PAA-ANALYZERS PRINCIPLE OF OPERATION

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Leo Caldwell
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1 PAA-ANALYZERS PRINCIPLE OF OPERATION The PAA process analyzer comprises a control unit and an analyzing section which includes the reaction chamber and measuring cell, reagent pumps and associated valves and tubing. The control unit monitors the analysis cycle, computes and displays the results, both locally and remotely via the current output or optional serial digital interface. In addition, two limit switches are controlled in accordance with their preprogrammed limit values. Therefore the analyzer not only analyses the value of the measured variable but also has the capability of providing a control function for pumps, valves etc. for the process being monitored. The process analyzer performs an automatic discontinuous analysis of the desired chemical variable in the sample stream. The combined analysis cycle, which includes filling, draining and rinsing of the reaction, chamber and measurement cuvette is controlled by a microprocessor. The sampling takes place through activation of the appropriate sample inlet solenoid valve. Since the reaction chamber/measurement cuvette is filled to a constant volume via an overflow port, a pressure of at least 0.1 bar must be present at the sample inlet valve. The maximum pressure tolerated by the system is 10 bar. For higher sample pressures it is advisable to install a pressure-reducing valve upstream of the sample inlet valve. In addition it is possible to shorten the factory preprogrammed time intervals for the flushing and filling of the cuvette to compensate for elevated sample inlet pressure. These measure results in a shorter analysis time and sample usage by the analyzer. The time required for the different steps during the analysis cycle namely, flushing, filling, measuring and draining is dependent on the analysis being performed by the analyzer.the shortest analysis cycle is set with sample interval set to zero. The PAA process analyser is equipped with a powerful microcomputer in the control unit. An intricate software program allows the operation of the backlit large-screen liquid crystal graphic display. This permits display of an abundance of information with respect to current analysis, operatoring status of the analyser and historical information regarding the last twenty analyses in the form of a graphical display of the measured results. In addition there are two digital limit contacts for external control of the analyser (e.g. remote start-up or shut-down of the unit). The system may be used for complex analysis and may be connected to auxiliary flushing dilution or sampling units. The analyser is supplied ready for service in a stable bisectional steel cabinet, with a window for viewing of the microprocessor control unit. Page 1

2 THE PROCESS PHOTOMETER Photometric analysis measures the absorption of light from a beam passing through the solution, by colored compounds present in the sample. The concentration of the absorbing species is calculated from the intensity of the absorption taking place. Through adding of specific reagents to a solution containing the unknown dissolved material, light absorbing colored compounds may be formed by reaction between the dissolved species and the active ingredient of the added reagent. The color intensity (i.e. light absorption at a defined wavelength) increases with increasing concentration of the measured dissolved compound. The on-line photometer sets the internal zero of the instrument against the sample in the reaction chamber/measuring cuvette before addition of reagent takes place. This compensates for background color and the presence of suspended material in the sample and discoloration or fouling of the cuvette windows. Between 1 and 3 reagents (depending on the analysis being carried out) are added in sequence to the sample in the reaction chamber/measuring cuvette. A predetermined time is required between the addition of reagents to the sample to ensure the intermediate reactions and the completion of color formation. After sufficient reaction time, the absorption of the light of a beam passing through the measuring cuvette is measured. The calculation of the concentration of the analyzed substance follows immediately after analysis of the colored species and is displayed on the display screen of the control unit on completion of the measurement. THE PROCESS TITRATOR Titration means, to find an unknown amount of a dissolved substance by dosing a suitable reagent of known concentration to alter a chemical start condition to a defined final condition.the volume of reagent needed for this reaction is measured and then calculated the amount of dissolved substance The process titrator is able to dose precise volume of reagent into the measuring cell, and to examine the reaction of the substance with a suitable indicator. This indicators may be specially fitted color-indikators for analysis or defined altering of chemicalphysical measured quantity during chemical reaction (conductivity,ph-value). The electronic control system detects during adding precise volume of reagent the progressive conversion of the substance. When the alteration of substance finally comes to the end, the analysis stops and the result is calculated of the amount of reagent needed. Page 2

3 DRAWINGS OF THE ANALYSER AND THE DISPLAY Figure 1 DIAGRAM OF THE PROCESS ANALYSER Top Part: Control unit with keyboard, graphic LCD Bottom Part: 1. Solenoid operated valves 2. Sample inlet needle valve 3. Reaction chamber / measuring cell 4. Reagent bottles 5. Reagent pumps ; = the number depends on the respective parameter Page 3

4 Figure 2: Keyboard for operation and graphic display (Example: Iron analyser) Figure 3: Graphic display, operation mode: automatic Analysis (A) (Example: Iron analyser) Figure 4: Graphic display, operation mode: manual/service operation (H) Page 4

