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Page 59

22.3  pH Temperature Compensation 

The slope of a pH sensor (section 22.2) is affected by temperature. This 
effect is compensated for either by using an Automatic Temperature 
Compensation (ATC) probe or by entering the sample temperature 
manually.  Figure  22-3 shows the slope of a pH sensor at various 
temperatures. 

-600

-400

-200

0

200

400

600

0

7

14

Millivolts

Electrode
Potential (mV) at
0 oC (54mV/pH)

Electrode
Potential (mV) at
50 oC (64mV/pH)

Electrode
Potential (mV) at
100 oC
(74mV/pH)

pH Electrode Response, as a Function of Temperature

pH

 

Figure 22-3 

22.4  Checking the reference junction of a pH sensor 

If pH readings are inaccurate or unstable, the reference junction of the 
sensor may be blocked. The following test can be performed to 
determine if the reference junction of a pH sensor is making adequate 
contact with the sample solution. 
1.  Calibrate the 

WP-91

, as per section 5. 

2.  Dilute 1 part of pH6.88 buffer with 9 parts of distilled water. 
3.  Measure the pH of the diluted buffer. The result should be 7.06 +/-

0.02 pH. 

4.  If the value obtained is outside of these limits, then clean the 

reference junction, as per the instructions supplied with the pH 
sensor. 

5.  Re-calibrate the 

WP-91

 and repeat the test. 

6.  If the value obtained is still outside 7.06 +/-0.02 pH, then the sensor 

should be replaced. 

 

Summary of Contents for WP-91

Page 1: ...troduction has a diagram and explanation of the display and controls of the WP 91 It also contains a full listing of all of the items that you should have received with your WP 91 Please take the time to read this section as it explains some of items that are mentioned in subsequent sections 3 Main Section The main section of the handbook provides complete details of the WP 91 including operating ...

Page 2: ...7 3 Calibration Messages 21 8 Salinity Correction 22 9 Altitude or Atmospheric Pressure Correction 23 9 1 Selecting Altitude or Pressure Correction 23 9 2 Changing the Altitude or Pressure Correction value 24 9 3 Notes 25 10 Good Laboratory Practices GLP 26 10 1 To recall GLP information on the display 26 10 2 Failed Calibration 28 10 3 Printing GLP Information to the RS232 Port 29 10 4 Instrument...

Page 3: ...WP 91 47 19 Instrument firmware version number 47 20 Troubleshooting 48 20 1 General Errors 48 20 2 Dissolved Oxygen Troubleshooting 49 20 3 pH and mV Troubleshooting 50 20 4 Temperature Troubleshooting 51 21 Dissolved Oxygen Sensor Fundamentals 52 21 1 Operating Principle 52 21 2 Maintenance Of The Membrane 53 21 3 Probe Storage 53 21 4 Notes On Units Of Dissolved Oxygen 54 21 5 Equilibrium Condi...

Page 4: ...Page 4 1 Introduction 1 1 WP 91 Display and Controls ...

Page 5: ...S232 port optional See section 13 2 Only used within the menu system on the WP 91 Press to access the user friendly menu system which makes the WP 91 easy to operate and The and keys are used for calibrating temperature readout section 7 1 setting the clock section 16 1 setting the automatic logging period section 12 and displaying GLP information section 10 1 The key is also used to initialise th...

Page 6: ... 123221 3 Combination pH Sensor 1m 121207 4 Combination Redox Sensor 1m 121262 5 Temperature Sensor 1m 121253 when Dissolved Oxygen sensor not in use 6 RS232 Serial Interface Option includes cable 130039 7 WinTPS Communication software for Windows 95 Later 130086 8 Hard Carry Case 130059 9 Battery charger lead for 12V cigarette lighter socket 130046 10 Solar Panel 130012 11 RS232 Printer 130031 Ot...

Page 7: ...Temperature 10 0 to 120 0 O C 0 1 O C 0 2 O C Oxygen ranges subject to sensor performance Additional Dissolved Oxygen Specifications Salinity Correction User set from 0 to 50 0 ppK NaCl Barometric Pressure Correction User set from 800 to 1100 HPa Altitude Correction User set from 0 to 5000m Temperature Compensation Dual automatic temperature compensation system DO2 Sensor Span Range 70 to 135 DO2 ...

