Hanna Instruments HI 21 User Manual

HI 21/HI 22 SeriesHI 21/HI 22 Series

HI 21/HI 22 Series

HI 21/HI 22 SeriesHI 21/HI 22 Series

Wall-mounted,

Microprocessor-based

Process pH/mV Meters

Instruction Manual

Dear Customer, Thank you for choosing a Hanna Product. Please read this instruction manual carefully before using the

instrument. It will provide you with the necessary information for the correct use of the instrument, as well as a precise idea of its versatility.

If you need additional technical information, do not hesitate to e-mail us at

These instruments are in compliance with the

tech@hannainst.comtech@hannainst.com

tech@hannainst.com.

tech@hannainst.comtech@hannainst.com

directives.

TABLE OF CONTENTS

PRELIMINARY EXAMINATION . . . . . . . . . . . . . . . . . . 4

GENERAL DESCRIPTION . . . . . . . . . . . . . . . . . . . . . 4

FUNCTIONAL DESCRIPTION . . . . . . . . . . . . . . . . . . 6

MECHANICAL DIMENSIONS . . . . . . . . . . . . . . . . . . 7

SPECIFICATIONS HI 21 & HI 22 . . . . . . . . . . . . . . . 8

INSTALLATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

MODELS

The models HI 21XYZ are pH controllers, while the models HI 22XYZ are ORP controllers.

HI 21111 pH Controller with single setpoint, ON/OFF

control, analog output

HI 21211 pH Controller with dual setpoint, ON/OFF

control, analog output

HI 21221 pH Controller with dual setpoint, proportional

and ON/OFF control, analog output

HI 21222 pH Controller with dual setpoint, proportional

and ON/OFF control, RS485 output

HI 21523 pH Controller with control through analog

output, PID control, analog and RS485 output

HI 22111 ORP Controller with single setpoint, ON/OFF

control, analog output

HI 22122 ORP Controller with single setpoint, propor-

tional and ON/OFF control, RS485 output

SETUP MODE . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

CONTROL MODE . . . . . . . . . . . . . . . . . . . . . . . . . 17

IDLE MODE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

ANALOG OUTPUT . . . . . . . . . . . . . . . . . . . . . . . . 26

RS485 COMMUNICATION & DATA LOGGING . . . . 28

CALIBRATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

LAST CALIBRATION DATA . . . . . . . . . . . . . . . . . . . . 48

START-UP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

FAULT CONDITIONS AND SELFTEST PROCEDURES . . 52

pH VALUES AT VARIOUS TEMPERATURES . . . . . . . . . 56

ELECTRODE CONDITIONING AND MAINTENANCE . . 57

TAKING REDOX MEASUREMENTS. . . . . . . . . . . . . . 61

ACCESSORIES . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

WARRANTY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

CE DECLARATION OF CONFORMITY. . . . . . . . . . . 70

written consent of the copyright owner, Hanna Instruments Inc., 584 Park East Drive, Woonsocket, Rhode Island, 02895 , USA.

PRELIMINARY EXAMINATION

Remove the instrument from the packing material and examine it carefully to make sure that no damage has occurred during shipping. If there is any noticeable damage, notify your Dealer or the nearest Hanna Customer Service Center immediately.

Note Save all packing materials until you are sure that the instru-

ment functions correctly. Any damaged or defective items must be returned in their original packing materials together with the supplied accessories.

GENERAL DESCRIPTION

The product is a real time microprocessor-based pH/ORP controller. It provides accurate measurements, flexible ON/OFF, proportional or PID control capabilities and dual alarm signals.

The system is composed of a case inside which the signal conversion circuitry, the microprocessor circuitry and the output power drivers are contained.

MAIN FEATURES OF DIFFERENT MODELS

• Display: large LCD with 4 ½ 13 mm digits and 3 ½ 7.7 mm digits.

• LEDs: three (HI 22) or four (HI 21) LEDs are provided for signaling the energizing of relay 1 (a yellow LED), relay 2 (a yellow LED in HI 21 Series only) and alarm relays (a green and a red LED).

