VICI ITC - Manual (Page 15)

1. The measuring junction should be of the lowest mass practicable for the appli-
cation. Simply put, the higher the mass, the more time required for the junction to
reach the temperature of its surroundings.
2. The measuring junction should be placed as close as possible (thermally) to
the heater. Whenever there is doubt about proper location of a thermocouple,
follow these suggestions:
a. Place the junction directly between the heater and the object to be heated,
as close to the heater as possible.
b. In a stirred air or liquid bath, place the junction immediately downstream
from the heater.
In addition to the more common considerations, there are a few important specific
notions regarding thermocouples to be used with the ITC.
•
Electrical contact. If the measuring junction is in electrical contact with an
object, that object must be connected to AC ground. For example, this would
require a heater block to be grounded unless the thermocouple is electrically
insulated from it. (The junction must float or be grounded.)
•
Thermocouple resistance. The following data describes the thermocouples
normally shipped with the ITC:
ITC-K: 10 ft., 28 gauge, 40 ohm , ANSI Type K
2.3 Setpoint (Set
°
C)
Loosely defined, the setpoint denotes the desired temperature within the heated
zone. However, the user should be aware that the denoted setpoint is not neces-
sarily the temperature at which the zone will stabilize.
To be more precise, the setpoint denotes the temperature at which power will be
applied 50% of the time. It is entirely possible that the zone will require more or
less than 50% power to maintain stability. As a consequence, the zone tempera-
ture will settle above the setpoint if less than 50% power is required, and below
the setpoint if more than 50% power is needed.
Essentially, this characteristic offset is brought about by the proportional power
control method used in the ITC, coupled with the thermal characteristics of the
user configured heated zone. Without prior knowledge of the zone’s heat input vs.
heat loss properties, the only certainty is that the zone temperature will stabilize
somewhere within the proportioning bandwidth. Exactly where the temperature
settles, can be optimized by adjustment of the proportioning bandwidth. (Refer to
Section 2.4.)
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