Dry-well Calibrator
Operation
15
Heater Power
The temperature controller controls the temperature of the well by pulsing the heater on and off. The
total power being applied to the heater is determined by the duty cycle or the ratio of heater on time to
the pulse cycle time. By knowing the amount of heating, the user can tell if the calibrator is heating up
to the set-point, cooling down, or controlling at a constant temperature. Monitoring the percent heater
power allows the user to know the stability of the well temperature. With good control stability the
percent heating power should not fluctuate more than ±5 % within one minute.
The heater power display is accessed in the secondary menu. Press
SET
and
EXIT
simultaneously
and release. The heater power displays as a percentage of full power.
100.0 C
Well temperature
S
+
E
Access heater power in secondary menu
SEC
Flashes SEC for secondary menu and then displays the heater power
12.0P
Heater power in percent
To exit out of the secondary menu, press
EXIT
. To continue on to the proportional band setting function
press
SET
.
Proportional Band
In a proportional controller such as this, the heater output power is proportional to the well temperature
over a limited range of temperatures around the set-point. This range of temperature is called the
proportional band. At the bottom of the proportional band, the heater output is 100 %. At the top of the
proportional band, the heater output is 0. Thus, as the temperature rises the heater power is reduced,
which consequently tends to lower the temperature back down. In this way the temperature is
maintained at a constant level.
The temperature stability of the well and response time depend on the width of the proportional band. If
the band is too wide, the well temperature deviates excessively from the set-point due to varying
external conditions. This is because the power output changes very little with temperature and the
controller cannot respond very well to changing conditions or noise in the system. If the proportional
band is too narrow, the temperature may swing back and forth because the controller overreacts to
temperature variations. For best control stability, the proportional band must be set for the optimum
width.
The proportional band width is set at the factory as printed on the Report of Calibration. The
proportional band width may be altered by the user if desired to optimize the control characteristics for
a particular application.
