tp maintain +40°F (+5°C) (i.e. to keep water from
freezing) in a minimum ambient condition of
-40°F (-40°C) with APC control method, follow this
procedure:
The “Maintain” Temperature is set at +40°F (+5°C),
above which the heat trace would be de-energized,
or “Off”.
The “Minimum Ambient” temperature (at which
power is on 100%) is programmed to be -40°F
(-40°C).
(Note: the difference between the Maintain and
the Minimum Ambient temperature defines the
“Control Band” (aka “Control differential” or “dead
band”), across which the time “on” and “off” is
established do deliver the heat proportionally. The
reduced power delivery results in overall energy
consumption when compared to Ambient “On-Off”
Control.)
Example of Ambient Proportional Control with
Mechanical Relays (APC-M):
Maintain +40°F (+5°C) (i.e. to keep water from freezing)
in a minimum ambient condition of -40°F (-40°C)
with APC control method, follow this procedure:
The “Maintain” Temperature is set at +40°F (+5°C), at
which the heat trace would be de-energized, or “Off”.
The “Minimum Ambient” temperature (at which
power is on 100%) would be programmed to be a
value of -40°F (-40°C).
In this example, assume the reading temperature is
at 0°F (-17°C).
% Temperature = (Maintain Ambient – RTD
Temperature) / (Temperature Span) * 100%
% Power = (40 – 0) / (80) * 100% = 50%
Heater On-Off Cycle is fixed at 20min, so 50% power
would represent10 minutes “on” and 10 minutes “off”
(Note that the difference between the Maximum
and the Maintain Ambient temperature defines
the “Control Band”, across which the time “on” and
“off” is established do deliver the heat proportionally.
The reduced power delivery results in overall energy
consumption when compared to Ambient “On-Off”
Control.)
4.2.2: Control Method: On-Off
In this case the heat trace is fully “on” when the
temperature falls below the “Maintain”. It is fully
“off” when the RTD temperature rises above the
�Maintain”. (For line-sensing control this is referred to
as the �Maximum” temperature.)
Example of "On-Off" Ambient Sensing Control:
When configured for Ambient “On-Off” Control,
one or more RTD’s is used to sense ambient
temperature, typically in a shaded area near the
control panel.
In this case, the heat trace operates at 100% power
whenever the RTD temperature drops below the
“Maintain” temperature. (To winterize water lines
Ambient Proportional Control with 100% Power at the
minimum ambient setting of -40°
APCM when ambient temperature calls for 50% of power
for desired Maintain Temperature
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