NPI salinometer guidelines#
Warning
A bunch of useful content collected here, but this is not finished!
Actual sampling section is thin.
Additional example videos to be added.
Things are a bit messy; Needs a readover
By ØF
Then by PD/others to validate/clear up misunderstandings
Based on notes from Paul Dodd, Tore Hattermann, and Øyvind Foss
Tip
Use the menu on the upper right to navigate through the documentation
Some general notes#
Salinity is a key oceanographic parameter, and obtaining high-quality measurements of salinity is a key reason why we do CTD casts at sea. Salinity is calculated from measurements of the electric conductivity of the seawater along with temperature and pressure (which are also measured by the standard CTD).
Compared to temperature and pressure sensors, conductivity measurements are prone to drift and outliers. The main reason for this is that the conductivity reading is proportional to the volume of water in the “conductivity cell” where the measurement is made[1]. Any contamination of the cell can therefore disturb the accurate reading of conductivity. The typical culprit in oceanographic measurements is growth of biological organisms within the cell, but other contaminants like oil can also disturb the measurement. Issues can also arise due to problems with the mechanical pump pumping water across the sensor, or problems with the sensor itself.
As a result, it is very important to thoroughly clean and rinse the conductivity cells during the cruise[1]. In addition, CTD salinity measurements should be compared with independent laboratory measurements of salinity based on in-situ water samples collected along with the ship CTD data in order to check the measurements and ensure that the sensors are performing well. The main objective of this comparison is to validate the CTD-based measurements, but if major discrepancies are discovered, measures may be taken to fix the conductivity sensors, or corrections may be applied to the CTD salinity data to better align them with the “true” values measured in the laboratory.
Whenever possible, we aim to do these laboratory salinity measurements on board. With some planning (and if the salinometer works correctly), the calibrated CTD dataset can be ready at the end of the cruise.
Underway salinity comparisons can also help identify issues with conductivity while there is still time to take measures ad solve problems..
Personnel hours are reuired for training, collecting water samples, setting up the salinometer, performing the laboratory analysis, and comparing CTD and laboratory data. Make sure to acccount for this when planning the cruise!
Collecting samples for the salinometer#
Planning and collecting water samples#
Plan in advance how to obtain salinity samples for calibrating the CTD casts.
If your cruise stations are all in shallow waters, consider whether you can afford “calibration casts” in deep waters at the start and end of the cruise.
Start calibration cast is more critical than end cast.
In general, consider that Niskin bottles average over ~5 m. For accurate comparisons with CTD, we therefore want to sample salinities in areas of the profie where salinity varies slowly with depth (typically below the pynocline, and usually below ~200 m depending on the hydrography).
Collect at minimum one sample per cast to >200 m.
For deep (<1000 m) stations, collect a salinity sample at least every 1000 m.
Preparing salinity sampling bottles#
Rinse bottles prior to use in warm but not too hot water (if the water is too warm it will damage the inner coating).
Tap water can be used only if limestone is not affecting the groundwater, if it does use distilled milli-Q water.
Do not use bottles with a damaged inner coating (seen as a slightly coloured thin film inside the bottle).
After cleaning, store bottles upside down in the crates to avoid deposition of particles.
To minimise the need for re-cleaning, place the crates in a plastic bag before transport/storage on open pallets.
Sampling from the Niskin bottles#
Rinse bottles (fill with water from the rosette, shake gently, empty the bottle and fill again) three times during sampling before collecting the sample.
Fill the sample bottle to the shoulder, leaving some air to allow for thermal expansion.
Dry the bottle top with lab towel before sealing the bottle insert and lid.
There is no massive rush to get screw taps on.
Seal the bottle by instering a plastic insert and screwing on the outer lid.
Dry the mouth and neck of the bottle after sealing.
This prevents salt crystals from forming on the top of the bottle, potentially contaminating the sample when it is opened for analysis later.
Leave bottles for > 24h in the room where salinometer is located for temperature stabilization before processing.
Setup of the salinometer#
Overview of the salinometer#
The recommended instrument for measuring salinity from bottle samples is the Guildline Portasal Salinometer Model 8410A, or “Portasal”.
We have had mixed results with an alterative, all-automatic instrument, the Optimare.
The salinometer makes high precision conductivity comparisons between an unknown water sample and a reference standard seawater sample, either displayed as a conductivity ratio or in practical salinity units.
Preparing a salinometer room#
Choose preferably a small, undisturbed room with:
Stable air temperature (a well-working air conditioner could possibly do the job)
Easy access to MQ/distilled water
A sink for waste water/rinsing of bottles.
Have a bucket ready for waste water.
Laboratory salinometer sampling#
(Refer to the overview of the Portasal salinometer for an overview of what the different terms refer to).
Preparing to process new sampling batch#
Process one crate at a time, not bottle by bottle.
Processing one crate takes about 45 minutes.
Make sure to have a log sheet ready before analysis
Check the room temperature (note on log sheet).
Check the bath temperature (note on log sheet)
Shake the bottles - ideally 10 minutes before beginning.
Seawater standard#
Sampling procedure#
Gently wipe any water off the sampling tube with a kimwipe. Briefly turn on the pump to suck any remaining water out of the tube (📹 Clean the sample tube).
Find the bottle you want and prepare the sample.
Write down the bottle sample number in the log sheet.
Take the lid off the bottle. Pry the white plastic lid off (you may want to use a knife or your fingernails - just make sure not to contaminate the sample.)
Wipe any water droplets off the rim of the bottle. If there are salt crystals along the edge, make sure to wipe them away (don’t push them into the bottle).
Put the bottle in the sampling slot below the pump, with the sampling tube inside the bottle.
Turn on the pump (always to 2), and flush the cell. This clears the old sample and sucks in water from the new one.
Repeat the process - flush the cell three times.
Think about how many times it fills, not how many times you press the button.
When the cell has been filled with water from your sample bottle three times, you are ready to take a measurement.
On the third filling, watch the cell carefully - there should be no bubbles in the cell. If there are bubbles, you may have to flush and refill again.
Note
When the salinometer operates correctly, you should see occasional flashing of lights within the cell. This means that the heating element is working correctly. If you never see any lights, or if the lights are constantly on or blink incessantly, there may be something wrong with the heating element, or the bath temperature may be too far away from the desired value.
Tip
In general, the measurements themselves are quick - it is the flushing that takes time.
Tip
As you become used to the process, you may find it saves time to:
Read the salinity value from the display
Flush the cell
Write the value down in the log sheet while the cell refills
Flushing the cell#
To flush, put a finger over the hole (below where it says FLUSH). This will cause air to flow into the cell, flushing the water out.
The water exits through the tube attached to the CELL DRAIN tube outlet - hopefully into a bucket you have placed at the end of the tube.