• Relays: Electromechanical type, working through NO, COM and NC contacts; 1 or 2 output relays for acid or base dosage; 1 output relay for alarm condition.

• RS485 isolated communication link.

• Calibration and Setup procedures allowed only through an unlock password.

• Calibration: for HI 21 Series in 1, 2 or 3 points with buffers 4.01, 7.01 and 10.01 pH (25 °C); for HI 22 Series in 1 or 2 points at 0, 350 and 1900 mV.

• Temperature compensation of the HANNA standard buffers (for HI 21 Series only).

• Temperature compensation of the pH reading (for HI 21 Series only).

• Manual temperature setting when the temperature probe is not inserted or temperature exceeds the upper range.

• Internally recorded last calibration data (non-volatile EEPROM memory): calibration date and time, pH offset, pH slopes, number of calibration points and correspondent pH values (for HI 21 Series only) or calibration date and time and the mV calibration points used (for HI 22 Series only).

• Input: pH electrode with BNC connector.

• ±5 VDC output for amplified electrodes (max 10 mA).

• Output:

- isolated 0-1 mA, 10 KW maximum load (optional);

- isolated 0-20 mA, 750 W maximum load (optional);

- isolated 4-20 mA, 750 W maximum load (optional);

- isolated 0-5 VDC, 1 KW minimum load (optional);

- isolated 1-5 VDC, 1 KW minimum load (optional);

- isolated 0-10 VDC, 1 KW minimum load (optional).

• Real time clock.

FUNCTIONAL DESCRIPTION

FRONT PANEL

1. Liquid Crystal Display

2. LCD key exits from setup and calibration modes and reverts back to normal mode (in idle or control phases with the measurement on the display). In HI 21 series, during pH calibration, alternately displays pH buffer value or current temperature

3. CAL DATA key last calibration data viewing (enters and exits)

4. SETUP key enters setup mode

5. CAL key initiates and exits calibration mode

6. ñ key increases the blinking digit/letter by one when selecting a parameter. Advances forward while in last calibration data viewing mode. Increases the temperature setting when temperature probe is not inserted

7. ð key moves to the next digit/letter (circular buffer) when selecting a parameter. Same as ñ key during last calibration data viewing mode

8. ò key decreases the blinking digit/letter by one when selecting a parameter. Reverts backward while in last calibration data viewing mode. Decreases the temperature setting when temperature probe is not inserted

9. CFM key confirms current choice (and skips to the next item)

10. BNC Socket

11. LEDs

CONNECTIONS PANEL

1. 4-pin RS485 terminal (not for HI 21XY1 and HI 22XY1)

2. Analog output (not for HI 21XY2 and HI 22XY2)

3. Connections for Pt 100 temperature sensor

4. ±5V power supply output

5. Connection for Potential Matching Pin

6. Connection for electrode reference

7. Power supply input (1 Potential Earth, 2 Neutral, 3 Line)

8. Alarm terminal (1 F.S. Closed, 2 COM, 3 F.S. Open)

9. Relay 1 - first dosing terminal (not for HI 21523) - (1 Normally Open, 2 COM, 3 Normally Closed)

10. Relay 2 - second dosing terminal (HI 212XY models only) - (1 Normally Open, 2 COM, 3 Normally Closed)

Unplug the meter before starting any electrical connections.

MECHANICAL DIMENSIONS

FRONT VIEW BOTTOM VIEW

SPECIFICATIONS HI 21 & HI 22

INSTALLATION

Range 0.00 to 14.00 pH (HI 21 Series only)

±2000 mV (HI 22 Series only)

-9.9 to 120.0 °C

Resolution 0.01 pH (HI 21 Series only)

1 mV (HI 22 Series only)

0.1 °C

Accuracy ±0.02 pH (HI 21 Series only)

(@20°C/68°F) ±2 mV (HI 22 Series only)

±0.5 °C

Typical EMC Deviation ±0.05 pH (HI 21 Series only)