Page 8: ...Page 8 Temperature Compensation Automatic 0 to 100 O C ...

Page 9: ...rt but is not displayed Battery Saver On Auto switch off after 5 minutes Off Continuous use Bar Graph display of battery charge level Readout of battery voltage available for troubleshooting Good Laboratory Practices Date Time and Value of last Dissolved Oxygen pH and Temperature calibration are stored along with Altitude or Pressure setting at time of calibration This information can be recalled ...

Page 10: ...H not available in mV mode F4 Sal or F4 Altitude or F4 Pressure or F4 Comp F1 Sal see sections 8 9 F2 Altitude or F2 Pressure F2 Notepad F1 Recall F2 Erase F3 Print F4 Prog F1 Min F2 Sec F3 Hr Set F3 Mode F1 ppM Man Sal Comp F1 OFF or F2 ON F2 Sat F3 Gas F4 mV or F4 pH F4 Setup F1 Batt F1 OFF F2 ON F2 Clock F3 GLP F1 Recall F3 Print F4 Baud F1 300 F2 1200 F3 9600 These functions available when RS2...

Page 11: ...0 M 7 00 25 0 12 00 00 2 F2 Sat Saturation readout Displays Saturation Dissolved Oxygen pH mV and Temperature readings simultaneously See section 16 2 for details on how to alternate temperature and date 100 0 S 7 00 25 0 12 00 00 3 F3 Gas Gaseous readout Displays Gaseous Oxygen pH mV and Temperature readings simultaneously See section 16 2 for details on how to alternate temperature and date 20 9...

Page 12: ...Page 12 2 The decimal point is replaced by a if a Dissolved Oxygen pH or Temperature calibration has failed sections 4 5 7 or if the unit is initialised section 18 ...

Page 13: ... 100mL of distilled water A 50g bottle of Sodium Sulphite powder is supplied with a new Dissolved Oxygen sensor for this purpose part number 123302 b Allow the reading to stabilise at or near zero This may take 2 3 minutes c Select Oxygen Calibration F1 Cal F1 Oxy d Press the key to calibrate e A will not be removed from the display after a Zero Calibration 7 Rinse the Dissolved Oxygen sensor in d...

Page 14: ...Page 14 9 The WP 91 is now calibrated ready for Dissolved Oxygen measurement ...

Page 15: ...e membrane is replaced In applications where there is a low level of dissolved oxygen a zero calibration may have to be done weekly 2 An air calibration should be performed at least weekly or when the membrane is replaced Of course more frequent calibration will result in greater confidence in results 3 The salinity correction value is ignored during zero and air calibration There is therefore no ...

Page 16: ...zero value of the sensor Zero Cal OK Zero 0 0 2 If an Air calibration has been successfully performed the WP 91 will display the following message and the span value of the sensor Air Cal OK Span 100 0 3 If an Air calibration has failed the WP 91 will display the following message and the failed span value of the sensor Air Cal Fail Span 65 0 ...

Page 17: ...librated 5 Remove the wetting cap from the pH sensor 6 Rinse the pH and Dissolved Oxygen Temperature sensors in distilled water and blot them dry 7 Ensure that you are using the primary buffer for which the WP 91 has been set see section 17 Place both sensors into a small sample of pH6 88 or pH7 00 buffer so that the bulb and reference junction are both covered as per the diagram over the page DO ...

Page 18: ...10 00 Buffer so that the bulb and reference junction are both covered as per the diagram in step 8 above DO NOT place the sensors directly into the buffer bottle NOTE pH9 23 and pH10 00 buffers are highly unstable Avoid using these buffers if possible Discard immediately after use 12 Select pH Calibration F1 Cal F3 pH 13 When the reading has stabilised press the key to calibrate The will now be re...

Page 19: ...ail Asy 1 50 or 1 Point Cal Fail Asy 1 50 3 If a 2 point calibration has been successfully performed the WP 91 will display the following message and the asymmetry and slope of the sensor 2 Point Cal OK Asy 0 10pH then 2 Point Cal OK Slope 100 0 4 If a 2 point calibration has failed the WP 91 will display the following message and the failed slope value of the sensor 2 Point Cal Fail Slope 130 0 o...

