Main Nitrate Instrument Components

The NAS-2EN instrument is a colorimetric mechanical device for the measurement of discrete samples. The NAS-2EN instrument is manufactured by WS Ocean Systems Ltd and has been commercially available for over 8 years. It has evolved from an original design developed at the Scottish Office Marine Laboratory, Aberdeen.

Figure 1

Diagram of mechanical structure of a NAS-2E (Hydes et.al, 1999).

1. Stepper motor. 2. Multi-port valve 3. Syringe 4. Colorimeter  5. Syringe drive motor.  6. Control electronics

 

 

The NAS-2EN instrument is constructed of five main components (Figure 1) namely an eight way rotary valve, a motor driven syringe, a colorimeter, reagent housing, and an electronic housing unit. The instrument is about 80cm long, 21cm in diameter and weights ~10Kgs without reactives. The chemical system is essentially pressure balanced as all the chemical circuitry is at ambient pressure and external to the electronic instrumentation. The ones used in ANIMATE are prepared to withstand depth of 1000 meters therefore some parts are oil filled.

 

A pressure sealed “cap” encloses the stepper motor. A reagent housing is then attached to this “cap”. The reagents are stored in plastic “transfusion” type bags within this container and connected via tubing to a corresponding valve on the multi-port valve unit (Figure 2)

Control and retrieval of data can be done by direct connection of a PC-computer through a RS232 communications port on the instrument, or can be done remotely by means of telemetry.  Instructions can be given either by a menu driven program, or by means of a command language.  The software allows for control of these protocols by the user.  For example, longer periods between the collection of samples can be set up or the details of the procedure can be altered - such as the relative volumes of samples and reagents, or the duration of the reduction step in the copper-cadmium column.

 

The colorimeter consists of a narrow linear capillary tube with a LED source at one end and a photodiode detector at the other.  An additional detector is placed adjacent to the source to monitor ant changes in LED intensity directly.

NAS – 2EN

 

 

Pre-Deployment

 

Before instrument is deployed the main housing “0” rings need to be greased and checked for any debris (this could cause leaking). It is also advisable to grease the connectors on the power cable and the prongs on the communication connector before putting on the blank plug.

 

1. Communicating with Instrument

 

• Connect cable to communication connector on Nas instrument and to the serial port of the computer.

 

• Start-up the Nas software.

 

• Apply Power to instrument (12V DC). You now have between 15 and 20 seconds to connect to the instrument. If you do not connect during this time the instrument will go into logging mode and you will not be able to connect with it. If this happens disconnect power and then re-connect power.

 

• To connect to the instrument select the “connect” button in the Nas software.

 

 

2. Cadmium Column.

 

• Place about a 1cm plug of glass wool at one end of a 15cm 3.2 mm (ID) plastic tube.

 

• Fill tube with 3.2g of Skalar cadmium granules.

 

• Place a second plug of glass wool at other end of tube.

 

• Connect a nipple connector at each end of the tube and then connect one end of the column to the brown port. Connect about a 75cm of narrow bore (blue) tubing into the other end. This end is left open to the sea.

 

 

3. Nitrate Reagents

Sulphanilamide:  Dissolve 5g in a mixture of 50 ml concentrated hydrochloric acid and 50 ml distilled water then dilute to 1l with distilled water, then add 1ml BRIJ-35 wetting agent (30% solution).

 

Napthylethelynedihydrochloride: Dissolve 0.5g in 1l distilled water. Add 1ml BRIJ-35 wetting agent (30% solution).

 

Ammonium chloride: Dissolve 50g in 2 litres of distilled water.

 

On-board-standard: Make up standard from stock solution in 40‰ sodium chloride. For ANIMATE two standards of different concentration are used in order to have two independent values for each sample. The first standard corresponds to the green valve and contains about the lowest nitrate concentration that the instrument will probably measure; the second is included in the purple valve and will contain the highest nitrate concentration expected to measure at the site. The standards are therefore dependant on the environment in which the instrument is to be deployed in.

 

4. Connect up Chemicals

 

• Place chemical bags into top hat and feed the connecting tubes thought the holes in the top hat so that you can then connect tubes to the ports on the chemical manifold valve. N.B. Remember that when the bags empty during the deployment they will collapse inside the top hat. Therefore there is a danger that if the tubes have not been fully pulled through to the outside of the top hat then they may kink and therefore block.

 

• Connect the bags to the following ports

 

Red                  Ammonium Chloride         

Orange            Sulpha. Reagent

Yellow            NEDD Reagent

Green               On-board-standard (I put the lowest)

Mauve              On-board-standard (I put the highest)

 

The blue port is connected to the colorimeter and a tube of about 30-50cm is attached to the free end of the colorimeter which is the output of the instrument to sea.

 

The grey port is the input so therefore connect a tube to this port with a filter connected at the other end.

 

The cadmium Column is connected to the brown port on one side and open to the water with a filter connected at the other end.

 

• Make sure that all the chemicals are running freely using the “motor” page in the Nas software.

 

 

5. Macro

The instrument is not going to be used in its standard mode as set up by the factory. The instrument is going to be used in Macro Mode. The macro is a small control program which has been written to instruct the instrument when to move either the motors or when to take readings of the sample in the colorimeter. The macro that is to be used is called the Master Nitrate or similar name depending on the site.

 

To run the instrument in this mode:-

 

·        Run Nas Software

 

·        Connect to instrument (see 1.)

 

·        Select macro page of WS program

 

·        Tick macro box

 

·        Select macro to be used (Master Nitrate Macro)

 

·        Upload macro to instrument by selecting upload button. As the macro uploads you will be able to see all the commands used in the macro scrolling down the display window.

