It is virtually impossible to predict the behavior of a Nitrate ISE over wide ranges of temperature
and conductivity, so you are strongly advised to calibrate the ISE at a temperature near that of
the water you intend to monitor.
Nitrate electrodes suffer interference from positive ions, especially potassium and sodium. For
instance, 1000 mg/L of sodium ions or 10 mg/L of potassium ions will cause at least a 1 mg/L-N
increase in the Nitrate reading. In fresh water, potassium is the primary interference, raising
Nitrate values by 0.1-0.2 mg/L-N in average 200 µS river water. In brackish or marine waters,
sodium can become the primary interference because of its high concentration; sea water reads
up to 12 mg/l-N Nitrate even if there is no Nitrate present.
It’s best not to let your ISE dry out, so place a small amount of tap water in the storage cup to
ensure 100% humidity.
Nitrate requires a two-point calibration:
1) Rinse your sensors several times with the standard you’ll use for calibration.
2) Fill the calibration cup with enough standard to cover both the ISE and reference
electrodes.
3) Follow the Manta 2 Control Software’s calibration instructions.
Again, you should calibrate with temperature of your standard as near as possible to the
temperature of the water you intend to monitor.
E16 Total Dissolved Gas (TDG)
The TDG sensor is a pressure transducer (the same one used for the 10-meter depth sensor)
attached to a “membrane”. This membrane is a long piece of thin-wall, silicone tubing whose job
is matching gas partial pressures inside the tube with those of the surrounding water. The sum of
those partial pressures is measured by the transducer, and that’s the TDG of the water.
Aside from keeping the membrane as clean as possible without tearing the tubing, the TDG
sensor requires no maintenance. When the membrane is torn or is just too dirty, the membrane
assembly must be replaced. Simply unscrew the old membrane and screw on a new membrane.
Screw it on finger-tight, plus 1/4 turn.
Silicone rubber is chosen for the membrane material because gases pass through silicone
readily. This means that response time for silicone is much faster than if the membrane were,
say, Teflon. However, if the membrane is soaked in water for more than a few hours, the silicone
absorbs just enough water to slow the gas transfer considerably. This is not usually a problem for
unattended monitoring applications (the TDG doesn’t change very quickly anyway), but can be
annoying if you are doing daily spot-checks. In that case, it’s best to dry out the membrane
between stations by using only a few drops of water in the storage cup instead of a few ounces.
F The Manta 2
F1 Manta 2 Components
Please see the illustration in Section A1.
F2 What the LED lights mean?
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