Table of Contents

INTRODUCTION.. 3

1. INSTRUMENTATION.. 5

1.1 Instruments and Materials. 5

1.1.1 Autosal instruments. 5

1.1.2 Plastic and glass bottles. 6

1.1.3 Substandard batches. 9

1.1.4 Data entry instruments. 9

1.2 Operators. 10

1.3 Maintenance. 11

1.3.1 Regular maintenance. 12

1.3.2 Repair 13

2. METHODOLOGY.. 14

2.1 Autosal Operation. 14

2.1.1 Duplicate measurements. 14

2.1.2 Bath and ambient temperature range. 16

2.1.3 Good Autosal operator habits. 18

2.2 Documentation. 19

2.2.1 Measurement Sheets / Run Log. 19

2.2.2 Written reports. 20

2.3 Data Processing. 21

2.3.1 Changes in processing programs. 21

2.3.2 Corrections. 22

3. STANDARDIZATION.. 24

3.1 IAPSO Standard Seawater 24

3.1.1 Standardization methods with IAPSO standard seawater 25

3.1.2 Autosal electronics drift 26

3.1.3 IAPSO batch comparisons. 30

3.2 Substandard Seawater 30

3.2.1 Substandard history. 32

3.2.2 Frequency of substandard measurement 36

4. QUALITY CONTROL. 38

5. CONCLUSION / SUGGESTIONS. 46

6. REFERENCES: Appendices. 48

 

 

INTRODUCTION

For over fourteen years, the Hawaii Ocean Time-series (HOT) project has been successfully building and maintaining a database for observing and interpreting physical and biogeochemical variability in oligotrophic waters around the Hawaiian Islands.  The HOT project has established an extensive compilation of data, collected at nearly monthly intervals, which are readily available to the public and to the scientific community (http://www.soest.hawaii.edu/HOT_WOCE/dataftp.html).  The Physical Oceanography (PO) component of the HOT project has been studying the relation of water mass variations to gyre fluctuations, determining the needs and methods for monitoring currents at Station ALOHA (A Long-term Oligotrophic Habitat Assessment) and developing a climatology of short-term physical variability (http://www.soest.hawaii.edu/HOT_WOCE/).  Thus, it is extremely important to maintain standardized methods for effective sampling and analysis in order to provide reliable data to the scientific community. 

 

Salinity sample measurements, one of the components for which the PO group is responsible, are extremely important in that 1) they provide primary calibration for CTD profiles and thermosalinograph data, 2) they comprise a database of water salinity characteristics at various depths in the water column for an extended period of time at Station ALOHA, and 3) they provide quality control on other Niskin bottle water samples for other measurements.  Standardized methods in sampling, analysis, documentation, and calibration are crucial in setting a context for extracting meaningful signals from a time-series that has been in effect for as long as the HOT Project.  In this report, we have reviewed the history of the HOT salinity database in order to quality-control sections of data with confidence parameters.  We have reviewed past and present methods and will recommend guidelines and standards for optimal procedures.  As a comprehensive history analysis, this report is organized into four parts: Instrumentation (Section 1), Methodology (Section 2), Standardization (Section 3), and Quality Control (Section 4).

 

The Instrumentation section (Section 1) presents the history of salinity sample measurement equipment and documentation of Autosal maintenance over the years.  We present changes in salinity measurement instruments and materials as well as changes in instrument operators.  We also stress the importance of documentation in the Autosal Run Log by the Autosal operator, including any physical, electrical, or chemical repairs done on the Autosal instrument, which should be entered into the Maintenance Database by the Electronics Technician. 

 

In the Methodology section (Section 2), we evaluate the overall methodology of the salinity bottle sampling and data analysis process.  Here we discuss which of the methods employed in the past have remained, been updated, or been discontinued, and ways in which we can improve organization and eliminate confusion in the future.  Some of the important procedures that need to be standardized include methods in Autosal operation, documentation, and data processing.

 

The Standardization section (Section 3) is a subset of the Methodology section in that they both relate to specific methods in the salinity measurement and analysis process.  The Standardization section, however, is specific towards standardization and calibration methods using IAPSO and substandard seawater (collected in a 50 liter carboy from 1000 m at Station ALOHA).  We also examine the significance of Autosal electronics drift throughout the years.  A close look at standardization methodology and Autosal drift may give clues to whether adjustments are needed for specific groups of data.

