Reset (Integral); Rate (Derivative); Curve - Hach EC1000 Instruction Manual

Process ph/orp system

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4.3.1 Reset (Integral)

Please see Figure 8 for a diagram of the controller output response (integral only) to a step

change on the input. The integral action is used to drive the loop so that there is zero long

term error. For example, on a proportional controller where the steady state pH is too high,

the integral component of the output starts ramping up until the increased chemical feed

brings the pH down to the set point. The higher the setting, the faster the loop reaches

steady state, but it should not be set faster than the delay times in the loop. If the setting is

too high, the loop goes into oscillation (this generally tends to be a slower oscillation than

that caused by a gain or derivative setting that is too high). The integral action is turned off

by setting it to zero.

Please see Figure 8 for a diagram of the controller output response (derivative only) to a

step change on the input. The derivative action reacts to rate of change of the input and

attempts to counter fast changes. The derivative action is turned off by setting it to zero.

The curve mode entry has three settings (linear+, linear-, curve). For most applications

either linear+ or linear- is used. Linear+ is selected when the control pump is set up so that

an increasing controller output signal causes the pH to increase. Linear- is selected when

the control pump is set up so that an increasing controller output signal causes the pH to

decrease.

The use of the curve selection can improve the control loop if the characteristics of the

sample are repeatable over time. For optimal control, the loop gain (controller gain x

process gain) should remain constant, but for most samples the process gain changes over

the pH range.

Please see Figure 10. The process gain (defined as: ∆ pH /∆ % reagent addition) at point 1

through point 5 is represented by the slope of the line between each point and the setpoint.

For this example, the slope of the line (process gain) changes considerably depending on

the pH of the sample. If this curve is entered into the EC1000, the controller can

compensate for the change in process gain to keep the loop gain constant.

To implement the titration curve, take a sample of the process water, and titrate with the

reagent (acid and/or base used in the process). Record the pH value (Figure 11). Enter this

curve into the instrument as percent reagent addition vs. pH. The final step, after entering the

titration points, is to go back to the curve mode entry, and select the curve setting.

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