Hanna-instruments HI 23 Manual do Utilizador descarregar pdf (Página 14)

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4. The deviation, Ti and Td can be calculated from the following:

• Deviation = Tx * max. slope (EC/TDS)

• Ti = Tx / 0.4 (minutes)

• Td = Tx * 0.4 (minutes).

5. Set the above parameters and restart the system with the

controller in the loop. If the response has too much over-

shoot or is oscillating, then the system can be fine-tuned

slightly increasing or decreasing the PID parameters one

at a time.

Note Connecting an external device (e.g. chart recorder) to the

controller, the procedure is easier and doesn’t need to hand

plot the process variable (EC or TDS).

ALARM RELAY

The alarm relay functions in the following manner:

During alarm condition, the relay is de-energized. When

not in alarm condition, the relay is energized.

Example: High alarm set at 1400 µS

Low alarm set at 600 µS

An hysteresis will eliminate the possibility of continuous se-

quences ‘energizing/de-energizing’ the alarm relay when

the measured value is close to the alarm setpoint. The alarm

hysteresis amplitude is 1.5% of full scale.

FS•O = NC (Normally Closed)

De-energized Relay

COM

FS•C = NO (Normally Open)

Energized Relay

The user can vary five different parameters, i.e. the setpoint

(S1 or S2), the deviation (D1 or D2), the reset time, the rate

time and the proportional control mode period T

(from 1

to 30 minutes).

Note User can disable the derivative and/or integrative action

(for P or PI controllers) by setting Td = 0 and/or Ti = MAX

(Ti) respectively through the setup procedure.

SIMPLE TUNING PROCEDURE

The following procedure uses a graphical technique of ana-

lyzing a process response curve to a step input.

1. Starting from a solution with an EC or TDS value quite

different from the dosed liquid, turn on the dosing device

at its maximum capacity without the controller in the loop

(open loop process). Note the starting time.

2. After some delay (T

) the EC or TDS starts to vary. After

more delay, the EC or TDS will reach a maximum rate of

change (slope). Note the time that this maximum slope

occurs and the EC or TDS value at which it occurs. Note

the maximum slope in EC or TDS per minute. Turn the

system power off.

3. On the chart draw a tangent to the maximum slope point until

intersection with the horizontal line corresponding to the initial

EC or TDS value. Read the system time delay Tx on the time

axis.

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