VICI ITC - Manual (Page 14)

11 Heater Receptacle, 120V AC only
Rear panel mounted AC receptacle; two-wire plus ground; connects heater
via standard 16 or 18 gauge three-wire power cord (not supplied).
12 Heater Receptacle, 220V AC only
Rear panel mounted AC receptacle; two-wire plus ground; connects heater
via power cord (black and white to heater, green to ground). Power cord is
not supplied.
13 Top Cover Retaining Screws
Remove these screws to gain access to interior of ITC.
2.2 Thermocouples
Thermocouples, when used properly, are a very expedient and reliable means of
sensing temperature. In this section, we will attempt to help the user avoid certain
general and specific pitfalls in thermocouple usage with the ITC.
Thermocouple measuring junctions are fabricated by joining two dissimilar metals.
A type K thermocouple is formed from chromel and alumel. In theory, the thermo-
couple is functional so long as the two metals remain in contact. (This does imply
that a measuring junction can be formed by twisting two wires together. We would
point out that the junction will not be suitable for any real application, however.)
Maintaining the integrity of the measuring junction is of prime importance. This
means that for a given application, thought must be given the junction’s maximum
attainable temperature, corrosion resistance to its environment, and mechanical
strength.
Commercially available thermocouples are usually joined by welding. This
produces a junction in which the maximum temperature and corrosion resistance
properties are those of the metals themselves. For applications below 400
o
C, a
quite serviceable junction can be formed by twisting the bare ends of the wire
together, and then securing with silver solder. For applications above 400
o
C, the
junction should be welded. In the case of silver soldered junctions, we would
again point out that the environment and maximum temperature must not be
harmful to the solder.
It is important to note that considerations pertaining to junction integrity are also
applicable to the
insulation around each wire. As stated earlier, a new junction is
formed each time the two thermocouple wires come into contact. Obviously,
unplanned junctions are to be avoided.
In matters concerning the thermocouple, measuring junction mass, thermal
conductivity of the controlled medium and placement can greatly affect controlled
temperature stability. In Section 1.326, an example was given illustrating tempera-
ture instability. It was pointed out that stability is obtained by supplying heater
power proportional to the need. At this point, it is important to recall that the
thermocouple is responsible for telling the controller what the need is. Most
importantly, any change in temperature must be reported without appreciable
delay. This causes instability, regardless of how craftily the correction is carried
out. This notion of minimizing delay is carried to fact by observing two rules:
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