pH PROBES
· Principle of Operation for pH
The sensor operates in principle as if it contained two 'batteries' whose voltages are measured and transmitted by electronic amplifiers. One battery is formed by the ground electrode and the glass process electrode. The voltage of this battery is a function of the solution pH. The other battery is formed by the same ground electrode and the standard electrode which contains a pH electrode in a chemical standard of fixed pH value. The voltage of the second battery is subtracted from the voltage of the first battery. The result is a differential pH measurement, the final signal being that of an pH electrode in the process compared to a pH electrode in a chemical standard solution.
A temperature sensitive resistor inside of the sensor automatically compensates the pH measurement for temperature variations by adjusting the output of the sensor.
Fouling reduces the effectiveness of a probe sensor, it should be cleaned promptly. Probe cleaning is a very important function of preventive maintenance. Improperly cleaned probes can cause mistreatment. Probes should be cleaned and calibrated to the instrument at least once a week, or more if conditions warrant.
A preventative maintenance cleaning and calibration program to meet the needs of each individual system should be initiated on the startup of the system. The frequency of the cleaning and calibration process is dependent on the type of foulant to be removed. In any event, we recommend that the probes be cleaned a minimum of at least once a week, and calibrated as required with two point buffer solutions. The need for immediate calibration will be indicated during the cleaning process when the probe is placed in the borax solution during the cleaning process. Borax in solution has a pH of 9.2
Equipment or Material Required
· 1 - 3 to 5 gallon pail for water
· 1 - 3 to 5 gallon pail for acid cleaning
· and/or 1 - 3 to 5 gallon pail for borax calibration verification.
Steps:
· Make up water rinse pail with clean warm water.
· Make up acid clean pail with acid solution.
A. 1 qt. muriatic acid diluted to 5 gallons water. (20/1 ratio recommended)
· Remove probe from the tank holder.
· Rinse probe in water.
· Gently wash probe in acid solution. Allow the probe to stabilize for 2-3 minutes.
· Rinse probe in water solution.
Borax 20 Mule Team calibration verification
· Place 2-3 cups of Borax 20 Mule Team in to the borax pail.
· Place probe in Borax buffer solution. Allow the probe to stabilize for 2-3 minutes.
· Check pH of solution with portable meter.
A. Verify pH of solution (9.2)
· If the pH is <= 9.1 or => 9.3:
A. Acid clean the probe again and re-calibrate using the two buffer method of calibration.
· Replace probe in the tank.
· Access Main Menu on the MMI.
· Press F(key) for system operations.
· Press Calibrate Probes in the Systems Operations menu.
· The screen will then display the Chem Pumps menu.
A. ON F(key)
B. OFF F(key)
C. CALIBRATE F(key)
· Press F(key) (Calibrate) button.
· The menu will then display:
· Start Calibrate?
A. YES F(key)
B. NO F(key)
· Press F(key) to calibrate the probes.
· The menu will then display:
"CHOOSE THE PROBE TO CALIBRATE"
· (Location of the probes that you may choose from will appear in this area of the screen.)
· Example: The menu may display.
A. Batch #1 = F(key)
B. Neut. #1 = F(key)
· Press the appropriate F(key) to proceed to than tank for calibration.
· The menu will then describe what tank you are working in:
· Example Menu:
A. Calibrate Probe Batch #1 pH 4"
B. Calibration Steps Using pH 4 Buffer"
· Steps:
A. Clean Probe
B. Rinse pH Probe in Water Pail
C. Place Probe in pH 4 Buffer Pail
D. Press Enter After pH Probe Has Been Put in The Buffer
Note: During the 1st step of cleaning the probe refer to the operations manual for making up a 10 to 15% solution of muriatic acid
· After pressing enter at Step 4 the menu will show:
A. "Please Wait"
Note: Processing will take 15 seconds.
· After processing data the screen will display:
A. "pH 4 buffer calibration completed"
B. "Going to pH 10 buffer calibration now"
C. "Press any key to continue calibration"
· The screen will then display:
A. "Calibrate probe Batch #1 pH 10"
B. "Calibration steps using pH 10 buffer"
· Steps:
A. "1) Rinse pH probe in water pail"
B. "2) Place Probe in pH 10 Buffer Pail"
C. "3) Press any key to continue calibration"
· After step three the menu will display:
A. "-Please wait-"
B. "Processing Data"
Note: Processing will take 15 seconds.
· The menu will then display:
A. "-pH 10 buffer calibration completed-"
B. "pH probe BT-1 completed"
C. "Press any key to complete calibration for BT-1
At this point when you press any key the menu will go back to to the original menu to allow another probe to be calibrated, or you may press "ESC" to escape to the System Operations menu when you are through. If you press another probe to calibrate, the MMI will guide you through the steps again for the next probe.
Method
· In order to change the set points of the probes you must access the systems operations menu. A statement will be displayed on the screen.
