APPLICATION NOTE NO. 2D
Instructions for Care and Cleaning of Conductivity Cells
Revised March 2014 
 

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This application note presents recommendations for cleaning and storing conductivity sensors. In the past, Sea-Bird had recommended cleaning and storing conductivity sensors with a Triton X-100 solution, and cleaning conductivity sensors with an acid solution. Our research leads us to recommend adding the use of a dilute bleach solution to eliminate growth of bio-organisms, and eliminating the use of acid in most cases.

The application note is divided into four sections:

• General discussion
• Identifying damaged or severely fouled conductivity cells
• Rinsing, cleaning, and storage procedures
• Cleaning materials

Since any conductivity sensor’s output reading is proportional to its cell dimensions, it is important to keep the cell clean of internal coatings. Cell electrodes contaminated with oil, biological growths, or other foreign material will eventually cause low conductivity readings. To control growth of bio-organisms in the conductivity cell, follow these rinsing and cleaning recommendations:

• Bleach is extremely effective in controlling growth of bio-organisms in the conductivity cell. Sea-Bird recommends cleaning the conductivity sensor in a dilute bleach solution.
• Triton X-100 is a mild, non-ionic surfactant (detergent), valuable for removal of surface and airborne oil ingested into the CTD plumbing as the CTD is removed from the water and brought on deck. Sea-Bird recommends rinsing and cleaning the conductivity sensor in a Triton solution.
• White vinegar, which is 5 - 8% acetic acid, may be used to remove minor mineral deposits on the inside of the cell.

No adverse effects have been observed as a result of dry storage, if the cell is rinsed or soaked with fresh, clean water before storage to remove any salt crystals. This leads to the following conductivity cell storage recommendations:

• Short-term storage (< 1 day, typically between casts): If there is no danger of freezing, store the cell with a dilute bleach solution in Tygon tubing looped around the cell. If there is danger of freezing, store the cell dry, with Tygon tubing looped around the cell.
• Long-term storage (> 1 day): Since conditions of transport and long-term storage are not always under the user's control, store the conductivity cell dry, with Tygon tubing looped around the cell ends. Dry storage eliminates the possibility of damage due to unforeseen freezing, as well as the possibility of bio-organism growth in water inside the cell. Filling the cell with a Triton X-100 solution for 1 hour before deployment will rewet the cell adequately.

The Tygon tubing looped around the ends of the conductivity cell, whether dry or filled with a bleach or Triton solution, has the added benefit of keeping air-borne contaminants (abundant on most ships) from entering the cell.

Every conductivity calibration certificate has a frequency output for zero conductivity, obtained from a cell thoroughly rinsed in distilled or de-ionized water, with all the water shaken out (dry cell). A zero conductivity frequency that has changed by more than a few 10ths of a Hertz may indicate a cell that is damaged or considerably out of calibration. Noisy readings (± a few 10ths of a Hertz) indicate a dirty cell; follow the procedure for Cleaning Severely Fouled Sensors to clean a dirty cell.

Note: See Cleaning Materials below for discussion of appropriate sources / concentrations of water, Triton X-100, bleach, white vinegar, and tubing.

CAUTIONS:

• The conductivity cell is primarily glass, and can break if mishandled. Use the correct size Tygon tubing; using tubing with a smaller ID will make it difficult to remove the tubing, and the cell end may break if excessive force is used. The correct size tubing for use in cleaning / storing all conductivity cells produced since 1980 is 7/16" ID, 9/16" OD. Instruments shipped prior to 1980 had smaller retaining ridges at the ends of the cell, and 3/8" ID tubing is required for these older instruments.
• Do not put a brush or object (e.g., Q-Tip) inside the conductivity cell to clean it or dry it. Touching and bending the electrodes can change the calibration; large bends and movement of the electrodes can damage the cell.
• If a dissolved oxygen (DO) sensor is plumbed to the CTD -- Before soaking the conductivity cell for more than 1 minute in Triton X-100 solution, disconnect the tubing between the conductivity cell and DO sensor to prevent extended Triton contact with the DO sensor membrane (SBE 43) or optical window (SBE 63). Extended Triton contact can cause sensitivity [slope] changes, usually temporary, to the sensors. For rinsing, cleaning, and storage, see Application Note 64 for the SBE 43 or the SBE 63 manual for the SBE 63.
• IDO and ODO MicroCATs (37-SMP-IDO, SIP-IDO, IMP-IDO, 37-SMP-ODO, SIP-ODO, IMP-ODO) have an integrated dissolved oxygen sensor. Do not following the rinsing, cleaning, and storage recommendations in this application note; extended Triton contact with the DO sensor membrane (IDO MicroCATs) or DO sensor optical window (ODO MicroCATs) can cause sensitivity [slope] changes, usually temporary, to the sensors. See the applicable MicroCAT manual.

Active Use (after each cast)

1. Rinse: Remove the plumbing (Tygon tubing) from the exhaust end of the conductivity cell. Flush the cell with a 0.1% Triton X-100 solution. Rinse thoroughly with fresh, clean water and drain.

