What’s Monitored


Tips for trainers teaching about measuring Dissolved Oxygen

Pre-training thoughts:

  • Review WAV methods prior to training-NOTE DIFFERENCES WITH OUR METHODS FROM HACH'S:
    1. We have monitors use two sample vials of the fixed sample so that each drop of sodium thiosulfate is equal to 0.5 mg of oxygen in the sample.
    2. We have the monitors add starch to the sample part way through the analysis, so they are titrating the sample from blue to clear rather than yellow to clear for better end points.
    3. We ask monitors to determine the percent saturation of the oxygen. It's a good idea to familiarize yourself with the procedures to determine this on the D.O. fact sheet.
  • Review the chemistry of what's happening in the reaction prior to the training. Roger Bales of the University of Arizona with the GLOBE Program in Arizona has created a web page with some great information about the chemical process.

  • It sometimes helps monitors to get the idea of how the Hach kits work (and saves some demo time) if you set up a dummy sample (A) to which you have already added chemicals #1 and #2 and in which you have allowed the floc to settle twice after shaking as many times. You can also set up a second dummy sample (B) from which you can take a sample to titrate. When you demonstrate the sample preparation to the monitors in training, you can add chemicals #1 and #2 into a newly collected sample, and then move right on to adding chemical number 3 into dummy sample A. You can then immediately move onto the titration step using dummy sample B. The monitors will have a chance to see the true time of the test when they perform it themselves following your demonstration.

  • Go to your training site prior to the training to pick an area that will work well for assessing dissolved oxygen during training or work with the training coordinator to learn what section of stream will be used for the dissolved oxygen measurement during the training session. Consider depth of water (can trainees enter the water as they'll be asked to do when monitoring), flow (safety concern as well as affect on D.O. that you could teach about), and accessibility of the site (steepness of streambank, etc.).
  • Be sure to have enough copies of the data sheet for use at the training as well as for use by monitors at their own sites.

Teaching tidbits:

  • Discuss the importance of dissolved oxygen in the water (all life depends on it for survival), how it enters water (atmospheric inputs, flow, and plant action), and what things (natural and human) cause dissolved oxygen to fluctuate (such as time of day, nutrient and sediment inputs, plant action, temperature, etc.).

  • Because we ask monitors to assess the percent saturation of the oxygen in the water, it's important to note the connection between temperature and dissolved oxygen. A great example that people can relate to is soda and the amount of carbonation in it - a warmer liquid (soda) has less gas able to be dissolved in it, thus we feel the soda is flat when it's warm, and thus, warmer water has the ability to hold less gas -oxygen- than colder water.

  • Please have monitors use a peanut butter jar sampling device to collect their samples. Here's how to use them:
    1. Put the open glass D.O. sample bottle into the open peanut butter jar
    2. Place the rubber stopper onto the peanut butter jar to seal the glass D.O. bottle in. The center pen in the rubber stopper should enter the glass D.O. sample bottle and the other pen should emerge from the top of the rubber stopper (on the outside of the peanut butter jar sampler).
    3. Then either plunge the sampler into the stream following directions in the fact sheets regarding depth of water and how to approach the sample site, or first attach the sampler to a pole if you are unable to enter the stream, then follow directions to collect the sample.
    4. Allow the glass D.O. bottle to fill and the peanut butter jar to mostly fill before removing the sampler from the water.
    5. Remove the glass D.O. bottle from the peanut butter jar and place the glass stopper in it, but allow excess water to fill to the rim of the D.O. bottle (to be used to help avoid getting bubbles in the sample once chemicals are added).

  • If you get an air bubble in your sample, first try recollecting the sample. If there are still bubbles, try collecting the sample one more time. This time, when you try capping be especially careful to drop the stopper from about a ¼ inch above the sample bottle directly into the neck. This should help reduce possibility of getting bubbles in the sample when stoppering it. (The cap is pointed and if put into the bottle directly up and down, it should force out the bubbles in the bottle. If you still get an air bubble, use an eyedropper filled with water from the stream to put additional water into the sample bottle to help flush out the bubble.
  • When adding the DO #1 and DO #2 reagents, don't delay the capping and shaking. If the reagents aren't mixed promptly they may stick to the bottom of the bottle.

  • When adding drops of sodium thiosulfate to titrate D.O., please ensure that people are holding the eye dropper vertically so that they are feeding a complete drop into the sample each time.
  • Please go through data entry on the data sheet with the volunteers - to make sure they know to use two vials of sample for titrations and to ensure they understand how to do the conversion to % saturation and why it's important.
  • Be sure to give monitors a copy of the data sheet for use at the training as well as for use at their own sites. All data sheets can be printed from this site here.
  • Dissolved oxygen extremes, particularly in mid-day (below 2.0 mg/L and above 17 mg/L) may indicate problems within the stream and monitors should first redo the measurement to determine if their finding is repeated, then might consider reporting their findings to the DNR.

  • If people are interested, some ways dissolved oxygen could be measured is 1) to try to measure it at the same time each sample date (relative to sunrise-say ½ hour after sunrise) or 2) to sample dissolved oxygen throughout a given sample day at set intervals- to get an idea of the cyclic nature and peaks and pits of the measurement at the site.

Errata in Fact sheet:

  • Page 3, Collecting the Sample #4: This should read: "Facing upstream, slowly lower the bottle..."

  • Page 3, Testing for Dissolved Oxygen Note: This note should read: "If you see any air bubbles trapped in the sample bottle during steps 2-4, discard the sample and start over." It is common to get bubbles when you add DO3, and the sample is fixed, so oxygen content will not change after DO1 and DO2 are added, so this is ok.

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