Page 20: ...s shown on the left 25 0 20 0o c Set F1 Cal 6 When the reading on the right has stabilised press the and keys until the reading on the left shows the same temperature as the mercury thermometer 7 Press the key to calibrate the temperature readout The will now be replaced by a decimal point if calibration was successful Alternatively press the key to abort temperature calibration 7 2 Calibration No...

Page 21: ...sfully performed the WP 91 will display the following message and the offset value of the probe Calibrate OK Offset 1 0O c 2 If a temperature calibration has failed the WP 91 will display the following message and the failed offset value of the probe Calibrate Fail Offset 10 5O c ...

Page 22: ...selected then the above key sequence becomes F1 Cal F4 Comp F1 Sal 3 The current salinity correction value is now displayed 36 0 ppK Sal Set F1 Save 4 Press the and keys until the display shows the desired salinity correction value 5 Press the key to save the salinity correction value Alternatively press the key to quit and retain the current setting 6 When manual Salinity correction is selected a...

Page 23: ...e or Pressure mode menu is now displayed F1 Altitude F3 Pressure F4 OF F The arrow indicates the current selection 4 Press to select Altitude correction Press to select Atmospheric Pressure correction Press to switch the Altitude or Pressure correction system OFF 5 If or was selected then the WP 91 now asks for the altitude or pressure 0 m Alt 12 Set F1 Save or 1013 HPa Set F1 Save Press the and k...

Page 24: ...i e F1 Cal F4 Altitude or F1 Cal F4 Pressure If Manual Salinity Correction is currently selected then the above key sequence becomes F1 Cal F4 Comp F2 Altitude or F1 Cal F4 Comp F2 Pressure 3 The WP 91 now asks for the altitude or pressure 0 m Alt Set F1 Save or 1013 HPa Set F1 Save Press the and keys to set the desired Altitude or Pressure 4 Press the key to save the Altitude or Pressure value 5 ...

Page 25: ... for most Dissolved Oxygen measurements 3 If Altitude correction is switched on an A is added to the WP 91 display in normal measurement mode 4 If Atmospheric Pressure correction is switched on a P is added to the 5 WP 91 display in normal measurement mode 6 Saturation and Gaseous readings are normalised to sea level 0m and 1013HPa when altitude or atmospheric pressure compensation is in use 7 ppM...

Page 26: ...LP menu F4 Setup F3 GLP 3 Select F1 Recall from the menu 4 The instrument model firmware version number and instrument serial number are displayed along with a prompt describing how to scroll through the GLP information WP91 V1 2 S1234 More Back 5 Press the key to sequentially scroll through the GLP information for all parameters Press the key to scroll back to previous data The sequence of inform...

Page 27: ... 11 10 OR Pressure 1013HPa 31 12 03 11 10 pH Asy 0 10pH 31 12 03 11 20 pH Slope 100 0 31 12 03 11 30 Temp Offset 1 0o c 31 12 03 11 30 Exit Altitude OR Pressure are displayed only if either one was switched on at the time of the last calibration pH Asymmetry and pH Slope calibration data is displayed only when the pH mode is switched on ...

Page 28: ...results of the last successful calibration These calibration values are still used if further measurements are taken without recalibrating The following are examples of failed calibrations Oxy Zero 0 0 00 00 00 00 00 Oxy Span 100 0 00 00 00 00 00 pH Asy 0 10pH 00 00 00 00 00 pH Slope 100 0 00 00 00 00 00 Temp Offset 1 0o c 00 00 00 00 00 ...

Page 29: ...32 port F4 Setup F3 GLP F3 Print 4 The GLP information is sent to the RS232 port in formatted ASCII text for example WP91 V1 2 S1234 31 12 03 12 00 Oxygen Zero 0 0 31 12 03 11 00 Oxygen Span 100 0 31 12 03 11 10 Oxygen Altitude 5000m 31 12 03 11 10 Oxygen Pressure 1013HPa 31 12 03 11 10 pH Asy 0 10pH 31 12 03 11 20 pH Slope 100 0 31 12 03 11 30 Temperature Offset 1 0oC 31 12 03 11 40 ENDS Notes 1 ...

