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EXPERIMENT NO:-1 Experiment: Study of Temperature process control using PID control.
Apparatus used: •
38-600 Temperature Rig
•
38-490 Digital Display Module
•
38-421 Pulse Flow Transmitter
•
Patch cords
Theory: The basic of this experiment is to control the temperature of the process with the use of heat exchanger. This process contains temperature process rig. The temperature rig has two isolated water circuits. The primary circuit which is used normally as heat source comprises: •
A heater
•
A circular pump
•
A servo valve for flow control
•
A pulse flow meter
•
A header tank
•
A heat exchanger The secondary circuit contains a heat exchanger and a cooler.
The primary circuit is self contained and has to be filled before the system is used. The secondary circuit is normally supplied via flexible hoses, from the Basic Control Rig which is set up to provide a controlled flow. An alternative arrangement is to use a Temperature Auxiliary Control Pack 38-480 to provide a control flow from a mains water tap. Alternatively if we have a 38-610 forced Air Cooling Unit, the water from the Temperature Rig can be cooled and re-circulated.
Role of Thermistor:
2 •
• • • •
•
•
The Thermistor Temperature Transmitter is a device which takes temperature information from the thermistors (T1 - T5) and transmits it to the Process Interface (PI). A thermistor is a device, the electrical characteristics of which alter in a predictable way with a change of temperature. The resistance of a thermistor is a function of the temperature around it, or 'ambient' temperature. The Thermistor Temperature Transmitter reads the resistance value and converts it to a 4-20 mA signal with respect to actual temperature. By converting to the 4-20 mA current signal format, communication is no longer restricted to short distances, a concern when dealing with large process plants. Also by using this format signals and equipment become standardized, removing the need for special interfaces. When using the thermistor and transmitter combination, temperature measurements are carried out to monitor a process parameter. This parameter is monitored and used to determine the control effort that should be applied to control the process correctly. In this experiment Thermistor Temperature Transmitter (TTT) is used. TTT is calibrated against the (38-490) Digital Display Module (DDM). Once calibrated, the TTT can be used to accurately monitor the temperature measured by two different thermistors.
There are five such devices included with the Temperature Process Rig. They are positioned to measure the temperature at five points around the secondary and primary flows. •
•
In the primary flow they are positioned before (T1) and after (T2) the heat exchanger. This is obviously crucial in observing the cooling effect of the heat transfer. In the secondary flow they are also positioned before (T3) and after (T4) the heat exchanger. The fifth device is placed at the output (T5) of the radiator in order to show the temperature of the flow before and after cooling has taken place.
Heat Exchanger: A major element in the topic of process control is the heat exchanger. These devices can be found in so many configurations that a person who has been simply introduced to the science of heat exchangers can be quite perplexed in trying to determine which of the almost limitless types available, many apparently satisfying the required heat transfer duty, should be used. For example, designs which incorporate tubes are only a subset of the many heat exchangers available. However, often the most critical step in the analysis of a heat exchanger is the determination of the overall heat transfer coefficient, U. This, in turn, involves the application of convection and (or) phase change correlation's to find the surface coefficients, h and uses these with the areas, A1 and A2 and wall resistance R , to find the result of the following equation : w
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The determination of pressure drop should be evaluated, as this also is an important design aspect. Some heat exchangers that perform extremely well thermally may however require a very high pumping power. Therefore it is a compromise between these constants when satisfying the specification.
The Secondary Flow: •
•
•
Domestic heating systems often consist of a series of radiators designed to extract energy from hot water being pumped through them. The situation sometimes occurs whereby one or more of the radiators is partly filled with air instead of water. The air does not transfer heat to the metal of the radiator as effectively as the water. This can be demonstrated by the time taken for the element of an electric kettle to become too hot in the absence of water. The air around the element does not remove the energy from it fast enough to prevent over heating. The cooling radiator supplied as part of the Temperature Process Rig can sometimes fall victim to the same problem. Air can be introduced into the system
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•
•
in a number of ways through pumps and joints. This air can find itself trapped in the upper part of the cooling radiator, where it will remain until bleeding can be carried out. Bleeding involves the removal of air from a fluid system by whatever method. The type of domestic system mentioned earlier is usually bled from a small 'tap' on the offending radiator. Air is pushed out under the system pressure until water begins to be expelled. The tap is closed and the radiator is free of air.