5 Figure 5: Menu-system for parameter -selection and parameter setting (P) Page 5

6 THE KEYBOARD AND DISPLAY The figure 2. illustrates the universal keyboard and display for the PAA on-line analysers. The keyboard on the right comprises 11 waterproof, membrane switches. The display on the left of the illustration is a backlit liquid crystal graphic display. The first line of membrane switches in the keyboard illustrated above are used to select the operating mode of the analyser. These are the Automatic (A), Service (H) and Program (P) buttons. The four buttons in the second row are the keys used to move the cursor when the analyser is in program mode. The first two keys in the third row are used to increase or decrease the numerical value of a user defined programmable parameter or the logical selection of the status of a parameter (e.g. MIN function active or inactive). The E (enter) key is used to fix the changed parameter into the RAM dedicated to this purpose in the microcomputer. The Res. (reset) key is used to clear the userdefined parameter settings and return the analyser to the preset factory standard values. The Reset key is only activated, for security reasons, once the correct user-defined code is entered. Attention: Basic instructions for input/selection/changing of parameters Keys have to pressed at least one second to make sure, that the instrument recognises the key pressing. A menu item can be selected by using the curser keys. A menu item can be changed by using the numerical keys, signed by an arrow with index n (e. g. n). If a menu item has been selected or changed at the display, it has to be sendet to the computer by pressing Enter (E). For some menu items there exists a hotkey function, which is marked in brackets. The selecting and sending (E) procedure kann be done by simply pressing the key which is shown inside the brackets (). OPERATIONAL MODES The control of the analyser allows three basic modes of operation, which on one hand allows setting the system for the required analysis and on the other hand day to-day operation of the analyser. According to the operating mode selected, three different sets of information may be displayed on the screen. Interaction between the operator and the analyser is possible in the service and programming modes i.e. the values on the screen may be altered, applicable to the control of the analyser. During the automatic operation mode, historical and current measurement values and operating parameters are displayed. DESCRIPTION OF THE MEASUREMENT AND CONTROL SECTION All the components comprising the microprocessor-controlled electronics of the process analyser are mounted in a compact housing in the top part of the cabinet.. In addition to the control of the analysis of the samples during the complete measurement cycle comprising rinsing of the reaction chamber/measuring cuvette, sampling, reagent addition, the microprocessor also monitors the status of the measuring cuvette. Four values for maximum or minimum limit switching of two potential-free contacts may be entered to facilitate the control function of the analyser. It is also possible to set an hysteresis function for the four values. All user-defined parameters retained by the microprocessor in the event of a power failure. Page 6

7 INPUT OF PARAMETERS/PROGRAMMING On selection of the Program (Parameter input) button, the analyser operator is prompted to enter the user defined 4-digit code. The input must be carried out within 90 seconds of the request appearing of the screen by operating the directional and numerical cursor (indexed arrows) buttons on the keypad. If the code is not entered within the prescribed time, the analyser defaults to normal i.e. Automatic operating mode. Once the code is selected, the Enter button is pressed. All the operational parameters which may be altered by the user are availabel on the screen when the correct code is entered and the menu items with come into consideration are selected. Movement about the parameter input fields of the screen is effected by operating the directional cursors of the top line in the 2-line section of the keypad. The numerical values of the parameters are increased (or decreased) by using the numerical cursors (n-indexed arrow keys) in the bottom line of keys. Once all the input fields on the parameter input screen have been altered according to the values defined by the operator, the changes are captured by the microprocessor by pressing the E (Enter) key. The menu returns then to selection item of the menu tree before to show to the operator, that his changes are effective and stored in the RAM storage now. Any further changes are made by following the procedure described above and saving the new parameter set by pressing E when the changes are made. Following the alteration procedure, the automatic mode of operation is reaccessed by simply pressing the A (Automatic) key. FUNCTIONAL DESCRIPTION OF RELAY CONTACTS 1/2 AND ALARM Minimum setting: Relay is activated when the measured value falls below a minimum preset value Maximum Setting: Relay is activated when the measured value exceeds a present maximum value Off: Relay remains inactivated and has no limit value Alarm contact: The relay will switch if there is an alarm, e.g.: The remaining running time is less than set The remaining reagent is less than set There is a failure in the photometric system There is no sample water in the measuring chamber The sample water is too murky The measured value is higher than the upper limit that was set ANALOG SIGNAL OUTPUT The signal output delivers a current of 0 to 20 ma or 4 to 20 ma. This signal may be used to supply analysis information to remote data capture stations e.g. a linerecorder or for control purposes. The maximum ohmic load permissible is 500 ohm, and provided this is not exceeded the output signal is proportional to the preset measurement range. The analog output range is preset in the programming mode (see section above). To avoid resolution errors in the output signal, the measurement range must be > 6% of the dynamic range and the start of the measurement output signal must be programmed range less the measurement span.when programming the analog output signal range, the end value (i.e. span) must be greater than the start value otherwise the output will deliver a constant 20 ma. Similarly should the start value be greater than the end value, than the signal output will deliver 0 (or 4) ma. EXTERNAL SWITCHING The process analyser has an external switching function. Switching is made possible by means of a potential-free switch across terminals 18 and 19. The instrument is delivered with a shorting bridge across these terminals. By activating the external switch, the current analyses cycle is interrupted and the analyser enters the standby mode. On closing the external switch, the analyser resumes automatic operation from step 1 in the analysis cycle. This is equivalent to a normal start from key A. By using this input function the analyser may be started by external controls. Additionally it is possible to start the analysis externally by closing this contact and openung it again. If the limit relays 1 and/or 2 are switched on they will be switched off after 15 minutes during the run of this externally started analysis. Page 7

