would continue to rise – even though all power had been disconnected – until the range coil cooled down.
This problem of overshooting the set point during initial warm-up is the major difficulty with process
controllers. Overshooting the set point is minimized in two ways by your J-KEM controller – but first
let's finish the range analogy. If you had turned the range off just as the water temperature reached 80
o
C,
the final temperature probably would not exceed 82
o
C by the time the range coil cooled down, because
the volume of water is so large. In most situations a 2
o
C overshoot is acceptable. But what if you were
heating 3 tablespoons (45 mL) of water and turned the stove off just as the temperature reached 80
o
C. In
this case, the final temperature would probably approach 100
o
C before the range cooled down. A 20
o
C
overshoot is no longer acceptable. Unfortunately, this is the situation in most research heating
applications. That is, small volumes (< 2 L) heated by very high efficiency heating mantles that contain
large amounts of heat even after the power is turned off.
Your controller handles the problem of ‘latent heat’ in the heating mantle in two ways:
1)
The controller measures the rate of temperature rise during the initial stages of heating. It then
uses this information to determine the temperature at which heating should be stopped to avoid
exceeding the set point. Using the range analogy, this might mean turning the power off when the
water temperature reached 60
o
C and allowing the latent heat of the burner to raise the water
temperature from 60 to 80
o
C. This calculation is done by the controller and is independent of the
operator. The next feature of the controller is directly under operator control and has a major
impact on the amount of overshoot on initial warm-up.
2)
Again referring to the range analogy, you'd obtain better control when heating small volumes if
the range had more than two power settings; Off and High. J-KEM’s patented power reduction
circuit (8) serves just this function. It allows the researcher to reduce the power of the controller
depending on the amount of heat needed. This circuit can be thought of as determining whether
the heating power is Very low (1-10 mL), Low (10-100 mL), Intermediate (50-500 mL),
Medium (300 mL-2 L), or High (> 2 L). The proper power setting becomes instinctive after
you've used your controller for a while. For additional information see Section 3.6.
4.2
Controlling the Heating Mantle Temperature Directly. In a normal heating setup, the thermocouple
is placed in the solution being heated. The controller then regulates the temperature of the solution
directly. The thermocouple could alternately be placed between the heating mantle and the flask so that
the controller regulates the temperature of the heating mantle directly, which indirectly regulates the
temperature of the solution.
Advantages to this method include:
1. The temperature of any volume (microliters to liters) can be controlled.
2. Temperature control is independent of the properties of the material being heated (e.g.,
viscosity, solid, liquid, etc.).
3. Air and water sensitive reactions can be more effectively sealed from the atmosphere.
The temperature controller must be programmed for use with an external thermocouple before this
procedure is used (see following procedure). The following step-by-step procedure programs the
controller to regulate heating mantle temperature. If you switch back and use the controller with the
thermocouple in solution, Procedure 1 in Section 3.9 will program the controller for heating mantles. For
all other heaters, see tuning instructions in Section 2.
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