Operation of the Cooler: • • •
• •
• •
•
The main reason for the cooler on the Temperature Process Rig (TPR) is to drop the temperature of the heated return fluid (secondary flow). The overall effect of this process is to prevent the secondary flow circuit (water in the tank of the BPR) from heating up too quickly. This is achieved using a cooler, which consists of a radiator and a fan unit, commonly known as an Air blow Water Cooler. The radiator itself comprises an aluminum structure of heat dissipating fins, whereby the fluid to be cooled passes behind. In order to increase the cooling efficiency, a fan is attached to the rear of the radiator to draw air through the radiator dissipating the heat from the fins. It must be noted that coolers of this type can only reduce the temperature to a minimum degree equal to the ambient air temperature. However with respect to the TPR due to the size of the cooler this would actually take a considerable amount of time depending upon the temperature of the BPR fluid. It is therefore shown that the cooler is only intended to provide a degree of cooling to the BPR (if connected). However in industrial applications, a cooler may be the primary source (only source) for cooling a process, in which case its specification would be critical to the dissipation required. Coolers of this type tend to be relatively large with respect to their function. In this particular case the cooler is switched on to demonstrate its efficiency in cooling the secondary flow before returning to the sump tank of the BPR.
In the overall controlling of the process, the roles of the controllers are very important. The functions of the controllers and their role in the process are described briefly described here. Proportional Control: In ON/OFF control, small deviations from the desired value cause just as much movement of the correcting element as large deviations. However, it is frequently convenient to arrange that the position of the correcting element is directly related to the deviation. This is known as proportional control. The following diagram shows that,
5 within the proportional band, there is a unique correcting element position for every value of the controlled condition. Suppose water passing through the secondary circuit of a heat exchanger is being heated by the hot water primary circuit. If the temperature of the secondary circuit is being controlled by regulating the primary flow circuit, via a proportional controller, the primary circuit valve would be half open when the temperature is correct and the flow of water through the secondary was normal. This is point A on the following diagram.
Should the temperature rise or fall, then the primary valve will open or close, in an attempt to restore the temperature to the desired value. If, however, the flow of water in the secondary be suddenly increased, then the primary flow must also be increased to maintain the temperature desired value. The primary valve will open but, as the temperature begins to rise towards the desired value, the valve will gradually close. PI Control of Temperature: Integral control produces a rate of movement of the correcting element proportional to the deviation. Therefore if integral action is added to proportional action, assuming linear operation, the position of the correcting element will be proportional to the magnitude and duration of the deviation. As long as there is offset, the correcting element (valve) will continue to open or close at a rate proportional to the deviation until the desired value is attained. Although offset is eliminated, the combination of proportional with integral action brings two disadvantages: 1. The process takes longer to stabilise than with proportional control only.
6 2. For negative deviation, integral action shifts the whole proportional band above the desired value until the controller condition reaches it. This means that the correcting element does not start to close until it is too late to prevent overshoot. The following diagram shows the output from a controller set to PI following a sudden deviation.
PID Control of Temperature: The derivative component of the control effort enables a controller to recognise a rapidly changing error and take extra action to account for it. By applying a control effort that is not simply directly proportional to the error, the response of the plant has been improved. This ability to recognise a rapid change in the rate of change of the error in a system is very important in many situations. A massive increase in the core temperature of a nuclear reactor, caused by a failure elsewhere in a plant for example, could result in meltdown. By applying a very large control effort, the time taken to reverse the direction of the system (towards failure) can be reduced. It is producing an over compensation for the rapidly changing error to halt its progress. But it is not only overcompensation that a derivative action offers to a system. As the measured value of a system approaches its set point, the rate of change of error will decrease as the proportional action reduces. This reducing error rate will produce a negative control contribution from the derivative term, reducing the control effort further. This applies a breaking effect to the control effort, and reduces the chance of overshoot. The derivative action will pull a system away from failure by producing an overly large control effort, and slow down its approach to the set point with the aim of preventing overshoot. Practical Procedure:
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• • • • • • • •
The water in the primary flow circuit should be about room temperature (20 -30.). The Process Interface (PI) and the Process Controller (PC) should be switched on and then the patching diagram is completed. In this experiment the Process Controller is only being used as an interface to the computer. The positions of all the thermistors (1 to 5) are located and their significance with respect to the flow circuits is determined. Now observed the readings of T1 (TTT is set to position A). Now by clasping hand around T1 (gripping tightly) , both the TTT reading and the chart recorder are recorded. Next it is switched to B and the temperature is observed which should be the same as T1. Then switch to A-B. This will give the temperature difference between A and B (T1 and T2).
