Fluke 7008, Руководство пользователя, Страница: 49 / 78

Controller Operation
9.5 Secondary Menu
9
9-7
9.5 Secondary Menu
Functions which are used less often are accessed within the secondary menu.
The secondary menu is accessed by pressing SET and EXIT simultaneously and
then releasing. The first function in the secondary menu is the heater power
display.
9.6 Heater Power
The temperature controller controls the temperature of the bath 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. This value
may be estimated by watching the red/green control indicator light or read
directly from the digital display. By knowing the amount of heating to the bath the
user can tell if the bath is heating up to the set-point, cooling down, or controlling
at a constant temperature. Monitoring the percent heater power will let the user
know how stable the bath 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.
+
Access heater power in secondary menu
12 Pct Heater power in percent
To exit out of the secondary menu press “EXIT”. To continue on to the
proportional band setting function press “SET”.
9.7 Proportional Band
In a proportional controller such as this the heater output power is proportional to
the bath 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 bath 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 bath depends on the width of the proportional
band. See Figure 8. If the band is too wide the bath temperature will deviate
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.