±4 mV (HI 22 Series only) ±1.0 °C

Analog Output 0 to 1 mA, 0 to 20 mA, 4 to 20 mA

0 to 5 VDC, 1 to 5 VDC, 0 to 10 VDC

Digital Output RS485 Power Supply Input ±5V (for amplified electrodes) Power Supply 230 ±10% VAC or 115 ±10% VAC, 50/60 Hz Power Consumption 15 VA Over Current Protection 400 mA, 250V, FAST FUSE Relays 1 and 2 Electromechanical relay SPDT contact outputs,

5A-250 VAC, 5A - 30 VDC (resistive load) (HI 211YZ, HI 212YZ and HI 221YZ)

Fuse protected: 5A, 250V FAST FUSE

Alarm Relay Electromechanical relay SPDT contact output,

5A - 250 VAC, 5A - 30 VDC (resistive load) Fuse protected: 5A, 250V FAST FUSE

HI 21 and HI 22 offer a multitude of possibilities, from single and dual setpoints to ON/ OFF, proportional or PID dosage, isolated outputs with user-selectable zoom, bidirectional RS485, recorder outputs in mAmps and Volts.

In addition, HI 21 and HI 22 are both equipped with the exclusive differential input.

In a system with poor grounding, it is possible to have a ground current flowing through the reference junction. This can cause a rapid degradation of the electrode. The Hanna differential input reduces the likelihood of ground loops.

See the diagram for a recommended installation.

Environment 0-50 °C; max 85% RH not condensing Protection IP 54 Dimensions 221 x 181 x 86 mm (8.7 x 7.1 x 3.4’’) Weight approximately 1.4 Kg (3.1 lb.)

••

• Power Supply: Connect a 3-wire power cable

•• to the terminal strip, while paying attention to the correct line (L), earth (PE) and neutral (N) terminal connections.

Power: 115VAC - 100 mA / 230VAC - 50 mA.

Line Contact: fused inside 400 mA.

PE must be connected to ground; leakage current 1

mA.

••

• Electrode: Connect the pH or ORP electrode to the BNC

•• socket (#10 at page 6). To benefit from the differential input, connect the proper electrode wire (if available) or a cable with a potential matching pin (grounding bar) to the relevant terminal (#5 at page 7).

Note When it is not possible to immerse the Potential Matching Pin

together with the pH electrode in the solution, disable the differential input by connecting the Connection for Potential Matching Pin (#5 page 7) with the Connection for Electrode Reference (#6 on page 7) with a jumper wire.

••

• Pt 100 Terminals: these contacts (#3 at page 7) connect

•• the Pt 100 temperature sensor for automatic temperature compensation of pH measurement. In the case of shielded wire, connect the shield to pin 4.

In the case of a 2-wire sensor connect the Pt 100 to pins 1 and 3, and short pins 2 and 3 with a jumper wire.

If the Pt 100 has more than 2 wires, connect the two wires of one end to pins 2 and 3 (pin 2 is an auxiliary input to compensate for the cable resistance) and one wire from the other end to pin 1. Leave the fourth wire unconnected, if present.

••

• Power Supply Output: these terminals provide +5V and -

•• 5V DC (max current: 10 mA) signals to supply power to amplified electrodes.

Note All cables connected to the panel should end with cable lugs.

SETUP MODE

HI 21 and HI 22 offer a multitude of possibilities from ON/ OFF, proportional or PID dosage to analog recorder output and from alarm to selftest features.

The Setup Mode allows the user to set all needed characteristics of the meter.

The setup mode is entered by pressing SETUP and entering the password when the device is in idle or control mode.

Generally speaking, if the password is not inserted the user can only view the setup parameters (except for password) without modifying them (and the device remains in control mode). An exception is certain setup items, or flags, which can activate special tasks when set and confirmed.

Each setup parameter (or setup item) is assigned a twodigit setup code which is entered and displayed on the secondary LCD.

The setup codes can be selected after password and CFM are pressed. When CFM is pressed, the current setup item is saved on EEPROM and the following item is displayed. Whenever LCD is pressed, the device reverts back to control mode. The same is true when CFM is pressed on the last setup item.