Page 30: ...H mV Temp where S1234 is the serial number The serial number is display when recalling the GLP information section 10 1 The serial number is included on the printout of GLP information section 10 3 10 5 Additional GLP Features Another GLP requirement is to record the date and time of every reading The WP 91 does this for you when readings are recorded either with the Notepad function section 11 or...

Page 31: ...eat steps 1 2 as often as required The maximum number of readings that can be stored in the Notepad is 150 11 2 Recalling Records from the Notepad To recall records from the Notepad onto the WP 91 display 1 Select the Notepad menu F2 Notepad 2 Select F1 Recall from the menu 3 Record number 1 is now displayed An s is displayed if manual Salinity correction was switched on when the reading was recor...

Page 32: ...n when the reading was taken For example 31 12 12 00 00 36 0 K 1 F2 Dat 31 12 12 00 00 1000m 1 F2 Dat 6 Press to move forward through the records Press to move backward through the records Press and hold the or keys to roll rapidly through the readings 11 3 Erasing Records from the Notepad To erase all records from the Notepad 1 Select the Notepad menu F2 Notepad 2 Select F2 Erase from the menu 3 ...

Page 33: ...n the cable for long term use if required 2 Connect the other end of the RS232 cable to an RS232 Printer or to COM1 or COM2 of a PC 3 Ensure that the baud rate for the printer or PC and the WP 91 are the same If necessary alter the baud rate of the WP 91 see section 13 1 The WP 91 uses XON XOFF protocol Ensure that the printer is set accordingly 4 Select the Notepad menu F2 Notepad 5 Select F3 Pri...

Page 34: ...ct whether this period is in minutes seconds or hours Press to save the period as minutes Press to save the period as seconds Press to save the period as hours eg If the period was set to 05 followed by then the WP 91 will automatically log a record every 5 seconds 5 If the optional RS232 port is fitted the WP 91 will ask if the records are to be logged into the Notepad or sent directly to the RS2...

Page 35: ... The WP 91 will beep each time a record is sent to the RS232 port 8 Press to stop automatic logging Notes 1 The clock must be set before the WP 91 will allow automatic logging to start The message Clock Not Set is displayed if the clock is not set 2 The Battery Saver function section 14 is disabled while the meter is in Automatic Datalogging mode to stop the meter switching off while logging data ...

Page 36: ...abled if the automatic logging period is set to greater than zero see section 12 Records can be sent directly to the RS232 port rather than stored in memory during automatic datalogging See section 12 for details 13 3 RS232 Configuration The WP 91 RS232 configuration is 8 bits No Parity 1 Stop Bit XON XOFF Protocol 13 4 Communication and Statistical Software Communication between the WP 91 and a P...

Page 37: ...Page 37 Information on how to use the software and import data is provided in the manual provided with the WinTPS CD ROM ...

Page 38: ...ll logged records from the WP 91 memory The data ends with the message ENDS cr Erase logged data E cr Erases all logged records from the WP 91 memory Returns the message ERASED cr to confirm that the records have been erased Request status information S cr Returns the model name firmware version number instrument serial number and number of logged readings in memory eg WP91ssV1 2sS1234s9999 cr whe...

Page 39: ...nit description CCCCCC is the Altitude OR Pressure correction value Maximum 6 characters right justified uuu is the unit description either ms ss s for Altitude or HPa for Atmospheric Pressure PPPPPP is pH or mV Data Maximum 6 characters right justified uuu is the unit description either pHs s for pH or mVs s for mV TTTTTT is the Temperature Data Maximum 6 characters right justified oCs s is the T...

Page 40: ...gen Altitude 5000m 31 12 03 11 10 Oxygen Pressure 1013HPa 31 12 03 11 10 pH Asy 0 10pH 31 12 03 11 20 pH Slope 100 0 31 12 03 11 30 Temperature Offset 1 0oC 31 12 03 11 40 ENDS Notes 1 Either Altitude OR Pressure is sent depending upon which was selected the last time the instrument was calibrated If the Altitude or Pressure Correction was switched off at the time of the last air calibration then ...