Patching Diagram:
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Observation table: T1= Temperature before entering heat exchanger. T2= Temperature after coming out of heat exchanger T3= Temperature of water. T4= Temperature of exhaust. TEMPERATURE T2
SET MEASURED PROPORTIONAL Ti(sec) Td(sec) POINT VALUE BAND (%) 40 40.3 38 26 12
EXPERIMENT NO-2
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Name of the experiment: PRESSURE PROCESS RIG
Objective of the experiment: 1) To control pressure with or without air receiver 2) To control pressure with or without air receiver when different valves (i.e.,
V1,V2,V3,V4,V5,V6) are opened or closed
Apparatus required: 1. 2. 3. 4. 5.
38-200 process interface. 38-200 process controller. 38-714 pressure control module. 38-461 pressure transmitter. 38-490 digital display modules.
The equipment requires a supply of clean compressed air.
Theory: Pressure Process Control System is a single loop pneumatic control system which allows study of the principles of process control pressure as the process variable to be controlled. The Pressure Process Rig consists of a low pressure air circuit supported on a bench-mounted panel, making it suitable for individual student work or for group demonstration.
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The circuit includes: • • • • • • • • • • • • •
Input supply filter/drier. Input converter. Pneumatically operated control valve. 2 Regulators. 4 Manual valves. 6 Gauges. Sight flow meter. Orifice Block with changeable Orifice Plates. Differential Pressure Sensor. Process Pressure Sensor. 27 liter Air Receiver Tank 20 psi Safety Relief Valve. Diffusers.
11 The system includes those pneumatic control components of interest to the process industries. The design allows the study of component operation and connection to the electrical control devices through the use of pressure/ current transducers. The unit consists of a pipeline on which are mounted a Pneumatic Control Valve, Orifice Block and pressure tapings. The flow discharges directly to the atmosphere or via an Air Receiver to vary the process lag. The valve is operated from a Current to Pressure converter, and sensors for direct and differential pressure facilities measurement of pressure and flow respectively. Process Rig Controller: The unit is designed to operate with the 38-200 and 38-300 Process Interface and Process Controller to configure open or closed loop control circuits. The pneumatic instrumentation comprises an I/P Converter and Pneumatic Control Valve. The I/P Converter accepts a 4-20 mA control signal from the 38-200 Interface and converts this to a 3-15 psi pneumatic signal which operates the control valve. Rig Control Valves: The control valve comprises the diaphragm actuator which positions the stem of a plug type valve. An indicator on the valve shows the actual position on the valve. A set of manual valves V1,V2 and V3 allow a rear mount air receiver to be connected in series or in parallel with the process pipe to change the response of the system (to vary the process lag). The air receiver incorporates a pressure relief valve. Step changes may be applied to the process by bleeding air through an additional diffuser by opening and closing the valve V4. The Discovery software contains an integral Chart Recorder and configuration program.
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Patching diagram:
Fig: Pressure Process Rig Procedure: 1. Connect the equipment as shown in the patching diagram with the air receiver in series with the process pipe. 2. V2, V4 & V6 to close. 3. Set R1, R2, V1, V3 & V5 to open. 4. Adjust R1 to give 25psi on G1. 5. Adjust R2 to give 10psi on G5 with the pneumatic control valve open. The chart recorder is connected to the pressure transmitter output/controller input and controller output/current to pressure converter input 4-
13 20mA loops to provide a record of the response. The set point of the system has been set to 50%. Observe the response of the system when valve V4 is open and close to give a disturbance. Isolate the air receiver from the process pipe by opening V2 and closing V1& V3. Apply a disturbance to the new configuration and observe the change in response.
