Stirring; Heater; Temperature Controller; Cutout - Fluke 7381 User Manual

Calibration Bath
General Operation

Stirring

Stirring of the bath fluid is important for stable temperature control. The fluid must be mixed well
for good temperature uniformity and fast controller response. The stirrer is precisely adjusted for
optimum performance.

Heater

The temperature controller precisely controls the Bath heater to maintain a constant bath
temperature. To control the power, the Bath periodically switches on a solid-state relay that turns
the heater on for a certain interval.
The front-panel red/green control indicator shows the state of the heater, see Table 1.

Temperature Controller

A unique hybrid digital/analog temperature controller controls the Bath temperature. The
controller offers the tight control stability of an analog temperature controller and the flexibility
and programmability of a digital controller.
A platinum resistance sensor in the control probe monitors bath temperature. The signal is
electronically compared with the programmable reference signal, amplified, and then passed to a
pulse-width modulator circuit that controls the amount of power applied to the heater.
The Bath operates within the temperature range given in the specifications. To protect against
solid-state relay failure or other circuit failure, the microcontroller automatically turns off the
heater with a second mechanical relay if the Bath temperature exceeds a certain amount above
the set-point temperature. As a second protection device, the controller has a separate
thermocouple temperature monitoring circuit that shuts off the heater if the temperature exceeds
the cutout set-point.
Use the controller to set the Bath temperature with high resolution, set the cutout temperature,
adjust the proportional band, monitor the heater output power, and program the controller
configuration and calibration parameters.
When you set the controller to a new set-point, the Bath heats or cools to the new temperature.
Once the Bath reaches the new temperature, allow 15 minutes to 20 minutes for the temperature
to stabilize. There may be a small overshoot or undershoot of about 0.5 °C.

Cutout

For protection against software or hardware fault, shorted heater triac, or user error, the Bath has
an adjustable heater cutout device that shuts off power to the heater if the Bath temperature
exceeds a set value. This protects the heater and Bath materials from excessive temperatures
and, most importantly, protects the Bath fluids from heating beyond the safe operating
temperature. This protection prevents hazardous vaporization, breakdown, or ignition of the
liquid. From the front panel. Set the cutout temperature below the upper temperature limit of the
fluid and no more than 10 degrees above the upper temperature limit of the Bath, see Cutout Set-
point.
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