There are two conditions that can cause this issue with the Vernier Optical DO Probe. If you are measuring the rate of photosynthesis in the lab using an artificial light source, it could be light contamination. If you are doing a field experiment or measuring primary productivity, then your sample may be supersaturated.

1. Supersaturated samples

Please note that the saturation level is based on pressure and temperature of the water. The value of 100% saturation is also based on the theoretical maximum based on mixing with air, which is at 20% O2.

A water sample can become super saturated. This is actually not uncommon in fast moving streams and riffles. If you place the probe directly in a riffle, it may supersaturate. Just move it back to where the water is moving at a slower rate etc.

Algae and aquatic plants can also cause supersaturation. Remember from above that the value 100% is based on atmospheric air which contains 20% Oxygen gas. The process of photosynthesis produces pure oxygen. For example, if you are measuring primary productivity you typically expose a water sample to a light source for 24 hrs. The sample is in a closed environment. If you have relatively large source of algae or other plant material that can perform photosynthesis in the sample of water then the sample can become super saturated.

DO levels above 100% saturation are not uncommon in ponds and other small bodies of water during the summer. This is typically the result of algal blooms etc.

The original model (ODO-BTA) only goes up to just over 100% saturation. Switch the sensor to the percent saturation setting. If the reading is above 100% then this is the problem.

The newer model (after January 2015) goes up to 300%. If you purchased your sensor before January 2015 you can get it upgraded.
Can I upgrade my Optical DO Probe?

2. Light Contamination

If you are trying to measure Photosynthesis using the ODO-BTA and your probes reaches a maximum and then falls to zero, then your light source may be shining into the ODO-BTA.

The ODO-BTA uses fluorescence to measure the DO level in the water. A blue light is used to excite the membrane. Emitted red-light then travels to a light detector in the probe. This means that a light sensor is located inside the device.

If you are using a very bright light source to stimulate photosynthesis, the light may interfering with the probe. The problem can be alleviated by following a few simple steps.

Check to see if the tip of the probe is placed in line with the light source. If you raise or lower the probe so the tip is not lined up with the light source the problem should go away.

If the problem persists, you can use the protective case that the ODO-BTA came in to fix this problem. Simply put the case back on the ODO-BTA. Remove the end cap of the case. This contains the sponge. Next, place a piece of black tape around the tip of the case. If you have the tape go from the bottom of the case to the second line on the case (which is located just above the temperature compensation pin on the ODO-BTA) you will be able to block light from getting to the tip of the sensor.

You can now place the sensor back in tour solution to measure photosynthesis.

You can also use the ODO guard to block extraneous light from interfering with the ODO-BTA.