• If not rinsed between uses, salt crystals may form on the conductivity cell platinized electrode surfaces. When the instrument is used next, sensor accuracy may be temporarily affected until these crystals dissolve.

2. Store: The intent of these storage recommendations is to keep contamination from aerosols and spray/wash on the ship deck from harming the sensor’s calibration.

• No danger of freezing: Fill the cell with a 500 – 1000 ppm bleach solution, using a loop of Tygon tubing attached to each end of the conductivity sensor to close the cell ends.
• Danger of freezing: Remove larger droplets of water by blowing through the cell. Do not use compressed air, which typically contains oil vapor. Attach a loop of Tygon tubing to each end of the conductivity cell to close the cell ends.

Routine Cleaning (no visible deposits or marine growths on sensor)

1. Agitate a 500 – 1000 ppm Bleach solution warmed to 40 °C (wrist warm) through the cell in a washing action (this can be accomplished with Tygon tubing and a syringe kit -- see Application Note 34) for 2 minutes. Drain and flush with warm (not hot) fresh, clean water for 5 minutes* or until rinsed thoroughly.
2. Agitate a 1%-2% Triton X-100 solution warmed to 40 °C (wrist warm) through the cell many times in a washing action (this can be accomplished with Tygon tubing and a syringe kit). Fill the cell with the solution and let it soak for 1 hour. Drain and flush with warm (not hot) fresh, clean water for 5 minutes* or until rinsed thoroughly.

* If you do not have a large supply of fresh water: As a minimum, flush the cell with enough warm, fresh, clean water to fill the cell five times, or until rinsed thoroughly.

Cleaning Severely Fouled Sensors (visible deposits or marine growths on sensor)

1. Repeat the Routine Cleaning procedure up to 5 times.

1. Thoroughly rinse in distilled or de-ionized water, and shake out all the water. With the conductivity cell dry, take and record a raw conductivity reading (in Hz). Compare to the zero conductivity frequency on the calibration sheet. The output should be within a few tenths of a Hz of the zero conductivity frequency. If not, proceed to Step 2.

2. Clean with a white vinegar solution or diluted HCl solution; see the Cleaning Materials section of this application note for details.

If the zero conductivity is still outside the expected range, the conductivity cell may require factory cleaning.

Long-Term Storage (after field use)

1. Rinse: Remove the plumbing (Tygon tubing) from the exhaust end of the conductivity cell. Flush the cell with a 0.1% Triton X-100 solution. Rinse thoroughly with fresh, clean water and drain. Remove larger droplets of water by blowing through the cell. Do not use compressed air, which typically contains oil vapor.
2. Store: Attach a loop of Tygon tubing to each end of the conductivity cell to close the cell ends and prevent contaminants from entering the cell.

• Storing the cell dry prevents the growth of any bio-organisms in water in the cell, thus preserving the calibration.

3. When ready to deploy again: Fill the cell with a 0.1% Triton X-100 solution for 1 hour before deployment. Drain the Triton X-100 solution; there is no need to rinse the cell.

Water

De-ionized (DI) water, commercially distilled water, or fresh, clean, tap water is recommended for rinsing, cleaning, and storing sensors.

• On ships, fresh water is typically made in large quantities by a distillation process, and stored in large tanks. This water may be contaminated with small amounts of oil, and should not be used for rinsing, cleaning, or storing sensors.

Where fresh water is extremely limited (for example, a remote location in the Arctic), you can substitute clean seawater for rinsing and cleaning sensors. If not immediately redeploying the instrument, follow up with a brief fresh water rinse to eliminate the possibility of salt crystal formation (salt crystal formation could cause small shifts in calibration).

• The seawater must be extremely clean, free of oils that can coat the conductivity cell. To eliminate bio-organisms in the water, boil the water or filter it with a 0.5 micron filter.

Triton X-100

Triton X-100 is Octyl Phenol Ethoxylate, a mild, non-ionic surfactant (detergent). Triton X-100 is included with every CTD shipment and can be ordered from Sea-Bird, but may be available locally from a chemical supply or lab products company. It is manufactured by Avantor Performance Materials (see http://avantormaterials.com/commerce/product.aspx?id=2147509608). Other liquid detergents can probably be used, but scientific grades (with no colors, perfumes, glycerins, lotions, etc.) are required because of their known composition. It is better to use a non-ionic detergent, since conductivity readings taken immediately after use are less likely to be affected by any residual detergent left in the cell.

100% Triton X-100 is supplied by Sea-Bird; dilute the Triton as directed in Rinsing, Cleaning, and Storage Procedures.

Bleach

Bleach is a common household product used to whiten and disinfect laundry. Commercially available bleach is typically 4 % - 7% (40,000 – 70,000 ppm) sodium hypochlorite (Na-O-Cl) solution that includes stabilizers. Some common commercial product names are Clorox (U.S.) and eau de Javel (French).

Dilute to 500 – 1000 ppm. For example, if starting with 5% (50,000 ppm) sodium hypochlorite, diluting 50 to 1 (50 parts water to 1 part bleach) yields a 1000 ppm (50,000 pm / 50 = 1000 ppm) solution.

