Feature pH sensors
Focusing on pH sensors
Fred Kohlmann, analytical product business manager at Endress+Hauser, guides us through when to do a pH sensor calibration versus a calibration check; how to properly clean pH sensors; and how to actually carry out pH sensor calibration
ll pH readings are supposed to be taken and accepted only when the pH sensor is clean, as a conta- minated pH sensor may yield an incorrect reading. The sensor, however, must be clean before calibration is carried out. But, once a pH sensor is installed in the process and operating, how do you determine when it is time to take the sensor out of the process and do a cleaning, or a calibra- tion? Do you perform a calibration, a cleaning, or both together, or does one just perform a calibration check in buffers, etc.?
A
This can be quite confusing, espe- cially when the operational practices and procedures documented by your company’s Quality Control or Environmental Practices department may not be specific enough when they describe the procedure or the timing on when to conduct the pH calibration and maintenance. Inversely, the procedures may be too specific, detailing many more procedures and operations than are actually required.
In practical terms, users must
develop their own maintenance and calibration schedule. This is accom- plished by taking the pH sensor out of the process after a set amount of time, perhaps after a day or two to perform a visual inspection of the sensor. If after inspection you find no debris or fouling on the electrode and reference
10
surfaces with the naked eye, rinse the sensor off in distilled water and perform a buffer check. To perform a buffer check, place the sensor into the calibration buffers you typically use and note the readings. If the readings are within the tolerances defined by your operational proce- dures, it is not necessary to perform a calibration. For example, let us use ±0.2 pH as your tolerances for pass/fail of a pH sensor reading in a calibration buffer. If the sensor reads within this value, in the offset (7 pH) and span buffers, (4 pH), the sensor needs no further action and can be reinstalled into the process. A calibration is not necessary. Repeat this exercise every few days until you see a change in either the level of debris/foulant on the electrode and reference surfaces, or more than the ±0.2 pH deviation as shown in the example above. To a certain extent, the above proce- dure sets the benchmark for time between cleaning and calibration. Now, you need to determine whether the sensor needs just a cleaning or a clean- ing and re-calibration. This is done easily by first making sure the sensor is clean. (Refer to the section on Cleaning pH sensors). It may be as easy as rinsing the sensor in water or as complicated as using acid or caustic solutions to remove the particular contaminate buildup that has occurred.
Once in the
application, access to the pH sensor is not always ideal
Should the above steps not yield results that are within your pass/fail criteria, it is time for a sensor cleaning.
Cleaning pH sensors
For the sensor to maintain an accurate reading of the process pH, it must remain clean – specifically, the glass measuring electrode can not become coated; and the reference electrode assembly must also not become coated, plugged or otherwise contaminated by the process solution. If the pH sensor has a slight coating or scaling, this might be removed using a water jet from a faucet or spray bottle. More entrenched coat- ings may require the use of a gentle acid brush or toothbrush to carefully remove the coating.
Depending on the nature of the scale or coating, you may find it necessary to dip the sensor in a hot water solution containing dishwashing
Contaminant Alkaline or Scale Acidic Coatings Oil, Grease, Organic Cleaning Solution 5% hydrochloric acid or vinegar Weak caustic (less than 4% NaOH)
Detergent, or if coating is tenacious, an organic solvent compatible with sensor material
measurement & sensors directory 2012-2013
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92