8 SCHEME: VALVES, PUMPS AND STREAMS OF LIQUIDS Page 8

9 General Technical Specifications Backlit liquid crystal graphic display 240 x 128 dots (124 x 70 mm) with the following displays: date, time and value of the last analysis date, time and value of the current analysis remaining volume for reagents 1 to 3 estimated remaining operational time (before reagents are depleted) information regarding the current step in the analysis cycle value and current status of the two limit settings graphical display of the last twenty analyses user-defined parameters may be programmed on input of a code RAM data storage with battery back-up for programmed userdefined parameters automatic selection of five analysis intervals between samples based on user-defined threshold measurement values reset function to factory preprogrammed settings Operating Modes: Manual: Automatic: Service: Program: manual start of an analysis cycle continuous repetition of the analysis cycle according to the intervals predetermined by the threshold measurement values individual valves, stirrer and reagent pumps may be operated from the keyboard input of alarm values and analyses parameters by means of the cursor control keys External Control: One digital input. (external analyser start-up or shutdown. Current Output: 0/4-20 ma, max. load 500 ohm. Programmable 6-100% of the measurement range. Begin 0-94% of the range. Limit Controls: two programmable potential-free limit contacts for maximum and minimum settings. Max. load 250V AC,3A Power Supply: 220V AC/50 Hz (-15 to +10%) Power Consumption: Approx. 17 VA Environmental Protection: IP65 (Microprocessor enclosure) Connections: screw connection terminal rail, 1,5 mm 2 Temperature: 5 to 45 C, at consumption of reagents within 6 months The analytical part contains all components that are necessary for analysis e.g.: electromagnetic valves, photometric measuring chamber with stirrer and PTFE-covered stir bar, max. 3 dosing pumps for reagents including bottles, tubes and pipes and connecting PE-pressure-tubes 6/4 mm, PN 6 A second sample inlet is optional The system is ready to use and completely installed in a two-part steel plate cabinet (1,5 mm steel sheet, WxHxD 600x1000x400 mm; pebble grey RAL 7032, protected according to IP 65, norm cabinet (total weight: about 60 kg Page 9

10 ELECTRIC CONNECTIONS OF THE TERMINAL BLOCK Term. 20,21 Current exit 1 for analysis 1 / sample 1 max. ohmic resistance: 500 Ohm Term. 22,23 Current exit 2 for analysis 2 / sample 2 * max. ohmic resistance: 500 Ohm Term. 18,19 External start of analysis with potential free contact. closed: start Term. 3,4,5 Limit switch GW1 max. 250 V AC / 3A Term. 6,7,8 Limit switch GW2 max. 250 V AC / 3A Term. 9,10,11 Analysis is running Max. 250 V AC / 3A Term. 12,13,14 Alarm contact (combined alarm) Max. 250 V AC / 3A Term. 15,16,17 Relay for rinsing counter ions (only with titrators) Max. 250 V AC / 3A * only in instruments with two consecutive analyses or 2 samples PE,N,L1 Power supply: 230 V AC (-15% / +10%) / Hz Power Consumption: about 17 VA All electric connections are mounted on a terminal block on the back side of the upper part of the casing. You will get access to these connections by undoing the two screws on the right side of the display and turning the swivelling frame to the left. Connecting cables have to have a cross-section of 1,5 mm². Page 10

11 CONNECTION DIAGRAMM TYPE PAA Page 11

12 DIMENSIONAL DRAWING TYPE PAA Page 12

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