Page 41: ...le Open 2 In the Files of type pull down box choose Text Files prn txt csv 3 Navigate to the folder where your data file is stored and double click it to start the Text Import Wizard Note The default data folder for the WinTPS software is C My Documents WinTPS 4 In step 1 of the Text Import Wizard select Fixed width as per the sample screen below then press Next Continued over the page ...

Page 42: ... the scroll bar across to access Temperature Press Next after the column breaks have been inserted 6 Simply press Finish at step 3 of the Text Import Wizard TPS recommends that the data format for each column be set once the data is in spreadsheet format For help on formatting the data columns charting graphing or other operations please consult the Microsoft Excel help file Alternatively please c...

Page 43: ...e battery saver function for continuous use Press to enable the battery saver function The meter will switch itself off if no key has been pressed for five minutes Press to quit the battery saver menu and retain the current setting Notes 1 For troubleshooting purposes the battery volts can also be displayed in the battery saver menu Press to display battery volts 2 The symbol flashes when the batt...

Page 44: ...echarge the battery 1 Plug the battery charger solar panel or car cigarette lighter adaptor into the Charger RS232 socket DO NOT plug into the Temperature socket as this will damage the WP 91 2 Charge for approximately 8 hours for full capacity The WP 91 has special circuitry to prevent overcharging so the charger can be used continuously To ensure optimum battery life and capacity the WP 91 shoul...

Page 45: ...s Press the and keys until the minutes are correct 8 Check that the date and time are correct Press to save the settings If any changes are needed press the key to move left to the desired position Press to quit without resetting the clock Note The WP 91 does not test for a valid day of the month when setting the clock eg attempting to enter 31 02 04 is not corrected 16 2 Displaying or Hiding the ...

Page 46: ...retain the current setting 5 The display will now show the currently selected high pH buffer Select F1 9 23pH Buffer F2 10 0pH The arrow indicates the current selection 6 Press to select pH9 23 as the high pH buffer Press to select pH10 00 as the high pH buffer the display shows 10 0 for the latter but this buffer is stored as pH10 00 Press to quit buffer selection and retain the current setting 7...

Page 47: ...Temperature Note that the decimal points have been replaced with a to indicate that the unit requires recalibration 5 When the WP 91 is initialised a The manual salinity correction value is re set 36 0ppK See section 8 if you wish to change this value b The Altitude or Pressure correction system is switched off and the instrument assumes sea level 0m and 1013HPa c The automatically recognised buff...

Page 48: ...f off when the battery falls below 5 00 volts Meter displays the word OFF and switches off Battery is below 5 00 volts Recharge the battery If this fails check the charger If charger OK replace the battery Meter will not turn on 1 Battery is exhausted 2 Faulty Instrument Recharge the battery If this fails check the charger If charger OK replace the battery Return to factory for repair Battery does...

Page 49: ...refill sensor Gently pump the pressure compensation diaphragm to re flush the membrane Membrane should be smooth and convex with no wrinkles Re fit membrane if necessary Replace membrane and re fill sensor Return sensor to factory for repair or replacement Blackened Silver anode wire Sensor has been exposed to sulphides or other chemical poisoning Return to the TPS factory for cleaning and service...

Page 50: ...been set see section 17 Clean glass bulb as per instructions supplied with the sensor Attempt rejuvenation as per instructions supplied with the sensor If not successful replace sensor Dry in a warm place Replace buffers Unstable readings 1 Electrolyte chamber needs to be refilled 2 Reference junction blocked 3 Glass bulb not clean 4 Bubble in glass bulb 5 Faulty connection to meter 6 Reference ju...

Page 51: ...ulb or internal stem cracked Replace sensor 20 4 Temperature Troubleshooting Symptom Possible Causes Remedy Displays OVR o C when Dissolved Oxygen or Temperature sensor is plugged in 1 Faulty sensor 2 Faulty instrument Return sensor to factory for repair or replacement Return to factory for repair Temperature inaccurate and cannot be calibrated 1 Faulty connector 2 Faulty Dissolved Oxygen or Tempe...

