Dissolved Oxygen Meters

Q: How does the Clark dissolved oxygen probe work?

A: The Clark cell discovered by Dr. Clark in 1956 is an ampherometric cell that is polarised around 800 mV. Reduction of oxygen is achieved between 400 to 1200 mV. Hence the need for a voltage of around 800 mV. In the Clark cell this is provided externally by a battery source.
The Clark cell is built around the popular Ag/AgCl half-cell and a noble metal such as gold.

Clarks Table

Q: What parameter is being measured by the dissolved oxygen content of a solution?

A: The dissolved oxygen probe supplied with the Jenway meters does not measure the actual amount of oxygen in water, but instead measures the partial pressure of oxygen in the water sample. Oxygen's partial pressure is dependent on both the salinity and temperature of the sample.

Q: How does temperature affect dissolved oxygen measurements?

A: The temperature of a water sample has an inverse relationship to the solubility of oxygen. As shown below, an increase in temperature reduces oxygen content. This relationship is non-linear and has been established empirically.

Q: How does atmospheric pressure affect dissolved oxygen measurements?

A: Due to the fact that the oxygen pressure reduces at higher altitude, the solubility of oxygen reduces with increase in altitude (or reduction of atmospheric pressure).

Q: How does salinity affect dissolved oxygen measurements?

A: Salinity has an inverse relationship to oxygen solubility. As the salinity of the solution increases, the solubility of oxygen reduces. Results in mg/l.

Q: Why is there a "warm-up time" with Jenway's dissolved oxygen probes and how long is it?

A: In Clark cells, an external polarising voltage of approximately 800 mV is applied to the electrode. When the probe is disconnected, the power supply is cut off. On connecting the probe again, the user must wait for the probe to be polarised i.e. for the current loop to be stabilised. This warm up time is approximately 10 minutes.