VICI ITC User Manual
Page 10
The number of degrees between the upper and lower temperature boundaries is
referred to as the proportioning bandwidth. Proper adjustment of the bandwidth
will further enhance temperature stability within the heated zone. Some guidelines
for adjustment are found in Section 2.4.
1.34
Power Attenuation
In the preceding section, proportional power control was described as the process
by which the ITC applies a percentage of power proportional to the difference
between the setpoint and the controlled temperature. It is not unusual that this
technique, alone, will not yield acceptable temperature stability. Commonly, this
situation occurs when near ambient temperatures are desired of a thermal system
originally designed for higher temperatures. Consequently, the heater size (rated
output) is much too large for system demand.
In compensating for such difficulties, laboratory personnel often employ variacs
(variable output, step-down transformers) to attenuate the power delivered to the
heater.
Power delivered to the heater can be attenuated in increments of 10% by setting
the ITC’s front panel mounted ATTN pushbutton switch. This control performs
exactly the same function as the variac mentioned above. However, the method
by which the ITC performs this function differs considerably from variac operation.
Here’s how:
A variac provides a means of adjusting the voltage (and consequently, the
power) applied to the heater. The ITC varies the number of half-cycles
avail-
able
to be delivered by its power circuitry. For example, 100% is available
when the ATTN control is set to 0. If the attenuation is changed to 4 (40%),
only six of every ten half-cycles are available for delivery to the heater. As a
result, the heater output will be 60% of its full rating.
1
2
3
102
°
98
°
PROPORTIONING
WAVEFORM
POWER
APPLICATIONS
TIME
1 CONTROLLED TEMPERATURE RISING FROM AMBIENT;
NOTICE POWER APPLICATION IS PROGRESSIVELY REDUCED AS THE TEMPERATURE
RISES
2 OVERSHOOT; NO POWER APPLIED
3 EQUILIBRIUM;CONTROLLED TEMPERATURE HAS SETTLED AT APPROX. 40% POWER
Figure 4
8