Note

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This document actually seems to be the user manual from the manufacturer!

Parsed from an earlier document by ØF.

Part 1: Sampling#

Bottles: cleaning and storage#

  • Clean bottles before use:

    • Rinse bottles three times in warm, but not too hot tap water (only if limestone is not affecting the groundwater, if it does use distilled milli-Q water). If the water is too warm it will damage the inner coating.

  • Do not use bottles with a damaged inner coating (seen as a slightly coloured thin film inside the bottle). 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 strategy#

The default sampling strategy is to collect samples in regions with minimal vertical and horizontal salinity gradients. This is to ensure that the water trapped in the Niskin bottle (which includes water from a vertical range of several meters) is as similar as possible to the water measured by the CTD.

Where possible, collect samples at a variety of pressures and salinities in case the conductivity sensor has a pressure or salinity-dependant offset. Collecting samples at 500 or 1000 m intervals between 500 m and the bottom of the cast will typically provide useful samples.

Samples above 500 m are typically not useful for calibration due to steep vertical salinity gradients. Adjust the sampling depths to give about 5 samples per cast for the first and last few casts of each section. At the beginning of every section make a quality control. For comparisons with the observed CTD-profile, assume that the Niskin bottles average over 5 m of the water column.

Sampling procedure#

  • Rinse three times with the water from the Niskin bottle before filling the sample bottle to the shoulder, to allow for thermal expansion.

  • Dry the mouth and neck of the bottle after sealing with a disposable plastic insert.

    • This prevents salt crystals from forming on the top of the bottle, potentially contaminating the sample when it is opened for analysis later.

  • Screw on the outer lid, which secures the sealing plastic insert.

Part 2: Analysing samples#

  • Leave bottles in salinometer room for 24 h prior to the analysis to allow for the temperature to stabilize.

  • Preferably process one tray at a time (takes about 45 min).

  • Make sure to have a log sheet ready before the analysis.

1. Principle of the Method#

Water samples are analysed using a Guildline Portasal Salinometer Model 8410A, with an external peristaltic pump. 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.

2. Practical Salinity#

A seawater of Practical Salinity 35 has a conductivity ratio of unity at 15 degC (and 1 atmosphere pressure) with a potassium chloride (KCl) solution containing a mass of 32.4356 grams of KCl per kilogram of solution.

3. Reference Material#

IAPSO standard seawater.

For more details see: Bacon, S., F. Culkin, N. Higgs, and P. Ridout, 2007: IAPSO Standard Seawater: Definition of the Uncertainty in the Calibration Procedure, and Stability of Recent Batches. J. Atmos. Oceanic Technol., 24, 1785–1799, https://doi.org/10.1175/JTECH2081.1

4. Salinometer Room#

  • Choose preferably a small, undisturbed room with:

    1. Stable air temperature (a well working air conditioner could possibly do the job)

    2. Easy access to MQ/distilled water

    3. A sink for waste water/rinsing of bottles.

  • Have a bucket ready for waste water.

5. Analytical Procedure#

The sample conductivity ratio, \(R_t\), given from the salinometer is calculated as (from the manual, sections: 6.6.3.1-2):

\[R_t = \frac{G}{G_{STD} \times r_t}\]

where \(G\) is the conductivity of the sample, \(G_{STD}\) is the conductivity of the IAPSO standard seawater at 15ÂșC, and \(r_t\) is a correction factor based on the bath temperature set point, \(t\) (in centigrade):

\[r_t = 0.6766097 \ \ +\ \ 2.00564 \times 10^{-2} \times t\]
\[+ 1.104259\times 10^{-4} \times t^{2} \ \ -\ \ 6. 9698\times 10^{-7} \times t^{3} \newline\]
\[+ 1.0031\times 10^{-9} \times t^{4}\]

The standard conductivity at 15ÂșC, \(G_{STD}\), is determined during standardization:

\[G_{STD} = \frac{G}{R_{STD} \times r_t}\]

where \(G\) is the measured conductivity of the standard water, which is divided by the certified conductivity ratio (see label on IAPSO bottle), and finally corrected for the bath temperature, using \(r_t\) to give the equivalent conductivity of the standard seawater at 15ÂșC.

The manual recommends a standardization every 24 hrs, but samples can also be post-calibrated using measurements of standard seawater.

The instrument uses the PSS-78 algorithm for Practical Salinity to convert \(R_t\) to salinity. This calculation is valid in the range 2 < SP < 42. The analyser can choose to get readings as either conductivity ratios or as salinities. If the former is chosen, gsw_SP_salinometer.m can be used calculate samples to salinities in MATLAB.

5.1 Start-up steps#

Fill water bath (>30L):

  • Attach tubes to TANK DRAIN FILL (inflow for MQ, long tube) and TANK OVERFLOW (for outflow when water bath is filled, short tube).