The possible transitions in setup mode are the following:

ENTERING THE PASSWORD

• Press SETUP to enter the setup mode. The LCD will display “0000” on the upper part and “PAS” on the lower. The first digit of the upper part of the LCD will blink.

• Enter the first digit of the password by the ñ or ò keys.

1110

• Then confirm the displayed digit with ð and move to the next one.

• When the whole password has been inserted, press CFM to confirm it.

Note The default password is set at “0000”.

• The LCD will display “SET” on the upper part and “c.00” on the lower, allowing the user to edit setup parameters (see table below).

• Enter the code of the parameter you want to set, using the arrow keys as per the password procedure above (e.g. 41).

• Confirm the code by pressing CFM and the default or the previously memorized value will be displayed with the first digit blinking.

Note When the password is not inserted or a wrong password is

confirmed, the display will only show the previously memorized value, without blinking (read only mode). In this case, the value cannot be set. Press LCD and start again.

• Enter the desired value using the arrow keys and then press CFM.

• After confirmation, the selected parameter is displayed. The user can scroll through the parameters by pressing CFM.

In order to directly set another parameter, press SETUP again and enter the code or scroll to it by pressing CFM.

The following table lists the setup codes along with the description of the specific setup items, their valid values and whether password is required to view that item (“PW” column):

Code Valid Values Default PW

00 Factory ID 0 to 9999 0000 no 01 Process ID 0 to 9999 (models w/o RS485) 0000 no

0 t o 99 (models with RS4 85) 00 no

02 Control enable/disable 0: C.M. disabled 0 no

1: C.M. enabled

11 Relay 1 mode 0: disabled 0 no

(M1) 1: ON-OFF high setpoint

2: ON-OFF low setpoint 3: PID, high setpoint 4: PID, low setpoint

for HI 21523 only : 0: disabled

1: PID, high setpoint 2: PID, low setpoint

12 Relay 1 setpoint 0.00 to 14.00 pH 8.00 pH no

(S1) -2000 to 2000 mV 500 mV

13 Relay 1 hysteresis 0.00 to 14.00 pH 1 pH no

(H1) 0 to 4000 mV 50 mV

14 Relay 1 deviation 0.50 to 14.00 pH 1 pH no

(D1) 25 to 4000 mV 50 mV

15 Relay 1 reset time 0.1 to 999.9 minutes 999.9 mins no

(HI 21523 only)

16 Relay 1 rate time 0.0 to 999.9 minutes 0.0 mins no

(HI 21523 only)

21* Relay 2 mode (M2) same as relay 1 0 no

22* Relay 2 setpoint (S2) 0.00 to 14.00 pH 6.00 pH no

-2000 to 2000 mV -500 mV

* Available only in models with two relays

1312

Code Valid Values Default PW

23* Relay 2 hysteresis (H2) 0.00 to 14.00 pH 1 pH no

0 to 4000 mV 50 mV

24* Relay 2 deviation (D2) 0.50 to 14.00 pH 1 pH no

25 to 4000 mV 50 mV

25* Relay 2 reset time 0.1 to 999.9 minutes 999.9 mins no

(HI 21523 only)

26* Relay 2 rate time 0.0 to 999.9 minutes 0.0 mins no

(HI 21523 only)

30 Relay 3 high alarm (HA) 0.00 to 14.00 pH 9.00 pH no

-2000 to 2000 mV 600 mV HA>LA, HA>S1 or HA>S2

31 Relay 3 low alarm (LA) 0.00 to 14.00 pH 5.00 pH no

-2000 to 2000 mV -600 mV LA<HA, LA<S1 or LA<S2

32 Proportional control 1 to 30 min (

not for HI 21523) 5 min no

mode period 5 s to 30 min (HI 21523 only) 30 s no

33 Maximum relay ON time 1 to 60 min (

10 to 999 min (

HI 21523 only) 60 no

not for HI 21523) 60 no

(after which an alarm mode is entered) 34 Alarm mask time 00:00 to 30:00 00:00 no

(HI 21523 only)

40 Analog output selection 0: 0-1mA 2 no

1: 0-20 mA 2: 4-20 mA 3: 0-5 VDC 4: 1-5 VDC 5: 0-10 VDC

Note: this parameter refer to internal hardware configuration of the instrument. To

change it from the factory setting, see page 26.

