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CPC400 Series User’s Guide Chapter 3: Operation and Setup

Doc. 0600-2900-2000 Watlow Anafaze 73

As the temperature in the middle of the tank (loop 1) drops,

the output goes up proportionally and the set point of loop

2 goes up proportionally. Thus heat is added to the system

at the element even though the temperature near the ele-

ment may have been at the desired temperature.

With proportional control, when loop 1 is at set point, its

output is 0 percent, and the set point of loop 2 is equal to

the desired temperature 150°F. If the temperature of the

loop 1 drops below 149°F, the deviation results in a propor-

tional output of 10 percent. This results in an increase to

the set point for loop 2 equal to 10 percent of the set point

range. In this case the range is 40°F (190 - 150°F = 40°F),

and 10 percent of 40°F is 4°F.

So when the temperature at loop 1 drops 1°F, the set point

of loop 2 increases by 4°F until the output of loop 1 is 100

percent and the set point of loop 2 is 190°F. At this point,

further decreases of the loop 1 process variable have no ad-

ditional affect on loop 2. Figure 3.12 illustrates this rela-

tionship.

Figure 3.12 Relationship of Secondary Loop

Set Point to Primary Loop Process

Variable in Cascade Example

Setting Up Ratio Control

Ratio control allows the process variable of one loop (mas-

ter loop), multiplied by a ratio, to be the set point of another

loop (ratio loop). You can assign any process variable to de-

termine the set point of a ratio loop.

By adjusting the ratio control parameters, you can adjust

the influence that the master loop process variable has on

the set point of the ratio loop

Set Point of the Secondary Loop

(Engineering Units)

Process Variable of Primary Loop (ºF)

Heat Output of Primary Loop

(Percent of Full Scale)

50%

100%

150ºF

145ºF

140ºF

190ºF

170ºF

150ºF

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