Sea-Bird Scientific SUNA V2, Руководство пользователя

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controls the length of time the sensor is in standby mode before it returns to low power
standby mode.
For best accuracy, regular dark measurements are necessary to compensate for the
change in temperature. Select a dark-to-light data collection rate based on either the
number of samples or the duration.
Note:
The lamp turn-off temperature is 35 °C. The lamp should not operate at temperatures above 35 °C.
When the lamp reaches the turn-off temperature, the sensor overrides the user-configured mode of
operation. The sensor does 5 cycles of 5-light to 5-dark sample collection, then does 1-light to 10-
dark cycles until the lamp temperature is below the turn-off temperature.
Contact the manufacturer for information to safely change the turn-off temperature.
7.3 Theory of operation
7.3.1 Background
The SUNA is a chemical-free nitrate sensor for fresh, brackish, and salt water. The
sensor is based on the In-Situ Ultraviolet Spectroscopy (ISUS) that was developed at
MBARI (cf. Kenneth S. Johnson, Luke J. Coletti, Deep-Sea Research I, 49, 2002,
1291–1305).
The sensor lights the water sample with its deuterium UV light source and measures this
with its spectrometer. The difference between this measurement and a prior baseline
measurement of pure water is the absorption spectrum.
Absorbance characteristics of natural water components are in the calibration file of the
sensor. The Beer-Lambert law for multiple absorbers makes the relationship between the
total measured absorbance and the concentrations of individual components. Based on
this, the sensor gives a best estimate for the nitrate concentration with multi-variable
linear regression.
If the "Integration Time Adjustment" configuration parameter is set to "On" or "Persistent,"
the sensor makes measurements with a spectrometer integration time that is 20 times as
long as the normal integration time. This increases the signal-to-noise ratio in faint light
conditions and lets the sensor operate in optically dense conditions. When the optical
density decreases, the sensor goes back to the normal spectrometer integration time.
7.3.2 Description of nutrient units
Nutrient units express the amount of something, usually moles or mass, relative to the
volume it is in. Many researchers and scientists use micromoles per liter (µM/L), a unit
that is independent of mass and useful for stoichiometric calculations.
Most fresh water monitoring programs and many researchers use units of milligrams per
liter. This unit is almost always expressed as milligrams of relevant atoms per liter—for
example, milligrams of nitrogen (N) per liter, rather than milligrams of nitrate per liter.
Although nitrate, NO
3
, is the most prevalent form of nitrogen, this unit is frequently used
as a means of easily keeping track of total nitrogen loading. Because milligrams per liter
is a mass-based unit and the mass of N and NO
3
are different, this difference is very
important to prevent mistakes. Milligrams per liter is also typically referred to as parts per
million (ppm), the mass of N relative to the mass of water.
The SUNA V2 sensor measures dissolved nitrate and displays units in micromolar (µM)
or milligrams of nitrogen per liter (mg/N/L). The SUNA V2 does not display milligrams of
nitrate per liter (mg/L or mgNO
3
/L ).
7.3.3 Nitrate concentration
Nitrate processing uses the 217–240 nm wavelength range, which is approximately
35 spectrometer channels. The precision of the nitrate concentration is related to the
number of absorbers into which the measured absorbance is decomposed. High
absorbance conditions introduce inaccuracies into the nitrate concentrations. Channels
with an absorbance of more than 1.3 are not included in the processing. If less than
approximately 10 channels remain, the sensor cannot give a nitrate concentration. The
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