Page 12 - Customer Needs and Use Assessments (CNUA) of pH Sensors

ACT In Situ pH Sensors Customer Needs and Use Assessment............................................................................ 11
12. Do you have records of your calibrations?
Thirty-seven respondents answered this question: 75.7% (28) do have calibration records on file; 24.3% (9)
either do not keep calibration records, do not but plan to keep records, or have to check first to see if they keep
records on file.
13. Do you have a sensor on a mooring?
This was a very challenging question to translate from the list of 26 who responded and commented; 16 skipped
this question. Five respondents claimed not to have a pH sensor mounted on a mooring. The remaining 21
respondents moor and calibrate pH sensors, with the exception of one user (spectrometric) who feels that the
calibration is inherent in the properties in the dye used. Deployment and calibration intervals ranged on the
scale of years, months, weeks, and days, as respondents tended to answer both mooring and calibration interval
questions using these same time scales. No user worked at scales finer than the scale of a day.
Respondents answered both questions in units of years, months, weeks, and days. Assuming a year is 365
days, a month is 30 days, and averaging ranges when given, a rough estimate of both deployment length and
calibration interval can be approximated. The overall (a) length of deployment is 561 ± 1202 days with a (b)
interval of calibration of 115 ± 136 days. On average, calibration intervals appear to occur at 1/5th the length of
a deployment.
Because the data cover such broad time scales, the overall averages and resulting large standard deviations only
echo the point that users deploy and calibrate over widely different intervals. We analyzed the patterns and
time scales used in the answers and cast the results below into more meaningful data in the context of how our
audience uses in situ pH sensors. A very noticeable pattern emerges.
Those who moor at a given time scale tended to calibrate at that same time scale (65%) or they calibrated on
next more frequent scale (25%). For example, those who moored pH sensor on a yearly time scale calibrated
also within the scale of year; and, if not yearly, then on monthly time scales. Those who had moorings that
lasted months tended to calibrate on monthly intervals, or on weekly time scales, and so on. Two users (10%)
were the exception to this rule and had moorings out for 1-15 years but calibrated every 2-4 weeks. Generally
speaking, however, it is perhaps useful to recognize that among 90% of those surveyed, moored deployments and
calibrations take place over similar time frames regardless of whether these time frames are narrow or broad.
This does raise the fundamental question of how often an in situ pH sensor should be calibrated and how long
they can last in the field and still yield meaningful data?
The same question answered from the overall data above is broken down into yearly, monthly, weekly and daily
categories below. In summary, the answer to both questions of mooring length and the interval of calibration is
not straightforward. Even when subdivided into finer time scales, the variability in answers is often as large as
the means.
If yes, what is
(a) the length of deployment?
Yearly: 4.1 ± 4.4 years (8)
Monthly: 4.7 ± 2.4 months (9)
Weekly: 3.0 ± 1.4 weeks (2)
Daily: 6.3 ± 6.1 days (2)
Overall: 561 ± 1202 days