41 Analog output 0.00 to 13.00 pH 0.00 pH no

lower limit -2000 to 2000 mV -2000 mV (O_VARMIN) (O_VARMIN < O_VARMAX-(1.00pH or 50mV))

42 Analog output 1.00 to 14.00 pH 14.00 pH no

upper limit -2000 to 2000 mV 2000 mV (O_VARMAX) (O_VARMIN < O_VARMAX- (1.00pH or 50mV))

60 Current day 01 to 31 from RTC no 61 Current month 01 to 12 from RTC no 62 Current year 1997 to 9999 from RTC no 63 Current time 00:00 to 23:59 from RTC no 71 Baud rate 1200, 2400, 4800, 9600 9600 (RS485) no 90 Display selftest 0: off 0 yes

1: on

91 Keyboard selftest 0: off 0 yes

1: on

92 EEPROM selftest 0: off 0 yes

1: on

93 Relays and LEDs selftest 0: off 0 yes

1: on

94 Watchdog selftest 0: off 0 yes

1: on

99 Unlock password 0000 to 9999 0000 yes

Note The process controller automatically checks to ensure that

the entered data matches other related variables. If a wrong configuration is entered, “WRONG” blinks on the LCD to prompt the user. The correct configurations are the following:

If M1=/ 0 then S1 If M2=/ 0 then S2

For models other than HI 21523: If M1= 1 then S1-H1

If M1= 2 then S1+H1 If M1= 3 then S1+D1 If M1= 4 then S1-D1 If M2= 1 then S2-H2 If M2= 2 then S2+H2 If M2= 3 then S2+D2 If M2= 4 then S2-D2 If M1= 1 and M2 = 2 then S1-H1 If M1 = 2 and M2 = 1 then S2-H2 If M1 = 3 and M2 = 2

>S2+H2, S2>LA, HA>S1+D1;

then S1

<HA, S1>LA; <HA, S2>LA;

>LA;

<HA;

<HA; >LA; >LA;

<HA;

<HA; >LA;

>S2+H2, S2>LA, HA>S1;

>S1+H1, S1>LA, HA>S2;

1514

If M1 = 2 and M2 = 3 then S1+H1 If M1 = 4 and M2 = 1 then S1 If M1 = 1 and M2 = 4 then S1–H1 If M1 = 3 and M2 = 4 then S1 If M1 = 4 and M2 = 3 then S2

For HI 21523 only: If M1 = 1 then S1+D1

If M1 = 2 then S1–D1 where the minimum deviation (D1 or D2) is 0.5 pH (for HI

21) or 25 mV (for HI 22).

<S2, S1>LA, HA>S2+D2;

<S2–H2, S1–D1>LA, HA>S2;

>S2, S2–D2>LA, HA>S1;

>S2, S2–D2>LA, HA>S1+D1;

>S1, S1–D1>LA, HA>S2+D2;

<HA;

>LA;

Note The password cannot be viewed when SETUP is pressed with-

out entering the original password first. The default password is set at “0000”. In the event that the user forgets the password, this can be reset to “0000” by pressing and holding CFM and then pressing LCD and CAL DATA at the same time when the pH controller is in normal operating mode (idle or control with measurement displaying).