 

The Quality Control section (Section 4) includes the methods used to determine the quality of the salinity samples by flagging them as good, suspect or bad. We also include plots of salinity values for samples below 3000 dbar, to give an indication of the salinity variability in the deep and bottom water.

 

A final section with Conclusions and Suggestions (Section 5) contains a summary of this report, including recommendations for improving the salinity data quality.
1. INSTRUMENTATION    

1.1 Instruments and Materials

1.1.1 Autosal instruments

A timeline shows the salinity measurement instrument used for each HOT cruise (Figure 1).

 

 

Figure 1. Instruments used to measure salinity samples during the HOT project cruises 1 through 149. The horizontal axis indicates the cruise number.

During HOT-1 through HOT-26, salinity samples were measured by Ted Walsh at the Analytical Services Lab on the 5th floor of MSB using an AGE Minisal 2100 salinometer.  The “Salinity Measurements” section in the World Ocean Circulation Experiment (WOCE) Operations Manual (Stalcup 1991, Appendix A) recommends Guildline Autosal salinometers as being the only type of instrument that is capable of achieving the required WOCE accuracy for salinity measurements. 

 

The HOT project, originally a part of WOCE, purchased the Guildline Autosal 8400A salinometer (SN: 58296, UH Decal: 065QV) in 1990.  Prior to switching over to the Autosal 8400A, comparisons were conducted between Autosal 8400A and Minisal measurements using substandards and duplicate salinity samples from HOT-22 through HOT-26.  From HOT-27 through HOT-107, Autosal 8400A was used to measure primary salinity samples.  For the first six cruises (HOT-27 to HOT-32), the Minisal was used to measure duplicate salinity samples for further comparison (Kennan 1992, “Summary of Comparisons between Guildline Autosal and AGE Minisal,” Appendix B).  Around 1993, an older model of the Autosal 8400A (Autosal 8400, UH Decal: 916RF) was purchased by the Joint Institution of Marine and Atmospheric Research (JIMAR) and was given to the Roger Lukas group.  This Autosal 8400 was used to rummage for parts and as a troubleshooting guide when the Autosal 8400A encountered problems.  The Autosal 8400 was formally disposed of and removed from inventory in June 2001.  After HOT-80 (March 1997), an adjustment was made on the Autosal 8400A to reset the standardization knob, which reached its minimum setting value and could not turn any further.  After the standardization knob was reset, we continued to use the Autosal 8400A until HOT-107. 

 

The Autosal 8400B (SN: 63903, UH Decal: 924SP) was purchased in February 1999 using NSF shipboard equipment funding and has been used to measure primary and duplicate salinity samples since HOT-108 until present day.  On September 19, 2002, a comparison experiment between Autosal instruments 8400A and 8400B was conducted by Daniel S. Fitzgerald (see his experiment notes, Appendix C) after Autosal 8400A was repaired in the lab by the Electronics Technician.  He concluded that the Autosal 8400A is functioning correctly and that it is possible to utilize the Autosal 8400A in case of an emergency with the Autosal 8400B.

 

1.1.2 Plastic and glass bottles

Previous salinity reports and records suggest that while Ted Walsh measured salinity samples on the Minisal, salt samples were collected in 250 ml high-density polyethylene plastic Nalgene bottles.  Evaporation is the single most important factor that affects salinity samples in plastic bottles.  Though parafilm was used as additional sealer for plastic bottles, evaporation still occurred through the sides of the bottles.  Along with the switch to the Autosal 8400A, the group also started using 200 ml flint glass bottles.  The glass bottles, equipped with screw caps and Poly-Seal caps to prevent leakage and evaporation, meet WOCE specifications more readily than the plastic bottles.  The first record of glass bottles being used is on October 4, 1990 by Sean Kennan during his test of evaporation in plastic bottles using glass bottles as reference samples (Kennan 1991, “Evaporation of Plastic Bottles,” Appendix D).  Kennan reports that samples in plastic bottles not measured within 15 days after the cruise must be corrected by an evaporation rate of 1 mpsu/14 days or 0.071 mpsu/day.  Thus, all samples collected in plastic bottles should have been corrected if measured on or later than the fifteenth day after the cruise (due to our own threshold of making corrections for differences >1 mpsu). A review of these corrections is presented below.