Tygon Tubing

Sea-Bird recommends use of Tygon tubing, because it remains flexible over a wide temperature range and with age. Tygon is manufactured by Saint-Gobain (www.tygon.com). It is supplied by Sea-Bird, but may be available locally from a chemical supply or lab products company.

Keep the Tygon in a clean place (so that it does not pick up contaminants) while the instrument is in use.

White Vinegar

White vinegar is a common household product used to dissolve mineral deposits, and may be used to remove minor mineral contamination of the conductivity cell. Commercially available white vinegar is typically 5 – 8% acetic acid (CH₃COOH) in aqueous solution; verify that there are no oils or other ingredients.

1. Prepare for cleaning:

1. Place a 0.6 m (2 ft) length of Tygon tubing over the end of the conductivity cell.
2. Clamp the instrument so that the cell is vertical, with the Tygon tubing at the bottom end.
3. Loop the Tygon tubing into a U shape, and tape the open end of the tubing in place at the same height as the top of the glass cell.

2. Clean the cell:

1. Pour weak white vinegar solution (1 part white vinegar, 2 parts water) into the open end of the tubing until the cell is nearly filled. Let it soak for 2-3 minutes only.
2. Drain the solution from the cell and flush for 5 minutes with warm (not hot), clean, de-ionized water.
3. Rinse the exterior of the instrument to remove any spilled solution from the surface.
4. Fill the cell with a 1% Triton X-100 solution and let it stand for 5 minutes.
5. Drain and flush with warm, clean, de-ionized water for 1 minute.
6. Carefully remove the 0.6 m (2 ft) length of Tygon tubing.

3. With the conductivity cell dry, take and record a raw conductivity reading (in Hz). Compare to the zero conductivity frequency on the calibration sheet. The output should be within a few tenths of a Hz of the zero conductivity frequency. If not, repeat Steps 1 and 2 with a moderate white vinegar solution (1 part white vinegar, 1 part water) and repeat the test. If still outside the expected range, repeat with a full strength white vinegar solution and repeat the test. If still outside the expected range, return to the factory or see Hydrochloric Acid (HCl) below.
4. Prepare for deployment, or follow recommendations above for storage.

Hydrochloric Acid (HCl)

Many years ago, Sea-Bird recommended cleaning with a hydrochloric acid (HCl) solution to eliminate bio-organisms or mineral deposits on the inside of the cell. However, bleach cleaning has proven to be effective in eliminating growth of bio-organisms, and is much easier to use and dispose of than acid. Data from many years of use shows that mineral deposits are an unusual occurrence. Sea-Bird recommends that, in most cases, hydrochloric acid should not be used to clean the conductivity sensor. In rare instances, it may still be required for mineral contamination of the conductivity cell. Sea-Bird recommends that you return the equipment to the factory for this cleaning. Information below is provided if you cannot return the equipment to Sea-Bird.

CAUTIONS:

• SBE 37-IMP, 37-SMP, 37-SIP, 37-IMP-IDO, 37-SMP-IDO, 37-SIP-IDO, 37-IMP-ODO, 37-SMP-ODO, 37-SIP-ODO MicroCAT; SBE 49 FastCAT; SBE 52-MP Moored Profiler CTD; or other instruments with an integral, internal pump -- Do not perform acid cleaning. Acid cleaning may damage the internal, integral pump. Return these instruments to Sea-Bird for servicing if acid cleaning is required.
• SBE 9plus, 25, or 25plus CTD -- Remove the SBE 4 conductivity cell from the CTD and remove the TC Duct before performing the acid cleaning procedure.
• All instruments that include AF24173 Anti-Foulant Devices -- Remove the AF24173 Anti-Foulant Devices before performing the acid cleaning procedure. See the instrument manual for details and handling precautions when removing AF24173 Anti-Foulant Devices.

WARNING! Observe all precautions for working with strong acid. Avoid breathing acid fumes. Work in a well-ventilated area.

The acid cleaning procedure for the conductivity cell uses approximately 50 - 100 cc of acid. Sea-Bird recommends using a 20% concentration of HCl. However, acid in the range of 10% to full strength (38%) is acceptable.

1. Prepare for cleaning:

1. Place a 0.6 m (2 ft) length of Tygon tubing over the end of the cell.
2. Clamp the instrument so that the cell is vertical, with the Tygon tubing at the bottom end.
3. Loop the Tygon tubing into a U shape, and tape the open end of the tubing in place at the same height as the top of the glass cell.

2. Clean the cell:

1. Pour 10% to 38% HCl solution into the open end of the tubing until the cell is nearly filled. Let it soak for 1 minute only.
2. Drain the acid from the cell and flush for 5 minutes with warm (not hot), clean, de-ionized water.
3. Rinse the exterior of the instrument to remove any spilled acid from the surface.
4. Fill the cell with a 1% Triton X-100 solution and let it stand for 5 minutes.
5. Drain and flush with warm, clean, de-ionized water for 1 minute.
6. Carefully remove the 0.6 m (2 ft) length of Tygon tubing.

3. Prepare for deployment, or follow recommendations above for storage.

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