Fluke 7312, User Manual

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9.5 Secondary Menu
Functions, which are used less often, are accessed within the secondary menu.
The secondary menu is accessed by pressing the “SET” and “EXIT” buttons si-
multaneously 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 di-
rectly from the digital display. By knowing the amount of heating 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 lets the user know
the stability of the bath. 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 the “SET”
and “EXIT” button simultaneously and release. The heater power is displayed
as a percentage of full power.
S+E
Access heater power in secondary menu
12 Pct
Heater power in percent
To exit out of the secondary menu, press the “EXIT” button. To continue on to
the proportional band setting, press the “SET” button.
E
Return to temperature display
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 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 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 5). If the proportional band is too wide, the bath 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 con-
troller cannot respond very well to changing conditions or noise in the system.
7312 TPW Maintenance Bath
User’s Guide
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