pH
The pH calibration should be verified every day the instrument is used. However, a new pH sensor may be
capable of holding its calibration for several days.
CALIBRATION TIPS
1. If using a pH sensor in a 6051010 or Quatro cable, calibrate the sensor in port 1 prior to calibrating the
sensor in port 2. The sensor in port 2 uses the reference of the sensor installed in port 1. Therefore, it is
important to verify that the port 1 sensor is working properly before calibrating the port 2 sensor. See pH
Troubleshooting Tips for additional info.
2. The pH sensor can be calibrated with up to six calibration points.
3. Calibration can be accomplished in any buffer order.
4. pH 7 buffer should be used regardless of how many calibration points you use; however, it does not have to
be the first point.
5. In most cases, a two-point calibration is all that is required (4 and 7 or 7 and 10). You can bracket the
expected in-situ pH values. Use a three-point calibration with 4, 7 and 10 if the in-situ pH values are
unknown or if you expect the in-situ values to be on both sides of the pH scale.
6. Rinse the sensors and cal cup with a small amount of pH buffer. Fill the cup so that the pH sensor tip and the
temperature sensor are submerged in buffer.
7. If necessary, highlight the Calibration Value and enter the pH value of the buffer solution. Note: The Pro
Plus has auto buffer recognition which can be set to USA (4, 7, 10) or NIST (4.01, 6.86, 9.18) buffer values in
the pH Sensor Setup menu.
8. Record the pH millivolts for each calibration point. The acceptable mV outputs for each buffer are shown
below.
pH 7 mV value = 0 mV +/- 50 mV
pH 4 mV value = +165 to +180 from 7 buffer mV value
pH 10 mV value = -165 to -180 from 7 buffer mV value
•
A value of +50 or -50 mVs in buffer 7 does not indicate a bad sensor.
•
The mV span between pH 4 and 7 and 7 and 10 mV values should be
≈
165 to 180 mV. 177 is the
ideal distance. The slope can be 55 to 60 mV per pH unit with an ideal of 59 mV per pH unit.
•
If the mV span between pH 4 and 7 or 7 and 10 drops below 160, clean the sensor and try to
recalibrate.
9. Wait for the pH to stabilize in the each buffer and then press enter to accept each calibration point.
10. Rinse the sensor and cal cup with a small amount of the next buffer between calibration points.
11. After pressing enter to accept your last calibration point, press cal
to complete the calibration. Otherwise
you will continue calibrating up to 6 calibration points.
12. If you receive a warning message stating that the calibration is questionable, do not continue with the
calibration. Instead, select ‘No’ and investigate what is causing the questionable results. If you accept a
questionable calibration, your pH readings will be erroneous. Typical causes for this error message include:
incorrect Sensor/Port setup in the instrument, a dirty sensor or bad buffer solution.
13. After accepting a good calibration, navigate to the GLP file and check the pH Slope and Slope % of ideal. A
good slope should be between 55 and 60 mVs while the ideal is 59 mV. If the slope drops below 53, the
sensor should be reconditioned and recalibrated.
TROUBLESHOOTING TIPS
Typical working life for pH sensors is approximately 12-24 months depending on usage, storage and
maintenance. Proper storage and maintenance generally extends the sensor’s working life.
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