CONTROL MODE

The control mode is the normal operational mode for these meters. During control mode HI 21 and HI 22 fulfill the following main tasks:

• convert information from pH/ORP and temperature inputs

• control relays and generate the analog outputs as deter-

• RS485 management (if the feature is included). In addition, HI 21 and HI 22 can log working data through

RS485 connection. This data includes:

• pH, mV and

• last calibration data;

• setup configuration (also from PC). The status of the meter is shown by the LED’s on the right

to digital values;

mined by the setup configuration, display alarm condition;

C measured values;

STATUS LEDs

Control Alarm Alarm LED (green) Relay LED (yellow) Red LED

OF F ---- O N O FF O N

ON OFF ON ON or OFF OFF

ON ON OFF ON or OFF Blinking

Note When a wrong setup value is con-

firmed, the pH controller does not skip to the next setup item but remains in the current item displaying a flashing “WRONG” indicator until the parameter value is changed by the user (the same is also true for the setup code selection). In some circumstances, the user cannot succeed in setting a parameter to a desired value if the related parameters are not changed beforehand; e.g. to set a pH high setpoint to 10.00 the high alarm must be set to a value greater than pH 10.00 first.

Note For code numbers 40, 41, 42, the output is related to pH or mV units

depending on the model (pH process meters or mV process meters). Items 41 and 42 are not available in HI 215YZ .

Meter exits control mode by pressing SETUP or CAL and confirming the password. Note that this command generates a temporary exit. To deactivate the control mode definitively, set CONTROL ENABLE to “0” (item # 02).

RELAY MODES

Once enabled, the relays 1 and 2 can be used in four different modes:

1) ON/OFF, high setpoint (acid dosage) (not for HI 21523);

2) ON/OFF, low setpoint (base dosage) (not for HI 21523);

3) Proportional or PID (HI 21523 only), low setpoint (base

dosage, if available);

4) Proportional or PID (HI 21523 only), high setpoint (acid

1716

An upper boundary is imposed for acid/base dosage time when relays are energized continuously, i.e. when relay works in ON/OFF mode or in PID mode but in the latter case only if the relay is always ON. This parameter can be set through setup procedure. When the maximum boundary is reached, an alarm is generated; device stays in alarm condition until relay is de-energized.

ON/OFF CONTROL MODE

Either for mode 1 or 2 (base or acid dosage) the user has to define the following values through setup:

• relay setpoint (pH/mV value);

• relay hysteresis (pH/mV value). Connect your device to the COM and

NO (Normally Open) or NC (Normally Closed) terminals.

The ON relay state occurs when relay is energized (NO and COM connected, NC and COM disconnected).

The OFF relay state occurs when relay is de-energized (NO and COM disconnected, NC and COM connected).

The following graphs show relay states along with pH measured value (similar graph can be derived for mV control).

As shown below, a high setpoint relay is activated when the measured pH exceeds the setpoint and is deactivated when it is below the setpoint value minus hysteresis.

Such a behavior is suitable to control an acid dosing pump. A low setpoint relay as can be seen from the following graphs is energized when the pH value is below the setpoint and is de-energized when the pH value is above the sum of setpoint and the hysteresis. The low setpoint relay may be used to control an alkaline dosing pump.

dosage, if available).

OFF

Setpoint – Hysteresis

Setpoint

OFF

Setpoint

P.I.D. CONTROL MODE (HI 21523 ONLY)

PID control is designed to eliminate the cycling associated with ON/OFF control in a rapid and steady way by means of the combination of the proportional, integral and derivative control methods.

With the proportional function, the duration of the activated control is proportional to the error value (Duty Cycle Control Mode): as the measurement approaches setpoint, the ON period diminishes.

The following graph describes the pH process controller behavior. Similar graph may apply to the mV controller.

During proportional control the process controller calculates the relay activation time at certain moments t etc. The ON interval (the shaded areas) is then dependent to the error amplitude.

With the integral function (reset), the controller will reach a more stable output around the setpoint providing a more accurate control than with the ON/OFF or proportional action only.

The derivative function (rate action) compensates for rapid changes in the system reducing undershoot and overshoot of the pH value.

During PID control, the ON interval is dependent not only to the error amplitude but even to the previous measurements.

Definitely PID control provides more accurate and stable control than ON/OFF controllers and it is best suitable in system with a fast response, quickly reacting to changes due to addition of acid or base solution.

t0+T

Setpoint + Hysteresis

t0+2T

t0+3T

, t0+Tc, t0+2T

1918

An example of how the response overshoot can be improved with a proper rate action setting is depicted in the following graphic.