Page 52: ... the plastic is 0 025mm intermediate density polyethylene sheet PTFE Teflon can be supplied for special applications It must be realised that using membranes of very different thicknesses will result in an error in the temperature compensation that is applied in the instrument for the membrane permeability This coefficient here 4 2 o C at 25o C is for this thickness polyethylene A polarising volta...

Page 53: ...tored with the tip in water If the membrane does dry out gently pump the pressure compensating diaphragm approx half way up the probe This will re flush the gap between the gold cathode and the membrane with fresh electrolyte DO NOT USE A SHARP OBJECT AS THIS MAY RUPTURE THE DIAPHRAGM For long term storage of several weeks or more 1 Remove the membrane 2 Hold the sensor with the tip facing downwar...

Page 54: ...le as ocean water is only 3 6 saline It does however stress the importance of correct interpretation of the salinity etc The Clark Sensor measures the partial pressure of oxygen diffusing through a membrane The current is a linear measure of this partial pressure assuming liquid flow conditions are met With air at sea level the 20 9 oxygen exerts about 15 5 cms mercury standard pressure Water in e...

Page 55: ...ed by manual entry of the salinity of the sample This must first be measured with a good quality salinity meter such as a TPS model MC 84 or a WP 84 21 5 Equilibrium Conditions Whilst Saline Water has a lower ppM than does Fresh Water it does not mean it necessarily has less oxygen biologically available Both have 100 Saturation presuming no Chemical Oxygen Demand C O D Biological Oxygen Demand B ...

Page 56: ...ing current once the flow is sufficiently high Tests with this sensor have shown that flow rates above 0 2 litres minute past the membrane give results indistinguishable from those with appreciably higher flow rates 5 litres minute Fluctuations in readings due to air bubbles passing through the membrane are however a different matter With the type of sensor to be used with this instrument very lit...

Page 57: ...ce junction A saturated solution of KCl is used to make contact with the sample It is vital that the KCl solution has an adequate flow rate in order to obtain stable accurate pH measurements 22 1 Asymmetry of a pH Sensor An ideal pH sensor produces 0 mV output at 7 00 pH In practice pH sensors generally produce 0 mV output at slightly above or below 7 00 pH The amount of variance from 7 00 pH is c...

Page 58: ...n practice pH sensors usually produce slightly less than this The output of a pH sensor is expressed as a percentage of an ideal sensor For example an ideal sensor that produces 59mV pH unit has 100 Slope An sensor that produces 50 15mV pH unit has 85 Slope see Figure 22 2 600 400 200 0 200 400 600 0 7 14 Millivolts 85 Slope at 25 oC 50 15mV pH 100 Slope at 25 oC 59mV pH Response of pH Electrode a...

Page 59: ... 22 3 22 4 Checking the reference junction of a pH sensor If pH readings are inaccurate or unstable the reference junction of the sensor may be blocked The following test can be performed to determine if the reference junction of a pH sensor is making adequate contact with the sample solution 1 Calibrate the WP 91 as per section 5 2 Dilute 1 part of pH6 88 buffer with 9 parts of distilled water 3 ...

Page 60: ...ximately pH7 00 depending on the current calibration settings 5 If the WP 91 is operating correctly the reading should be totally stable with the wire firmly in place If not the meter requires servicing 6 Now carefully disconnect the wire from the centre pin only make sure the other end of the wire remains connected to the outside frame of the connector 7 The reading should steadily drift away fro...

Page 61: ... derived from the equipment Shipping damage is not covered by this warranty PLEASE NOTE A guarantee card is packed with the instrument or electrode This card must be completed at the time of purchase and the registration section returned to TPS Pty Ltd within 7 days No claims will be recognised without the original guarantee card or other proof of purchase This warranty becomes invalid if modifica...

Page 62: ...ime phone number Your company name ORDER number and return street address A description of the fault Please be SPECIFIC Note Please Repair does NOT describe a fault Your equipment will be repaired and returned to you by air express where possible For out of warranty units a repair cost will be calculated from parts and labour costs If payment is not received for the additional charges within 30 da...

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