Observation table: 1) To control pressure with or without air receiver
Type
Proportiona l Band (%)
Integral Action
Derivative Action
Set Point (%)
Measured value (%)
Flow Pneumatic meter Control (lt/min) Valve (%)
With air receiver
200
12
6
65
63.8-65.2
7.9
75
Without air receiver
250
55
35
52
51.6-51.5
19
25
2) To control pressure with or without air receiver when different valves (i.e.,
V1,V2,V3,V4,V5,V6) are opened or closed
With air receiver: V1
V2
V3
V4
V5
V6
Proportiona l band (%)
Tr (sec)
Ti (sec)
Measured Set value (%) point(%)
ON
OFF
ON
OFF
OFF
ON
243
28
27
50.1-52
50
ON
OFF
ON
OFF
ON
ON
123
31
30
49.6-49.8
50
ON
OFF
ON
ON
ON
ON
173
23
19
53.6-53.7
53
Without air receiver: V6
Proportiona l band (%)
Tr (sec)
OFF OFF OFF
ON
152
24
24
53.6-53.8
53
ON
OFF OFF
ON
ON
162
27
24
51-51.2
52
ON
OFF
ON
ON
185
30
6
35.4-35.6
35
V1
V2
OFF
ON
OFF OFF
V3
V4
ON
V5
Ti Measured Set (sec) value (%) point
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EXPERIMENT NO-3 Name of experiment: STUDY OF FLOW PROCESS CONTROL.
Objective: To control the flow of liquid using PID control.
fig: PID control of flow of liquid using sensor.
Apparatus required: 1. Basic process Rig 38-100 2. Digital Display Module 38-190 3. Float Level transmitter 38-101 4. Process interface 38-200 5. Process controller 38-300 6. Patch cords 7. Pulse flow transmitter 38-401
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THEORY: A small change in the control effort will after the position of the servo valve only slightly, but this will affect flow rate quite considerably. It can be said that the process plant has a very high gain; a small change in the input (control effort to servo position) produces a large change in the output (flow rate). If the deviation between measured value and set point is changing rapidly this will produce a large and so that care must be taken when selecting parameters values the expression below is that made previously for PID control law. Uc =Up + Ur + Ud =k [e+ (1/Tr) + ∫edt +Td (de/dt)]
The greater PB is (so that smaller the system gains k), the smaller the overall control effort will be for a measured deviation. This allows for a greater range of derivative configuration to the control before it produces the oscillation. In the practical, the PID values have been set such that the least performance is achieved out of the system. We can observe changes, nothing the system response on the chart recorder. In the second part of the practical we can investigate the effect of the deviation action on the overall system. We will be able to change Td from 0 to 5 seconds, in 1 sec division. Although this sounds very small, it will be enough of a variation to observe all consequences derivation action in the control effort, both good and bad. We will be able to switch derivative action ‘ON’ & ‘OFF’ as we did integral action earlier. By providing the same deviation (changing the set point) in the most convenient way of design this I to PI and PID algorithm, we will be able to observe the difference between them. There will be control bars on screen to change the set point, Tr and Td. We will not apply Td>5 sec. As these will already be instability at this level, although there is of a control bar on screen for PB gain, we are still able to change it using the manual keys on the front of 38-300.
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Patching diagram:
Procedure: i. ii. iii.
iv.
Complete the patching diagram. Ensure the control action set to reserve and then at the input filter FLT.1 is set to 1 second. Fully open MV3.When initializations is complete switch ON the pump and switch the controller to automatic mode. Thus the practical begins in PI mode, with optimum values set for PI. The derivative action is logged with the onscreen button. When the derivative component time is variable with the control bars. Derivative action improves the response of controller to a rapidly changing error and provides a breaking effect. When the measured variable approaches the set point.
17 v.
