Oxygen Sensor

Feb 28, 2010

The oxygen sensor can be used to make measurements of the level of free oxygen in air or dissolved oxygen in water.

The free oxygen in air mode is used to measure changes in oxygen levels during combustion or in reactions that produce oxygen (hydrogen peroxide decomposition). The dissolved oxygen mode is useful in the study of photosynthesis. The mode is changed using the module setup option on the sensor module box in NeuLog™ software (see video) or the change range option on the Monitor Display Unit.

The operation of the oxygen sensor is demonstrated in the video below.

Oxygen Sensor from Allan Kerr on Vimeo.

The oxygen sensor is designed for use both in the school laboratory and in the field. It employs easy-to-use polarographic (Clark) technology and replaceable membranes are available. The electrode itself is constructed of Delrin® for durability. With its integral thermistor it provides dependable temperature-compensated measurements. The thermistor is housed in stainless steel and sealed on the electrode’s outer wall providing fast, accurate readings.

The installation and replacement of the membrane is quick and easy. Simply fill the membrane cap assembly with DO electrolyte and screw it into place. Two membrane cap assemblies are included with each sensor. The sensor can be stored in de-ionised water between measurements and overnight. For long term storage the membrane cap should be removed, rinsed in de-ionised water and stored dry.

Sensor Calibration

Calibration of the probe is simply achieved in open air, taking this as a standard level of 20.9%*. First connect the sensor to a voltage source (the USB Bridge plugged into a PC, Neulog’s Monitor Display Unit or Battery Unit) and wait until the reading is stabilized (about 2 minutes). Press the black button on the sensor box for about 3 seconds when the readings are stable. The sensor will then be calibrated at 20.9%.

Alternatively the sensor can be connected to a PC running the Logger Sensors software via the USB module. First click on the Module setup button on the Oxygen sensor’s Module box to open its Module setup window. Then click on the calibration icon.

*This is an assumed stable level in the Earth’s atmosphere at sea-level.

The oxygen sensor can be used in experiments from as short as 1 second and up to 31 days in duration. Sampling rates can be varied from 60 per minute up to 100 per second.

Experiment 1

After calibrating the sensor to 20.9%, I ran the following experiment. I started to breathe on the sensor and I had to breathe out quite strongly to get the oxygen level down to 20% in the 10 seconds of the experiment. The results are shown in the chart below.

Experiment 2

In this experiment we will investigate how the oxygen level in a closed environment drops as a burning flame uses up oxygen in its combustion. The experimental setup is shown below. I used a large soft drink bottle with the bottom cut off and a hole cut for the electrode. I used Blue Tack to seal the around the bottom of the bottle and around the oxygen electrode. I lit a small candle and placed the assembly over it for the experiment. I positioned the candle near the side as far away from the electrode position to avoid heating up the electrode. Although the picture below does not show this it is how I did the experiment when I found the top of the bottle heating up.

The results and observations are quite interesting. Over one minute the oxygen level in the air inside the bottle dropped from 20.9% to below 17% and the flame became weaker and weaker and finally went out after around 50seconds after which the oxygen level started to rise because I took the electrode out of the bottle. Although initially there was a linear drop in oxygen level this soon started was starting to flatten as you can see as the flame became weaker. On these findings you might conclude that the oxygen concentration needs to be above 17% or so to support combustion at least with our candle.

Experiment 3

In this brief experiment we looked at dissolved oxygen levels in cold and warm tap water. It is well-known the oxygen levels decrease as water temperature rises and we found the same here. Cold tap water had an oxygen level of 10.4 mg/l or parts per million (ppm) which is quite high and hot tap water 10.2 mg/l.

Specifications

Specifications for the electrode are as follows