A. "Changes Setpoints"= F (KEY)
Note: F (KEY) can be any key from F1 to F20.
When you press the F(KEY) the menu will display a new screen that will give you the current reading for the low and high set points for each probe that you have in the system. If you wish to change the setpoint for a particular treatment tank or process, press the F (KEY) for that high or low set points value you wish to change. The screen will show a box to enter a numerical value.
Example: If you want to change the low set point on Neutralization Tank #1 to 7.00 from 7.50, you will press the low set point key on Neutralization Tank #1, the box will show empty in the next screen and you will push 700 on the numeric key pad and hit enter. This will automatically enter the new value of 7.00
Maintenance of pH and ORP Probes
Check the glass electrode periodically to see that it is not fouled. If it is fouled, wipe with a soft cloth, taking care not to break the glass electrode. If the glass electrode cannot be wiped clean, try soaking a few minutes in hydrochloric acid (10%). Tar or greases can usually be removed with acetone. Oil is best removed with dishwashing detergent.
The other metal electrodes on the probe need not be as clean as the glass electrode, but should not be grossly fouled.
Note: Never use abrasive cleaners on the probe or glass electrode.
Replacing Standard Cell Buffer/Salt Bridge
The sensor's standard cell buffer and salt bridge should be replaced if calibration cannot be accomplished after cleaning the sensor.
· Remove the hex-shaped salt bridge by turning it counter-clockwise with a 9/16" socket or nut driver to initially loosen it. Take care not to damage the protruding process electrode (glass). Discard the old salt bridge.
· Replace the standard cell buffer in the chamber of the standard cell.
· Pour out the aged standard cell buffer. Thoroughly flush standard electrode chamber with distilled water.
· Fill standard electrode chamber with fresh standard cell buffer.
· Install the new salt bridge. Inspect the O-ring for imperfections and replace if necessary. Turn the salt bridge clockwise until finger tight. Then tighten with a 9/16" socket or nut driver (approximately 1/4 turn).
· DO NOT OVERTIGHTEN!
Cautions to be Observed on pH and ORP Probes
The sensing electrode at the base of the probe is made of glass and can break easily. Do not subject the glass electrode to shocks or other mechanical abuse. Before installing the probe, remove the red plastic cap on the standard cell by twisting clockwise.
Note: Store the red plastic cap in a safe place for further use. If the probe is out of solution for more than a few days, put a few drops of water in the cap and replace it over the standard electrode by twisting clockwise.
Note: Do not use the probe in nitric acid solutions stronger than 1%. Consult factory before using the probe in very strong solvents such as ethylene dichloride, etc.
A few simple measurements can determine if the sensor is operating properly. A multimeter and two pH buffer solutions (pH 7 and pH 4 or 10) are needed.
Clean the sensor. If the instrument and sensor cannot be calibrated, replace the standard cell buffer and salt bridge. If the measuring system still cannot be calibrated after replacing the standard cell buffer and salt bridge, perform the following test:
· Connect a DC milliammeter in series with the sensor and the instrument (or a +24 VDC source)
· Disconnect the sensor's RED (+) wire from instrument and connect it to milliammeter (+) input.
· Connect the milliammeter(-) input to the instruments (+) input terminal.
· Place sensor in pH 7 buffer. Allow temperature of sensor and buffer solution to equalize at room temperature. Read and note the mA value. This reading is called the 'offset' and it should be between 11 and 13 mA. If not, discontinue the test and replace the probe, or, if under warranty return to DMP. If it is between 11 and 13 mA, the sensor 'offset' is within factory-specified limits and the sensor 'span' should now be checked.
This is done with the following steps:
· Check the sensor 'span' by measuring the mA value with the sensor in either pH 4 or pH 10 buffer. Keep the milliammeter connected as previously described.
Span check in pH 4 buffer:
· Rinse sensor with water and place in pH 4 buffer. Allow temperature of sensor and buffer solution to equalize at room temperature. The 'span' reading should be 2.37 to 3.10 mA lower than the noted 'offset' reading.
Example: Suppose 'offset' reading in the pH 7 buffer is 11.50 mA. Then the 'span' reading should be between 8.40 and 9.13 mA to be within factory-specified limits.
Span check in pH 10 buffer:
· Rinse sensor with water and place in pH 10 buffer. Allow temperature of sensor and buffer solution to equalize at room temperature. The 'span' reading should be 2.37 to 3.10 mA higher than the noted 'offset' reading.
Example: Suppose 'offset' reading in the pH 7 buffer is 11.50 mA. Then the 'span' reading should be between 13.87 and 14.60 mA to be within factory-specified limits.
If the calculated 'span' reading conforms to either pH 4 or pH 10 buffer test limits, the sensor is working properly. If not, take the probe out of service, replace and call DMP for warranty information.