APPLICATION NOTE NO. 32
Application note in pdf format
Sea-Bird's SBE 13 and 23 dissolved oxygen sensors can be configured with either a Beckman or a Yellow Springs Instruments (YSI) oxygen element. This application note applies only to YSI types.
The YSI 5739 probe has been mounted using a pressure equalization scheme that allows use to depths of 2000 meters (6500 ft). Before the sensor is tested and shipped, the probe is filled with electrolyte. Then, a high-sensitivity membrane is installed, and it is calibrated.
A kit (SBE Part Number 24110) containing 30 high-sensitivity membranes and replacement KCl electrolyte solution is shipped with the oxygen sensor. Additional supplies of these items are available from Sea-Bird or Yellow Springs Instruments. Routine maintenance on this sensor involves changing the membranes and electrolyte and keeping the sensor clean using a non-ionic detergent. During periods of extended storage, YSI advises that the oxygen membrane be kept in a moist environment. For units with a pump, a small amount of water trapped in the plumbing system should suffice. On units that do not have a plenum installed (for use with a pump), a white plastic screw-on cap is provided for sensor storage. Remove the screw-on sensor guard, put a small amount of water (several drops) in the cap, and screw the cap on to the sensor. Located at the end of this application note is a copy of the instructions that YSI sends with the oxygen probes.
Application Note 13-1 contains information on the calibration of the instrument and the equation used by Sea-Bird software to calculate dissolved oxygen. The correct values of tcor, pcor, tau, and wt for the YSI sensor are:
tcor = -0.033
pcor = 1.50e-4
tau = 2.0
wt = 0.85
Obtaining high quality oxygen measurements is only possible when careful attention is paid to the calibration and operation of the oxygen sensor. Whenever possible, independent measurements of oxygen should be obtained. When the oxygen sensor is mounted on a profiling instrument without a pump, a sufficient fall rate (0.75 to 1 meter/second) must be maintained to flush water past the sensor to avoid oxygen depletion at the membrane surface. In applications where the oxygen sensor is pumped, it is possible to obtain a measurement while the sensor is stationary.
To change the electrolyte and membrane the following steps should be followed. The Sea-Bird assembly drawing for the YSI sensor (drawing number 40356) and Figure 4 in the YSI instructions are referenced in these instructions.
1. Remove the plenum from pumped oxygen sensors or the sensor guard from units that are not pumped.
2. There are two styles of housings used to hold YSI sensor elements. To determine which style of housing you have, locate the black, pressure-compensation diaphragm, shown on drawing 40356, on the side of the white plastic housing. Old style housings have a white disk, shown as the DO access cap on drawing number 40356, above the pressure-compensation diaphragm. New style housings do not have the DO access cap, and are smooth above the black diaphragm. To facilitate changing the membrane, remove the DO end cap from the housing using the following specific instructions for your housing type.
OLD STYLE HOUSINGS: Using a 3/32 hex driver, remove the four or six retaining screws that hold the DO end cap to the DO housing (drawing 40356). Then gently lift the end cap off the sensor assembly.
NEW STYLE HOUSINGS: There are 6 Phillips head screws in the white plastic housing. Remove the 3 screws in the end cap closest to the sensor membrane, above the pressure compensation port. DO NOT remove the other 3 screws, which are below the pressure compensation diaphragm and closest to the aluminum pressure housing. Gently lift the end cap off the sensor assembly. This will expose the electrolyte diaphragm and facilitate the replacement of the oxygen membrane.
3. If you have an OLD STYLE HOUSING, remove the DO access cap by gently pulling with a twisting motion. The removal of this cap exposes the electrolyte diaphragm on the YSI probe.
4. Gently remove the membrane O-ring and the membrane itself, taking care not to touch the gold cathode.
5. Empty the electrolyte chamber by turning the sensor over and gently pumping the electrolyte diaphragm with the eraser end of a pencil or Q-tip. (YSI Figure 4A). NOTE: There are 2 diaphragms on the housing. The lower, pressure-compensation diaphragm, visible on the side of the housing before disassembly, should NOT be pumped. The electrolyte diaphragm is the diaphragm closer to the sensor membrane, which was exposed in steps 2 and 3.
