ThermoWorks 9141 HART SCIENTIFIC FIELD DRY-WELL TEMPERATURE CALIBRATORS - Manual (Page 30)

the percent heater power will let the user know how stable the well temperature
is. With good control stability the percent heating power should not fluctuate
more than
±1% within one minute.
The heater power display is accessed in the secondary menu. Press “SET” and
“EXIT” simultaneously and release. The heater power will be displayed as a
percentage of full power.
100.0 C
Well temperature
S
+
E
Access heater power in secondary menu
SEC Flashes for secondary menu and then displays the
heater power
12.0 P
Heater power in percent
To exit out of the secondary menu press “EXIT”. To continue on to the propor-
tional band setting function press “SET”.
7.7 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 bot-
tom of the proportional band the heater output is 100%. At the top of the pro-
portional 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 fairly constant
temperature.
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 will deviate
excessively from the set-point due to varying external conditions. This is be-
cause 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 be-
cause the controller overreacts to temperature variations. For best control stabil-
ity the proportional band must be set for the optimum width.
The proportional band width is set at the factory to about 15.0°C. The propor-
tional band width may be altered by the user if he desires to optimize the con-
trol characteristics for a particular application.
The proportional band width is easily adjusted from the front panel. The width
may be set to discrete values in degrees C or F depending on the selected units.
The proportional band adjustment is be accessed within the secondary menu.
27
7 Controller Operation
Proportional Band