  • Connect inflow tube with MQ/distilled water tap (avoid microorganisms from growing, do not use tap water).

    • Important to not use too much pressure here.

  • Check water level both by releasing inflow tube and by checking the level through the monitor glass.

  • When done, keep the TANK OVERFLOW vent open (i.e. keep tube on).

  • Attach the other tube to CELL DRAIN.

Power up the salinometer:

  • Open the metal lid on the back

  • Connect the cable

  • Switch on the machine (button on the back).

Set the bath temperature

  • Measure room temperature and press T set on the instrument to set the water bath temperature to 2 degrees above the room temperature.

    • Adjust the value using the up and down arrows and press ENTER when satisfied.

Rinse/flush the cell with MQ

  • Prepare a bottle with MQ and insert the thin sample tube.

  • Check the tube of the peristaltic pump and massage it if necessary.

  • Power up the pump and set pump speed to 2.

    • This value should always be 2 when the pump is on, this will optimize the precision.

  • Flush cell with at least 100 ml MQ.

  • To empty the cell, use the built-in air pump, i.e. turn on the FLOW RATE (pretty noisy) and use a fingertip to block the air vent.

Note that the bath temperature can take several hours to reach the set temperature. Wait until the next day or at least 6 hours. To check the bath temperature press TEMP, this value must be within 0.02ÂșC of the set bath temperature. The heater lamps should be flashing when the bath temperature is set properly. If not, check the heater lamps (9).

If the bath temperature is satisfactory, it is possible to start analysing samples. This can be done either by jumping directly to section 8 or by starting with the reference (7.2) and zero (7.3) calibrations, the standardization (7.4), to finish with the samples. A standardization may be useful if the accuracy of the instrument is poor, but not necessary since measurements of standard seawater can be used to do a post-calibration.

If the instrument will be running over several days, the room and bath temperatures should be checked daily. It is ok for the bath temperature to be within 1 degree below to 3 degrees above the room temperature but aim for 2 degrees above.

5.2 Reference Calibration#

  • The instrument must have been powered up for a minimum of 3 hours after the bath temperature begins regulating.

  • Ensure the cell is empty.

    • The cell should be evacuated if it contains a sample. To do this turn on the FLOW RATE and press your finger on the air vent, FLUSH. Switch off the FLOW RATE when done, this switch should be OFF during the reference calibration.

  • Set FUNCTION switch to STDBY and press REF key.

    • After approximately 8 seconds the display will read -REFERENCE  xxxxx and update for 16 seconds.

    • Next, the display will read +REFERENCE  xxxxx and update for 7 seconds, and then the display will read REFERENCE for 8 seconds.

    • This cycle will repeat until any key is pressed.

  • The -REFERENCE and +REFERENCE must stabilize to between 19750 to 19999.

  • The -REFERENCE and +REFERENCE values must agree to within ±2 counts after 10 cycles and must remain stable to within ±2 counts over 10 minutes.

5.3 Zero Calibration#

  • Set FUNCTION switch to ZERO and press COND key.

  • When the zero conductivity ratio measurement is stable, press ZERO key.

    • \(\Rightarrow\) Display reads ZERO  x.xxxxx

    • This value should not exceed ±0.00075.

  • If satisfied that this number is not drifting press the COND key.

    • Display should then read RATIO 0.00000.

  • If this reading is not stable either restart the Reference Calibration or wait another half day as the bath temperature may not have stabilized.

  • Set FUNCTION switch to STDBY.

  • Note the Ratio reading, example 1.32348.

5.4 Standardization of the Salinometer#

Start from here if the salinometer has been left on and has gone through the reference and zero calibrations. The manual states that the standardization should preferably be done every 24 hrs for an accuracy of 0.002 ppt. Note: 1) the instrument often has such an accuracy without the calibrations/standardization steps; 2) measurements of IAPSO standard seawater can be used to post-calibrate the samples instead of using the internal instrument calibration.

  • Take a bottle of IAPSO standard seawater, remove gradients within the bottle by gently rocking it.

  • Wipe sample tube and insert intosample, set pump speed to 2 to fill the cell and turn on FLOW RATE to flush 3 times using the air vent, FLUSH.

    • Make sure that there are no air bubbles when the cell is refilled.

  • Set FUNCTION switch to READ and press COND key.

  • Allow ratio to stabilize and note the measurement.

  • Switch to STDBY and repeat the flushing and filling of the conductivity cell.

  • Switch back to READ and allow the measurements to stabilize.

Repeat until measurements agrees with previous flushing.

Warning

Once the following steps commence DO NOT FLUSH the cell and DO NOT move the FUNCTION switch from READ.

  • Press STD key.

    • Display reads STD STANDARDIZE.

  • Press ENTER key.

    • Display prompts COND  NO x.xxxxxx.

