8.5
Temperature Controller
The bath temperature is controlled by Hart Scientific’s unique hybrid digi-
tal/analog temperature controller. The controller offers the tight control stability
of an analog temperature controller as well as the flexibility and programmabil-
ity of a digital controller.
The bath temperature is monitored with a platinum resistance sensor in the con-
trol probe. The signal is electronically compared with the programmable refer-
ence signal, amplified, and then passed to a pulse-width modulator circuit
which controls the amount of power applied to the bath heater.
The bath is operable within the temperature range given in the specifications.
For protection against solid-state relay failure or other circuit failure, the
microcontroller automatically turns off the heater with a second mechanical re-
lay anytime the bath temperature is more than a certain amount above the
set-point temperature. As a second protection device, the controller is equipped
with a separate thermocouple temperature monitoring circuit which shuts off
the heater if the temperature exceeds the cutout set-point.
The controller allows the operator to set the bath temperature with high resolu-
tion, set the cutout, adjust the proportional band, monitor the heater output
power, and program the controller configuration and calibration parameters.
The controller may be operated in temperature units of degrees Celsius or Fahr-
enheit. The controller is operated and programmed from the front control panel
using the four key switches and digital LED display. Remote digital operation
with the controller is possible via the standard RS-232 serial port. The control-
ler may be optionally equipped with an IEEE-488 GPIB digital interface. Oper-
ation of the controller using the front control panel is discussed in Section 9,
Controller Operation. Operation using the digital interfaces is discussed in Sec-
tion 10, Digital Communication Interface.
When the controller is set to a new set-point the bath heats or cools to the new
temperature. Once the new temperature is reached, the bath usually takes 10-15
minutes for the temperature to settle and stabilize. There may be a small over-
shoot or undershoot of about 0.5°C.
35
8 General Operation
Temperature Controller
