Page 13 - Customer Needs and Use Assessments (CNUA) of pH Sensors

ACT In Situ pH Sensors Customer Needs and Use Assessment............................................................................ 12
(b) the interval of calibration?
Yearly: 1.6 ± 1.2 years (4)
Monthly: 3.6 ± 2.1 months (8)
Weekly: 3.4 ± 1.2 weeks (5)
Daily: 4.1 ± 3.0 days (3)
Overall: 115 ± 136 days
14. Do you have a sensor on a profiler? How often is it used, and how often is it calibrated?
Twelve survey participants answered this question; 30 skipped this question. Five of the respondents do not
profile; 7 do. Actual responses are given randomly below in the order of how often the profiler is used; how often
it is calibrated.
once a year; every use
frequently; frequently
every 5 days; 4 years
2 hourly for 30 minutes; monthly
2 days a month; annually
1-4 times a year; 1-4 times a year
1 time per week; Daily when in use
As in Question 13, profiling occurs on scales of years, months, weeks, and days. By its nature, we expect profiling
to yield data on the scales of hours and minutes, but actual profiling intervals appear to vary widely among users.
In 4 cases the time frame for calibration is less than or matches the profiling intervals; however, in contrast to
moored applications, calibration intervals are much longer in 3 of the cases. In summary, as with Question 13,
the answer to both questions of profiling frequency and the interval between calibrations is not straightforward.
Converting the responses into units of days and averaging the data does not yield meaningful answers. Again,
this raises the question how frequently in situ pH sensors have to be calibrated?
B. Specifications
15. What is the typical pH range for your application?
Forty-one responded to this question with only one respondent skipping it. The low end of the typical users’ pH
scale fell between a minimum of 4.00 and a maximum of 7.80 with an average pH of 6.60 < 1.09 and median of
7.00. The high end of the range had a minimum pH of 8.00 and a maximum of 11.00, an average pH of 8.47 ±
0.71, and a median of 8.25.
16. What level of overall uncertainty do you require for this application?
0.1: 31.6% (12)
0.01: 39.5% (15)
< 0.01: 28.9% (11)
Other. Please describe:
The comments left by 4 respondents are echoed in the response statistics above, but they indicate finer intervals
or more defined levels of uncertainty: 0.1-0.01, 0.02 – 0.05, 0.05, and 0.005.