RATE ACTION COMPENSATES FOR RAPID CHANGES t

PID TRANSFER FUNCTION

The transfer function of a PID control is as follows:

with Ti = Kp/Ki, Td = Kd/Kp, where the first term represents the proportional action, the

second is the integrative action and the third is the derivative action.

Proportional action can be set by means of the Proportional Band (PB). Proportional Band is expressed in percentage of the input range and is related to Kp according to the following:

Kp = 100/PB.

Kp + Ki/s + s Kd = Kp (1 + 1/(s Ti) +s Td)

100%

Controller

output

In HI 21 and HI 22 the proportional action is set directly as “Deviation” in pH and mV units respectively. Relation between Deviation (D) and PB is:

D = Range * PB/100 Each setpoint has a selectable proportional band: PB1 for

setpoint1 and PB2 for setpoint2. Two further parameters must be provided for both setpoints:

Ti = Kp/Ki, reset time, measured in minutes Td = Kd/Kp, rate time, measured in minutes.

Ti1 and Td1 will be the reset time and rate time for setpoint1, while Ti2 and Td2 will be the reset time and the rate time for setpoint2.

TUNING A PID CONTROLLER

The proportional, integrative, derivative terms must be tuned, i.e. adjusted to a particular process. Since usually the process variables are not completely known, a “trial and error” tuning procedure must be applied to get the best possible control for the particular process. The target is to achieve a fast response time and a small overshoot.

Many tuning procedures are available and can be applied to HI 21. A simple and profitable procedure is reported in this manual and can be used in almost all applications.

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 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 analyzing a process response curve to a step input.

Note Connecting an external device (chart recorder or PC) to the

controller, the procedure is easier and doesn’t need the use of hand plotting the process variable (pH or mV).

(from 1 to

Proportional Band

Error

2120

1. Starting from a solution with a pH or mV value different

from the dosed liquid (at least a 3 pH or 150 mV difference) 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 the pH or mV starts to vary. After more

delay, the pH or mV will reach a maximum rate of change (slope). Note the time that this maximum slope occurs and the pH or mV value at which it occurs. Note the maximum slope in pH or mV 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 pH or mV value. Read the system time delay Tx on the time axis.

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

• Deviation = Tx * max. slope (pH or mV)

• 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 overshoot or is oscillating, then the system can be fine-tuned slightly increasing or decreasing the PID parameters one at a time.

Example:

the chart recording in the figure aside was obtained continuously dosing an alkaline solution to a weak acid solution in a tank. The initial settings will be:

Max. slope = 3 pH/5 mins = 0.6

pH/min

Time delay = Tx = approx. 7 mins

Deviation = Tx * 0.6 = 4.2 pH Ti = Tx / 0.4 = 17.5 mins Td = Tx * 0.4 = 2.8 mins

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.

FS•C = NO (Normally Open)

Energized Relay

COM

FS•O = NC (Normally Closed)

De-energized Relay

Example: High alarm set at 10 pH

Low alarm set at 4 pH

An hysteresis will eliminate the possibility of continuous sequences ‘energizing/de-energizing’ of the alarm relay when the measured value is close to the alarm setpoint. The hysteresis amplitude is 0.2 pH in HI 21 and 30 mV in HI 22. Moreover the alarm signal is generated only after a user selectable time period (alarm mask) has elapsed since the controlled value has overtaken one alarm threshold. This additional feature will avoid fake or temporary alarm conditions.

Note If the power supply is interrupted, the relay is de-energized as

if in alarm condition to alert the operator. In addition to the user-selectable alarm relays, all HI 21 and

HI 22 models are equipped with the Fail Safe alarm fea- ture.

The Fail Safe feature protects the process against critical errors arising from power interruptions, surges and human errors. This sophisticated yet easy-to-use system resolves these predicaments on two fronts: hardware and software. To eliminate problems of blackout and line failure, the alarm function operates in a “Normally Closed” state and hence alarm is triggered if the wires are tripped, or when the power is down.

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