  • Enter the value on the label of the IAPSO standard seawater bottle (press SHIFT 2 keys down together to enter a decimal point).

    • Display prompts BATCH NO xxxx.

  • Enter the batch number on the label of the IAPSO standard seawater bottle (press SHIFT down with either ↑ or ↓ to enter a letter).

    • Display reads ENTER WHEN READY.

  • Press ENTER key

    • Measuring
 will briefly appear on the display followed by STANDARD x.xxxxx (e.g. 4.22000).

  • When the displayed number is stable, note the value and press COND key.

    • This will terminate the STD and display the conductivity ratio using the new calibration value (this should match the value on the label of the IAPSO standard seawater bottle).

  • \(\Rightarrow\) The instrument is ready to analyse samples, set FUNCTION switch to STDBY and continue at 8.1.

If the instrument is to be left for less than 12 hours:

  • Leave the sample bottle in the sample bottle holder

  • Set the FUNCTION switch to STDBY

  • Turn off the peristaltic pump and FLOW RATE switch.

If the machine is to be left for over 12 hours:

  • Set the FUNCTION switch to STDBY

  • Remove the sample bottle

  • Fill the cell with distilled water flushing at least 3 times, and leave the cell full

  • Turn off the peristaltic pump and FLOW RATE switch.

6. Analysing samples#

Shake all bottles properly before starting with the analysis. Note the air and set bath temperature on the log sheet.

  1. Always start with the IAPSO standard seawater bottle. Be careful to fill in the correct information in the log sheet. If a standardization was doner ight before and enough liquid is left, FLUSH the cell and continue at 8.5, otherwise start at 8.2.

  2. Wipe the tubing

  3. Rock the bottle gently to eliminate newly formed gradients. Open the bottle and wipe the top to hinder any salt crystals from falling into thesample. Put the tube into the bottle. Set pump speed on 2 and turn on the FLOW RATE to empty cell from previous sample using the airvent, FLUSH.

  4. Fill and FLUSH the conductivity cell three times. Make sure there are no bubbles in the cell. If there are, FLUSH again.

  5. After the third FLUSH, when the cell is filled, take a reading by turning the function switch to READ, press the SAL key. Record the result on the log sheet. It is also possible to press COND to note the conductivity ratio instead/as well. Set FUNCTION switch to STDBY before emptying the cell.

  6. FLUSH the cell again and take another reading. Repeat for 3-5 readings. Avoid taking readings when air bubbles are in the cell, FLUSH instead and redo.

  7. When done with the last reading, i.e. the last three readings agree within the expected precision, leave sample in the cell and turn off the peristaltic pump as well as the FLOW RATE switch.

  8. Remove the sample bottle.

  9. Repeat steps 2-8 for each sample. Preferably analyse all samples in the crate.

  10. Finish with a new bottle of IAPSO standard seawater bottle to be able to correct for potential drift.

  11. Clean the cell with fresh MQ, let it flow through for some time.

When satisfied, empty cell and turn off all pumps. Turn off the instrument, unplug it, and close the metal lid on the back. Remove tube from DRAIN CELL and attach it to TANK DRAIN FILL instead. Make sure that there is enough volume left in the bucket before emptying the water bath. The bath and cell should be empty during storage to avoid freezeup.

Check for any damaged bottles/inserts/caps and note down on the log sheet. If the accuracy is poor the salinometer may need to be re-standardized. If the problem persists, cease any analysis and seek further advice on likely instrument faults.

7. If heater lamps are not flashing despite 24 hours of adjustment#

  • Set the bath temperature to 36ÂșC.

    • Both the heater lamps should light if the bath temperature is below this value.

    • If either of the heater lamps has failed, the message HEATER 1 FAILURE or HEATER 2 FAILURE will be displayed within 15 seconds.

  • Set the bath temperature to 15ÂșC.

    • Observe that both heater lamps will remain off if the bath water temperature is above 15ÂșC.

  • Reset the bath temperature to 2 degrees above room temperature.

  • Wait for a minute.

  • Press TEMP.

    • The display TEMP tt.tttÂșC must be within 0.02ÂșC of the set temperature.

  • Press ↑

    • TH1 TEMP tt.tttÂșC will be displayed.

  • Press ↑

    • TH2 TEMP tt.tttÂșC will be displayed and must be within 0.04ÂșC of TH1 TEMP.

    • Press ↓ twice to display TEMP in ÂșC.

If any of these checks fail see the manual for more details.

8. Precision, Bias and Limit of Determination#

  • The Guildline Portasal Model 8410A has a measurement range from 0.004 mS/cm to 76 mS/cm with a resolution of 0.0003 mS/cm.

  • The measurement can be made at a bath temperature selectable from 15°C to 38°C in steps of 1°C.

  • The accuracy of the instrument is better than 0.003 equivalent Practical Salinity Units.