Hanna Instruments PH 502 User Manual

pH 502/mV 602 Series

Panel-mounted,
Microprocessor-based
Process pH/mV Meters

Instruction Manual

MODEL IDENTIFICATION

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 [e-mail].

These instruments are in compliance with the directives.

The models pH502XYZ are pH controllers, while the models
mV602XYZ are ORP controllers.

The meaning of the last three letters is according to the fol­lowing scheme:

© 1999 Hanna Instruments

All rights are reserved. Reproduction in whole or in part is prohibited without the
written consent of the copyright owner, Hanna Instruments Inc., 584 Park East Drive,
Woonsocket, Rhode Island, 02895 , USA.

2

TABLE OF CONTENTS

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

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

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

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

SPECIFICATIONS pH 502 & mV 602 . . . . . . . . . . . . . 8

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

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

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

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

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

RS 485 COMMUNICATION . . . . . . . . . . . . . . . . . . 28

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

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

STARTUP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 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

3

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 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 17 mm digits and 3 ½
10 mm digits.

• LEDs: three (mV 602) or four (pH 502) LEDs are provided
for signaling the energizing of relay 1 (a yellow LED), relay
2 (a yellow LED in pH 502 Series only) and alarm relays (a
green and a red LED).

• Relays: 1 or 2 output relays for acid or base dosage.
The models with Electromechanical Relay(s) have COM,

NO and NC contacts. The models with Solid State Relay(s)
have COM and NO contacts.

1 output relay for alarm condition (COM, NO and NC
contacts).

• RS485 isolated communication link (optional).

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

• Calibration: for pH 502 Series in 1, 2 or 3 points with
buffers 4.01, 7.01 and 10.01 pH (25 °C); for mV 602

4

Series in 1 or 2 points at 0, 350 and 1900 mV.

• Temperature compensation of the HANNA standard buff­ers (for pH 502 Series only).

• Temperature compensation of the pH reading (for pH 502
Series only).

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

• Last calibration data internally recorded (non-volatile EE­PROM memory): calibration date and time, pH offset, pH
slopes, number of calibration points and correspondent
pH values (for pH 502 Series only) or calibration date and
time and the mV calibration points used (for mV 602 Series
only).

• Input:pH electrode with BNC connector.

• ±5 VDC output for amplified electrodes.

• Output:

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

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

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

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

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

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

• Real time clock.

5

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 pH 502 series, during pH
calibration, alternately displays pH buffer value or current
temperature

3. SETUP key enters setup mode

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

5. CAL key initiates and exits calibration mode

6.  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.  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

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

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

10. LEDs

6

REAR PANEL

Models with 3-contact

electromechanical output relay(s)

1. 6-pin RS485 terminal (not for pH502XY1 and mV602XY1)

2. Analog output (not for pH502XY2 and mV602XY2)

3. Power supply input

4. Alarm terminal

5. Relay 2 - second dosing terminal (pH5022XY and pH5024XY models only)

6. Relay 1 - first dosing terminal

7. Connections for Pt 100 temperature sensor

8. Connection for electrode reference

9. Connection for Potential Matching Pin

10. BNC Socket for pH or ORP electrode

11. ±5V power supply output

Models with 2-contact
solid state output relay(s)

Unplug the meter before starting any electrical connections.

MECHANICAL DIMENSIONS

FRONT VIEW SIDE VIEW

7

SPECIFICATIONS pH 502 & mV 602

Range 0.00-14.00 pH (pH 502 Series only)

±2000 mV (mV 602 Series only)

-9.9 to 120.0 °C

Resolution 0.01 pH (pH 502 Series only)

1 mV (mV 602 Series only)

0.1 °C

Accuracy ±0.02 pH (pH 502 Series only)

(@20°C/68°F) ±2 mV (mV 602 Series only)

±0.5 °C

Typical EMC Deviation ±0.2 pH (pH 502 Series only)

±10 mV (mV 602 Series only)
±0.5 °C

Installation Category II
Power Supply 230 ±10% VAC or 115 ±10% VAC, 50/60 Hz
Power Consumption 15 VA
Over Current Protection 400 mA 250V FAST FUSE
Max. Oscillation Frequency 4 MHz
Relays 1 and 2 • Electromechanical relay SPDT contact outputs,

5A-250 VAC, 5A - 30 VDC (resistive load)
(pH 5021YZ, pH 5022YZ and mV 6022YZ)

Fuse protected: 5A, 250V FUSE

• SSR (Solid State Relay) 1A - 250VAC (pH 5023YZ ,
pH 5024YZ and mV 6023YZ)

Fuse protected: 2A, 250V FAST FUSE

Alarm Relay Electromechanical Relay SPDT contact output,

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

Environment 0-50 °C; max 85% R.H. non-condensing
Enclosure single case ½ DIN
Weight approximately 1.6 kg. (3.5 lb.)