 

The first cruise that utilized glass bottles for some casts seems to be HOT-23 (February 1991).  These samples were measured on both the Minisal and the Autosal 8400A and the results are summarized in a report by Sean Kennan (Kennan 1991, “Summary of Salinity Measurements from HOT-23 (2/1 to 2/6, 1991),” Appendix E).  After switching to glass bottles during HOT-24, the bottles were numbered in sequential order from #1 to #408.  When there were not enough glass bottles during a cruise, plastic bottles were used, mostly for thermosalinograph and duplicate samples.  HOT-82 shows new glass bottles (#501 and up) being used; however, plastic bottles were still used for thermosalinograph samples.  HOT-84 (June 1997) is the first cruise in which only glass bottles (#1 to about #700) were used for all casts and since then, only glass bottles have been used.  Since bottles should be replaced every eight to ten years according to the WOCE Operations Manual, the group started using a second set of bottles (#501 to #980) during HOT-140 (September 2002).   Table 1 below categorizes the bottle types used for sets of HOT cruises and whether evaporation corrections were applied to the respective sets of data.       

 

 

 

1.1.3 Substandard batches

Secondary lab-standard (substandard) seawater batches were used from the beginning of the HOT program in order to monitor drift of the Autosal within and between measurement sessions.  Substandard batches were first used sometime around HOT-23 (February 1991).  The seawater used for these batches was collected from 1000 m at Station ALOHA, stored in a glass carboy topped with mineral oil to prevent evaporation, and covered with a black plastic bag to prevent biological growth in the seawater.  Appendix G is a record of Sean Kennan’s calculations on how much mineral oil is needed for the glass carboy.  A maintenance record on July 16, 1991 (Valenciano and Rii 2003, Autosal History Maintenance Database Report, Appendix H) suggests that air pumps on the Autosal 8400A malfunctioned possibly due to mercuric chloride, which had been put into the substandard batch along with mineral oil to inhibit biological growth.  Use of mercuric chloride ceased starting batch #4 (December 1991).  Details on substandard seawater batch use are discussed in Section 3.2. 

 

1.1.4 Data entry instruments

After HOT-24 (March 1991), the group started using a ‘C’ program called ‘autosal’ to enter handwritten Autosal output data into the computer (Appendix I).  This interactive program, originally written by Janice Sato, automatically converted the Autosal’s electronic reading (2 x K15 conductivity ratio) into salinity units. The data were entered by a pair of operators (usually two student assistants) that would double-check the typed values to prevent data entry errors. The ‘autosal’ program prompted for the ambient and bath temperatures as well as the conductivity ratios of IAPSO Wormley seawater ampoules, substandards, and salinity samples, converting the conductivity units to salinity units using the bath temperature and the Autosal’s reading.  On July 18, 1991, Sean Kennan and Carl Chun revised the program to include the following: 1) to calculate the standardization offset from IAPSO measurements, 2) to incorporate this offset into subsequent calculations of salinity, 3) to make backups of data files during the run to avoid disastrous loss of data, and 4) to get time and date from the computer.  Originally, the program was on Personal Computers but was later moved to the Sun UNIX systems.  Additional improvements to the program were made by Sophia Asghar in November 1992, and June 1993.  Further updates and program improvements were made by Fernando Santiago-Mandujano in April 1995.   

 

On March 10, 2000, a few months after switching to using the Autosal 8400B for salinity measurements, an Autosal interface was purchased. The interface is used for direct downloads of conductivity data from the salinometer to a PC-based computer.  The program, which came with the interface (asal ), creates raw data files for subsequent data processing, as the salinity is measured by the Autosal.  The data are then transferred to the Sun UNIX system.  This interface was first used for HOT-114 salinity samples on May 2, 2000 (Figure 1).  The use of this interface eliminated the necessity for handwritten readings off the salinometer as well as manual data entry into a computer, thus increasing efficiency and decreasing operator errors.  For details on processing programs, see Section 2.3.1.  

 

1.2 Operators

 Table 2 below lists all Autosal operators, the model of the Autosal that they used, and the HOT cruises for which salinity samples were measured by each individual.