6. Assemble the following items needed for the next few steps: Q-tips, high-sensitivity membrane booklet, electrolyte refill bottle, and new membrane O-ring.
7. Hold the sensor assembly vertical and place a few drops of electrolyte solution in the top of the sensor. With a Q-tip or pencil eraser gently pump the electrolyte diaphragm to draw the electrolyte into the reservoir (YSI Figure 4A). Repeat this step until there are no more bubbles coming out of the reservoir. When no more bubbles appear, add one or two more drops of electrolyte to form a nice meniscus on the top of the sensor.
8. Referring to YSI Figure 4B, secure a high-sensitivity membrane between your thumb and the probe body. Try not to dislodge the meniscus of electrolyte solution. If the meniscus drips away, add a few more drops of solution. Take care to handle the membrane at the ends only.
9. With the thumb and forefinger of your free hand, grasp the free end of the membrane (YSI Figure 4C) and with a continuous motion stretch the membrane up, over, and down the other side of the sensor head (YSI Figure 4D). Stretching forms the membrane to the contour of the probe.
10. Secure the end of the membrane under the forefinger of your left hand while holding the probe. This may take some practice, as the membrane must remain taut. (YSI Figure 4E)
11. Gently place the O-ring on top of the membrane and place a drop of electrolyte solution in the center of the O-ring. This will help lubricate the O-ring as it is placed over the membrane. Roll the O-ring over the end of the probe, being careful not to touch the membrane surface (YSI Figure 4F). There should be no wrinkles in the membrane and no air bubbles trapped under the membrane. Some wrinkles may be removed by lightly tugging on the edges of the membrane beyond the O-ring. It may take some practice to acquire the coordination necessary to properly install a membrane. If the membrane gets wrinkled or punctured, take off the O-ring, discard the membrane, add a few drops of electrolyte to reform the meniscus, and try again.
12. Trim off the excess membrane with sharp scissors. Do not use an Exacto Knife! There is too great a risk of cutting the O-ring or slipping and puncturing the membrane. Check that the stainless steel thermistor tube is not covered by excess membrane (YSI Figure 4G).
13. For old style housings, reinstall the DO access cap with a slight twisting motion to seat the O-ring. Align the 4-40 hole in the access cap with the matching hole in the body.
14. Carefully place the DO end cap over the YSI sensor probe and line up the mounting holes with the matching holes in the DO housing. You may have to "fit" the end cap by turning it 90 degrees, pressing it into place, then turning it back to line up with the screw holes. Be sure that you don't turn the YSI sensor itself; on old style housings the electrolyte diaphragm on the YSI probe must remain aligned with the access hole in the DO housing.
15. OLD STYLE HOUSINGS: Install the four 4-40 x 1/2" screws that hold the DO end cap to the DO housing. Tighten these screws gently in an alternating pattern to avoid stripping the threads in the plastic housing. The DO end cap may not sit flush against the DO housing. This is normal, as there is no O-ring groove in the DO housing. Do not distort the plastic parts by over-tightening the screws in an attempt to make parts fit flush.
NEW STYLE HOUSINGS: Press the end cap down firmly on the housing until it seats against the housing. Replace the 3 Phillips head screws, being careful not to over-tighten them into the plastic housing.
16. Rinse the reassembled unit with distilled or deionized water to wash away electrolyte. Carefully reinstall the plenum.
Drawing 40356 YSI DO Sensor Assembly (click thumbnail to view full size)
YSI Instructions (click thumbnails to view full size)
![[HRule Image]](../images/div.gif){kind=small}
Sea-Bird Home Phone: 425-643-9866 Fax: 425-643-9954 E-mail: seabird@seabird.com