8

INSTALLATION

pH 502 and mV 602
offer a multitude of
possibilities, from single
and dual setpoints to
ON/OFF or PID dos­age, isolated outputs
with user-selectable
zoom, bi-directional
RS485, recorder out­puts in mAmps and
Volts.

In addition, pH 502
and mV 602 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.

9

• 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 400mA.

PE must be connected to ground; leakage current 1mA.

• Electrode: Connect the pH or ORP electrode to the BNC
socket (#10 at page 7).
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 (#9 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 (#9 page 7) with the Connection for Electrode
Reference (#8 on page

7) with a jumper wire.

• Pt 100 Terminals: these contacts (#7 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 signals to
supply power to amplified electrodes.

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

10

SETUP MODE

pH 502 and mV 602 offer a multitude of possibilities from
ON/OFF 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

• Enter the first value of the pass-

“0000” on the upper part and “PAS” on the lower. The first
digit of the upper part of the LCD will blink.

word by the  or  keys.

11

• Then confirm the displayed digit with
 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.

12

• 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 RS48 5) 0000 no

0 to 99 (models with RS485) 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 pH 5025YZ and mV 6025YZ 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

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

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

* Available only in models with two relays

13

Code Valid Values Default PW

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

-2000 to 2000 mV -500 mV

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

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

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 (models w/o SSR) 5 min no

mode period 5 s to 30 min (models with SSR) 30 s no

33 Maximum relay ON time 1 to 60 min 60 no

(after which an alarm mode is entered)

14

34 Alarm mask time 00:00 to 30:00 (mm:ss) 00:00 no

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

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))

Code Valid Values Default PW

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<HA, S1>LA;
If M2=/ 0 then S2<HA, S2>LA;

For models other than pH5025YZ and mV6025YZ:
If M1= 1 then S1-H1>LA;

If M1= 2 then S1+H1<HA;
If M1= 3 then S1+D1<HA;
If M1= 4 then S1-D1>LA;
If M2= 1 then S2-H2>LA;
If M2= 2 then S2+H2<HA;
If M2= 3 then S2+D2<HA;
If M2= 4 then S2-D2>LA;
If M1= 1 and M2 = 2
then S1-H1>S2+H2, S2>LA, HA>S1;
If M1 = 2 and M2 = 1
then S2-H2>S1+H1, S1>LA, HA>S2;

15

If M1 = 3 and M2 = 2
then S1>S2+H2, S2>LA, HA>S1+D1;
If M1 = 2 and M2 = 3
then S1+H1<S2, S1>LA, HA>S2+D2;
If M1 = 4 and M2 = 1
then S1<S2–H2, S1–D1>LA, HA>S2;
If M1 = 1 and M2 = 4
then S1–H1>S2, S2–D2>LA, HA>S1;
If M1 = 3 and M2 = 4
then S1>S2, S2–D2>LA, HA>S1+D1;
If M1 = 4 and M2 = 3
then S2>S1, S1–D1>LA, HA>S2+D2;

For pH5025YZ and mV6025YZ only:
If M1 = 1 then S1+D1<HA;

If M1 = 2 then S1–D1>LA;
where the minimum deviation (D1 or D2) is 0.5 pH (for pH502)

or 25 mV (for mV602).

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).

16

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 ava ilabl e in pH5025YZ and mV6025YZ.

CONTROL MODE

The control mode is the normal operational mode for these
meters. During control mode pH 502 and mV 602 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, pH 502 and mV 602 can log working data

through RS485 connection. This data includes:

• pH, mV and oC measured values;

• 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;

STATUS LEDs

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

OFF ---- O N OF F O N

ON OFF ON ON or OFF OFF

RELAY MODES

ON ON OFF ON or OFF Blinking

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).

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

1) ON/OFF, high setpoint (acid dosage) (not for pH5025YZ

and mV6025YZ);

2) ON/OFF, low setpoint (base dosage) (not for pH5025YZ

and mV6025YZ);

3) PID, low setpoint (base dosage, if available);

4) PID, high setpoint (acid dosage, if available).

17

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.

Note The NC contact is available with electromechanical relays only.

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.

18

ON

OFF

Setpoint –
Hysteresis

Setpoint

14

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.

ON

OFF

P.I.D. CONTROL MODE

Setpoint

Setpoint +
Hysteresis

14

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.

t

0

t0+T

t0+2T

c

t0+3T

c

c

During proportional control the process controller calculates
the relay activation time at certain moments t0, t0+Tc, t0+2T
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.

c

19

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

pH

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.

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

20

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.

100%

Controller

output

0

Proportional Band

Error

In pH502 and mV602 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 pH502 and mV602. 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 Tc (from 1 to
30 minutes for models with electromechanical relays and from
5 seconds to 30 minutes for models with SSR).

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).

21

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

from the dosed liquid (at least a 3 pH or 150mV 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

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