Fluke 9140 Dry-well Calibrator - Manual (Page 32)

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 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 propor-
tional band setting function press “SET”.
7.5.2 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 level.
27
7 Controller Operation
Secondary Menu
Proportional Band too Narrow Proportional Band too Wide
Optimum Proportional Band
Figure 5 Well temperature fluctuation at various proportional band settings