 

Once the macro is uploaded, the instrument will then follow the commands set out in the macro. The macro is made up of a number sub-macros namely the Comment, Analysis, S, T and R sub-macros.

 

Use the comments page for notes.

 

The Analysis sub-macro tells the instrument the sequence in which the sub-macros S, T and R are to be run.

 

The S sub-macro is the routine, which measures a seawater sample.

 

The T sub-macro is the routine, which measures the On-Board- Standards macro.

 

The R sub-macro is called the refresh sub-routine. This was created mainly for use with a wire cadmium reduction column. However, the Master Nitrate Macro uses this sub-macro to add one command, which is “pause”. This is used because we need to set the active time interval (time between samples) to different values depending on the sampling frequency, say for example six hours for this deployment. However, the longest active time interval that can be set in this instrument is 90 minutes. Therefore, if we set the following sequence in the Analysis sub-macro:- T, S, R, R, R, R, and make the active time interval 60 minutes a seawater sample and an on-board standards will be measured every six hours.

 

N.B.  Any changes to macro must be uploaded to the instrument.

 

6. Set Time and Set Data Pointer.

 

• Run Nas software.

 

• Connect to instrument (see 1.).

 

• Select Date & Time page of Nas software.

 

• Set time either manually or by setting the time to the computer’s time.

 

• Then Select Data page of Nas software.

 

• Press the re-set memory button. N.B. All the data stored previously in the instrument will be deleted (not however any of the settings). Therefore if you wish to keep a copy of that data before re-setting it then select a filename and download data into that file.

 

7. Calibration

 

At least five standards of nitrate and nitrite according to the expected values are prepared and measured by chemical analyses. New reagents are also prepared to check them in the same way.

Once all the chemicals and standards are correct, they are used to make the calibration (in general between 0 and 30 mMol) for the sensor as compared to the autoanalyser SkalarÓ measurements of the same standards. If the results are not satisfactory (R² =0.99) the sensor is checked again and the calibration is repeated until reaching the adequate determination coefficient.

 

• Never leave the sensor ready to go to the ship with the piston to the top. Always use the motor to retract 80 steps and filled it with destilled water and remember to take it out before the deployment instructions once on board.

 

 

Deployment

 

• Take sample from On-Board Standards (to check OBS’ concentration).

 

• Place instrument into frame.

 

• Make sure that all the chemicals are connected and that the tubes are free and not snagging. Also check that the cadmium reduction column is connected to the brown port and that the free end of column is free and not snagged.

 

• Check that the tube and filter attached to grey port is free and is not snagged.

 

• Check that blue port is attached to colorimeter and that free end of colorimeter is free and not snagged.

 

• Connect Battery to instrument (check battery voltage reading before deployment, can do this by connecting power cable to battery and then take voltage readings from the prongs of the connector at the other end of the cable).

 

• Connect to instrument with computer and run Nas software (remember you have between 15 and 20 seconds to do this before the instrument will automatically start logging).

 

 

• Check that instrument is in macro mode. When you connect to the instrument a number of commands are display on the display screen such as date and time etc. If macro mode has been selected you will see M1 displayed.

 

If you see M0 then it is in factory mode and you need follow number 5 of pre-deploy check. Also if the instrument has been running on the bench make sure that the Analysis page on the Master Nitrate Macro has been re-set to

 T, S, R, R, R, R

 

If you have to edit the macro remember that you have to upload the macro once more or the instrument will ignore any changes that you have made.

 

• Check that the date and time are correct.

 

• Check data pointer is set at 0. You will see the command D0 on display if the memory has been re-set. If you see the D followed by a number greater than 0 then the memory has not been re-set.

 

• Select Logging page of Nas software.

 

• Set the active time interval to 60 minutes (for example, in ANIMATE is site dependent).

 

• Set delay time. This is the time from which the instrument is set into logging mode until it starts to operate. A maximum value of 90 minutes (for example, in ANIMATE is site dependent) can be set.

 

• Press the “write settings” on the logging page.

 

• Press the “set logging” button to set the instrument into logging mode.

 

• Disconnect Computer AND REMEMBER TO PLUG IN THE BLANK PLUG TO THE COMMUNICATION PORT.

 

• Get instrument into water before it starts to operate. If the instrument does start then not too much damage will be done to the column however if there is a delay of more than 2 hours then you will have to reconnect the computer and “pull out” then “push back in” the power cable to re-set instrument. Start instrument in to logging mode once again when ready to deploy.

 

 

 

Recovering Instrument.

 

• Rinse and dry off instrument.

 

• Remove blank plug from the communication port and connect computer to instrument.

 

• Re-set power by disconnecting then reconnecting power lead. You will only be able to do this if there is power left in the battery. If the battery is flat then you will have to connect an external power supply (12V DC) to the instrument.

 

• Run Nas Software.

 

• Select Data page of software

 

• Enter a filename into which the data is going to be downloaded into.

 

• Make sure the box next to the option “Download to File” is ticked.

 

• Press the download data button. The data will then download. Please note that this can take a few minutes. Also there is a delay between the time that when the data has been downloaded until the data is written to the file. It can look as if the instrument has “hung”. It has not hung, just give it time.

 

• It is also worthwhile to download the diagnostic data as well. Therefore, enter a new file name then press the diagnostic data button to download this data into the new file. DO NOT RE-SET MEMORY AT THIS STAGE.  It is better to make sure you have at least two good copies of the data before doing this.

 

• Take a sample from each On-board-standard bag (to check OBS’ calibration); to do so, go to Analyses and do a “T” macro.

 

 

 

 

 

Maintenance

 

Before re-deploying instrument make sure that you follow the maintenance section of the Nas manual. The 0 rings and probably the syringe/piston piece will need to be replaced.