Integrating the overall performance of the system with respect to set point changes, remembering that the system has been configured for optimum performance. Note for this part of the experiment only the set point will be changed. Once we have a feel for how the system performs, experiment with the full range of Td and observe the response of the system to set point changes. We will find that it is very easy to produce instability in the system, both in a steady state situation & also when attempting to reach the set point.
vi.
Observation table: Serial No
Set point
1. 2. 3. 4.
50 60 68 75
Flow Meter range (%) 49.8 to 50.6 59.4 to 60.5 66.7 to 68.7 73.8 to 74.9
Control valve opening (%) 11 24 38 49
PB (% ) 101 123 153 192
Tr(Tc) (sec)
Td(sec)
13 19 22 20
5 4 5 7
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Experiment No.-4 Experiment name: Level control of coupled tank system.
Objectives: Our aim is to control the water level in different tanks under various conditions and set point.
Apparatus used: 1. UCI (Universal Control Interface) 2. PSUPA (Power Supply Unit and Power Amplifier) 3. Digital display module 4. Patch Cords
Description of the process: The Coupled Tanks Unit consists of 4 tanks placed on a rig. Fifth reservoir tank is placed at the bottom. In the reservoir two submersible pumps are placed, which pump the water on command to the tanks. The water flows freely to the bottom tanks through the configurable orifice. The way the water flows through the setup can be configured in many ways with manual valves labeled (MV1, MV2, MV3, MV4 ). Configuration with valves allows for dynamics coupling introduction and disturbances generation giving vast possibilities of control. Apart from the mechanical parts the coupled tank system is equipped with power supply unit and power amplifier (PSUPA) and the universal control interface (UCI). The UCI serves as an interface between the PC and the PSUPA. Further more it can be used as a controller. The PSUPA unit amplifier the water pressure-level signals and passes them an analog signal to the UCI and PC. The pumps control signal can be sent from the PC through the UCI and PSUPA or from the UCI to the PSUP
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Diagram of coupled tank:
Fig-Coupled tank mechanical unit
PID CONTROLLER: A PID Controller consists of three blocks—proportional, integral and derivative. Each of PID controller blocks (P, I and D) plays an important rule. A PID controller calculates an error value as the difference between a measured process variable and desired set point. The equation governing the PID controller is as follows-
U(t)=kpe(t)+ki∫t0e()d+ kd d/dt e(t) The proportional block is mostly responsible for speed of the system reaction. However for oscillatory plants it might increase the oscillations if the value of P is set to be too large. The integral part is very important and assures zero error value in the steady state, which means that the output will be exactly what we want it to be. Nevertheless the integral action of the controller causes the system to response slower to the desired value changes and for system were very fast reaction is very important it has to be omitted .Certain nonlinearities will also cause problem for the integration action. The Derivative part has been introduced to make the response faster. However, it is very sensitive to noise and may cause the system to react very nervously. Thus, very often it is omitted in the controller design. Derivative part output filtering may reduce the nervous reaction but also slows the response of the controller down and sometimes undermines the sense of using the Derivative part at all proper filtering can help to reduce the high frequency noise without degrading the control system performance in the lower frequency band.
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Procedure: 1. Connect the UCI 2. Make sure that the PC connects to UCI. Proper connection manage should appear in the ‘Connection Console’ window on LCD. 3. Select the UCI control mode from the working mode selection menu. 4. From the list of possible control schemes select the water level control in tank1/tank 3. 5. In the middle of control panel the plots of the measurements and control signal are presented. 6. On the right, the displays are presented with the numerical representation of the measurement. 7. On the left, a panel is placed for control parameters setting and variation. 8. At the top the main menu is displayed. 9. While the experiment is running the controller parameters and the set point value can be change. 10. If we are sure the configuration is correct we may start the control algorithms, by pressing the ‘starts simulation’ button. 11. Observe the water level being controlled by the simple PI controller. We may change the set point character using the provided interface. The controller parameter can also be varied.
