Nextron MASTERTRACE MS-2101, Руководство оператора

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1.7
MS-2101 Chapter 1 Product Overview
Figure 1.6 On/Off Control with Deadband
Heat Trace Curve
Monitoring low and high current alarms on self-regulating
heat tracing cable is difficult. Choosing a high current
alarm setting based on cable characteristics near the
heater setpoint temperature will produce nuisance alarms
during startup. Choosing a low current alarm setting other
than below the current draw of the cable at heater setpoint
temperature produces nuisance alarms. The heat trace
curve function allows you to program the cable character-
istics so that the controller can offset the current alarm
settings.
The heat trace curve is described by the following linear
equation:
W = aT + b
where, W is the Watt/ft at temperature T (in degree F), a
and b are the slope and offset of the linear curve.
If the Watt per foot value of the heat trace is Ws at
setpoint temperature Ts, and the Watt per foot value of the
heat trace is W at temperature T, the offset ratio to be
applied to the high/low current alarm level is Ws/W.
That is, if the high/low current alarm level is set to Is, the
current draw of the cable at setpoint temperature of Ts,
then the high/low current alarm level at operating tem-
perature T should be corrected to (Ws/W)*Is to compen-
sate the effect of operating temperature on the allowed
maximum/minimum heater current. The heat trace curve
is shown in figure 1.7.
If no heat trace curve is used, the offset ratio is set to 1
and no correction to the high/low current alarm level is
done.
Figure 1.7 Heat Trace Curve