Observation Table:
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For Tank 1Set point
P
I
D
20
0.896
0.052
0.001
EXPERIMENT NO:5 Name of the experiment: To study pH control
Objective of the experiment: To study the ON/OFF control of pH process control. Apparatus used: •
digital display module
•
patch cord
•
effluent holding tank
•
reagent holding tank
•
pH probe
Description of the process: The 38-716 pH process rig is designed to demonstrate the problems associated with the control of processes for treatment of industrial effluents. The pH rig comprises two independently pumped fluid circuits mounted on a bench- top panel which allows the study of the principles of process control using the pH of the mixed effluent and reagent fluids as the process variable. The constituent components of the 38-716 are similar to those which can be found in many industrial situations. The rig is specifically designed to withstand attack from strong acid and alkaline fluids. The effluent holding tank, reagent holding tank. Reaction vessel, treated fluid tank, circulating pumps, variable area flow meters, manual flow control valves,
22 solenoid valves, motorized servo control valve and appropriate pipe work and fittings are mounted on a support frame which is designed to stand on a firm level bench top. The rig is designed to be operated in conjunction with the 38-200 Process Interface and 38-300 Process Controller.
Fig.1: a pH rig The effluent to be treated is stored in the effluent tank and fed, via a filter, to a constant speed high pressure circulating pump. The pump automatically switches off if back-pressure becomes too great, i.e., at approximately zero flow. The switch-off point is adjustable on the pump. The effluent passes through a flow meter/ manual needle-valve combination and a manual/electric on-off solenoid valve SV1 before entering the reaction vessel. The reaction vessel incorporates a double-paddle contra-rotating agitator which is driven by two fixed speed electric motors and gearing. The agitator and solenoid valves are all supplied with power by connecting a 24 V dc supply from the 38-200 to the appropriate socket on the side of the 38-716. The pH rig components: 1) Control valve:
23 The control valve, in the reagent feed circuit, has been developed to provide a small scale demonstration of an infinitely variable control valve. The needle-type valve is driven by a dc electric motor is provided by the 38-200.
2) pH probe: The pH probe incorporates an integral connecting lead which terminates in a DIN plug and connects to the pH Transmitter, 38-471 where the pH value is converted into a 4-20 mA current signal. A 7-way DIN lead (provided) from the 38-471plugs into one of the four process connections on the 38-200 and allows the 4-20 mA signal to be used for control or monitoring purposes. The current output from the 38-471 may be calibrated using the ‘zero’ and ‘span’ controls on its front panel. A small screwdriver may be used for this purpose. A set of capsules to make up buffer solutions is also supplied for the purpose of calibration. When dispatched, the transmitter is set up to give an operating range of 49 pH equating to a 4-20mA output. The transmitter may be re-calibrated to correspond to a different range if required.
pH processes: The On-Off Control features: The parts of the process interface(PI) that have not yet been discussed are the On-Off Control features, including 0-5 V Schmitt trigger with variable hysteresis loop (providing variable threshold voltages) and logic inputs to the switched ac and 24V dc outputs. These are located next to the power unit-circuit breaker arrangement. The On-Off Control found on the Process Interface can be used to control a process without using the 38-300 Process Controller. The later assignments explore the use of on-Off Control using the microphones controlled 38-300.
Patching diagram:
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The patching diagram is completed before performing the experiment. To calibrate, the following steps are done: The current source knob is turned fully anti-clockwise, to the 4 mA setting. Noting the value of the current in the loop shown by the DDM. The zero setting is adjusted, using a small screwdriver, so that the value of the current shown on the DDM is exactly 4mA. The current source is calibrated against the DDM. The DDM is not a highly accurate measurement device, but the purpose here is to illustrate the process of calibration and to provide sufficient accuracy to the practical that follow. In this respect, the DDM is satisfactory. If greater accuracy is required, a digital millimeter may be used instead. PC: The 25-way D type connector is connected from the PC serial port to the left hand connector on the 38-300 Process Controller rear panel. The Connection switch (38-300 rear panel ) is ensured that it is set ON. And the Termination switch (38-300 rear panel) is set to ON.
Adding effluent and reagent and removing treated fluid:
25 Before starting an experiment it is ensured that the effluent (left) tank is filled with 0.05M Sodium Hydroxide and the reagent tank (right) with 0.05M Hydrochloric Acid. It is ensured that the treated fluid tank is emptied into a vessel of adequate capacity at the conclusion of each experiment and is emptied at the start of an experiment.
